Stress-cycle fracturing technology suitable for tight oil reservoirs in the Ordos Basin

Type Ⅱ reservoirs of Chang 7 tight oil reservoir in the Ordos Basin are characterized by low brittleness index, larger horizontal bidirectional stress difference and undeveloped natural fractures, so complex fractures can be hardly created by means of conventional mixed water fracturing and the single-well production rate is low. To deal with this situation, a stress-cycle fracturing technology was developed. In this paper, its stimulation mechanisms were clarified by means of physical stimulation experiments, and the technological optimization and tool matching were completed. In this process, the liquid of high proppant concentration is pumped by using the stress-cycle fracturing tool, and the net liquid is injected into the annulus. The proppant concentration and displacement at the bottom hole is controlled in real time according to the formation response and pressure change in the process of fracturing. Cyclic stress is applied to the reservoir so that the reservoir is suffered from the fatigue failure and fracture network is formed. The stress-cycle fracturing technology was test on site in 37 wells. Its average construction size is equivalent to that of conventional mixed water fracturing, but its construction displacement is only half of the conventional mixed water fracturing when inter-fracture pressure increasing has been realized many times. And compared with the reference well, the effective stimulated reservoir volume is increased by 44.5% and the single-well daily oil production is increased by 1.6 t.