Shale gas desorption behavior and carbon isotopic variations of gases from canister desorption of two sets of gas shales in south China

• Published in: Marine and petroleum geology Vol. 113; p. 104127
• Main Authors: Ma, Yong, Zhong, Ningning, Yao, Lipeng, Huang, Haiping, Larter, Steve, Jiao, Weiwei
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-03-2020
• Subjects: Desorption; Gas composition; Methane carbon isotopes; Pore structure; Shale gas; Gas composition; Methane carbon isotopes; Desorption; Pore structure; Shale gas
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2019.104127

Four shale samples from the Lower Silurian Longmaxi Formation and the Lower Cambrian Shuijingtuo Formation in the eastern part of Chongqing, SW China, were subjected to canister desorption experiments at reservoir and elevated temperatures (up to 90 °C) to investigate shale gas release behaviors and their variations in chemical and isotopic compositions of desorbed gases. Both the ratios of gas released at reservoir temperature to the total desorbed gas at reservoir and elevated temperatures and the desorption rates suggest that the Longmaxi shale gas is more easily released than the Shuijingtuo shale gas. Scanning electron microscope (SEM) observation, along with low temperature CO2 and N2 adsorption results, suggests that organic matter hosted pores with diameters of 2–100 nm are rich in the Longmaxi shales, whereas well-developed micropores (<2 nm) are characteristics of the organic matter in the Shuijingtuo shales. High proportions of organic matter hosted micropores and high TOC contents, coupled with a poorly connected organic matter hosted pore network, result in the dominance of adsorbed gas in the Shuijingtuo shales and subsequent difficulty for gas release. Both the CO2/CH4 and C2H6/CH4 ratios of the incrementally collected desorbed gas, increase with the extent of desorption, whereas the C2H6/CH4 ratio increases first and then decreases after a peak. The changes in chemical composition of desorbed gas with desorption process are mainly caused by both the difference of adsorption capacity of shales and the varying molecular sizes of CH4, C2H6 and CO2. Carbon isotopic values of incrementally collected desorbed methane become rapidly 13C-enriched as desorption proceeds at reservoir temperatures, and up to 13.7–16.2‰ and 9.1–10.2‰ for the Longmaxi and Shuijingtuo shales respectively. At elevated temperatures (60 and 90 °C), a similar trend was observed but the desorbed gas is initially 13C-depleted. Isotopic fractionation and mass balance calculations of lost gas and desorbed gas suggest that the in situ δ13CCH4 values of shale gas were approximately represented by the δ13CCH4 values of desorbed gas collected at the 5th hour of canister desorption at reservoir temperature. This corresponds typically to a stage when 42–70% of potentially available gas is released from the shale. Thus shale gas content and gas chemistry data from canister desorption of freshly obtained core samples provides valuable geochemical information for understanding shale gas production behavior in the field.
•Shale gas released from canister desorption at different temperatures was analyzed.•Both OM pore structure and TOC content control shale gas desorption behavior.•Sorption affinity and molecular size affects chemical composition of released gas.•Desorbed gas after 5 h approximately represents the δ13C1 value of in situ gas.