Microscopic Characterization Morphology And Conductivity Evaluation Of Non-planar Fractures In The Shale Formation

The hydraulic fracture conductivity of shale reservoirs is influenced by a combination of formation fluids,proppant,main fractures,and natural weak surfaces such as natural fractures and laminae.The classical hydraulic conductivity model is derived under the premise of a single flat and straight fracture,without considering the influence of natural fractures.In fact,the hydraulic fracture is not a single flat and straight fracture but has a non-planar morphology.The morphology of the fracture itself,the distribution of proppant,and the communication with the natural fracture are the direct influencing factors of the inflow capacity,which play an important role in improving oil and gas production.In this paper,the following works are carried out for the microscopic characterization morphology and conductivity evaluation of non-planar fractures in the shale formation:(1)Microscopic characterization morphology of non-planar fractures in the shale formationPhysical simulations of hydraulic fracturing using downhole cores and 3D laser scanning of hydraulic fracture surfaces are carried out to obtain fracture surface point cloud data and reconstruct fracture morphology.Based on Delaunay triangular mesh segmentation and Gabriel mesh growth criterion,the fracture surface is partially repaired to improve the integrity of the fracture surface 3D morphology reconstruction.The geometry and distribution pattern of micro-convex bodies on the fracture reference surface is studied by integrating the results of the multi-index evaluation,and the nonplanar characteristics of the fracture are quantified and characterized,and the influence of the laminar direction and the distance of the fracture from the wellbore on the microroughness of the fracture surface is analyzed.(2)The conductivity of shale self-supported fracturesTo study the flow within self-supported fractures containing the clustered distribution of micro-convex bodies,a flow around multi-circular cylinders model is developed to evaluate the flow disturbance within self-supported fractures.Based on the microscopic quantitative characterization of the fracture non-planar morphology,the fracture inflow capacity is linked to the fracture face micro-convex geometry,and a computational model of the self-supported fracture inflow capacity related to the fracture face micro-convex geometric parameters under different stress conditions is established,and the micro-convex distribution method for the optimal inflow capacity is analyzed and proposed.(3)The conductivity of shale propped fractures with local accumulation of proppantBased on the Hertz elastic contact theory and the Navier-Stokes equations,a flowsolid coupling model of local proppant fracture infiltration capacity considering the fracture non-uniform closure and the flow transition of Darcy-Poiseuille is established and its infiltration capacity is analyzed,taking into account the proppant particle accumulation and stagnation effects.A model for calculating the local support fracture infiltration capacity related to the geometric parameters of proppant clumps and ground stress state is established,and the optimal clump size and optimal clump spacing for local placement of proppant are optimized.(4)The overall conductivity of non-planar fractures in shale formationA model of non-planar fractures considering the coexistence of supported and unsupported fractures is established to evaluate their support effects.Based on the calculation model of the conductivity of self-supported fractures and locally supported fractures,a rock matrix-discrete fracture coupled flow model is established,and an evaluation method of the overall conductivity of non-planar fractures considering equivalent hydraulic fracture width and main fracture flow is proposed.The influence of geological features and engineering conditions on the overall conductivity of non-planar fractures is analyzed and verified with the actual geological engineering in the FX area,and suggestions to improve the overall inflow capacity of non-planar fractures in shale are put forward.