Optimizing Fracture Stimulation in Low-Permeability Oil Reservoirs in the Ordos Basin

Abstract

Abstract Huge oil reserves are located in low-permeability reservoirs in the Ordos basin in China. These reservoirs have three key characteristics: low reservoir quality (10 to 13% in porosity), low reservoir permeability (0.3 to 2.0 md), and low reservoir pressures (pressure gradients of 0.0075 to 0.009 MPa/m). Fracture stimulation is essential for commercial production in these reservoirs. To better understand fracture growth behavior and to optimize treatment designs, microseismic monitoring and fracture modeling were performed for pilot wells in key areas. In some low-permeability oil reservoirs in the basin, fracture closure stress sometimes could not be obtained from commonly used pump-in/shut-in tests because pressure decline is extremely slow. Closure stress is one of the important parameters for fracture modeling. To obtain closure stress in these reservoirs, conventional pump-in/flow-back (PIFB) tests were modified and employed. Once the closure stress in the payzone was determined, the closure stresses in adjacent and/or bounding layers were estimated from sonic log and lithology data. Fracture modeling analysis was then conducted using a calibrated fracture model, which must match both the observed net fracture pressures and the fracture dimensions from microseismic mapping. This approach led to significant improvements in fracture treatment designs in these tight-oil reservoirs. The first improvement was to maximize effective fracture lengths in fracture designs. A novel staging technique was also developed to stimulate large pay intervals using a stress diversion technique without mechanical isolation. Larger treatments using a hybrid fracturing technique were recently implemented to stimulate ultralow-permeability oil reservoirs. Production results using the new stimulation techniques were significantly better than traditional techniques in the region. This paper discusses how an integrated, engineered approach was applied to stimulate tight-oil reservoirs in the Ordos basin and demonstrates the benefits of an integrated approach to developing marginal, unconventional liquid hydrocarbon reservoirs.