Study On Shale Percolation Characteristics Change Regularities Under Hydraulic Fracturing

Due to argillaceous and carbonate content,different brittle and plastic characteristics were presented in shale reservoir,and distribution of natural fracture networks were complex,therefore,accurately determine the evolution and connected rules of micro-pores and micro-fractures in shale reservoir was the key for shale fracturing and shale gas development.In this paper,shale reservoir micro-pores and micro-fractures double-medium percolation model and statistical method for percolation probability of rock mass were established based on site percolation grid model.A new damage variable was defined by the change of percolation probability.From the perspective of energy,shale rock mutation rupture model and energy release equation were established based on percolation theory.By conducting CT scanning experiment on rock samples of different brittle characteristics before and after fracturing,and RFPA2 D to simulate fracture propagation and the change of percolation probability of shale rock mass at different stages of fracturing,which were used to analysis the influence of different brittle characteristics for the change of percolation probability,and the change rules of shale percolation characteristics before and after fracturing were got,which have great significance on the study of describing the substantive characteristics of shale fracturing fracture network evolution.The main research contents are:(1)Based on site percolation grid model,3D micro-pores and micro-fractures double-medium percolation model was established.The principle of CT scanning experiment was introduced,and CT scanning experiment for shale was conducted.Scanning pictures were divided into grids with corresponding coordinates.3D vector quantity matrix expression form for pores,fractures and rock matrix was established.Doing path addressing for rock mass by computer programs and find percolation connected groups.Comparing the sizes and quantities of these percolation connected groups and rock mass percolation probability was got.CT scanning experiment on rock samples from Liaohe L-93 block and sorting the pictures,3D digital rock samples were reconstructed.Statistics show that percolation probability of nature shale rock samples is 5.12%.(2)The pores and fractures in rock mass were regarded as a kind of injury.A new damage variable was defined by the change of percolation probability and shale rock mutation rupture model were established.If the energy changes curve gradually passes through the original point,which means rock mass rupture is continuous and percolation transition didn't happen.Energy aggregation and release didn't happen,too.If the energy changes curve from a point below suddenly jump to the top,which means mutation rupture happened during the fracturing and percolation transition(percolation threshold),energy aggregation and release happened,which prompt fractures extension and evolution,and energy release equation of shale rock mutation rupture was established.(3)Lab hydraulic fracturing test on shale samples of different brittleness and CT scanning on shale samples after fracturing were conducted and then calculate the percolation probability before and after fracturing.Finite element simulation of hydraulic fracturing on 3D reconstructed digital rock samples was conducted,and the changes of X axis displacements were observed.Results show that if the changes of the square of X axis displacements were larger than the area of the grid,which means the grids were connected;and the area of connected group changed,ie.percolation probability changed.(4)The results of the lab hydraulic fracturing test and the finite element simulation were identical with each other.The variation trend of percolation probability and fracturing curve were the same.The change of percolation probability(percolation threshold)and the mutation point of fracturing curve were corresponding.Results show that during hydraulic fracturing period,shale rock mutation rupture and fracture evolution could be expressed by the change of percolation probability.The stronger the rock brittleness,the more obvious of percolation characteristics,which means the more obvious of the characteristics of fracture damage evolution which were resulted from rock mutation rupture.With the enhancement of rock brittleness,macroscopic fracture forming process was no longer a single fracture's extension,but the process of micro cracks bifurcation,convergence and connection.