Investigation of Gas Adsorption in Shale Reservoirs

Abstract

Unlike conventional reservoirs, there is a considerable amount of gas stored as an adsorbed phase in most shale reservoirs in North America. Injection of CO2 into shale formations is found to be a promising method for extraction of hydrocarbons and CO2 can be also stored as an adsorbed phase in shale reservoirs, which can mitigate the global warming issue. Therefore, it is necessary to understand rock wettability and water contact angles of shale reservoirs under reservoir conditions and obtain accurate adsorption isotherms for estimation of the total gas-in-place (GIP), and determination of the proportions of free gas and adsorbed gas, which can help understand gas transport behavior and predict gas production from shale reservoirs. In this thesis, we present an investigation of water/brine contact angles as functions of temperature, pressure, salinity, ion types, and gas contents by Molecular Dynamics (MD) simulations. It is found that temperature has profound effects on water contact angles below a critical temperature at an intermediate pressure and a CO2-water-shale organic matter system turns from a neutrally-wet state to a CO2-wet state at the critical pressure of CO2. Only a slight increase in water contact angles is observed with increasing salinity, and an increase in the CO2 fraction of gas mixtures can increase water contact angles at the same pressure and temperature. Moreover, we use Grand Canonical Monte Carlo (GCMC) simulation to investigate suitable experimental conditions to obtain an accurate void volume of pores by helium expansion tests and provide a method that can be used to obtain an adsorbate-based pore volume for estimation of adsorption isotherms. Considering an economic effect and measurement accuracy, determining a void volume by helium expansion tests within a moderate pressure range at 500 K is suggested. Additionally, by using a CH4-based volume for calculating excess adsorption amounts, a negative excess adsorption amount of methane at high pressure is not observed. Kerogen swelling in shale formations can be induced by gas adsorption, which further results in changes in pore volumes and influences determination of adsorption isotherms. Gas adsorption in kerogen and the corresponding sorption-induced swelling are also investigated by hybrid GCMC/MD simulations in this thesis. A unified relationship between volumetric strain and an absolute adsorption amount is developed for different adsorbates and a theoretical model for calculating excess adsorption isotherms coupling swelling is also proposed. Excess adsorption isotherms generated by our model are consistent with excess adsorption data calculated by a variable pore volume, which shows a large discrepancy compared to that determined using constant volumes, especially at high pressures.