Improved Calculation Method of Undervalued Free Gas in Shale: A Case Study of Well Z302 Rock in Wufeng-Longmaxi Formation, Sichuan Basin

Shale gas content data always plays an important role in the process of shale resource evaluation. However, free gas is always underestimated in the evaluation of the gas content. As a result, there are always contradictions between the on-site desorption data and actual production data. This phenomenon will lead to the evaluation process of shale gas resources; some of the potential shale gas blocks are considered to be of no development value. Through the on-site shale desorption and organic geochemical experiment on shale of Ordovician Wufeng-Silurian Longmaxi Formation in Well Z302, Sichuan Basin, the following conclusions are drawn: (1) the desorbed gas release in a sequence of N2, followed by CO2, CH4, and C2H6; (2)the δ13C of methane and ethane showed different fractionation characteristics during on-site desorption, and ethane showed “desorption hysteresis” during desorption; (3) the shale gas is formed by oil cracking under the condition of high overmaturity, and the parent material of the oil-generating gas is mainly type II; (4) in the late stage of gas desorption, carbon isotope rollover occurred due to the stronger diffusion ability of 12C-rich methane and the reversal phenomenon caused by residual enrichment of 13C methane isotopes; (5) the transition model of gas abundance during shale desorption is established. At the same time, the separation method of free gas and adsorbed gas in the shale desorption process is determined. Finally, a new calculation method of the ratio of free gas and adsorption gas is proposed to solve the problem that free gas is underestimated in the evaluation of shale gas resources.