Parameter Optimization for CO₂ Water-free Energy-storing Fracturing

In CO₂ water-free energy-storing fracturing, the fracturing fluid generates fractures of high flow conductivity in formations. By interacting with crude oil, the fracturing fluid also shows some special stimulation characteristics that are different with those of the conventional fracturing fluids, such as viscosity reducing, volume expansion and phase mixing etc. Thus, the optimization design process of the CO₂ water-free energy-storing fracturing fluid is different with that of the conventional fracturing fluids. In CO₂ waterfree energy-storing fracturing fluid design, the interaction between crude oil and CO₂ is tested to obtain the minimum miscibility pressure. Using the relation pattern between the length of fractures caused by liquid CO₂ fracturing and the physical parameters of the reservoir, the fracture parameters can be determined. Using 3-D reservoir numerical simulation method, the sweep area of CO2 and the area of miscible zone can be determined based on the value of minimum miscibility pressure, the fracture parameters and the injection pressure of the bottom of the wellbore, and finally the optimum amount of CO₂ can be determined. Using FracProPT, a virtual 3-D fracture simulation software, with the friction loss along the flowline, the operational flow rate can be determined from the perspective of hydraulic power efficiency and economic benefits and the prerequisite of ensuring the safety and stability of wellhead and downhole string. Using the formation temperature-pressure computational model established after fracturing operation, the area of the miscible zone after well shut-in can be simulated, and from the simulation it can be determined that the optimum well shut-in time is the time spent for the miscible zone to become maximized, that is to say, the time spent when the bottom hole pressure after fracturing is greater than the minimum miscibility pressure. CO₂ water-free energy-storing fracturing technology has been successfully applied in Jilin Oilfield to fracture tight reservoirs. Oil production rate was obviously enhanced after reservoir fracturing. 6 wells fractured with the CO₂ water-free energy-storing fracturing technology have oil production rates that are at least one time of those wells fractured with conventional fracturing technology. Study results have shown that CO₂ water-free energy-storing fracturing is able to substantially enhance oil recovery, and the method for the parameter optimization design of the CO₂ water-free energy-storing fracturing is reasonably practical.