Hydraulic fracturing is an important means to develop tight reservoirs. However, many drawbacks have been exposed in the hydraulic fracturing process for a tight reservoir. For example, the clay minerals in the fractures are soaked and hydrated by fracturing fluid, which results in high pollution and poor transformation effect of the near-well area and seriously affects the flow capacity of reminding oil and the fracturing effect of tight reservoirs. This paper proposes a fracturing method that uses CO2 as the fracturing fluid to transform the seepage capacity and quickly supply energy for a tight reservoir. The EOR mechanism of CO2 fracturing has also been discussed by analyzing the physical properties change of CO2 in reservoir conditions. Three kinds of advantages can be shown in the fracturing process with CO2 fracturing fluid. Firstly, the fractures of CO2 fracturing are more complex than those caused by hydraulic fracturing, which greatly enhances the seepage capacity of a tight reservoir after CO2 fracturing. Secondly, a quick reservoir energy supply can be found in the process of CO2 injection, because of the high diffusion ability and good solubility of CO2 in oil of tight reservoir pores. Thirdly, as an inert gas, CO2 can reduce the influence of water sensitivity on reservoir rock, which can effectively resist the clay swelling pollution to oil production. The numerical simulation method of CO2 fracturing is used to optimize the supercritical CO2 fracturing parameters. According to the simulation results of CO2 fracturing, the optimized fracture half-length in the tight reservoir is 130 m, the fluid conductivity ability of a sand-filled fracture is 30 μm2·cm, the injection rate of CO2 fracturing fluid is 5.5 m3/min, and the energy storage time of a CO2 injection well is 7 days in the tight reservoir with permeability 0.35 mD. Compared with before fracturing, the actual CO2 fracturing wells in the study block also have an obvious oil increase effect, with a daily oil increase of 3.2 tons and a cumulative annual oil increase of 1650 tons.
Keywords tight reservoir, CO2 fracturing, numerical simulation, parameter optimization