Study On Optimization Of Carbon Dioxide Fracturing Parameters For Low Permeability Gas Reservoirs

CO2 fracturing is one of the reasonable and effective measures for low permeability gas reservoirs,and has gradually become the focus of research.The volumetric fracturing technology of large-displacement and large-volume CO2 has been applied to the development of tight oil and gas and shale oil and gas at home and abroad.Because of its remarkable effect,it is expected to use in the development of unconventional oil and gas reservoirs including low-permeability gas reservoirs.Promote.In the CO2 fracturing process,CO2 experiences different states-liquid state,supercritical state,and gaseous state.Liquid CO2 enters the formation as a fracturing anhydrous phase.Because the low-permeability gas reservoir is generally a high-temperature and high-pressure environment,CO2 changes from liquid to supercritical CO2(SCCO2,Supercritical CO2).After fracturing,it is discharged from the fomration.When the ambient temperature and pressure are below its critical temperature(31.1?)and critical pressure(7.38 MPa),CO2 is converted into a gas state.In low-permeability gas reservoirs,the analysis of CO2 phase and physicochemical properties with temperature and pressure changes,study of CO2 fracturing influencing factors,and optimization of reasonable and effective CO2 fracturing parameters are of great significance for mechanism analysis and on-site construction.Based on the characteristics and properties of CO2 phase behavior,this paper combines temperature and pressure models between wellbore and reservoir,and develops temperature pressures through numerical simulation methods for the wellbore(injection)and reservoir(fracture)stages of low-permeability gas reservoir CO2 fracturing.The law of variation,fracturing parameters(ie,reservoir parameters and injection parameters)has been studied to obtain the following conclusions and understanding:(1)Taking into account the wellbore and formation heat losses,using the Stars module,simulated the temperature and pressure changes in the CO2 injection process,and output the parameters such as temperature,pressure,density,viscosity,composition along the wellbore.The results show that the physical properties of CO2 are sensitive to temperature and pressure.The coefficient of variation near the low temperature and critical pressure is small,indicating that CO2 is easy to compress under this condition and it is easier to enter the natural micro-cracks.In addition,the expansion of the temperature after the increase in temperature,so the liquid CO2 pressurization to make seams stronger.In the initial stage of CO2 injection,the density is high,and the viscosity is small,which is favorable for the injection of fracturing fluid.During fracturing to shut-in,the CO2 density decreases and the viscosity increases,which is favorable for fracturing and joint formation and late flowback.At the same time,the supercritical CO2 is similar to the liquid density,the viscosity is close to that of the gas,and the diffusion coefficient is large,so the formed fracture conductivity is better.In summary,the liquid CO2 fracturing pressurization reforming ability and returning capacity are stronger than other fracturing fluids.However,due to the low CO2 viscosity,severe filtration loss,and difficulty in carrying sand,the CO2 fracturing has an effect on the injection displacement and injection volume.Dependency is serious.(2)A single-well coupling model was established to simulate the temperature and pressure field of the wellbore and reservoir,and the phase change of the CO2 fracturing process was analyzed:the ground and the wellhead were liquid,and in the early stage the wellbore was also liquid,and then increased with temperature and pressure.It is converted into a supercritical state;the formation and the fracture are in a supercritical state;in the process of backflow,the wellbore is in a supercritical state and the ground is in a gaseous state.(3)The sensitivity of CO2 fracturing reservoir parameters was simulated.The results show that the effective thickness of the reservoir and the matrix permeability have a great influence on the pressure and density,and have a negative correlation with it,but have little effect on the temperature and viscosity.(4)Simulated the sensitivity of CO2 fracturing injection.The results show that with the increase of injection volume,wellbore temperature and wellbore pressure increase,and CO2 along the crack direction and both sides of the increase in the area,indicating that the long slit must reach a certain degree The amount of injection.In addition,pressure in the reservoir propagates along the direction of the fracture and on both sides of the wave,especially in the direction of the fracture,and the high pressure is in the fracture.The greater the injection volume,the higher the pressure and the larger the range.However,when the injected amount reaches a certain value,the excessive accumulation of CO2 in the near-wellbore reservoir does not have a significant effect on the further extension of the fracture,which results in a large amount of waste,and therefore it is necessary to optimize the appropriate injection amount.(5)The sensitivity rule of CO2 fracturing injection displacement was simulated.The results show that the injection displacement has a great influence on the pressure,and with the increase of injection displacement,the wellbore pressure becomes higher,the temperature becomes lower,the density becomes larger,and the viscosity changes.The seam is shorter,the pressure is higher,as well,the pressurization effect is more obvious.However,as the injection displacement increases,the friction in the wellbore rises linearly,so that a suitable displacement is required.(6)After CO2 injection,it is beneficial to return later,and the higher the CO2 injection is,the more favorable it is to return.During the flowback process,the daily production volume of single wells with direct water injection and fracturing is 11m3/d,and the daily production of the first injection CO2 fracturing wells is 48m3/d,and the return flow is increased by 4 times compared with the former.For the low-permeability tight sandstone gas reservoir in the text,the optimal CO2 injection volume is more than 300m3,and the displacement is about 5m3/min.