Research On Seismic Inversion Method Of Coalbed Methane

The onshore coalbed methane reserve of China is the third largest in the world.Although its exploration has been set up for a short period,it shows great potentialities in recent years.The exploration and development can effectively not only reduce gas accidents in coal seam mining and the emission of energy waste gas,but also opti mize domestic energy structure.Geophysical inversion technology can identify the rich areas and improve the production of coalbed methane.The inversion application shows considerable economic value and significance for coalbed methane development.Compared with traditional reservoirs,the common post-stack inversion method is difficult to obtain good results due to the relative thin coalbed methane reservoir.For example,the acoustic impedance inversion is represented by pulse inversion showing simple and quick calculation.But its multiple solutions with inadequate constraint condition make the results less predictive.Although the stochastic inversion based on Geostatistics considers the randomness of underground geological body,more wells in the study area are necessary to control the inversion.Based on model,the log constrained inversion shows quick computation and high precision.But the results are largely rely on accurate initial model.According to the thin reservoir and little log data in the study area,this paper uses log constraint inversion method based on model to study its efficiency.The first step is selecting the seismic attributes that are sensitive to lithology.The logging-seismic attribute joint reconstruction method can highlight the anomalies between coal seam and surrounding rock.In this way,a good initial model can be used to the next inversion.Secondly,introducing seismic phase-constrained post-stack inversion to this method,it is efficient to reduce the multi-solution of inversion with the help of priori information such as specific geometry and internal structure in seismic reflection.This process can highlight the information of target thin layer.Under the Bayesian framework,the next step is choosing an adaptively modified regularization operator according to the realtime state of the inversion iteration.This operator can unify the stability and accuracy of post-stack inversion result.This method can predict thin coal seams layer successfully.The result from this coal-bed methane inversion method mentioned in this paper keeps good consistence with logging data.So this method is appropriate for inversion of spatial distribution of coalbed methane reservoir.Based on post-stack inversion results,it is realizable to deducing the recursion formulas of the pre-stack three-parameter inversion and determining optimal approximate equations by Zoeppritz and its approximate formulas.Finally,the mutually constrained method between empirical formulas and rock physical characteristics can improve the accuracy and stability of pre-stack inversion result.Combining the pre-stack and post-stack inversion results,the rich coalbed methane can be divided and optimized furthermore.Appling this method mentioned above to Qinshui coalfield A block,the predicted thickness of thin coal seam layer is 1 meter using the post-stack inversion.The agreement of blind well log test reaches up to 87%.Pre-stack inversion locates the gas containing province in coal seam reservoir based on post-stack inversion.This inversion results are in accord with the result from gas test.At last,with the help of the analysis of post-stack and pre-stack inversion results,the divided favorable zones of coalbed methane enrichment can guide the exploration in the next step.