Research And Application Of Seismic Prediction Method For Brittle Index Of Coalbed Methane Reservoir

Emerging unconventional natural gas resources in recent years are promising and have considerable benefits.The development and utilization of CBM reservoirs has multiple benefits such as environmental protection and safety,and can be the most reliable supplementary energy source for conventional natural gas.However,CBM reservoirs in China are generally characterized by low pressure,low permeability and low saturation inhomogeneity,and generally rely on fracturing to increase permeablility for reservoir reconstruction,Namely most CBM wells are basically worthless for industrial exploitation if no production enhancement measures are applied.For coalbed methane reservoirs with such characteristics,hydraulic fracturing reformation work is essential.Therefore,it is very important to select a reasonable fracturing location for the exploitation and utilization of coalbed methane reservoir.The brittleness index of rocks is an important reference data for the selection of fracture zone of CBM reservoir and the prediction and evaluation of sweet spot area,which is an important parameter for the evaluation of CBM reservoir.Petrophysical analysis technique is an important tool for reservoir physical parameters description,and is a "bridge" between seismic elastic parameters and reservoir physical parameters.This paper firstly constructs a petrophysical modeling process for CBM reservoirs based on the petrophysical equivalent medium theory,combined with the significant characteristics of CBM reservoirs that have both free and adsorbed CBM,and constructs a petrophysical modeling process for CBM reservoirs.The petrophysical model of CBM reservoir shows that the gas content of CBM reservoir is negatively correlated with its density,longitudinal velocity and transverse velocity: with high gas content,the reservoir has low density,low longitudinal velocity and low transverse velocity;with low gas content,the reservoir has high density,high longitudinal velocity and high transverse velocity.Then,the formulae for calculating the brittleness index of CBM reservoirs were compared and analyzed,and the production of brittleness quantity version of elastic parameters of CBM reservoirs was carried out,and the joint effects of rock porosity and mineral components on elastic properties and brittleness characteristics were studied,and the brittleness index prediction formulae applicable to CBM reservoirs were selected on the basis of this comprehensive analysis.Finally,based on the actual seismic data,a pre-stack simultaneous inversion is carried out and the brittleness index of CBM reservoir is predicted.Comparison of the actual gas production results shows that the distribution results of the brittleness index are good indicative of the gas production in CBM reservoir.Therefore,it is feasible to study the brittleness index of CBM reservoirs based on seismic method,and it has important theoretical guidance and practical application value for the exploration and development of CBM reservoirs.