Characteristics and origin of in-situ gas desorption of the Cambrian Shuijingtuo Formation shale gas reservoir in the Sichuan Basin, China

Abstract

In-situ shale gas extraction provides direct evidence in response to gas-bearing evaluations and productivity predictions regarding shale gas reservoirs. The desorption characteristics and origin of in-situ shale gas extraction in the lower Cambrian Shuijingtuo Formation of the Sichuan Basin are studied via in-situ gas desorption tests. Tests are conducted at both reservoir (35°C) and high temperatures (90°C). The results indicate that most in-situ shale gas is quickly desorbed within 300min at both the reservoir temperature desorption stage and the high temperature desorption stage. In both instances, the in-situ gas desorption rates decrease rapidly over time. In addition, the desorbed gas content and desorption rate at the reservoir temperature desorption stage are both clearly lower than the equivalent measures at the high temperature desorption stage. The in-situ desorbed gas from the reservoir temperature desorption stage is mainly free gas; the in-situ desorbed gas from the high temperature desorption stage, on the other hand, is mainly sorbed gas, which is the dominant form in the in-situ shale gas. The percentage of in-situ desorbed gas at these two stages is mainly controlled by gas adsorption, pore volume, and specific surface area. The desorption rate of in-situ shale gas is likely controlled primarily by average pore size and methane adsorption.