In-situ gas content evaluation of producing coalbed methane reservoirs based on electromagnetic radiation anomalies

Abstract

In-situ gas content evaluation in coalbed methane (CBM) reservoirs is one of the most critical issues in the fields of gas disaster forecasting, stable production and resource control. Traditionally, the methods for the CBM assessment of coal reservoirs have relied on laboratory evaluation, well testing, logging and seismics, which are very expensive and inefficient in terms of accurate regional investigation. This paper proposes a non-contact in-situ evaluation method based on electromagnetic radiation (EMR) anomaly. Electrical-mechanical coupling models have been theoretically investigated based on the Super-Low Frequency (SLF) electromagnetic radiation (EMR) generating process. Model simulation and field survey results have further demonstrated that in-situ EMR anomalies’ collection with a level of 0.01–1000 nT on the surface is feasible. Furthermore, using the BD-6 SLF detector with built-in processing algorithms such as background field removal and the ensemble empirical mode decomposition (EEMD), the natural-source EMR extraction method is proposed in support of statistical models established to estimate the in-situ reservoir pressure. Using nonlinear cubic polynomials, an ideal model of reservoir pressure and the normalized EMR signals is built, satisfying theoretical predictions (the maximum R-square value is above 0.83). The “Reservoir Pressure (RP)-EMR"-based approach for evaluating gas content in relation to producing reservoirs is then further suggested. In the study area, the proposed methodology is effectively used with a good ability for the local CBM resource monitoring of 6.4 × 1010m3. From August 2011 to October 2011, the estimated gas production is 6.3 million m3 (the actual gas production is 5.9 million m3). Therefore, this promising method is expected to be one of the most effective ways for industrials to realize the CBM evaluation in producing reservoirs.