Evaluation Method Research On Coalbed Methane Reservoirs

With the acceleration of world economic development, conventional oil and gas resources cannot meet the needs of society. Therefore, coalbed methane(CBM), shale gas and other unconventional energy sources become particularly important. With rich coal resources in our country, the research of CBM reservoirs will bring significant benefits to the society. Heterogeneity of CBM reservoirs is strong and the pore structure is complex. The CBM reservoirs are high in adsorbed gas and low in free gas and dissolved gas. These characteristics result in the difference of well-logging evaluation method between CBM reservoirs and conventional gas reservoirs, and the evaluation method is still in the exploratory stage.Based on mathematical statistical method, rock volume model theory and the characteristics of CBM reservoirs, applied geophysical logging techniques to carry on the study of logging interpretation method to evaluate CBM reservoirs, a complete logging evaluation system was formed. It has theoretical and practical significance to the logging evaluation of reservoirs whose main composition is adsorbed gas, and this research provides supports for the improvement and development of CBM reservoirs.Aimed to solve issues above, the research was carried from the sides of the correction of collapse influence, automatic hierarchy, lithology identification, coal component model and content calculation, reservoir parameters evaluation, the simulation of CBM adsorption process and the prediction of gas content. The main research contents are as follows:To solving collapse cases of coal and obtaining high quality density log data, Monte Carlo simulation method was used to simulate the count rate changes on near and far detectors, establish the formula of calculating coal density and make correction cross-plots for collapse influence of coal density log. It provided a solution for the correction of the coal collapse of density data.According to coal logging response characteristics, with the curves sensitive to coal, extrema variance clustering method was used to realize automatic hierarchy. Cross-plot and GRNN methods were used to identify lithology. The preferred method will be selected as lithology identification method in coal logging evaluation system.The volume model with four components was established which contains fixed carbon, volatile matter, ash and water, and volume fractions were optimized by PSO method.Deep and shallow lateral resistivity logging data were used to calculate coal fracture porosity and fracture permeability. Density data after collapse correction was used to calculate coal total porosity and the ash content was used to predict total permeability.Numerical simulation was used to research the process that coal adsorbs methane gas, construct coal structural model and the methane molecule model. An isothermal adsorption model of CBM reservoirs was established to simulate the adsorption law of methane gas. Multiple linear regression and coal isothermal adsorption model approach were used to predict adsorbed gas content. Free gas content was calculated by Archie formula."Coalbed Methane Reservoir Logging Evaluation System" was established based on the results above. Firstly, correcting collapse influence to density logs. Secondly, automatic layering using extrema variance clustering method with natural gamma curve, acoustic curve and density curve, and identifying lithology with GRNN neural network methods. Thirdly, calculating coal components, including fixed carbon, ash, volatile matter and water. Finally, obtaining CBM reservoir parameters and predicting gas content. A complete evaluation system was established by integrating these processing functions and it was applied to the actual datas. The development of the "Coal Analysis and Log Evaluation of CBM Software" in 2015 had won the national computer software copyright.( 2015SR122479).Based on the current research status of logging evaluation method on coalbed methane reservoirs, the innovations of this paper as following: Monte Carlo numerical simulation method was used to research the law of collapse effect on density logging, and a correction chart was established; A four-components volume model was established, the content of each component was optimized by PSO method; Numerical simulation was used to research the process that coal adsorbs methane gas, according to the simulation result, gas content of CBM was predicted.