Risk Management and Control for CO2 Waterless Fracturing

Abstract The fluid is seen with the high pressure and low temperature during the CO2 waterless fracturing operation, which is prone to accidents such as casualties and environmental pollution. In this paper, a complete operation safety management approach and a contingency plan are developed, with respect to the CO2 waterless fracturing. For the fracturing equipment and support facility, high-pressure-resistant, low-temperature-tolerant specialized wellhead, valve and surface pipeline are required. The pipe wall thickness should exceed the N80 tubing criterion, and the relief line has to be anchor-fixed. In order to prevent suffocation caused by CO2, all vehicles and personnel must stay windward. All safety valve outputs should be arranged leeward, facing no personnel operation and equipment placement. Crews must appropriately wear protection that prevents suffocation and cold injury. While operating valves, they should stand beside the valve and operate steadily, so as to avoid object strike induced by the dry ice. The contingency plan corresponding to the operation condition is developed. In the case of CO2 leakage, the operation should be stopped immediately, personnel will be evacuated to the open area on the windward side, and the air blower at the field should be turned on. As for the occurrence of gaseous CO2 in the surface process, one has to identify the gasification position in a timely fashion and adjust the vent valve of the blender and the surface pipeline and the wellhead valve correspondingly. If gaseous CO2 occurs inside the wellbore, the countermeasures of close surveillance, stabilizing pressures by injecting CO2 at a small rate, and pressure relieving after switching off pumps are implemented, depending on the severity of the case. Upon the arrival of the blender pressure at 2.4 MPa, the safety valve is open for venting. Synergistic pressure relief should be executed by opening the sand-input ball valve, gas valve and liquid valve, if the pressure continues to grow. So far the CO2 waterless fracturing has been successfully operated for over 20 times, in the tight oil reservoir, shale oil reservoir, low-pressure sensitive gas reservoir, etc. No safety accident has been reported, which demonstrates the rationality and operability of the proposed risk management and control approaches for the CO2 waterless fracturing.