ABSTRACT: The Ordovician carbonate reservoir in the Shunbei oilfield of china has a buried depth of more than 5000 m, with a reservoir temperature of around 160℃ and a reservoir thickness of more than 100 m. It is challenging to realize commercial production by conventional acidizing fracturing technology. To solve this problem, a novel fracturing technology is proposed. By applying buoyant agent (BA) to inhibit the upward propagation and pumping temporary plugging agent (TPA) to restrict the fracture length, the hydraulic fractures will propagate substantially downward to realize the highly-efficient stimulation of the extremely thick reservoir.
In this paper, the BA and the TPA were optimized and designed. The plugging strength evaluation experiments in the fractures were carried out. The pumping process parameters were optimized based on the experiments and numerical simulations which study the migration and placement mechanism of BA and TPA. The fracturing design scheme for the target reservoir was developed.
The results show that the floating beads with a density of 0.4-0.7 g/cm3 are suitable as high temperature resistant BA. The optimal matching particle size combination is 20-40 mesh: 100-120 mesh: 180-200 mesh=1:2:5.2, and a thickness of 16cm can meet the pressure holding requirement of 10 MPa. It is recommended to use the low-viscosity slick water to inject the BA with the injection rate of 3-5 m3/min, and the concentration should be controlled at around 5 %, the pump only needs to be stopped once after injection, and particles of three sizes were mixed and injected together. The new degradable TPA is able to handle the 160℃ reservoir temperature. The optimal combination is 40-60 mesh: 160-180 mesh = 1:1.3. A thickness of 14cm can hold up to 10MPa sealing strength. It is recommended to use the slick water to inject the TPA with the injection rate of more than 6m3/min, and the optimal concentration is 3 %-4 %. The 160-180 mesh TPA should be injected after the 40-60 mesh TPA injection. In order to obtain a fracture with a half-length and a height of more than 100 meters, both the BA and TPA need to be injected twice.
By performing the lab experiments and numerical simulations, a novel fracturing technology with significant downward propagation of fracture (up to 100 m or more) has been proposed. This new technology is ready to be deployed to develop the ultra-deep, ultra-high temperature and ultra-thick reservoirs.