Tight gas/tight oil reservoirs require fracture stimulation to achieve commercial rates of hydrocarbons. Fracturing operations involve pumping considerable volumes of proppant and water/gel into the reservoir. Rapid cleanup of fracturing fluids and residual proppant ensures the desired goals of the stimulation operations are achieved: enhancing the flow capacity of the well while minimizing the risk of proppant damage to surface equipment. It is normal for significant quantities of proppant and frac fluid to flow back after opening the well for cleanup. Frac fluid often contains broken cross-linked gel, which must be flowed back to ensure cleanup of the fracture and minimize plugging of the fracture face and the proppant pack.
In North America, it is not unusual for frac fluid recovery to be only a small fraction of the amount of injected fluid. Frac fluid recovery of 5 to 30 percent is not unusual and 50 percent is often considered excellent.
In the Sultanate of Oman, BP is currently achieving 50 to 90 percent frac fluid recovery, while pressure transient analysis indicates post-frac skin damage figures of -6 or better, indicating excellent stimulation effectiveness. Critically, no proppant has flowed through to the surface facility. These cleanup procedures have been conducted in wells with a variety of frac types including 450,000 to 1,000,000 lb cross-linked gel fracs and multiple 17,000 bbl slick water fracs.
An analysis of the procedures used to achieve these results will be presented in this paper. It indicates that allowing the frac to close and opening the well on a moderate choke, with re-direction of the post-frac fluid through effective sand management systems, followed by flowing the well at a managed drawdown against the reservoir has achieved excellent results. These results underpinned the decision to move forward in this multi-billion dollar development project.