The fractures in Permian stratum of Shunbei oil and gas field are well developed, the formation fracture pressure is low, and the leakage problem is very severe during the cementing jobs. It is difficult to solve this problem by using traditional low-density cement slurry and staged cementing process techniques. To solve this problem, a new type of thermo-sensitive plugging spacer and foam cement slurry were developed, and the cementing operation without leakage was successfully realized. The thermo-sensitive plugging spacer is developed based on the principle of "bridging-grading", consisting of thermo-sensitive memory expandable mesh, thermo-sensitive memory expandable pellet, rigid supporting particles, organic fibers and inorganic fibers, which the thermos-sensitive memory expandable mesh and thermo-sensitive memory expandable pellet can expand several times under downhole high temperature conditions. This thermo-sensitive plugging spacer system can meet the requirements of 8.5 MPa pressure-bearing for a fracture of 3 mm wide and can withstand a high temperature of 150 °C.
By developing high performance foaming agent and stabilizing agent, selecting larger-temperature difference retarder and accelerator, etc., a foamed cement slurry system is developed. The cement slurry density was adjustable in the range of 1.10 - 1.60 g/cm3, and the API fluid loss was less than 50mL at 120 °C. The compressive strength was good and the comprehensive performance was excellent. To solve the problem of large density difference of cement slurry caused by the compressibility of foam, a cement slurry aerating method was designed based on well depth, temperature, and pressure. The aeration volume of cement slurry could be adjusted in a real-time way according to well depth, cement slurry density, and pumping rate, so as to guarantee the uniform density of foamed cement slurry in the annulus.
Using the developed thermo-sensitive plugging spacer system and foamed cement slurry system, the leakage-proof cementing process has been formed for low-pressure leakage-prone formation in Shunbei oil and gas field. This technology has been applied for 2 wells, both the thermo-sensitive plugging spacer and foamed cement slurry returned to the ground successfully, and the cementing quality was excellent. This technology can provide a reference for the leakage-proof cementing job of fractured formations in other blocks.
