Pressure transient testing techniques such as pressure buildup, pressure drawdown, and constant rate injection have been used in petroleum industry for well performance evaluation and reservoir characterization. Conventional method of analysis usually assumes that permeability and compressibility of the reservoir formation are constant or a function of pore pressure. This assumption has limitations when applied to an oil sands reservoir because of the unconsolidated deformable nature of oil sands. Three injection tests were conducted in an oil sands reservoir at a depth of about 500 m. History matching of the field injection data using a fully coupled reservoir-geomechanical simulator demonstrates that the permeability and compressibility of oil sands are interrelated and effective stress dependent.
The basic principle of pressure transient testing techniques, which are prevalent in petroleum industry, is to create and observe changing wellbore pressures. Appropriate and comprehensive interpretation of recorded well testing data provides information into reservoir properties such as permeability and compressibility. Conventional analysis is based on the principle of mass conservation, assuming that the permeability, porosity and compressibility of fluid are dependent on the pore pressure only. This simplified assumption has limitations when applied to an oil sands reservoir because oil sands will deform subjected to fluid injection and withdrawal, thereby causing changes in pore pressure and total stresses. Therefore, in order to interpret the well testing data in oil sands reservoir, coupled diffusion-deformation analysis, which considers the principle of mass conservation and equilibrium, should be used.1,2,3 In this paper, a history matching of the pore pressure responses of three injection tests in an oil sands reservoir was carried out using a fully coupled reservoir-geomechanical simulator. This exercise provides some estimate on the flow (permeability) and deformation response of oil sands subjected to water injection.
Three injection tests were conducted in a cased well at Burnt Lake, Alberta. The well was completed with a diameter of 178 mm. The perforation zone is 5 m in the middle of the oil sands layer, which is 21 m thick, and has an overburden of 505 m. The overlying and underlying formations of the oil sands layer can be considered impermeable because it is capped and underlain by shale layers of 3 to 5 m. The oil sands layer has an initial pore pressure of 3.3 MPa and its in situ porosity is 33%. The void ratio of oil sands layer is 0.4893, which is the ratio of the volume of void to the volume of solid.
In each test, cold water was injected into the oil sands formation through the tubing at a controlled rate for some interval, and then the well was shut in allowing the bottom hole pressure to decay to its initial in situ state. The bottom hole pressure was monitored during the injection and shut-in periods. The injection rate was increased from test 1 to test 3. The injection rates of three tests are presented in Figure 1. The bottom hole pressures monitored during the well testing are shown in Figure 3.
