Models of Shale Gas Content in Open/Closed Shale-Gas Systems Under Geological Conditions

Abstract

Estimating shale gas content and understanding how it evolves during postgeneration evolution are major challenges to shale gas exploration in Southern China. The organic-rich shales in Southern China have experienced intensive uplift and erosion of overburden rock, which results in the present preservation conditions of oil-gas systems very complicated. However, in previous studies, the geological models of shale gas content were all in open shale-gas systems. In this study, based on the adsorption potential theory and SDR isotherm model, equations to calculate the adsorbed gas content (AGC), free gas content (FGC) and total gas content (TGC) are deduced. Then high-pressure methane adsorption isotherms at different temperatures are measured on organic-rich shale. The relationships between temperature with adsorption capacity and adsorbed gas density are applied to calculate the AGC, FGC and TGC under geological conditions. Finally, the models of shale gas content in closed/open shale-gas systems during postgeneration uplift and substance are researched. In open shale-gas systems, with the increasing of the burial depth, the dominant influence factor of AGC is the pore pressure at lower burial depth, and then is the temperature at the deeper burial depth. At deep burial depth, AGC almost have no difference under different pore pressure, while the FGC vary in a wide range. Such different effect of pore pressure on AGC and FGC can help us to assess the source of the gas liberated as shale reservoir pressure decreases through on-going production. At shallow burial depth, the total gas is dominant by adsorbed gas and the shale with higher pore pressure have a higher free gas ratio. In closed shale-gas systems, it is the mechanism of conversion between adsorbed gas and free gas to adjust pore pressure to keep TGC remaining constant. During uplift, although it exists the conversion of free gas to adsorbed gas, the FGC are still higher than that of at the equivalent depth in open shale-gas systems, which results in the formation of the overpressure in shale reservoir. Similarly, during substance, it is the adsorbed gas that converts to free gas and then the abnormal subpressure is formed. The pressure coefficient changes slowly at deeper depth, while changes quickly at shallow depth. Lacking the adsorbed gas, the pressure coefficient of the conventional reservoir varies quickly than that of the unconventional shale reservoir for a same uplift or substance depth.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018