Th P3 04
Study of Shale Wettability for CO2 Storage
N. Shojai Kaveh* (Delft University of Technology), A. Barnhoorn (Delft University of Technology), F.C. Schoemaker (University of Amsterdam) & K.H.A.A. Wolf (Delft University of Technology)SUMMARY
For a water-saturated cap-rock, which consists of a low-permeability porous material, the wettability of the reservoir rock-connate water- CO2 system and the interfacial tension (IFT) between CO2 and connate water are the significant parameters for the evaluation of the capillary sealing. Also, the amount of capillary-trapped CO2 depends on the wettability of reservoir rocks. The wettability of the rock matrix has a strong effect on the distribution of phases within the pore space and thus on the entire displacement mechanism and storage capacity. In this work, the equilibrium contact angles of water/shale system were determined with CO2 for a wide range of pressures at a constant temperature of 318 K by using the dynamic captive bubble method. The results reveal that intermediate-wet conditions and hence possible leakage of CO2 must to be considered at relatively high pressures, however, the salt concentration of the water in the shales plays an important role too. The results show that this estimate is highly dependent on the pore structure, fluid composition and pressure/temperature conditions of the reservoirs. These properties need to be first evaluated before estimates for shale capillarity is used.
1. Introduction
Over the last decades, concerns about greenhouse gas emissions and global warming have been risen (IPCC 2005), as greenhouse gases increase the average temperature of the earth. To control CO2
emissions different options are proposed, such as capturing and storing carbon dioxide (CO2) in
geological formations (CCS). Storage strategies include CO2 injection into deep saline aquifers,
(depleted) gas and oil reservoirs, and unmineable coal seams.
In deep saline aquifers and oil/gas reservoirs, CO2 injection takes places at depths of over 800 m,
where CO2 is in a liquid or supercritical phase. Here, CO2 density is about 50% to 80% of the water
density, which makes it more buoyant than other liquids present in the pore space (IPCC 2005). Therefore, CO2 penetrates through the porous medium until it reaches the top of the formation where
it is trapped by an impermeable layer of cap-rock (structural/stratigraphic trapping). The hypothesis is that the caprock acts as the primary seal to prevent undesirable migration and leakage (Chiquet et al. 2007). However, capillary leakage occurs when the pressure in the CO2-rich phase increases above the
minimum pressure which is required to initiate the displacement of brine within the caprock. Also a part of the CO2 moves through the porous medium and displaces reservoir fluid from the pores. Some
of the CO2 remains in the pore spaces as residual bubbles which are immobile (residual/capillary
trapping). Other trapping mechanisms are mineral trapping, where CO2 reacts with minerals in the
rock, and solubility trapping, where CO2 dissolves in the aqueous phase. In general, different trapping
mechanisms occur simultaneously (Basbug et al. 2007, Salimi et al. 2012, Shariat et al. 2012).
For a secure CO2storage, capillary trapping is one of the important mechanisms which occurs when
CO2 comes to be immobile in the rock pores by capillary forces. This mechanism involves partly
during the injection process, in residual trapping and also as a result of solubility trapping over the time. For a brine-saturated cap-rock, which consists of a low-permeability porous material, the wettability of the reservoir rock-connate water- CO2 system and the interfacial tension (IFT) between
CO2 and connate water are the significant parameters for the evaluation of the capillary-sealing
(Chiquet et al. 2007). Therefore, wettability evaluation is a step towards understanding the physics of this phenomena. Also, the amount of capillary-trapped CO2 depends on the wettability of reservoir
rocks. The wettability of the rock matrix has a strong effect on the distribution of phases within the pore space and thus on the entire displacement mechanism and storage capacity. Hence, reduction of the interfacial tension (IFT) may result in the mobilization of connate water (capillary trapping). Precise understanding of wettability behaviour is therefore fundamental when injecting CO2 into
geological formations to sequestrate CO2 and/or to enhance gas/oil production.
In the literature, a large amount of data related to research with respect to CO2 storage in depleted gas
reservoirs and aquifers can be found. However, there are a limited amount of literature data with focus on wetting properties of aquifers and gas reservoirs at high pressures and elevated temperatures (Alotaibi et al. 2010, Chalbaud et al. 2007, Chiquet, Broseta and Thibeau 2007, Chiquet et al. 2005, Chiquet et al. 2007, Espinoza and Santamarina 2010, Jiamin et al. 2011). Previous measurements by Chiquet et al. ( 2005) have shown that CO2 under geological pressures can alter the wettability from
water-wet to intermediate-wet. Thereby making it possible that leakage occurs, especially over longer time frames (periods of several thousands of years). Although the permeability of the shale is low, carbon dioxide breakthrough can still happen. Similar to a cleat system (Shojai Kaveh et al., 2011 and 2012), molecular diffusion of water in CO2 is small, while for a CO2 filled system it will be larger
(gas-wet system). Therefore, using a representative reservoir sample besides a reliable experimental procedure can provide valuable experimental data regarding the wettability behaviour in reservoir condition, which can be directly used as input for reservoir simulation.
The main purpose of this work is to examine the wettability of the shale with water and CO2 at in-situ
conditions, i.e. high pressures and elevated temperature. To this end, contact-angle measurements were conducted at a constant temperature of 318 K and at pressures varying between 0.2 and 15 MPa. To obtain a reliable and reproducible contact angle data, all experiments were conducted with fully-saturated aqueous phase, which exclude the effect of composition change and CO2 dissolution during
the measurements (Ameri et al. 2013, Shojai Kaveh et al. 2014). When the aqueous phase is not completely saturated with CO2, the injectivity and the gas distribution in the reservoir are not only
influenced by the rock properties but also by the diffusion of CO2 into the aqueous phase (Shojai
Kaveh et al. 2011, Shojai Kaveh et al. 2012). 2. Methodology
To study the wettability of the shale system, the pendant drop setup was adapted to perform captive-bubble contact-angle measurements (Figure 1). This setup makes it possible to measure CO2
-water-clay contact angles at high pressures and elevated temperatures. To avoid the temperature fluctuations, the entire set-up is placed in an insulated oven. At the start of the experiment the cell is filled with tap water in which a CO2 bubble is injected through a needle at the bottom of the set-up.
The visual observation of the bubble and substrate inside the pendant drop cell is possible through the glass windows on both sides of the cell. The detailed description of the experimental setup and procedure can be found at Shojai Kaveh et al. (2014).
Figure 1 Schematic view of the pendant drop cell for measuring water-CO2-shale contact angles
For contact angle measurement, the Muschelkalk Röt/Solling shale sample from the well Q4-8 in North Sea was used. Each slab has dimensions of 30 × 12 × 6.0 mm3.
3. Results and Discussion
Contact angle measurements (Figure 2(a)) show that consolidated shale from well Q4-8 at 2461m depth is water-wet since the contact angles are well below 90°. However, the pressure dependence of contact angle values on shale shows a gradual transition to more intermediate-wet configurations. Therefore, high pressures (>>140 bar), i.e. large burial depths may make shales eventually intermediate-wet. The reported data from Chiquet et al. (2005) also shows intermediate-wet configurations at high salt concentrations and at high pressures. From the capillary measurements, we also observed that the imbibition of shale (decreasing the amount of water saturation) and the above lying drainage curve (increasing the water saturation) indicate that the system is water-wet.
Accordingly, the results reported here would indicate that intermediate-wet conditions and hence possible leakage of CO2 must to be considered at relatively high pressures, however the salt
concentration of the water in the shales plays an important role too (Figure 2 (b)).
PDͲcell Rock Needle
Figure 2 (a) Contact angle of CO2/water/shale system as a function of pressure at a constant temperature of 318 K, and (b) Advancing and receding contact angles of quartz in brines for various salt contents as measured by Chiquet et al. ( 2005) compared with Q4-8, 2461m shale in tap water.
Using the contact angle values from the contact angle experiments, interfacial tension of the water-CO2 system from Shojai Kaveh et al. (2013), and a pore throat size in the order of ~40 nm (e.g.
Barnett shales), results in capillary pressure in the range of 5-10 bars. Thus, at reservoir pressure the contact angle may be significantly larger approaching an intermediate-wet system (angles close to 90°) which results in a small reduction in capillary entry pressure. For shales with very small pore throats (<10 nm), the capillary entry pressure may increase to ~50 bar for a 5 nm pore and 250 bar for a 1 nm pore throat. Taking into account the different methods for predicting capillary entry pressures for shales, it can be concluded that the capillary entry pressure is expected to be from ~5 bars to a couple of tens of bars. However, the results also show that this estimate is highly dependent on the pore structure, fluid composition and pressure/temperature conditions of the reservoirs. These properties need to be evaluated first before estimates for shale capillarity is used. Fluid flow into the cap rock is thus expected when the pressure difference between the water-rich shale and the CO2-rich
reservoir is in the range of 10’s of bars. Significantly below these pressure differences, the capillarity would prevent flow of CO2 into the cap-rock system.
Conclusions
In this work, the equilibrium contact angles of water/shale system were determined with CO2 for a
wide range of pressures at a constant temperature of 318K. The contact angle measurements were conducted with tap water fully saturated with CO2 to eliminate the effect of any changes in the
composition of the aqueous phase and to minimize the dissolution effect. The results reported in this study reveal that intermediate-wet conditions and hence possible leakage of CO2 must to be
considered at relatively high pressures, however the salt concentration of the water in the shales plays an important role too. The results also show that this estimate is highly dependent on the pore structure, fluid composition and pressure/temperature conditions of the reservoirs. These properties need to be first evaluated before estimates for shale capillarity is used.
Acknowledgements
The research reported in this paper is carried out as a part of the CATO2 project (CO2 capture,
transport and storage in the Netherlands). This research is conducted in the Laboratory of Geoscience
and Engineering at Delft University of Technology. Our grateful thanks to the technical staff of the Laboratory, particulary J. Etienne, M. Friebel, K. Heller and J. van Meel.
References
Alotaibi, M. B., Azmy, R. M. and Nasr-El-Din, H. A. [2010] Wettability Studies Using Low-Salinity Water in Sandstone Reservoirs.
Ameri, A., Shojai Kaveh, N., Rudolph, E. S. J., Wolf, K. H., Farajzadeh, R. and Bruining, J. [2013] Investigation on Interfacial Interactions among Crude Oil–Brine–Sandstone Rock–CO2 by Contact Angle
Measurements. Energy & Fuels 27(2), 1015-1025.
Basbug, B., Gumrah, F. and Oz, B. [2007] Simulating the Effects of Deep Saline Aquifer Properties for CO2
Sequestration. (10).
Chalbaud, C., Robin, M., Bekri, S. and Egermann, P. [2007] Wettability impact on CO2 storage in aquifers:
visualisation and quantification using micromodel tests, pore network model and reservoir simulations. International Symposium of the Society of Core Analysts, Calgary, Canada.
Chiquet, P., Broseta, D. and Thibeau, S. [2007] Wettability alteration of caprock minerals by carbon dioxide. Geofluids 7, 112-122.
Chiquet, P., Broseta, D. F. and Thibeau, S. [2005] Capillary Alteration of Shaly Caprocks by Carbon Dioxide. SPE Europec/EAGE Annual Conference. Society of Petroleum Engineers, Madrid, Spain.
Chiquet, P., Daridon, J.-L., Broseta, D. and Thibeau, S. [2007] CO2/water interfacial tensions under pressure
and temperature conditions of CO2 geological storage. Energy Conversion and Management 48(3), 736-744.
Espinoza, D. N. and Santamarina, J. C. [2010] Water-CO2-mineral systems: Interfacial tension, contact angle,
and diffusion-Implications to CO2 geological storage. WATER RESOURCES RESEARCH 46.
IPCC. [2005] Carbon Dioxide Capture and Storage IPCC, B. Metz, O. Davidson, H. d. Coninck, M. Loos and L. Meyer (eds.), 431.
Jiamin, W., Yongman, K. and Jongwon, J. [Year] Wettability alteration upon reaction with scCO2: Pore scale
visualization and contact angle measurements. Goldschmidt 2011.
Salimi, H., Wolf, K.-H. and Bruining, J. [2012] The influence of capillary pressure on the phase equilibrium of the CO2–water system: Application to carbon sequestration combined with geothermal energy. International
Journal of Greenhouse Gas Control 11, Supplement(0), S47-S66.
Shariat, A., Moore, R. G., Mehta, S. A., Fraassen, K. C. V., Newsham, K. E. and Rushing, J. A. [2012] Laboratory Measurements of CO2-H2O Interfacial Tension at HP/HT Conditions: Implications for CO2
Sequestration in Deep Aquifers. Carbon Management Technology Conference. Carbon Management Technology Conference, Orlando, Florida, USA.
Shojai Kaveh, N., Rudolph, E. S. J., Rossen, W. R., Hemert, P. v. and Wolf, K. H. [2013] Interfacial Tension and Contact Angle Determination in Water-sandstone Systems with Injection of Flue Gas and CO2. IOR 2013 -
From Fundamental Science to Deployment EAGE, Saint Petersburg, Russia.
Shojai Kaveh, N., Rudolph, E. S. J., Van Hemert, P., Rossen, W. R. and Wolf, K. H. [2014] Wettability evaluation of a CO2/water/bentheimer sandstone system: Contact angle, dissolution, and bubble size. Energy and Fuels 28(6), 4002-4020.
Shojai Kaveh, N., Rudolph, E. S. J., Wolf, K.-H. A. A. and Ashrafizadeh, S. N. [2011] Wettability determination by contact angle measurements: hvBb coal–water system with injection of synthetic flue gas and CO2. Journal of Colloid and Interface Science 364(1), 237-247.
Shojai Kaveh, N., Wolf, K. H., Ashrafizadeh, S. N. and Rudolph, E. S. J. [2012] Effect of coal petrology and pressure on wetting properties of wet coal for CO2and flue gas storage. International Journal of Greenhouse Gas
