Models of shale gas storage capacity during burial and uplift: Application to Wufeng-Longmaxi shales in the Fuling shale gas field

Abstract

Abstract Marine shales in South China, such as the Upper Ordovician Wufeng (O3w) and Lower Silurian Longmaxi (S1l) shales, have generally experienced a long-term burial and rapid uplift process. The variations of temperature and pressure caused by the complex tectonic movements have significant effects on the shale gas storage capacities. Based on the supercritical Dubinin-Radushkevich (SDR) equation and high-pressure methane (CH4) adsorption experiments, equations to calculate the shale gas storage capacities under geological conditions were established in this work. Using the derived equations, the gas storage capacities in two basic geological processes were modeled, namely, the burial process, which has an increased gas content, and the uplift process, which has a constant gas content. The two basic models were applied to investigate the evolution of the shale gas content of the O3w-S1l shales in the JY 1 Well in the Fuling shale gas field, Sichuan Basin, South China. Geological models of the gas storage capacities of the shales during burial show that the storage capacity of adsorbed gas (SCadsorbed gas) initially rapidly increases and then slowly decreases with the increasing burial depth. The storage capacities of free gas (SCfree gas) and total gas (SCtotal gas) increase with the increasing burial depth. Compared with SCadsorbed gas, SCfree gas significantly increases as the pore pressure increases. Such an obvious difference implies that free gas is the predominant gas form in the initial stage of shale gas production. During uplift, the gas content remains unchanged, resulting in a linear decrease in SCfree gas and a linear increase in SCtotal gas. Nonetheless, SCfree gas during uplift is still higher than SCfree gas during burial at the same burial depth. The modeling of the shale gas content evolution of the O3w-S1l shales in the JY 1 shows that from the maximum burial depth of 6200\u202fm to the present-day burial depth of 2415\u202fm, the free gas content declines by 0.89\u202fm3/t, from 2.30\u202fm3/t to 1.41\u202fm3/t, while the adsorbed gas content only increases by 0.58\u202fm3/t, from 0.47\u202fm3/t to 1.05\u202fm3/t. This difference indicates that only a small amount (0.31\u202fm3/t) of shale gas was lost during the uplift and that most (0.58\u202fm3/t) of the reduced free gas was converted into adsorbed gas. A unique aspect of this study is that we illustrate how adsorbed and free gas are converted when the total gas content remains constant. Moreover, the two basic geological models established in this study can also be applied to other shale gas systems with complex tectonic movements.