Characteristics of sorption and distribution of water in shale porosity are topics of great interest for evaluation of unconventional reservoirs. Also, the study of surface force interactions between bound water and pore surface at the nanoscale is significant for understanding the fate of residual treatment water in shale systems. In this work, thickness and stability of water film were investigated by water sorption isotherm on clay and shale samples. Meanwhile, a new approach based on surface forces (disjoining pressure), which result in the instability of adsorbed film transition into condensed bulk liquid, was developed to describe molecule/pore-wall interactions. Our experimental results directly demonstrated the evidence of capillary condensation in hydrophilic clay minerals, however, water would not entirely fill in shale nonopores even in high moist condition. This remarkable finding may be caused by the inaccessibility of water molecules into micropores of organic matter. In addition, water distribution characteristics significantly vary in different sized pores. As the pore size decreases, the scale of surface interaction increases, directly leading to a higher water saturation and a lower critical relative humidity for condensation. Therefore, under a moist condition with certain relative humidity (e.g. RH=0.98), the water distributed in different sized pores mainly as:
capillary water in the smaller pores (e.g. <6nm), and
water film in the larger pores (e.g. >6nm).
These inorganic porosities blocked by water may be unavailable for gas transport or adsorption capacity in actual shale system with initial water saturation.
Shale gas is a typical unconventional gas resource which is derived from the organic matter within the mudrock and kerogen through biogenic and/or thermogenic processes [1] The recoverable reserves of shale gas in the United States are estimated to be 24.4×1012m3[2], and the gas production from a particular shale reservoir will depend on its storage potential and transport properties [3~5]. Moreover, the initial water saturation or moisture within shale porosity under actual condition will be a significant factor that influence the evaluation of gas deliverability. Present studies showed that the methane adsorption capacity of shale samples in moist conditions would reduce about 40%~90% compared with dry conditions [6~7]. Meanwhile, the presence of pre-adsorbed water would obviously decrease the apparent permeability (or diffusion coefficient) and increase the stress sensitivity of shale matrix [8~10]. Therefore, Understanding how water is stored and distributed in shale system is essential information required for shale-reservoir gas in place estimation and gas production prediction.
