Research On The Flow Model Of Microcracks In The Early Stage Of Shale Fracturing

In the practice of shale fracturing,it was found that the flowback rate of shale fracturing fluid was lower than that of conventional reservoirs,and there was a new characteristic of backflow dynamics.At the initial stage of flowback,the single-phase flow of the liquid phase in the shale reservoir does not exist,and the fluid flow in the reservoir is gas-water two-phase flow.At present,scholars at home and abroad have established relevant models for the flowback of single-phase flow for tight oil and gas reservoirs,but for shale gas reservoirs,the single-phase flow model does not apply.Therefore,in this paper,in order to solve this problem,the early stage of shale reservoir return flow was studied,the crack model after fracture was modeled and analyzed,and the flow model based on LBM method was used to initially understand the initial stage of shale backflow.Instructs the flowback of fracturing fluid after shale fracture.This study selected the marine shale of the Longmaxi Formation in the Jiaoshi area of the Sichuan Basin.The pore characteristics,porosity,permeability,and mineral composition of the shale were first analyzed experimentally.Based on CT scan experiments,the core porosity of the Longmaxi formation in the Jiaoshi area was analyzed,and the core porosity changes after the water absorption experiment were analyzed.A digital core model was established using physical experiments.Threshold optimization method was used to optimize the threshold of block shale core samples.The principle of voxel connectivity was used to extract the spatial network model of microcracks.Thirdly,the 3D LBM of BGK format D3Q19 lattice model was established to simulate and calculate the shape of rough fractures.Flow model.The boundary conditions are:using the standard rebound format,using a square grid as the calculation grid,and processing the rough crack boundary with the "01 flag" to form a complete simulation calculation process.Finally,the simultaneous digital core physics model and the fluid flow model in the fracture construct the initial flow numerical model of shale flowback.Through relevant research,this article has mainly achieved the following conclusions and understanding:(1)High-precision CT scans of marine shales from the Longmaxi formation in the Jiaoshi area have acquired changes in the internal structure of the core after water absorption.The results show that the core induced new microcracking after water absorption,and the porosity of the core porosity has both Obviously increased.Based on the results of CT experiments,core samples were digitally reconstructed,and the reconstructed digital cores were highly consistent with the samples.(2)A three-dimensional LBM flow model for the D3Q19 lattice model was established.Based on the digital core,the initial flow of shale backflow was analyzed,and the shale gas backflow dynamics were recognized.That is.in the early stage of return flow,the fracturing fluid enters the shale pores.In the displacement gas,the displaced gas enters the large pore and forms a gas-water two-phase flow with the fracturing fluid.(3)According to different fracturing fluid types,the dynamic simulation of flowback was carried out.The saturation degree of the broken glue in the core was the lowest,ie,the amount of broken glue liquid was the most,the rate of returning was the highest,and distilled water was the lowest.(4)The calculation of the root research project scale simulates the flowback behavior of the fracturing fluid under different fracture reconstruction volumes.The results show that the larger the volume of the fracture,the more holes there are,and the smaller pores,the more More fracturing fluid stays in the small crack space and pores,which reduces the flowback rate of the fracturing fluid;(5)In the case of low pressure difference,the resistance of the gas to the fracturing fluid is small,and the liquid carrying capacity of the gas is poor.The liquid in the small pore can not be discharged together with the gas;however,in the case of high pressure difference Under the gas,the liquid-carrying capacity of the fracturing fluid becomes stronger and the saturation of the liquid phase in the seam decreases,but a large amount of fracturing fluid still exists in the seam.