Methodologies for Gas Content Evaluation in Deep Coal/Shale Formations: Assessment, Improvement, and Comparison

Le résumé fourni par la source

Gas‐in‐place (GIP) content is a critical indicator for evaluating deep coal/shale resource potential, typically categorized into lost gas content, desorbed gas content, and residual gas content. The lost gas content cannot be directly quantified and requires specific methods for assessment. Current lost gas content restoration methods exhibit significant discrepancies between estimated results and pressure‐holding coring measured values, yet the underlying causes of these errors remain not fully understood, which hinders the effective refinement of these methodologies. Study conducted field degassing experiments on shales from the Longmaxi Formation in the Sichuan Basin and coals from the Taiyuan and Shanxi Formations in the Ordos Basin (a total of 12 samples, including four pressure‐holding coring samples). We systematically compared and analyzed the Amoco curve fit method (ACF) and modified curve fit method (MCF) methods using experimental and numerical simulation results. The issue of excessive overestimation of lost gas content has been addressed through method improvements. The applicability of the improved method and the rationale behind parameter selection have been thoroughly discussed. The study reveals that: (1) The primary reason for the significant errors in the ACF and MCF methods is the neglect of the infinite series summation term in practical applications, which affects the lost gas content calculation and results in generally overestimated values. (2) A selection criterion for order n was established for the improved ACF and MCF methods. For deep coalbed methane (CBM) applications, the optimal order is n = 5 for the improved ACF method and n = 10 for the improved MCF method. When applied to deep shale gas, an optimal order of n = 20 is more appropriate for the improved ACF method, whereas the MCF method fails to effectively restore lost gas content in deep shale gas reservoirs. (3) Comparative analysis of the improved methods revealed that the improved MCF method demonstrates superior adaptability for lost gas content restoration in deep CBM, while the improved ACF method exhibits stronger applicability in deep shale gas. Validation using pressure‐holding coring data from deep shale samples demonstrated that the improved methods significantly enhanced calculation accuracy, reducing error ratios from 276.31%, 164.06%, 189.70%, and 365.55% to 1.41%, 46.58%, 45.49%, and 34.59%, respectively, compared to conventional methods. This study holds significant implications for resource potential evaluation, sweet spot selection, and development plan optimization of deep coal/shale gas.

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