Seismic Nonlinear Prediction Method And Application For Fractures In Coalbed Methane Reservoir

Coalbed methane(CBM)refers to hydrocarbon gas stored in coal seams and coal measures,and is a clean energy source.Accelerating the development and utilization of CBM is of great significance to safeguarding coal mine production safety and reducing greenhouse gas emissions.The results of previous studies indicate that seismic signals carry subsurface media information,fracture development information,fluid information,etc.,with obvious nonlinear characteristics.There exist nonlinear relationships among rock physics parameters(such as Young's modulus and formation pressure)of coal rocks,and the mechanical property parameters of coal rock have nonlinear characteristics.The coal seam has undergone many non-linear geological processes during the formation process,so the coal seam's structural characteristics and internal structure have obvious nonlinear characteristics.These nonlinear characteristics provide rock physics basis for nonlinear prediction of CBM reservoir.The CBM reservoir has a typical dual-porosity structure,and the fracture system of the coal seam has a great influence on the adsorption,desorption and penetration of the CBM.Therefore,it is very important to predict the fracture of the CBM reservoir.Firstly,the coal seam fractures are detected by seismic amplitude attributes,coherent attributes,and curvature attributes.At the same time,aiming at the nonlinear characteristics of CBM reservoir,this thesis proceeds to study the nonlinear prediction methods of CBM reservoir fractures.Testing fracture prediction methods using three nonlinear parameters,including scaling exponent,correlation dimension and catastrophe parameters.The results show that the nonlinear parameters can better reflect the development of fractures,in which the catastrophe parameters have the highest sensitivity to fracture development and are more accurate to characterize the boundaries of fractures.Therefore,nonlinear parameters can be used for fracture prediction.The study of nonlinear prediction of fractures in the YCN CBM exploration area has mainly achieved the following research results.In the process of coherent calculation,different analysis points should be analyzed with different length of time window to fully display the information of each frequency carried by the seismic signal,and accurately reflect the characteristics of the discontinuity of the geological body.The distribution characteristics of instantaneous center frequency can indicate the development of fault and fractures to some extent.The curvature attributes can be used to describe fractures produced by the rock formation or fractures associated with fault,and the curvature attribute is more detailed in the characterization of the geological details such as fractures.The scaling exponent is better for identifying weak reflection horizons on some seismic reflection profiles.Correlation dimension is better for identifying some geological anomalies and also depicts fracture development characteristics.The catastrophe parameters are sensitive to the discontinuity interface,and can effectively describe the boundary features of the fracture development area.The RGB image fusion method can fully reflect the development of fractures in the region.In this thesis,seismic attributes are used to predict the fracture of the coal seam.According to the nonlinear characteristics of the CBM reservoir,the nonlinear comprehensive prediction for the fractures is performed and the expected effect is achieved,which establishes the foundation for prediction and evaluation of CBM enrichment area.At the same time,it also provides a new approach for comprehensive fracture prediction.