Water Distribution Characteristic and Effect on Methane Adsorption Capacity in Shale Clays
Authors: Li Jing , Wu Keliu , Shi Juntai , Li Xiangfang , Li Yingying , Feng Dong , Zhang Tao , Xu Min , Bai Yangai 
Published: November 2015

Abstract
Methane adsorption in shale is result of gas-liquid-solid interaction rather than gas-solid interaction by considering the initial water saturation in actual condition. As an important constituent of inorganic matter, clay minerals can provide additional adsorption capacity due to high internal surface area. Under dry conditions, both inorganic and organic materials dominate methane adsorption content. However, under reservoir conditions, water always adsorb on clay particle surface, which will significantly reduce the adsorption capacity of methane. Thus, the experimental evaluation of adsorbed gas reserves with dry samples or other improper conditions will misestimate the total gas in place (OGIP). What's more, the commonly used Langmuir equation is not available in describing the complex gas-water competitive adsorption under different moisture conditions. Thus, the mechanism and mathematical model to describe gas-water-clay three phase interactions is badly needed.
In this paper, we analyze the interaction characteristics between methane, water film and clay base on adsorption theory, and results reveal that: methane adsorption on clay (dry) is a typical gas-soild interaction; however, methane adsorption on clay bound water film should belong to gas-liquid interaction. Based on our analysis, a united model is established to describe gas-water-clay interactions, in which, (i) gas-solid interface Langmuir equation is employed to describe methane adsorption on clay (dry); (ii) gas-liquid interface Gibbs equation, instead of Langmuir equation, is employed to describe methane adsorption on water film; (iii) water coverage coefficient was defined to describe the transition between gas-solid adsorption and gas-liquid adsorption; (iv) Langmuir equation and Gibbs equation integrated by water coverage coefficient is established to describe gas-liquid-solid interaction. Meanwhile, mathematical model is presented to quantify water films thickness bound on clay based on DLVO theory (by considering disjoining pressure in nanoscale water film).
The preliminary result shows that, the water saturation in shale clay pore mainly depends on relative humidity and pore size. Under a certain shale humidity system, water saturation is significant effected by pore size. And the pore size is smaller, the water saturation is higher. Otherwise, a capillary condensation phenomenon is also found in our work. Thus, the water saturaion distribute in different pores mainly as: (i) capillary water in the small pores; (ii) water film in the lager pores. Furthermore, considering the water distribution characteristic, the effect of moisture on methane adsorption capacity in shale clay is mainly for two aspects: (i) small pores blocked by water are invalid for methane adsorption, (ii) large pores bounded by water film change interaction characteristics for methane adsorption (from gas-solid interaction to the gas-liquid interaction). And the overall effect could reduce the adsorption capacity by 90% in our study.
The comparison presents the same trend between calculation results by our united model describing gas-water-clay interactions, and experimental result of methane adsorption on clay-rich shale under different mositure conditions by Chalmers (2012). Thus, our model is reasonable and available to describe water and methane competitive adsorption in shale inorganic mineral or clay-rich shale. Furthermore, our model can be applied to predict methane absorption capacity under different water saturation condition in shale system with a real pore size distribution. Our present work reveals mechanism of moisture effect on the methane absorption capacity and lays foundations of evaluating the GIP in shale system more accurately.