Research On Influence Mechanism Of Different Pulsation Parameters On Shale Flow Conductivity

Hydraulic fracturing technology is an efficient stimulation measure for oil and gas wells and has a wide application prospect.Pulsation hydraulic fracturing technology makes up for the deficiency of traditional hydraulic fracturing technology.It has been applied not only in coalbed methane,but also in unconventional oil and gas reservoirs such as tight oil and shale gas.Pulsating hydraulic fracturing is a technology that produces pulsating pressure by pulsating fluid at a specific frequency to fracture the reservoir rock.Pulsating hydraulic fracturing can significantly improve the recovery of unconventional tight gas in low-permeability,low-porosity formations by creating more complex fracture networks than conventional hydraulic fracturing.In the process of pulsation hydraulic fracturing,the variation of pulsation parameters has a significant influence on the formation of fracture network.Therefore,it is very important to understand the characteristics of fracture conductivity and fluid flow change in fracture under pulsation hydraulic fracturing to improve reservoir recovery.The purpose of this paper is to study the influence mechanism of pulsating parameters on the conductivity of straight fractures in shale.The physical and mechanical parameters and mineral composition of the black shale of Niutitang Formation of Lower Cambrian in Northwest Hunan are tested,the shale pulsating conductivity tests under different pulsating parameters are carried out,and its pulsating conductivity is analyzed and discussed.Finally,the following conclusions are obtained:(1)The black shale of Niutitang Formation contains a large number of brittle minerals,of which the content of quartz is the highest,reaching34.94%,followed by pyrite,reaching 22.15%.In addition,it contains a small amount of chlorite clay minerals,accounting for 6.53%.The true density of shale sample is 2.5×10~3 kg/m~3;Uniaxial compressive strength is 139.88 MPa;The average elastic modulus is 28.11 GPA;Poisson’s ratio is 0.28;The tensile strength is 18.3 MPa.(2)In this study,the flat fractured shale is selected to carry out the pulsating conductivity test of hydraulic fractures.The pulsation diversion test system mainly includes pulsation control device,pulsation liquid injection device,core holder and monitoring device.By adjusting the pulsation control device,the pulsation liquid injection device can be controlled to form pulsating fluid with specific frequency and duty cycle.(3)In the pulsation frequency test,the input pressure generally showed an increasing trend with the increase of the pulsation frequency.The change process was divided into two stages:rising and stable,and the amplitude of the pulsating pressure dropped rapidly.In the pulsation frequency range of 2-6 Hz,it reaches 93%.When the pulsation frequency is 8 Hz,the pulsating pressure wave in the fracture is the largest,the change value of the fit degree is the largest,and the flow conductivity is the best.The increase of initial pressure generally shows an increasing trend,and the pressure amplitude first increases and then decreases.When the initial pressure of pulsation is 6 MPa,the pulsating pressure wave in the fracture is the largest.Therefore,when the pulsation initial pressure is 8 MPa,the change value of the degree of conformity is the largest,and the flow conductivity is the best;in the pulsation duty cycle test,the input pressure increases slightly with the increase of the pulsation duty cycle,and the pressure amplitude decreases rapidly.When the flow rate is 20%-80%,the fracture seepage flow rate increases,and when the pulsating duty ratio is80%-100%,the flow rate decreases slightly from the state of pulsating hydraulic fracturing to steady flow.(4)The pulsation frequency model is fitted by a sine function;the relationship between the pulsation initial pressure and the conductivity is in line with the research results of the fracture seepage flow response under different pulsation initial pressures.Therefore,segmental fitting is used to establish the conductivity model.The conductivity model of empty ratio is established by power function fitting.The change of conductivity and the change of fit are in a linear relationship,and based on this,the stimulation effect of different pulsation parameters on fracture closure can be predicted.The final calculation results show that the prediction results of the pulsation frequency and the pulsation initial pressure are consistent with the experimental phenomenon,while the pulsation duty cycle test results are not ideal.(5)The sensitivity evaluation results show that increasing the initial pressure of pulsation is the most effective method to improve the conductivity of straight fractures.In addition,when the initial pressure of the pulsation is too high and it is difficult to continue to increase the flow conductivity,an ideal flow conductivity effect can be finally obtained by adjusting the appropriate pulsation frequency or increasing the pulsation duty cycle.The AHP analytic hierarchy process was used to quantify the influence of the three pulsation parameters on the conductivity,and the corresponding weights of pulsation frequency,pulsation initial pressure,and pulsation duty cycle were calculated to be 28.952%,64.635%,and6.413%,respectively.