Continental shale gas dynamic enrichment and evolution over geological time

Abstract

Shale gas content alters with geological time, which indicates that its formation has evolved and is dynamically enriched. In this paper, a numerical model is proposed to describe the dynamic enrichment and evolution process of shale gas over geological time. The model involves the coupled process of geological conditions (temperature and pressure), source rock quality (abundance, type and maturity), and reservoir characteristics (mineral composition and pore characteristics). By comprehensively considering the gas occurrence state, pore water content and overpressure, we divide the shale gas dynamic evolution process into six stages: dissolved gas stage, adsorbed gas stage, free gas stage, overpressure formation stage, overpressure maintenance stage, and overpressure reduction stage. As a case study of continental shale gas from the Shahezi Formation in the Xujiaweizi Fault Depression, the dissolved and adsorbed gas stages are of less significance to shale gas accumulation, despite their long life. In contrast, the free gas and overpressure formation stages with short geological times are of decisive significance for the efficient accumulation of shale gas, whereas the overpressure maintenance and reduction stage are of great significance to shale gas preservation. Moreover, the gas-in-place content simulated by this method is close to the estimated value of the process analysis method, which is higher than that estimated by the United States Bureau of Mine method and lower than that estimated by the Amoco curve fitting method.