Integrated method for nitrogen-assisted carbon dioxide fracturing and development of shale oil reservoirs

An integrated method for nitrogen-assisted carbon dioxide fracturing and development of shale oil reservoirs, comprising:

• Step 1:
• Fracture the target shale oil reservoir with nitrogen-assisted carbon dioxide;
• Step 2:
• After fracturing, firstly inject carbon dioxide gas into the target shale oil reservoir, and then inject nitrogen gas to push the carbon dioxide gas into the further location of oil reservoir;
• Step 3:
• Shut in the well, to ensure the injected carbon dioxide gas can be fully recombined into shale oil , expand the volume of shale oil, reduce the viscosity, and extract the light components of shale oil;
• Step 4:
• After shut-in, open the well to implement depletion production, and terminate the first cycle of production when the reservoir pressure is depleted to ½ of the original reservoir pressure;
• Step 5:
• After the first cycle of production, inject carbon dioxide gas into the target shale oil reservoir, and then inject nitrogen gas to push the carbon dioxide gas into the further location of the oil reservoir while increasing the reservoir pressure to be close to the original reservoir pressure, where the slug volume of carbon dioxide gas and nitrogen gas is 1.5 times of that in Step 3;
• Step 6:
• Shut in the well in the target shale reservoir for 1.5 times of that in Step 3;
• Step 7:
• After shut-in, open the well to implement depletion production, and terminate the second cycle of production when the reservoir pressure is depleted to ½ of the original reservoir pressure;
• Step 8:
• In the subsequent production process, the slug volume of the injected gas and the shut-in time are 1.5 times of those in the previous cycle, and Steps 5 to 7 are repeated for each cycle;
• The specific fracturing operation in Step 1 is:
• Inject 0.1 PV high-pressure carbon dioxide to form slugs in the early stage, and then inject 0.1 PV high-pressure nitrogen;
• Increase the injected gas pressure to rise the pressure in the wellbore to be greater than the shale oil fracture pressure, fracturing the target shale oil reservoir and injecting proppant into the fractures so that the fractures will not be closed, which is conducive to subsequent gas injection.