Study on Filtration Property of Hypercritical CO₂ Fracturing Fluid for Shale Reservoirs

Filter loss of fracturing fluid plays a major role in the geometry of fractures created by the fracturing fluid and the performance of fracturing. Reports on the filtration property of hypercritical CO₂ fracturing fluid used in shale formation fracturing have not been found at present. Experiments have been conducted on shale reservoir cores (buried at 1,300-2,300 m, with natural fractures developed) taken from the Longmaxi Formation in Sichuan Basin, using a self-made filtration simulator, to study the filtration property of CO₂ fracturing fluids with different initial phase states, at 55℃ and (confining pressure) 20 MPa, and different injection pressures and back pressures. Analyses of the experimental data indicate that in the conditions mentioned above, the filtration coefficient of CO₂ fracturing fluids is between 1.00×10^-4 m/min^0.5 and 48.17×10^-4 m/min^0.5, with the spurt loss being negative. The filtration rate of CO₂ fracturing fluid increases with increases in injection pressure, differential pressure, and the widths of fractures. The dominant factor affecting the filtration property of CO₂ fracturing fluid in different conditions is different; at hypercritical state (7.38 MPa, 31.1℃), the CO₂ fracturing fluid, because of its high viscosity, has a lower filtration coefficient.