Study of coal gas wettability for CO₂ storage and CH₄ recovery

Abstract

To quantify and rank gas wettability of coal as a key parameter affecting the extent of CO 2 sequestration in coal and CH 4 recovery from coal, we developed a contact angle measuring system based on a captive gas bubble technique. We used this system to study the gas wetting properties of an Australian coal from the Sydney Basin. Gas bubbles were generated and captivated beneath a coal sample within a distilled water-filled (pH 5.7) pressurised cell. Because of the use of distilled water, and the continuous dissolution and shrinkage of the gas bubble in water during measurement, the contact angles measured correspond to a 'transient receding' contact angle. To take into account the mixed-gas nature (CO 2 , CH 4 , and to a lesser extent N 2 ) of coal seam gas in the basin, we evaluated the relative wettability of coal by CH 4 , CO 2 and N 2 gases in the presence of water. Measurements were taken at various pressures of up to 15 MPa for CH 4 and N 2 , and up to 6 MPa for CO 2 at a constant temperature of 22°C. Overall, our results show that CO 2 wets coal more extensively than CH 4 , which in turn wets coal slightly more than N 2 . Moreover, the contact angle reduces as the pressure increases, and becomes < 90°at various pressures depending on the gas type. In other words, all three gases wet coal better than water under sufficiently high pressure.

The effect of rank, lithotype and roughness on contact angle measurements in coal cleats

“…The relative permeability behaviour of gas and water in the cleats of a coal seam gas (CSG) reservoir is dependent on many factors including fluid pressure, phase saturations, effective stress, the geometry of individual cleats, the interconnectedness of the cleat network, and the wettability of the coal (Saghafi et al, 2014;Su et al, 2001). Most laboratory studies that report relative permeability in coals measured by steady state or unsteady state core flooding methods derive a crossover point water saturation fraction (S w ) exceeding 0.5, which is usually interpreted as the coal being water wet or hydrophilic (Conway et al, 1995;Durucan et al, 2013;Ham and Kantzas, 2011;Purl et al, 1991;Rahman and Khaksar, 2007;Shen et al, 2011).…”

“…The most commonly used experimental methods include measurements on (i) a flat, smooth surface and include techniques like: captive bubble (Arnold and Aplan, 1989;Saghafi et al, 2014;Sakurovs and Lavrencic, 2011), tilted plate (Brady and Gauger, 1940;Yuan and Lee, 2013) and sessile drop (Drelich et al, 2000;Gutierrez-Rodriguez et al, 1984). (ii) powdered/pulverised coal.…”

Direct Measurement of Static and Dynamic Contact Angles Using a Random Micromodel Considering Geological CO2 Sequestration

“…Values of in-situ contact angles and local capillary pressure are not distinguishable [9]. In a different approach, static and dynamic contact angles have been vastly measured on a flat surface representing a specific mineral surface (i.e., silica, mica, or natural rock) using various methods such as sessile drop, captive bubble, Wilhelmy plate, and dual-drop dual-crystal (DDDC) methods [18][19][20][21][22][23][24][25][26][27][28][29]. High discrepancies in contact angles measured in literature have been observed.…”

The Effect of Water Occlusion on Gas Production in Coal

“…Experimentally measured relative permeability using coal cores in the laboratory under steady or unsteady methods detail a cross-point water saturation fraction in the relative permeability curve exceeding 0.5, implying coal is water wet (Conway et al, 1995a;Durucan et al, 2013;Ham and Kantzas, 2011;Purl et al, 1991;Rahman and Khaksar, 2007;Shen et al, 2011 (Kaveh et al, 2012;Saghafi et al, 2014), gas desorption (Ham and Kantzas, 2008;Ham and Kantzas, 2011), mineral matter (Gosiewska et al, 2002b) and liquid characteristics including ion concentration (Albijanic et al, 2010;Mishra et al, 2002) and pH (Chaturvedi et al, 2009). Coal rank is also another important factor that contributes to coal wettability, as high rank coal, such as anthracite are not water-wet, yet low rank coal like peat has both a high moisture content and is waterwet (Moore, 2012).…”

“…Coal rank is also another important factor that contributes to coal wettability, as high rank coal, such as anthracite are not water-wet, yet low rank coal like peat has both a high moisture content and is waterwet (Moore, 2012). As water-wet surfaces are more likely to trap water in smaller pores (both micropores and micro-cleats), the result is that sections of the cleat network may not allow Darcy flow, as well as capillary effects that hinder gas diffusion from the matrix until the pressure difference has sufficiently decreased (Saghafi et al, 2014;Sakurovs and Lavrencic, 2011). Teng et al (2016) identified these restrictions in the cleats as a water film threshold pressure gradient (S) that could be ruptured if the differential gas pressure was sufficiently large.…”