Polymer Gels for Water Control:
NMR and CT Scan Studies
Polymer Gels for Water Control:
NMR and CT Scan Studies
Proefschrift
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof.dr.ir. J.T. Fokkema,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen,
op maandag 8 september 2008 om 10:00 uur
door
Ghaithan Ahmad AL-MUNTASHERI
Master of Science in Chemical Engineering
King Fahd University of Petroleum & Minerals, Dhahran
Kingdom of Saudi Arabia
Dit proefschrift is goedgekeurd door de promotor: Prof. Dr. P.L.J. Zitha
Samenstelling promotiecommissie:
Rector Magnificus Voorzitter
Prof.dr. P.L.J. Zitha Technische Universiteit Delft, promotor Prof.dr. P.K. Currie Technische Universiteit Delft Prof.dr. S.J. Picken Technische Universiteit Delft
Dr.ir. J.A. Peters Technische Universiteit Delft Prof.dr. J. Bruining Technische Universiteit Delft
Prof.dr. H.A. Nasr-El-Din Texas A&M University, College Station, Texas, USA
Dr. R.S. Seright New Mexico Tech, Socorro, New Mexico, USA
Prof.dr. S.M. Luthi Technische Universiteit Delft, reservelid
The research described in this thesis was funded by Saudi Aramco. Most of this work was performed at the Dietz Laboratory, Faculty of Civil Engineering and Geosciences, Delft University of Technology, The Netherlands. The NMR measurements were performed at the Department of Applied Sciences of Delft University of Technology. The gas analysis experiments were performed at The EXPEC Advanced Research Center, Saudi Aramco, Dhahran, Kingdom of Saudi Arabia.
Copyright © 2008 G.A. Al-Muntasheri ISBN 978-90-9023224-9
Printed by Gildeprint B.V., Enschede, The Netherlands Cover design by A.G. Al-Shamrani
To:
My Parents
&
My Wife
This work resulted in the following publications:
PAPERS PUBLISHED IN REFEREED JOURNALS
Published:
Al-Muntasheri, G.A, Nasr-El-Din, H.A., Peters, J.A. and Zitha, P.L.J.: “Investigation of a High Temperature Organic Water Shut-off Gel: Reaction Mechanisms,” SPEJ, 11(4), pp. 497-504, 2006.
Al-Muntasheri, G.A., Nasr-El-Din, H.A., Peters, J.A. and Zitha, P.L.J.: “Thermal Decomposition and Hydrolysis of Polyacrylamide Co-tert-butyl Acrylate,” European Polymer J., 44, pp. 1225-1237, 2008.
Al-Muntasheri, G.A., Nasr-El-Din, H.A. and Zitha, P.L.J.: “Gelation Kinetics and Performance Evaluation of an Organically Cross-linked Gel at High Temperature and Pressure,” SPEJ, 13(3), published, 2008.
Accepted:
Al-Muntasheri, G.A., Zitha, P.L.J. and Nasr-El-Din, H.A.: “A New Organic Gel for Water Control: A Computed Tomography Study,” SPEJ, accepted for publication, 2008.
CONTRIBUTED PAPERS PUBLISHED IN CONFERENCE PROCEEDINGS
Al-Muntasheri, G.A., Nasr-El-Din, H.A., Peters, J.A. and Zitha, P.L.J.: “Investigation of a High Temperature Organic Water Shut-off Gel: Reaction Mechanisms,” paper SPE 97530 presented at the SPE International Improved Oil Recovery Conference in Asia Pacific held in Kuala Lumpur, Malaysia, 5–6 December 2005.
Al-Muntasheri, G.A., Nasr-El-Din, H.A. and Zitha, P.L.J.: “Gelation Kinetics of an Organically Cross-linked Gel at High Temperature and Pressure,” paper SPE 104071 presented at the First International Oil Conference and Exhibition held in Cancun, Mexico, 31 August-2 September 2006.
Al-Muntasheri, G.A., Nasr-El-Din, H.A., Al-Noaimi, K.R. and Zitha, P.L.J.: “A Study of Polyacrylamide-Based Gels Cross-linked With Polyethyleneimine,” paper SPE 105925 presented at the SPE International Symposium on Oilfield Chemistry held in Houston, TX, 28 February-2 March 2007.
Al-Muntasheri, G.A., Zitha, P.L.J. and Nasr-El-Din, H.A.: “Evaluation of a New Cost-Effective Organic Gel for High Temperature Water Control,” paper IPTC 11080 presented at the International Petroleum Technology Conference held in Dubai, UAE, 4-6 December 2007.
APPLIED TECHNOLOGY WORKSHOP
Al-Muntasheri, G.A., Nasr-El-Din, H.A. and Zitha, P.L.J.: “Insights and Development in Water Shut-off Gels for High Temperature Applications,” presented at the SPE Applied Technology Workshop of Chemical Methods of Reducing Water Production, San Antonio, Texas, 4-6 March 2007.
AWARDS
An appreciation letter from the Technology Vice President of Halliburton Energy Services was received in March 2008 for this research work.
Table of Contents
CHAPTER 1: INTRODUCTION
1.1 Water production... 1
1.2 Good vs. bad produced water... 3
1.3 Water production mechanisms ... 5
1.4 Chemical methods for remediation of excess water ... 7
1.5 Thesis objectives ... 8
1.6 Thesis outline ... 8
CHAPTER 2: POLYMER GELS FOR WATER CONTROL 2.1 Introduction...11 2.2 Polyacrylamide-based polymers ...11 2.3 Cross-linkers ...13 2.3.1 Inorganic cross-linkers ... 13 2.3.2 Organic cross-linkers ... 15 2.4 Thermal stability ...17
2.5 Choice of a suitable chemical system ...19
2.6 Summary ...20
CHAPTER 3: GELATION MECHANISMS OF THE POLYACRYLAMIDE tert-BUTYL ACRYLATE (PAtBA)/POLYETHYLENEIMINE (PEI) GEL SYSTEM 3.1 Introduction ...21
3.2 Scope and objectives...23
3.3 Experimental...23
3.3.1 Materials and methods ... 23
3.4 NMR measurements ...25
3.5.1 Polymers characterization
... 25
a. Analyses of NMR spectra ... 25
b. Viscosity measurements... 27
3.5.2 Effect of temperature on the degree of esterfication ... 28
3.5.3 Effect of temperature on the amide hydrolysis ... 32
3.5.4 Gelation tests ... 34
3.6 Summary ...38
CHAPTER 4: HYDROLYSIS KINETICS AND THERMAL DECOMPOSITION PRODUCTS OF PAtBA 4.1 Introduction ...39
4.2 Scope and objectives...39
4.3 Experimental...40
4.3.1 Materials and methods ... 40
4.4 Results and discussions ...42
4.4.1 13C NMR spectrum of PAtBA ... 42
4.4.2 Hydrolysis reaction of tert-butyl acrylate groups ... 45
4.4.3 Thermal decomposition of tert-butyl acrylate ... 47
4.4.4 Distribution of tBA on PAtBA... 50
4.4.5 Kinetics of hydrolysis and thermal decomposition reactions of tBA... 52
4.4.6 Kinetics of amide hydrolysis ... 54
4.4.7 Electrostatic effects ... 55
4.4.8 Structures of PHPA resulting from PAtBA hydrolysis/thermal decomposition... 59
CHAPTER 5: GELATION OF POLYACRYLAMIDE (PAM) CROSS-LINKED WITH PEI
5.1 Introduction ...61
5.2 Experimental studies...62
5.2.1 Materials ... 62
5.2.2 Methods ... 64
5.2.3 Determination of gelation time... 64
5.2.4 Equipment ... 65
5.3 Results and discussions ...65
5.3.1 Bottle tests ... 65
5.3.2 Effect of temperature on the gelation time... 67
5.3.3 Effect of salinity... 71
5.3.4 Effect of polymer concentration ... 72
5.3.5 Effect of the cross-linker concentration... 73
5.3.6 Effect of the initial degree of hydrolysis ... 75
5.3.7 Effect of initial pH... 78
5.4 Summary ...79
CHAPTER 6: GEL UNDER STRESS IN POROUS MEDIA 6.1 Introduction ...81
6.2 Theory...82
6.3 Experimental studies...87
6.3.1 Materials and methods ... 87
6.3.2 Porous media samples ... 88
6.3.3 Core-flow set-up and procedures ... 88
6.4 CT scan principles and apparatus ...91
6.4.1 CT scan principles ... 91
6.4.2 CT principles adapted for gel studies ... 92
6.5 Results and discussions ...94
6.5.1 Polymer adsorption... 94
6.5.2 CT-visualization of displacement of PAM/PEI gelants and weak gels... 96
a. Displacement of PAM/PEI gelants ... 96
b. Effect of shut-in temperature on the displacement of PAM/PEI weak gels and gelants ... 97
c. Pressure data ... 100
6.5.3 Yielding studies of strong PAM/PEI gels... 101
a. Pressure data... 101
b. Saturation profiles ... 102
6.5.4 Long term gel stability in porous media ... 105
6.6 Summary ...107
CHAPTER 7: GENERAL CONCLUSIONS General Conclusions ...109
Nomenclature ...113
Appendix A: Effect of Gelation Time on Gelants Placement in Porous Media.. ...117
Appendix B: Gel Displacement by Brine in Porous Media ...121
Appendix C: Adaptation of the Computed Tomography Technique for Gel Displacement Studies ...127
Appendix D: Beam Hardening Analysis...129
Appendix E: Optimization of Gel Strength for Yielding Studies in Porous Media...131
Bibliography ...135
Summary ...147
Samenvatting ...149
About the Author ...153
Chapter 1
Introduction
In this Chapter, the subject matter of the thesis work is introduced. A brief overview of worldwide water production is given. Available techniques for dealing with excessive water production are summarized. Then, the thesis objectives are stated. Finally, the outline of the thesis is provided.
1.1 Water production
Large volumes of water are produced in maturing oil and gas fields. A common term used in the oil industry to quantitatively describe water production is the water to oil ratio (WOR). WOR is defined by Equation 1: o w
Q
Q
WOR =
(1)where
Q
w andQ
o are the flow rates of water and oil in barrels/day, respectively. It has been reported that for each barrel of oil produced currently on a worldwide basis, there are three barrels of water produced (Bailey et al., 2000). In other words, from Equation 1 above, the average global WOR is 3. The excessive water production associated with the oil and gas operations imposes additional costs. It is estimated that the petroleum industry spends 40 billions of US dollars annually to deal with excessive water production (Bailey et al., 2000). Processes to handle produced water include: (1) lifting the excess water with the oil from the producing well (2) separation of the water from oil and gas in the gas oil separation plants (GOSPs) (3) the water needs to be de-oiled and filtered from suspended solids; finally, (4) water is often pumped back into water injection/disposal wells. Table 1.1 shows estimates of the costs per barrel of produced water for the mentioned processes during the water cycle. These data are based on a waterChapter 1: Introduction
rate of 100 thousands of barrels per day (MBPD) (Bailey et al., 2000). These costs include both Capital Expenditure (Capex) and Operational Expenditure (Opex).
Table 1.1: Average costs for processes involved in the water cycle based on a 100 MBWPD rate
(Bailey et al., 2000)
Process Cost, $/Barrel % of total production cost Lifting 0.098 20.5 Separation 0.072 15.0 De-oiling 0.097 20.0 Filtration 0.057 12.0 Pumping 0.125 26.1 Injection 0.03 6.4
Global water production data are summarized in Figure 1.1. The petroleum industry produced an average of 210 million barrels (33.4 million m3) of water per day (MMBWD) in the year 2000 (Bailey et al., 2000) which increased to 249.32 MMBWD (39.64 million m3) in 2005 (Khatib, 2007). For these rates, the WOR was fairly the same at a value of 3.0. Data from major oil companies on specific fields are not easy to access. However, a paper by Van Eijden et al. (2004) gives an idea about the water production within the Shell group. The authors reported that water production within the Shell group has increased from 2.2 million barrels (350, 000 m3) per day in 1990 to more than 6.3 million barrels (one million m3) per day in 2004 (Van Eijden et al., 2004). Data exist for specific wells in various fields operated by major oil companies. Wells in a large carbonate reservoir in Saudi Arabia (operated by Saudi Aramco) were reported to have 81% water cut (Mohammed et al., 1998). Kuwait Oil Company (KOC) and British Petroleum (BP)
Chapter 1: Introduction
reported a water cut of 80% for wells in Sabiriyah field in Kuwait (Clark et al., 2007). Total EP Qatar reported a water cut of 75% (WOR of 3) for Al Khalij field in Qatar (Pradie et al., 2007). In western Siberia, BP reported an average water cut of 80% (WOR of 4) for wells in the Ust Vakh field (Guerra et al., 2007).
It is clear from the data presented that large financial resources are devoted to handling/managing the excess water. With the growing demand on energy, there is a need for reducing the water production rates. That will avoid extra costs for the production of oil in addition to improving the oil recovery.
1.2 Good vs. bad produced water
Clearly, it is of very good economical value to minimize water production. The first question is which water needs to be blocked/minimized? In order to answer this question, a classification for good vs. bad water needs to be defined. Smith and Ott (2006) define good water as the water that brings a considerable amount of oil. For example, water used in injection wells can be considered as good water. A broader definition is “good water is any water that cannot be shut-off without affecting the oil production”. Bailey et al. (2000) reported a good illustrative example. Consider
Figure 1.1: Global water production data (Bailey et al., 2000 and Khatib, 2007)
0 50 100 150 200 250 300 2000 2003 2005 Year W a te r P roduc tion R a te , M M B W P D
Chapter 1: Introduction
an oilfield producing oil at a certain initial WOR. Figure 1.2 shows that as the field matures, the water production increases. This increase shown in cycle A is economically acceptable as long as the WOR economic limit is not reached. This limit shown as line B in Figure 1.2 is the limit beyond which the costs of producing oil are more than the profit made. In fact, the position of this economic limit (line B) on the WOR axis is also heavily dependent on the price of oil. Once, this limit is reached, there is a need to decrease the WOR. This is done by applying an improved oil recovery (IOR) method.
Figure 1.2: The effect of water control on improved oil recovery (Bailey et al., 2000)
This method should bring the WOR to lower values (under the economic limit) as shown in line C. Hence, the field produces again giving an additional recovery of oil (D). Therefore, improving the oil recovery increases the economic life of a certain oilfield. There are chemical and mechanical methods for the IOR process shown in cycle C of Figure 1.2. The use of a certain method depends on the nature of the water production mechanism. This will be discussed in the next section.
0 0 Oil production, bbl W O R ,
-WOR economic limit B D A C Added recovery 3
Chapter 1: Introduction 1.3 Water production mechanisms
The success of a water shut-off treatment strongly depends on the identification of the water production mechanism. Once the water production mechanism is known, a suitable solution can be proposed. In the following paragraphs, examples on water production problems will be given. Consider the well shown in Figure 1.3. In this well, there is a water cone reaching the perforated zone. This cone has brought a portion of water to a level above the Oil Water Contact (OWC).
This type of problem occurs when there is a relatively high vertical permeability and the well is produced at a high rate. If the cone is formed through a rock matrix, then, this type of problem is best solved by a mechanical method. For example, by re-perforating the well in the upper part of the oil producing zone at a distance well above the OWC. Another mechanical solution is to drill an additional drain hole into the top part of the oil layer. The use of a chemical method for solving matrix-water-coning problems requires a large volume of fluids. Also, even if the treatment is successful, water can re-establish flow by coning. For these reasons, chemical treatments were not recommended to solve this specific type of
Figure 1.3: Water coning in a vertical well Oil OWC Water Oil Perforations Well tubing Well casing
Chapter 1: Introduction
problem (Sydansk and Seright, 2007). In cases where the cone is formed by natural fractures or hydraulically induced fractures, a chemical treatment is more likely to give better results (Sydansk, 2007a). Figure 1.4 shows a well with fractures connecting the lower water layer to the perforated oil zone.
Figure 1.4: Excess water production due to natural fractures
When the fractures do not contribute to the oil production, they can be sealed by using a chemical treatment (a hydrophilic gel). Once set, the gel should be strong enough to resist extrusion through the fracture and the treatment volume should be large enough to seal the fracture completely. Another water production mechanism is shown in Figure 1.5 where water is produced from a bottom layer that is separated from the oil layer by shale. This is an open-hole wellbore with no vertical communication between the oil and water layers. A polymer gel is a more favourable solution here to seal the water producing zone.
In summary, the success rate of a water control job (chemical or mechanical) is strongly dependent on the correct characterization of the reservoir and the wellbore. The water production mechanism must be well-identified. This diagnosis will judge the type of treatment to be recommended.
Oil Perforations Well tubing Well casing Oil OWC Natural fracture Water Natural fracture
Chapter 1: Introduction
Figure 1.5: Water production from a bottom layer not in vertical communication with the oil layer
1.4 Chemical methods for remediation of excess water
Many methods for solving water production problems have been developed. This thesis concerns the investigation of polymer gels for reducing water production i.e., for water control in oil or gas wells. Hence, a description of this technology will be given in this section.
A polymer gel is based on a solution of water, a polymer and a cross-linker. This solution is prepared at the surface in batches or on the fly using flow mixers. Then, the solution (gelant) of a relatively low viscosity is injected through the wellhead or coiled tubing to the target zone. Note that in fractured reservoirs, a partially formed gel is injected (Sydansk et al., 2005). As the gelant flows to the target zone, a cross-linking (gelation) reaction takes place between the polymer and the cross-linker. This chemical reaction results in the formation of a 3-dimensional polymer network. The 3-dimensional structure should only form when the gel reaches the target zone. This structure is referred to as gel. The formed gel has the properties of an elastic solid. Once the well is put back on the production line, the gel forms a barrier to water flow. Different types of these gels and the nature of their chemistry will be discussed in the next Chapter. These polymer gel treatments
Oil Well tubing Well casing Oil OWC Water Shale Shale Water Oil
Chapter 1: Introduction
are referred to as conformance improvement treatments (CITs) (Sydansk, 1990). When totally blocking water is not feasible, other chemical technologies exist (partial blocking) where a chemical is used to decrease the permeability to water more than that to oil (Zaitoun and Kohler, 1988; Sydansk and Seright, 2007). This is done through the disproportionate permeability reduction effect (DPR) (Liang et al., 1995; Stavland et al., 2006). It should be mentioned that gels can also be applied in water injection wells to improve reservoir sweep efficiency. This is needed when oil is bypassed by the water due to high permeability channels/fractures. Hence, polymer gels are used to divert injected water flow into less permeable (less swept) layers.
1.5 Thesis objectives
There are many types of polymer gels available for use in the oilfield services market as will be reviewed in Chapter 2. Choosing a certain chemical for water control in a given oilfield depends on several factors like: permeability of treated formation, duration of the treatment, well type (producer vs. injection), reservoir temperature and formation brine salinity. We will identify a suitable polymer gel for high temperature water shut-off. The main objective of this work is to develop a fundamental understanding of the gelation mechanisms of this polymer gel system. Research devoted to the understanding of the gelation mechanisms of this system can also lead to the development of more cost-effective alternatives.
1.6 Thesis outline
This thesis is comprised of 7 Chapters. The first Chapter presented a general introduction of the subject matter. Chapter 2 gives a literature review of the available chemical systems for water control. Chapter 2 ends up with a choice of a chemical system for high temperature water shut-off. Most of the results and discussions of
Chapter 1: Introduction
this research are presented in Chapters 3 to 6. Chapter 3 gives insights into the gelation chemistry of the chemical system. Then, models governing the hydrolysis of the base polymer are given in Chapter 4. After that, a more cost-effective polymer is proposed. Chapter 5 investigates the thermal stability of the modified system in bulk as well as its gelation kinetics. The new system is examined in porous media in Chapter 6. Finally, the general conclusions of this work are reported in Chapter 7.
Chapter 2
Polymer Gels for Water Control
A brief survey of the existing polymer gels used in water control processes is given. The different types of polymers and cross-linkers are also presented. Based on the type of the cross-linker, a classification of polymer gels is made. An overview of the thermal stability of polymer gels is shown. The Chapter will conclude with the identification and discussion of a suitable gel system for use in high temperature environments, which will be studied in detail in this thesis.
2.1 Introduction
As mentioned before, polymer gels for water control are obtained by cross-linking a polymer in solution. The chemical gelling solution (gelant) is prepared by adding the polymer to water. Then, a linker is added to the solution. With temperature and time, a cross-linking (gelation) reaction takes place between the two components to form a 3-dimensional cross-linked polymer network. This structure (including the encapsulated water) is referred to as gel. Two main types of polymers have been used to prepare gels for water control. The first type is biopolymers and the second is synthetic polymers. Xanthan and scleroglucan are examples of biopolymers (Sydansk, 2007a). An example on the second class (synthetic polymers) is polyacrylamide (PAM). PAM is considered to be the most commonly applied type of polymers. Therefore, the discussion in this Chapter will focus on PAM and PAM-based copolymers. A description of PAM-based polymers and cross-linkers commonly used to reduce water production will be given in the following sections.
2.2 Polyacrylamide-based polymers
PAM has the chemical structure shown in Figure 2.1. It is a cost effective polymer which costs 2 to 4 $/kg (Morel et al., 2007). It can
Chapter 2: Polymer Gels for Water Control
be obtained in the form of a powder or an aqueous solution. With time and temperature, PAM undergoes auto alkaline hydrolysis producing partially hydrolyzed polyacrylamide (PHPA) and ammonia (NH3) according to the reaction shown in Figure 2.2.
Figure 2.1: Chemical structure of polyacrylamide
An important parameter to note from Figure 2.2 is the degree of hydrolysis of the PHPA. The degree of hydrolysis,
τ
, can be defined by the following equation:
x
y
y
+
=
τ
(1) wherey
is the molar concentration of the carboxylate groups on the PHPA chain in mol/l andx
is the molar concentration of the amide groups in mol/l. A method for determining this quantity,τ
, will be given in the next Chapter.Figure 2.2: Hydrolysis of amide groups at high pH conditions
The presence of the carboxylate groups (CH2CHCOO-Na+) is essential for the cross-linking reaction with certain cross-linkers. In addition,
τ
of the base polymer and its changes with temperature play a key role in the thermal stability of the produced gel. These aspects will be discussed thoroughly in the next sections.CH2 CH . C O CH2 CH . C O O H 2N CH2 CH . C O H2N CH . C O NH2 CH2 C H 3 n y x NaOH NH3 -Na+ + a CH2 CH . C O . NH2
Chapter 2: Polymer Gels for Water Control 2.3 Cross-linkers
The second key component of a gelling solution is the cross-linker. PAM-based polymers can be cross-linked either with inorganic or organic cross-linkers. In the next two subsections, commonly used cross-linkers will be reviewed.
2.3.1 Inorganic cross-linkers
Inorganically cross-linked polymer gels (ICP) result from the bonding between the negatively charged carboxylate groups on PHPA and the multivalent cation (Prud'homme et al., 1983; Sydansk, 1990; Sydansk and Southwell, 2000; Lockhart, 1991; te Nijenhuis et al., 2003). A widely used metallic cross-linker is Cr+3. The Cr+3/carboxylate cross-linking is believed to rely on coordination covalent bonding (Sydansk, 1990).
When deploying water control gels in high temperature reservoirs, there is a need to delay the cross-linking reaction to allow enough time for the gel to reach its target zone. One way to control the gelation time of Cr+3/polyacrylamide gelling systems is to introduce the chromium ion into solution as Cr+6. To the same solution, reducing agents like sodium thiosulfate (Na2S2O3) are added to reduce the Cr+6 (inert to polymer cross-linking) ion to the Cr+3 state. As a result, the cross-linking reaction takes place slowly, which enables field designers to place the gelling solution deeper into water producing zones of the reservoir. Another method of cross-linking reaction delay is to use complexes of Cr+3 like chromium acetate (Fulleylove et al., 1996) or chromium malonate (Lockhart et al., 1991). The degree of hydrolysis (
τ
) of PHPA can also be used to control gelation time. When a low degree of hydrolysis polymer is used, the cross-linking reaction can be delayed. PHPA/Cr+3 enjoyed a wide range of success in field applications. Sydansk and Southwell (2000) summarized a 12-yearsChapter 2: Polymer Gels for Water Control
experience of field applications with this system in oil and gas fields.
Note that Cr+3 can also be used to cross-link xanthan biopolymers (Jousset et al., 1990). Xanthan is not subject to shear degradation and does not loose viscosity in the presence of high salinity brines (Jousset et al., 1990). However, biopolymers are expensive and difficult to prepare for water shut-off field applications (Wellington, 1990).
One main disadvantage of the Cr+6-based cross-linkers is their relatively high toxicity. However, Cr+3 is much less toxic than Cr+6 and has been widely-applied in many parts of the world. Many systems utilizing chromium as a cross-linker (e.g., chromic chloride or chromic propionate) exhibit short gelation times at temperatures higher than 60oC (Lockhart, 1991; Albonico et al., 1994). In addition, Cr+3 can precipitate at high pH conditions. At elevated reservoir temperatures (more than 100oC), PHPA/Cr+3 gels (with low polymer concentrations) exhibit poor stability resulting in polymer hydrolysis and degradation of the polymer (Moradi-Araghi et al., 1987).
Table 2.1: Examples of inorganically cross-linked polymer gels
Polymer Cross-linker Reference
PHPA Aluminum citrate Al-Assi and Willhite, 2006 PHPA Chromium acetate Sydansk, 1990 PHPA Sodium dichromate Jordan et al., 1982 PHPA Zirconium lactate Omari et al., 2003
Other metallic cross-linkers used include aluminum added to polyacrylamide in the form of aluminum citrate. In high TDS water (> 30,000 ppm), the gel strength of this system was found to decrease substantially with time (Stavland et al., 1996). Titanium
Chapter 2: Polymer Gels for Water Control
and zirconium that have lower toxicity than Cr+3 metal ions are also used at times in near well-bore applications (Moffit et al., 1996). Several types of ICP systems are summarized in Table 2.1.
2.3.2 Organic cross-linkers
Organically cross-linked polymer gels (OCP) are known to be stable over a wider temperature range (Moradi-Araghi, 1991; Albonico et al., 1994; Hutchins et al., 1996; Hardy et al., 1999).This is possible because in this case the cross-linking is done via a covalent bonding which is more stable than coordinate covalent bonding and, hence, produces more thermally stable gels. The covalent bonds often involve the amide groups on the polymer backbone. A typical example of an OCP is the PAM/vinylpyrrolidone (PAM-VP) copolymer cross-linked with phenol and formaldehyde, which has been reported to be stable at 121oC for 13.3 years (Moradi-Araghi, 2000 and 1993). However, the toxicity of phenol and formaldehyde limited the broad use of this system in the field. Chemical alternatives for the phenol/formaldehyde system were also reported (Moradi-Araghi, 1994; Dovan et al., 1997). An OCP system which is based on high polymer loading was introduced (Morgan et al., 1997). This gel is based on the cross-linking of polyacrylamide/tert-butyl acrylate (PAtBA) copolymer with polyethyleneimine (PEI). PEI is reported to form stable gels (through covalent bonding) with different types of polyacrylamide-based copolymers. PEI is a polymer with a molecular weight of 70 kg/mol (Morgan et al., 1997). The chemical structure of PEI is shown in Figure 2.3.
Gels formed by the cross-linking of PEI are usually of high polymer loadings (7 wt%). Various PAM-based copolymers were reported to form stable gels when cross-linked with PEI. For
example, copolymers of acrylamide and
Chapter 2: Polymer Gels for Water Control CH2 CH . C O CH2 CH . C O O C CH3 CH3 CH3 NH2 x y
copolymers of acrylamide, AMPSA and N,N-dimethyl acrylamide (Vasquez et al., 2005).
Figure 2.3: Chemical structure of the PEI cross-linker
Figure 2.4 gives chemical structures of commonly used PAM-based polymers. PAtBA is the most widely applied copolymer with PEI.
PAM-NaAMPS PAM-VP
PAtBA
Figure 2.4: Structures of commonly used polyacrylamide copolymers
The ester groups on PAtBA (OC(CH3)3) are believed to be essential for cross-linking with PEI. Moradi-Araghi (2000) reported a review on field applications of various OCP gel. Examples on some OCP gels are given in Table 2.2.
CH2 CH . C O NH2 CH2 CH . C O NH C CH3 CH3 H2C SO3H x y CH2 CH . C O NH2 CH2 CH . N O x y 3 x CH NRR' H2C CH2 NR CH2 H2C NRR'
Chapter 2: Polymer Gels for Water Control 2.4 Thermal stability
It has been shown in the previous sections that gelant (solution of polymer and cross-linker) forms a gel in the pore space of the reservoir rock. When the treated well is brought back on production, water exerts a pressure on the blocking gel. Hence, the gel should withstand this pressure and block water flow for sufficient periods of time (6 months to several years). The temperature of the reservoir makes long term mechanical strength a challenge. It has been suggested that the hydrolysis of the base PAM polymer plays a key role in the thermal stability of the formed gel (Moradi-Araghi, 2000). At reservoir temperatures of 100 to 150oC, the amide groups on the PAM-based polymer undergo accelerated hydrolysis. Extensive hydrolysis causes degelation and/or syneresis (expulsion of the water from the gel structure).
Table 2.2: Examples of organically cross-linked polymer gels
Polymer Cross-linker Reference
PHPA Glyoxal Han et al., 1998
PAtBA Chitosan Reddy et al., 2003
PAM Hydroquinone/Hexamethyle- -netetramine (HQ/HMTA) Hutchins et al., 1996 PAM-NaAMPS & PAM –VP
Phenyl acetate/HMTA Moradi-Araghi, 1994
PAtBA PEI Morgan et al.,
1997
In fractured reservoirs, degelation and/or syneresis weaken the gel and reduce the effectiveness of the water shut-off treatment. Extensive hydrolysis becomes more serious in hard brines, with
Chapter 2: Polymer Gels for Water Control
high contents of Mg+2 and Ca+2 where polymer precipitation may also occur (Moradi-Araghi and Doe, 1987). This is because of the interaction between the negatively charged carboxylate groups on the PHPA chains with the positively charged divalent cations.
Polyacrylamide homopolymers were found to resist
hydrolysis and the subsequent precipitation at temperatures of 75oC (167oF) when used in brines containing 2000 ppm hardness levels (Moradi-Araghi and Doe, 1987). In order to enhance thermal stability of the base PAM homopolymer, other monomers are copolymerized with PAM. Adding these groups reduces tendency of hydrolysis (Moradi-Araghi et al., 1987; Doe et al., 1987). For
example, sodium-2-acrylamido-2-methylpropane sulfonate
(NaAMPS) is copolymerized with acrylamide (Moradi-Araghi et al., 1987). When PAM is copolymerized with NaAMPS at a weight ratio of 40/60 PAM-NaAMPS, the thermal stability of the copolymer is improved (structure is shown in Figure 2.4). It is reported that the resulting copolymer is efficient at 93oC (200oF) when used in seawater (total dissolved solids of 33,500 ppm) at a copolymer content of 1000 ppm (Moradi-Araghi et al., 1987). Another monomer was introduced to stretch the working temperature limit to 121oC (250oF) at the same salinity levels and polymer
concentrations. This monomer is vinylpyrrolidone (VP)
copolymerized with PAM at a weight ratio of 50 wt% (Doe et al., 1987). The presence of VP on the copolymer backbone enhanced the thermal stability of the copolymer (Figure 2.4). This is because of the considerably less conversion of amide groups to carboxylate groups by hydrolysis. Reported thermal stability data on the PAM-VP and PAM-NaAMPS are summarized in Table 2.3.
Another way of producing stable polymer gels is the use of higher polymer concentrations. When the PHPA concentration is increased to 7 wt%, PHPA/Cr+3 gels were reported to be stable at
Chapter 2: Polymer Gels for Water Control
temperatures up to 148.9oC (300oF) in Berea cores under pressure drops of 68.95 bars (1,000 psi) (Sydansk and Southwell, 2000). This indicates that stable gels can be obtained when higher polymer loadings are used.
Table 2.3: Summary of thermal stability data of PAM and PAM-based copolymers (1000 ppm of the polymer)
(Moradi-Araghi et al., 1987; Doe et al., 1987)
Polymer Type Temperature Limit, oC Salinity Level, ppm PAM 75 2,000 PAM-NaAMPS 93 33,500 PAM-VP 121 33,500
2.5 Choice of a suitable chemical system
The PAtBA/PEI system has been reported to be stable in porous media at 156oC for 2 months (Morgan et al., 1997). Studies of this system in porous media revealed its ability to propagate deeply into the rock matrix (Hardy et al., 1998). As a result, it enjoyed a lot of success worldwide as a total blocking system (Vasquez et al., 2006; Eoff et al., 2006). It can also be emulsified in oil and used as a selective water control agent by the disproportionate permeability reduction mechanism (DPR) (Stavland et al., 2006). Based on the positive testing results of the PAtBA/PEI OCP system for water control treatments, the PAtBA/PEI system will be investigated. Research will aim at the understanding of the gelation mechanism of the PAtBA/PEI gel system. This could be a chance for developing more cost-effective polymers that can be cross-linked by PEI. Note that PAtBA copolymer is not a very cost-effective option which costs around 7.7 $/kg.
Chapter 2: Polymer Gels for Water Control 2.6 Summary
The technology of polymer gels used in the water control operations was introduced. A classification for polymer gels based on the nature of the polymer/cross-linker bonding was reviewed. Examples were given on IOC and OCP gels. The Chapter concluded with the selection of the PAtBA/PEI OCP gel to be investigated in this thesis. Research in this thesis will aim at the understanding of the PAtBA/PEI system. The presence of the pendant ester groups on PAtBA has been believed to be essential for forming the stable gels of the PAtBA/PEI. The gelation mechanisms of the PAtBA with PEI will be the subject of the next Chapter. The understanding of the gelation mechanisms could open the door for new developments in this area. In other words, it might be possible to identify and/or develop more cost-effective polymers to be cross-linked with PEI. The effectiveness of any modified gel system is always a question that needs to be answered through performance assessment. Answers to these questions will be given in next Chapters. We will start by examining the gelation between PAtBA and PEI in the next Chapter.
Chapter 3
Gelation Mechanisms of the PAtBA/PEI
Gel System
This Chapter is devoted to the study of the gelation mechanisms of the chosen PAtBA/PEI system. Carbon-13 Nuclear Magnetic Resonance Spectroscopy (13C NMR) is used to elucidate structural changes on the PAtBA copolymer. Understanding the structural changes is a step towards better description of the gelation mechanism as will be discussed. [This chapter was published in a slightly different form in the Society of Petroleum Engineers’ Journal, 11(4), pp. 497-504, 2006]
3.1 Introduction
In Chapter 2, a suitable gel for high temperature water control was chosen. The gel is based on the cross-linking of polyacrylamide tert-butyl acrylate (PAtBA) copolymer with polyethyleneimine (PEI). This system was reported to be stable for 3 months at temperatures up to 156°C (312.8°F) (Morgan et al., 1997). It was first applied in a carbonate reservoir at nearly 130°C (266°F) and in sandstone reservoirs at 75 (167°F) and 82°C (179.6°F) (Hardy et al., 1999; Polo et al., 2004). Rheological studies concerning the gelation kinetics (Hardy et al., 1999; Al-Muntasheri et al., 2007a), viscoelastic properties of the final gel (Al-Muntasheri et al., 2007b), and gel strength in porous media (Zitha et al., 2002) of this system have also been reported. In addition, the performance of the PAtBA/PEI system in porous media was also examined under field conditions (Hardy et al., 1998; Okasha et al., 2001; Alqam et al., 2001; Vasquez et al., 2003). This chapter is concerned with the gelation mechanisms of the PAtBA with PEI. A gelation mechanism between PAtBA and PEI proposed by Hardy et al. (1999) involves the formation of covalent bonds between the carbonyl carbon at the ester group and an imine nitrogen from PEI (Figure 3.1). This mechanism was proposed to explain the gelation of the PAtBA and
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
PEI only at temperatures less than 95oC (295oF). Reddy et al. (2003) proposed a second mechanism (Figure 3.2) wherein the PEI nitrogens form covalent bonds with the carbonyl carbons at the amide group of PAtBA through a transamidation reaction.
Figure 3.1: Cross-linking reaction with PEI through ester carbonyl carbon (Hardy et al., 1999)
Figure 3.2: Cross-linking reaction with PEI through transamidation of the amide group (Reddy et al., 2003)
+ NRR' H2C CH2 NR CH2 H2C NRR' + . . C O NH2 C O NH2 . . n n x x n . . . C O NR H2C CH2 NR CH2 H2C NR C O . + 2R'NH2 + + . . C O O C CH3 CH3 CH3 n n x x n C O O . . C CH3 CH3 CH3 n NRR' H2C CH2 NR CH2 H2C NRR' . . C O NR H2C CH2 NR CH2 H2C . C O . NR + 2R'OC(CH3)3
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
In both cases the hydrolysis of the polymer is believed to play a key role in the gelation process, but detailed studies identifying either mechanism over the temperature range: 80 to 120oC (176 to 248oF) are lacking.
3.2 Scope and objectives
The objective of this Chapter is to investigate the gelation mechanisms of the PAtBA with PEI over the temperature range: 80 to 120oC (176 to 248oF). This is a first step towards a better understanding of the gelation of PAtBA/PEI system which will help developing more cost-effective polymers to be cross-linked by PEI. A non-ionic polyacrylamide (homopolymer) was also investigated, to provide a basis for comparison. Carbon-13 Nuclear Magnetic Resonance Spectroscopy (13C NMR) technique was used to reveal chemical and structural changes of the polymers, resulting from thermal treatment under alkaline conditions typical of those in PAtBA/PEI gelling solutions. This Chapter proceeds with the presentation of the experimental details and the discussion of results. Finally, a summary of the Chapter is given.
3.3 Experimental
3.3.1 Materials and methods
The copolymer (PAtBA) used to perform the experiments was supplied as an aqueous solution containing 20 wt% active material and was used as provided. The molecular weight of PAtBA disclosed by the supplier was in the range of 250-500 kg/mol. The second polymer was a polyacrylamide (PAM) having a molecular weight in the range of 250-500 kg/mol and was supplied as an aqueous solution with 20 wt% activity (as disclosed by manufacturer for both activity and molecular weight). It was supplied by SNF Floerger. The pH values of PAtBA and PAM were 4.1 and 4.0, respectively. The cross-linker was PEI with a molecular weight of 70
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
kg/mol and an active content of 30 wt% (as disclosed by the supplier). Since the pH of the neat PEI solution is high (pH=11.7), gelation of PAtBA/PEI occurs under alkaline conditions with pH values ranging from 9 to 11. Thus, hydrolysis of PAtBA was carried under alkaline conditions. A few drops of 1N NaOH solution were added to the polymer solution to adjust its pH to the desired value. Analytical grade tertiary butanol (tert-BUOH) with 99.99 wt% content was used to confirm the peak assignment in the NMR spectra as will be discussed later (in section 3.5.2).
Nitrogen gas was circulated through the polymer solutions before heating to remove oxygen, which is known to adversely affect polymer stability at high temperatures (Shupe 1981; Ryles 1983). A Lauda E100 heating bath was used with a digital temperature display having an accuracy of ± 0.1°C. Screwthread GL 18 SCHOTT type high thermal resistant glass tubes were used. They were sealed while heating.
Viscosity measurements were conducted using a Physica MCR 300 Rheometer. This rheometer is equipped with a high pressure cell that is only compatible with double gap cylinder geometry. The viscosity measurements were done on neat samples having a volume of nearly 2.3 cm3.
For the gelation tests reported in this Chapter, all gelling solutions were prepared at room temperature by adding the needed amount of polymer to distilled water. The solution was homogenized by stirring. Then, the needed amount of PEI was added. It should be mentioned that all gels prepared in this work contained 7 wt% of polymer and 0.3 wt% of PEI. In addition, initial pH values were around 10.5 for all gelling solutions. This is due to the high pH value of PEI. Then, gelling solutions were transferred into the Screwthread GL 18 SCHOTT high temperature glass tubes. All gels were observed while at temperature.
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System 3.4 NMR measurements
The available methods for determining the degree of hydrolysis of polyacrylamide copolymers were reviewed by Taylor and Nasr-El-Din (1994). 13C NMR spectroscopy was found to be the most suitable for this study because of its accuracy (despite the relatively low abundance of 13C).
13C NMR spectra were collected at 75.48 MHZ using a Varian Unity-300 spectrometer. Spectra were collected with reference to p-dioxane at 67.7 ppm. D2O was used as a lock signal. All data were collected at 25oC. Additional experimental parameters included the use of 4 seconds as a delay time, a 90o pulse flip angle (corresponding to a pulse width of 10 µs), 1000 scans and an acquisition time of 1 second. In this work, Nuclear Overhauser Enhancement (NOE) was overcome by using inverse-gated decoupling. Although methine carbons have lower relaxation times than the carbonyl carbons (Halverson et al., 1985; Al-Makhshi et al., 1994), the use of carbonyl carbons has been reported for quantitative determination of the degree of hydrolysis of polyacrylamides and their copolymers (Hutchinson and McCormick, 1986; Moradi-Araghi et al., 1988).
3.5 Results and discussions 3.5.1 Polymers characterization a. Analyses of NMR spectra
Peak intensities and chemical shifts were determined by deconvolution of the experimental spectra using Lorentzian line functions. Then, the degree of esterfication, E, and the degree of hydrolysis, τ, were determined through the following equations, respectively:
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
100
)
(
)
(
)
(
)
(
2 3 3 3 3 ) ( ) (×
+
+
=
= = = = NH O C CH OC O C OH O C CH OC O CI
I
I
I
E
(1)100
)
(
)
(
)
(
)
(
2 3 3) (×
+
+
=
= = = = NH O C CH OC O C OH O C OH O CI
I
I
I
τ
(2) where 3 3) ()
(
I
C=O OCCH is the resonance peak intensity of the carbonyl carbon at the tert-butyl acrylate group (176 ppm),2
)
(
I
C=O NH is the intensity of the carbonyl carbon at the amide group (177-180 ppm) and(
I
C=O)
OH is the intensity of the carbonyl carbon of the carboxylate group (181-184 ppm). The amide and carboxylate chemical shifts assignment agree with those reported in literature (Halverson et al., 1985; Moradi-Araghi et al., 1988). We are not aware of any earlier reports on the chemical shifts of the tert-butyl acrylate copolymerized with acrylamide. For this reason, the PAtBA is compared to other chemicals, which are analogous to PAtBA from a structural point of view. One such chemical is the sodium-2-acryalamido-2-propanesulphonate NaAMPS, copolymerized with acrylamide. For this copolymer, the peaks at 177.5-175.5 ppm were assigned to the carbonyl carbon at the NaAMPS group (Parker and Lezzi, 1993) which is consistent with the values assigned for the tert-butyl acrylate. Clearly, the different chemical nature between PAtBA and NaAMPS-PAM could make the comparison argument somewhat speculative, yet it is informative.The degree of esterfication (mol% of tert-butyl acrylate) of PAtBA was determined by 13C NMR using Equation 1 and was found to be 4.7 mol%. Equation 2 was used to reveal that this polymer has a degree of hydrolysis less than 0.1 mol%. The degrees of esterfication and hydrolysis of the raw PAtBA agree with those given in the literature (Reddy et al., 2003). The degree of hydrolysis
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
of PAM was less than 0.1 mol% as measured by 13C NMR using Equation 2 with 3 3) (
)
(
I
C=O OCCH= 0
. b. Viscosity measurementsFigures 3.3 and 3.4 show the viscosity of neat solutions of PAtBA and PEI, respectively, as a function of shear rate at different temperatures.
Figure 3.3: PAtBA viscosity vs. shear rate at different temperatures
Figure 3.4: PEI viscosity vs. shear rate at different temperatures
Due to the low molecular weights of PAtBA and PEI, there is no shear thinning behavior noted over the shear rates examined (10 to 1,000 s-1). The Andrade-Eyring equation can be used to describe the viscosity dependence of polymer solutions on temperature. The Andrade-Eyring is shown in Equation 3 below (Macosko, 1994):
0.01 0.1 1 10 10 100 1000 25 40 60 80 100 120 V isco si ty, P a. S Shear Rate, 1/s Temperature, oC PAtBA Pressure = 30 bars 0.01 0.1 1 10 10 100 1000 25 40 60 80 100 120 Temperature, oC V is c o s it y , P a .S Shear Rate, 1/s PEI Pressure = 30 bars
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
=
RT
E
C
ηη
0exp
(3)where
η
0 is viscosity in Pa.s,C
is a constant in Pa.s,E
η is the activation energy of viscous flow,R
is the universal gas constant in J/ (mol.K) andT
is the absolute temperature in oK. From Equation 3, a plot ofln(
η
0)
vs.1
/
T
should give a straight line whose slope isR
E
ηand an intercept of
ln(C
)
. The viscosity data of PAtBA are plotted as a function of1000
/
T
in Figure 3.5. Viscosity decreases with temperature with anE
ηof 21.9 kJ/mol. Similarly, viscosity of PEI decreases with temperature with anE
η of 27.0 kJ/mol. These values ofE
η are comparable to those reported in literature for similar polymer solutions (Macosko, 1994).Figure 3.5: Dependence of PAtBA viscosity on temperature 3.5.2 Effect of temperature on the degree of esterfication
In the mechanism shown in Figure 3.1, the presence of ester groups on the polymer backbone is essential for the nucleophilic attack of the imine nitrogen on PEI to form a cross-linking point. Hence, we start by examining hydrolysis of the ester groups. Figure 3.6 shows the degree of esterfication as a function of heating time
PAtBA Pressure = 30 bars Shear Rate = 57 s-1 Viscosity = 8E-05e2.6348(1000/T) 0.01 0.1 1 2.5 2.7 2.9 3.1 3.3 3.5 1000/Temperature, 1/K V is c o s it y , P a .s
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
at 80 and 105oC. These data are for PAtBA (20 wt%) samples with an initial pH value of 11.2. 13C NMR spectra were collected at 25oC. The data show that ester hydrolysis is about seven times much faster at 105
Figure 3.6: Ester hydrolysis at 105 and 80oC
than at 80oC. No traces of tert-butyl acrylate were observed after 6 and 43 hours of heating at 105 and 80oC, respectively. The fact that it took nearly 43 hours to decrease the degree of esterfication to zero is a possible explanation for the long gelation times encountered for this system at temperatures less than 100oC (Hardy et al., 1999; Al-Muntasheri et al., 2007a). This result also validates the mechanism depicted in Figure 3.1, where the cross-linking reaction takes place through the nucleophilic attack of the imine nitrogen (on PEI) on the carbonyl carbon of tert-butyl acrylate. Figure 3.7 shows the degree of esterfication as a function of time at 120oC. At this temperature, the tert-butyl acrylate is totally hydrolyzed in 70 minutes only. It is clear that the hydrolysis of tert-butyl acrylate strongly depends on temperature (Figure 3.8). According to the hydrolysis reaction of the tert-butyl acrylate shown in Figure 3.9, tertiary butanol (tert-BUOH) is produced from this reaction. Figure 3.10 shows a portion of the 13C NMR spectrum of
PAtBA Test Temperature = 25 oC Initial pH =11.2 0 1 2 3 4 5 0 10 20 30 40 50
Heating Time, hours 80 105 Series3 E s te rf ic a ti o n D e g re e , m o l% HeatingTemperature, oC
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
a PAtBA sample heated for 53 hours at 80oC under alkaline conditions. Peak assignment of the polymer backbone agrees with chemical shifts reported in literature for poly-
Figure 3.7: Ester hydrolysis at 120oC
-acrylamide polymers (Inoue et al., 1983; Truong et al., 1986). The two peaks observed at 30 and 70 ppm were assigned to the methyl and tertiary carbons of tert-BUOH, respectively. In order to confirm this assignment, a few drops of pure tert-BUOH were added to the same sample of Figure 3.10 and then 13C NMR data were collected again. The intensity of the peaks at 30 and 70 ppm increased accordingly as shown in Figure 3.11 supporting the assignment to tert-BUOH. Moreover, tert-BUOH concentration increases with increasing extent of ester hydrolysis.
Figure 3.8: Ester hydrolysis completion time as a function of temperature
The initial ratio of the intensity of C=O to that of the methyl carbon in the tert-BUOH (30 ppm) was 40. As the hydrolysis reaction
PAtBA Test Temperature = 25 oC Initial pH =11.2 0 1 2 3 4 5 0 20 40 60 80 100
Heating Time, minutes
E s te rf ic a ti o n D e g re e , m o l% HeatingTemperature = 120 oC 0 10 20 30 40 50 80 100 120 T im e , h o u rs Temperature, oC PAtBA Initial pH = 11.2
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
proceeds, this ratio decreased to less than 20 at the temperatures investigated. It has been reported that at temperatures larger than 100oC, tert-butyl acrylate produce isobutene gas (Figure 3.12) (Hardy et al., 1999). However, its presence could not be confirmed by 13C NMR measurements. Nevertheless, isobutene has a low boiling point (-6.9oC) at atmospheric conditions and could have evaporated from the heated samples (Lide, 2004). Detailed insights into the gases produced from the PAtBA reactions will be highlighted in Chapter 4.
Figure 3.9: Hydrolysis of tert-butyl acrylate group at high pH (Hardy et al., 1999)
Figure 3.10: 13C NMR spectra of PAtBA heated at 80oC for 53 hours OH C CH3 CH3 CH3 1 1 1 1 3 CH2 CH . C O NH2 CH2 CH . C O O C CH3 CH3 CH3 2 2 2 4 4 p -D io x a n e 30 30 40 40 50 50 60 60 70 70 Chemical Shift, ppm 3 3
+
. . C O O C CH3 CH3 CH3 m . . C O O m NaOH OH C CH3 CH3 CH3 . -Na+Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
Figure 3.11: 13C NMR spectra of the sample in Figure 3.10 with a few drops of
raw tert-BUOH
3.5.3 Effect of temperature on the amide hydrolysis
The above discussion focused on the hydrolysis of the ester groups. However, the 13C NMR spectra indicate also the progressive
transformation of the PAtBA into partially hydrolyzed
polyacrylamide (PHPA), which is due to the hydrolysis of the amide groups.
Figure 3.12: Thermolysis of tert-butyl acrylate group at high temperature (Hardy et al., 1999)
This is shown in Figure 3.13 where the total degree of hydrolysis is plotted as a function of time for 80, 105, and 120oC. These data were collected over a week time. PAtBA samples with the pH
+ . . C O O C CH3 CH3 CH3 m . . C O . m C CH2 H3C H3C NaOH O -Na+ OH C CH3 CH3 CH3 1 1 1 1 3 CH2 CH . C O NH2 CH2 CH . C O O C CH3 CH3 CH3 2 2 2 4 4 p -D io x a n e Chemical Shift, ppm 3 3 30 30 40 40 50 50 60 60 70 70
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
adjusted to 11.2 were heated for 8 to 12 hours over one week. 13C NMR spectroscopy was conducted on these samples.
Figure 3.13: PAtBA degree of hydrolysis as a function of time
Then, their spectra were analyzed according to the procedure outlined earlier (in section 3.5.1a). These data show that at 80oC, the degree of hydrolysis increases with time and then levels off to a plateau value of about 9.5 mol%. At 105oC the degree of hydrolysis increases with time and the tendency to a plateau is less pronounced. In this case, the degree of hydrolysis reached at the end of the week (after 168 hours) is nearly 19.6 mol%. At 120oC, the rate of increase of the degree of hydrolysis is higher and the corresponding maximum degree of hydrolysis is equal to 40 mol%; the lines in Figure 3.13 are drawn to guide the eye. Insights into the hydrolysis kinetics of the PAtBA and investigations of the produced gases will be presented in Chapter 4.
Interestingly, the data in Figure 3.13 suggest that hydrolysis of the amide group as shown in Figure 3.14, proceeds after the hydrolysis of the ester groups has been completed. This is particularly true for the 105 and 120oC cases where the hydrolysis of the ester is completed after 6.0 and 1.1 hours, respectively (Figures 3.6 and 3.7). Hydrolysis beyond these times can be
PAtBA Test Temperature = 25 o C Initial pH =11.2 0 10 20 30 40 50 0 40 80 120 160
Heating Time, hours
80 105 120 Heating Temperature, oC D e g re e o f H y d ro ly s is , m o l %
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
attributed to the amide groups. The situation is more intricate for the 80oC case where the total degree of hydrolysis was nearly 6.5 mol% after the complete hydrolysis of the ester and only increased to about 9.5 mol% after one week of thermal treatment.
Figure 3.14: Hydrolysis of amide groups at high pH conditions
The above discussion supports the idea that the transformation of PAtBA into PHPA involves the hydrolysis of both the ester and the amide groups. Further analysis is needed, however, to separate the contributions of hydrolysis of each group. Our observations are in good qualitative agreement with previous studies of the hydrolysis of the NaAMPS and vinylpyrrolidone/poly- -acrylamide copolymers using titration methods (Moradi-Araghi et al., 1987; Doe et al., 1987), reporting similar trends of the total hydrolysis degree with temperature.
3.5.4 Gelation tests
Based on the above discussion, an attempt was made to contribute to the validity of the cross-linking mechanisms presented in the introduction section. Three polymer samples (neat PAtBA) were heated at 80, 105 and 120oC for a sufficiently long time to ensure complete hydrolysis of the tert-butyl acrylate groups. In order to show how the zero esterfication degree was obtained, Figures 3.15 through 3.18 are presented. Figure 3.15 shows the 13C NMR spectrum of the raw PAtBA with an esterfication degree of 4.7 mol%. Figures 3.16 through 3.18 show spectra of the heated samples at 80, 105 and 120oC, respectively. It is clear from the
CH2 CH . C O CH2 CH . C O O NH 2 CH2 CH . C O NH2 CH . C O NH2 CH2 C H 3 n y x NaOH NH3 + -Na+
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
spectra of these samples that their degree of esterfication decreased to zero where the peak at 176 ppm disappeared.
Figure 3.15: 13C NMR Spectrum of raw PAtBA in the carbonyl region
(E = 4.7 mol%)
Figure 3.16: 13C NMR Spectrum of PAtBA heated for 1.2 hours at 120oC with
an initial pH of 11.2 (E = 0 mol%) 174 174 176 176 178 178 180 180 182 182 184 184 186 186 188 188 190 190 192 192 194 194 196 196 -Na+ CH2 CH C O CH2 CH HC C O O NH2 CH2 CH C O O C CH3 CH3 CH3 . 2 3 3 2 Chemical Shift, ppm Chemical Shift, ppm 1 2 174 174 176 176 178 178 180 180 182 182 184 184 186 186 188 188 190 190 192 192 194 194 196 196 198 198 -Na+ CH2 CH C O CH2 CH HC C O O NH2 CH2 CH C O O C CH3 CH3 CH3 . 2 1
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
Figure 3.17: 13C NMR Spectrum of PAtBA heated for 6 hours at 105oC with an
initial pH of 11.2 (E = 0 mol%)
Figure 3.18: 13C NMR Spectrum of PAtBA heated for 43 hours at 80oC with an
initial pH of 11.2 (E = 0 mol%)
As shown in Table 3.1, the first sample was a PAM with no thermal treatment. Second, third and fourth samples were PHPA resulting from PAtBA after the complete hydrolysis of the ester at 80, 105 and 120oC, respectively. Then, the polymer samples were cross-linked with PEI at the same temperature of hydrolysis. All of these gelling solutions contained 7 wt% of polymer and 0.3 wt% of PEI.
Chemical Shift, ppm 2 2 3 -Na+ CH2 CH C O CH2 CH HC C O O NH2 CH2 CH C O O C CH3 CH3 CH3 . 3 2 174 174 176 176 178 178 180 180 182 182 184 184 186 186 188 188 190 190 192 192 194 194 196 196 198 198 Chemical Shift, ppm 2 -Na+ CH2 CH C O CH2 CH HC C O O NH2 CH2 CH C O O C CH3 CH3 CH3 . 3 2 2 3 174 174 176 176 178 178 180 180 182 182 184 184 186 186 188 188 190 190 192 192 194 194 196 196 198 198
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
Visual observations indicated the formation of rubbery gels for both PAtBA and PAM cross-linked with PEI. The gels were stable when left under the same temperature and observed over a four days period. Reddy et al. (2003) reported the formation of lipping gels when cross-linking PHPA with chitosan at 87.7oC that is cross-linking as per the mechanism shown in Figure 3.2.
Table 3.1: Polymers used in gel studies
Initial Polymer Heating Time, hr T, oC τ, mol% E, mol% PAM - - <0.1% - PAtBA 72 80 7-9 0 PAtBA 24 105 7 0 PAtBA 24 120 12 0
Since, the interaction of PEI with the carboxylate anions on PHPA is weak and breaks completely at 90oC (Hardy et al., 1998), the gels formed at 105 and 120oC reported in this study were not the result of such interaction nor through the mechanism shown in Figure 3.1. This suggests that the mechanism in Figure 3.2 is the mechanism governing the gelation reaction in this case. Additional work is needed to examine the long term stability for PAM/PEI gels
Chapter 3: Gelation Mechanisms of the PAtBA/PEI Gel System
and their gelation time as a function of temperature. This will be discussed in Chapter 5.
3.6 Summary
Chemical and structural modifications in PAtBA copolymers that result from thermal treatments were revealed for the first time by 13C NMR spectroscopy. Quantitative analysis shows that thermal treatment reduces the degree of esterfication of the polymer. The rate of PAtBA copolymer hydrolysis in alkaline conditions increases significantly with temperature. At the temperature range considered in this Chapter, the hydrolysis of the ester group in the PAtBA produces tertiary butyl alcohol. The cross-linking of PHPA with PEI forms gels which are stable for several days over a range of temperatures. All gels prepared in this study were the result of the transamidation reaction shown in Figure 3.2, rather than the nucleophilic substitution shown in Figure 3.1.
Chapter 4
Hydrolysis Kinetics and Thermal
Decomposition Products of PAtBA
In this Chapter, the investigation of the hydrolysis and thermal decomposition of PAtBA is reported over a temperature range. Gas Chromatography (GC) is used to analyze gas reaction products of the PAtBA. Hydrolysis kinetics of PAtBA is also examined. The distribution of the tBA groups on the PAtBA copolymer chains was examined. [This chapter was published in a slightly different form in the European Polymer Journal, 44, pp. 1225-1237, 2008]4.1 Introduction
In the last Chapter, attention was paid to the gelation reaction between PAtBA and PEI. Hydrolysis of the PAtBA plays a key role in the elucidation of the reaction mechanism of the PAtBA with PEI because the presence of the tBA groups on the PAtBA is crucial to distinguish one mechanism from the other. Hence, detailed studies on the PAtBA hydrolysis kinetics and thermal decomposition products over a wide temperature range are lacking. To the best of the author’s knowledge, the only study reporting on the possible hydrolysis and thermal decomposition of PAtBA was that of Hardy et al. (1999). These authors proposed isobutene gas as a thermolysis product of PAtBA at temperatures greater than 100oC. It was also proposed that hydrolysis of the PAtBA produces tert-butanol (tert-BUOH) at temperatures less than 100oC under initial acidic conditions. However, direct evidence to support these reactions is lacking.
4.2 Scope and objectives
The objectives of the present Chapter are to: (1) predict the hydrolysis kinetics of the PAtBA under initial alkaline conditions over a wide temperature range (2) identify the thermal decomposition products of PAtBA as a function of temperature, and
