Study On Interaction Mechanism Between Unsaturated Shale With Fractures And Fluid

Large-scale hydraulic fracturing of shale gas is the main means to form industrial production.Horizontal well drilling with long horizontal sections and multistage fracturing are two principle technologies for the successful development of shale gas.Low flowback rates and high salinity of fracturing flow-back fluid show strong physical and chemical interaction between shale reservoir and working fluid.The flowback rates of shale gas wells in one field show striking difference.The mechanism and controlling factors of low flowback rates after hydraulic fracturing is not clear.The complexity of shale reservoir brings a series of fundamental theoretical challenges to analyse the interactions of fluid and shale,highlighting the fact that traditional theories and models are no longer applicable.It is an important research topic to master the special properties of shale gas reservoirs and to study the microscopic interaction mechanism and laws of fluid and shale gas reservoirs.The reservoir shale of the Jiaoshiba block in the Fuling shale gas field is used to study the characteristics of reservoir shale.Physical and chemical properties and shale-fluid interaction tests show that the dispersion and the expansion of reservoir shale is weak.The fracture of Wufeng-Longmaxi Formation shale is extremely developed,and the shale reservoir is characterized by ultra-low water saturation.The research on the physical,chemical and mechanical interactions between unsaturated shale with fracture and fluids is carried out in terms of ion diffusion,chemical osmosis membrane efficiency,capillary pressure and multi-field coupling.Based on the electrochemistry theory of a clay mineral-water interface,a computational model of ion diffusion considered water saturation is obtained by using the constitutive equation of multiphase porous media coupling.There is a significant different of structure between reservoir shale fracture and matrix pore.According to the effect of critical water saturation and porosity on ion diffusion,the ionic diffusion coefficient of shale fracture is larger than matrix pore under unsaturated conditions.The reservoir shale fracture and matrix ion diffusion coefficient increase with the increase of water saturation.At higher pore fluid concentration and higher water saturation,the ion diffusion coefficient is larger.Relatively,effect of the cation exchange capacity to the ion diffusion coefficient of reservoir shale matrix is small.The saturated shale membrane efficiency test was carried out based on the pressure conduction experiment.The shale developed with fracture could not act as a semi-permeable membrane,and the chemical coupling between the reservoir shale and fluid only acted on the matrix pore.The shale membrane efficiency computational model considering water saturation is obtained by using the constitutive equation of multiphase porous media coupling with current density,flux of the salt and the Darcy velocity.Under unsaturated conditions,the membrane efficiency of shale will decrease with the increase of saturation of water.At high partition coefficient of counterion in the Stern layer,high cation exchange capacity,and low solute concentration in pore fluid,the membrane efficiency of shale is relatively high.Using this model,the characterization method of membrane efficiency evaluation and the water-absorbing capacity by chemical osmosis per unit pore of shale was established.The experimental method of reservoir shale capillary pressure evaluation was established through the shale water activity experiment,and the relationship between capillary pressure and water saturation was obtained.The capillary pressure of the shale matrix was characterized by the capillary pressure under the condition of reservoir water saturation.The low flowback rate of shale gas wells is mainly caused by fracturing fluid retention in fracture and the water absorption of matrix shale under the condition of low water saturation.As the water-absorbing dynamics of reservoir shale matrix,capillary pressure and chemical osmosis determine the water absorption intensity per unit area of shale reservoirs.The fracture network of shale gas reservoirs composed of natural fractures and hydraulic alterations determine the water absorption area of shale gas reservoir matrix.The mechanical model considering the coupling effect of solute diffusion and the responsed pore pressure by chemical osmosis between shale gas reservoir and fracturing fluid was established,and the multi-field coupling of fracturing fluid and reservoir was studied.The law of the temporal and spatial variation of pore pressure and fluid solute concentration under chemical action shows that the shut-in time and permeability affect the coupling rate;the reservoir shale porosity and membrane efficiency affect the coupling strength,which include the distribution of induced pore pressure and the peak pressure.As the shale developed with fracture,the solute concentration varies greatly during the interaction of fracturing fluid and shale reservoir without chemical osmosis.The ion exchange is more intense and the induced pore pressure does not appear.This study elaborates on the microscopic mechanism of reservoir shale and fluid,which can contribute to provide some theoretical support for process optimization of shale gas production.