Research On Formation Mechanism Of Fracture Network In Coalbed Methane Reservoir Based On Stress Interference And Fractal Theory

As one of the unconventional oils and gas resources,the occurrence environment of coalbed methane(CBM)has the characteristics of low porosity,low permeability and even ultra-low porosity and ultra-low permeability.The industrial exploitation of CBM resources requires necessary measures for CBM reservoirs.However,the characteristics of natural fractures development of CBM reservoir has led to complex problems such as complex stress field environment,interaction of hydraulic fractures and natural fractures,and stress interference between fractures during the process of CBM reservoir reconstruction.Moreover,the fractal characteristics of matrix pores and the distribution of natural fractures will also have a significant impact on the formation and expansion of complex fracture networks in the process of coalbed methane reservoir reconstruction.In view of this,this thesis uses probabilistic statistical analysis and indoor core testing to obtain the rock mechanical parameter characteristics of coal and constructs a numerical model of the complex fault block tectonic stress field of CBM reservoirs and uses this model to evaluate the target block perform inversion of the in-situ stress field.Based on the rock mechanics parameters of coal and the distribution law of in-situ stress,a numerical model of staged fracturing of horizontal wells under homogenous stress interference and a numerical model of staggered fracturing of double horizontal wells under multi-source stress interference were constructed.The influence law of stress interference between fractures on fracture morphology and fracture parameters is analyzed.The discrete element method is used to construct the fractal propagation model of coalbed methane reservoir fractures considering the fractal matrix pores,and the fractal propagation law of fractures under the influence of different factors is analyzed.On the basis of the fracture propagation model under the stress interference between fractures,the interactive development environment of MATLAB-PYTHON-FINITE ELEMENT analysis platform is used to embed the natural joints with fractal characteristics in batches,and the fractures are analyzed in the CBM reservoirs with fractal natural joints.The influence of stress interference between fractures on fracture morphology,fracture parameters and fracture fractal dimension are obtained from the research.It is proposed to use the principle of underbalanced drilling to reform the CBM wells after drilling for shallow CBM reservoirs.Based on the process and mechanism of underbalanced reformation,the finite element numerical model and discrete element numerical model of coal and rock damage around the wellbore were constructed respectively,and the expansion law of coal rock fracture zone and the fractal expansion law under fine crushing state were analyzed.The research results show that the elastic brittleness of coal and rock is relatively low,and it will be further weakened under fracturing fluid soaking.In addition,the in-situ stress field inversion model constructed based on the tectonic stress field inversion theory and multiobjective constrained optimization method can obtain high-precision in-situ stress field distribution results and infer the influence of fault structure on the stress field.The crack propagation laws of the same-source stress interference model and the multi-source stress interference model show that the cracks under the same-source stress interference deflect to the outside,the cracks under the multi-source stress interference deflect to the inside.And under the action of stress interference between fractures,the fracture parameter curve will also show obvious "sudden rise" or "sudden drop" fluctuation characteristics.The calculation results of the fracture fractal propagation model of CBM reservoirs considering fractal matrix pores show that the higher the fractal dimension of matrix pores,the higher the fractal dimension of hydraulic fractures,and the easier it is to form complex fractures.The fractal dimension of hydraulic fractures.There is a negative correlation with the fracturing fluid pumping rate and the inhomogeneous coefficient of ground stress.The calculation results of the fractal fracture propagation model of CBM reservoirs with developed fractal natural joints and considering the stress interference between fractures show that the degree of stress interference between fractures in CBM reservoirs with developed natural joints is greater than that of CBM reservoirs with undeveloped natural joints.The higher the fractal dimension of natural joints,the higher the stress interference intensity between fractures and the higher the fractal dimension of hydraulic fractures,the easier it is to form complex fracture networks.The intensity of stress interference between fractures meets the law of first increasing and then decreasing with the increase of displacement,and is negatively correlated with the heterogeneous system of ground stress and the number of fracturing intervals.The fractal dimension of hydraulic fractures meets the law of first increasing and then decreasing with the increase of fracturing fluid pumping rate and in-situ stress non-uniformity coefficient,and is positively correlated with the interval of fracturing sections.When designing the fracturing construction plan,the fracturing parameters can be optimized according to the degree of stress interference between fractures and the correlation between the complexity of the fractures and the fracture parameters.For CBM reservoirs with shallow depths,the complexity of fracture zones obtained by using underbalanced reforming technology to reform CBM reservoirs is significantly higher than that of hydraulic fractures formed by fracturing reforms.