Characterising Coal Gas Wettability to Assess CO2 Enhanced Methane Recovery

Saghafi, Abouna¹ Pinetown, Kaydy²; Javanmard, Hoda¹
¹Energy Technology, CSIRO, North Ryde, NSW, Australia.
²Earth Science and Resource Engineering, CSIRO, North Ryde, NSW, Australia.

In water saturated coal gas reservoirs, the efficiency of methane (CH4) recovery and carbon dioxide (CO2) storage can be largely affected by the differential wettability of coal to gas and water. The wettability is not only affecting the breakthrough capillary pressure threshold but also affects the gas adsorption and desorption properties of coal. The change in these reservoir properties in turn modifies the rate of desorption of CH4 from coal.

A coal with poor gas wettability prevents the gas from spreading over the surface of the pore walls, and therefore, optimal contact between the free gas molecules in the pore system and coal is not achieved. Consequently, the non-wetting fluid (gas) does not penetrate into micro pores where the largest storage space for gas is located. Since wettability plays such an important role in the migration and storage of gas in coal, it is considered to be a major parameter in characterisation of coal seam gas reservoirs for storage potential. Characterisation studies may look into the conditions which allow coals to be sufficiently gas-wet so that the injected CO2 can penetrate into smaller pores for adsorption to occur.

The aim of this project is to quantify the relative wettability of a suit of coals to gases naturally occurring in coal seams including CO2 and CH4. Coals of various ranks and composition, collected from coalfields of the Sydney Basin in eastern Australia were studied. Wettability was quantified in terms of the contact angle of the gas-water interface with the coal surface. A CSIRO purposely built captive bubble system was used to undertake these measurements. The coal samples were studied to evaluate the wettability behaviours of coal seams, with equivalent depths of 200 to 1500 m. The results show that the contact angle of the water and gas interface with the coal changes as the gas-water system pressure and temperature changes. The results of this study are to be used in optimising the recovery CH4 and storage of CO2 in coal.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012