Capillary Pressure and Wettability Behavior of CO2 Sequestration in Coal at Elevated Pressures

Summary Enhanced coalbed-methane (ECBM) recovery combines recovery of methane (CH4) from coal seams with storage of carbon dioxide (CO2). The efficiency of ECBM recovery depends on the CO2 transfer rate between the macrocleats, via the microcleats to the coal matrix. Diffusive transport of CO2 in the small cleats is enhanced when the coal is CO2-wet. Indeed, for water-wet conditions, the small fracture system is filled with water and the rate of CO2 sorption and CH4 desorption is affected by slow diffusion of CO2. This work investigates the wetting behavior of coal using capillary pressures between CO2 and water, measured continuously as a function of water saturation at in-situ conditions. To facilitate the interpretation of the coal measurements, we also obtain capillary pressure curves for unconsolidated-sand samples. For medium- and high-rank coal, the primary drainage capillary pressure curves show a water-wet behavior. Secondary forced-imbibition experiments show that the medium-rank coal becomes CO2-wet as the CO2 pressure increases. High-rank coal is CO2-wet during primary imbibition. The imbibition behavior is in agreement with contact-angle measurements. Hence, we conclude that imbibition tests provide the practically relevant data to evaluate the wetting properties of coal. Introduction Geological sequestration (Orr 2004) of CO2 is one of the viable methods to stabilize the concentration of greenhouse gases in the atmosphere and to satisfy the Kyoto protocol. The main storage options are depleted oil and gas reservoirs (Shtepani 2006; Pawar et al. 2004), deep (saline) aquifers (Kumar et al. 2005; Pruess et al. 2003; Pruess 2004), and unmineable coalbeds (Reeves 2001). Laboratory studies and recent pilot field tests (Mavor et al. 2004; Pagnier et al. 2005) demonstrate that CO2 injection has the potential to enhance CH4 production from coal seams. This technology can be used to sequester large volumes of CO2, thereby reducing emissions of industrial CO2 as a greenhouse gas (Plug 2007). The efficiency of CO2 sequestration in coal seams strongly depends on the coal type, the pressure and temperature conditions of the reservoir (Siemons et al. 2006a, 2006b), and the interfacial interactions of the coal/gas/water system (Gutierrez-Rodriguez et al. 1984; Gutierrez-Rodriguez and Aplan 1984; Orumwense 2001; Keller 1987). It can be expected that in highly fractured coal systems the wetting behavior positively influences the efficiency of ECBM recovery. It is generally accepted that the coal structure consists of the macrocleat and fracture system (>50 nm) and the coal matrix (<50 nm). The macrofracture system is initially filled with water and provides the conduits where the mass flow is dominated by Darcy flow. The coal matrix can be subdivided in mesocleats (from 2 to 50 nm), microcleats (from 0.8 to 2 nm), and the micropores (<0.8 nm). The matrix system is relatively impermeable, and the mass transfer is dominated by diffusion. After a dewatering stage, CO2 is injected and flows through the larger cleats of the coal. Subsequently, CO2 is transported through the smaller cleats and is sorbed in the matrix blocks (Siemons et al. 2006a). Depending on the wettability of coal, we can distinguish the following gas exchange mechanisms:The coal is water-wet, and CO2 and CH4 diffuse in the water-filled cleats.The coal is CO2-wet or gas-wet, and countercurrent capillary diffusion can take place.The coal is gas-wet, and binary diffusion of CO2 and CH4 occurs. Capillary diffusion finds its origin in capillary pressure (Pc) effects, where Pc is defined as the pressure difference between the nonaqueous and aqueous phase. The storage rate for CO2 is much smaller if the microcleat system is water-wet. This is because of the small CO2 molecular-diffusion coefficient (DCO2 ˜ 2 x 10-9 m2/s). For CO2-wet conditions, a faster and more efficient sorption rate is expected and the molecular diffusion is much larger (i.e., DCO2 ˜ 1.7 x 10-7 m2/s at 100 bar) (Bird et al. 1960). Therefore, we assert that the wettability of coal is important for ECBM recovery applications. For this reason, we have undertaken an experimental study to investigate the wetting properties of two different coal types under reservoir conditions, measuring the capillary pressure between CO2 and water. The dissolution properties of CO2 in water (Wiebe and Gaddy 1940), the interfacial tension between water and CO2 (Chun and Wilkinson 1995), and the CO2 sorption (Siemons et al. 2003) play important roles in the interpretation of capillary pressure experiments. The CO2, will sorb on the coal and will cause a swelling-induced permeability decrease (Mazumder et al. 2006). The higher the pressure, the more CO2 can be sorbed and the more the coal swells (Reucroft and Sethuraman 1987). The largest amount of sorption-induced swelling in intact coal is approximately 4%. It is found that the swelling for ground coal is much higher than intact coal and has been reported to be in the order of 15-20%. The swelling causes a porosity reduction, thus the water saturation decreases. In the Background section, relevant literature about the wettability of coal and the capillary pressure is summarized. The Experimental Design section describes the experimental setup we have developed to measure the capillary pressure as a function of the CO2 pressure. Furthermore, we describe the sample preparation and experimental procedure. In the Results and Discussion section, the experimental results are presented and discussed. We end with Conclusions.