Coupled Reservoir Geomechanical Simulations For the SAGD Process

Abstract Conventional reservoir simulation may not fully characterize the recovery mechanism of the SAGD process in uncemented oil sands reservoirs. Uncemented oil sands exhibit significantly different geomechanical behaviour from that of their cemented counterpart. Therefore, to fully explore the SAGD recovery process, coupled reservoir geomechanical simulations are required. This simulation technique can characterize both the multiphase fluid flow and reservoir parameter variations based on certain stress-strain relationships of oil sands in the SAGD process. In this paper, sequentially coupled reservoir geomechanical simulations are conducted. It is based on both the reservoir simulator, EXOTHERM, developed by T.T. &Associates Inc. and the geotechnical simulator, FLAC, developed by Itasca Consulting Group Inc. This paper discusses the strategies and methodology of the coupled reservoir geomechanical simulations for the SAGD process based on these two simulators. Meanwhile, the results of the conventional reservoir simulations and the coupled reservoir geomechanical simulations are compared. These comparisons are conducted based on initial stress conditions of K0= 1.0. The simulation results show that differences exist between coupled reservoir geomechanical simulations and conventional reservoir simulations under certain SAGD operation conditions. Introduction Canada has huge oil sand resources. Development technologies of oil sand resources include world-class surface mining, SAGD, Vapex, and cold production. The SAGD process has proven to be a promising technology and will play a critical role in the development of oil sand resources. In addition to reliable reservoir characterization and production facilities, realistic predictions of production performance are an important component in the design of commercial-scale SAGD projects. The realistic predictions of SAGD performance by numerical simulation are an integral component in the design and management of a SAGD project. Conventional reservoir numerical simulation emphasizes multiphase flow in porous media but generally does not take the interactions between fluids and solids into account. Unfortunately, this treatment is not correct for oil sand material. Oil sands are locked sands(1), which have very high rates of dilation at failure. In Athabasca deposits, McMurray Formation oil sands display a high incidence of tangential contacts as well as common straight and interpenetrative contacts(2). Owing to the grain-to-grain contacts observed in locked oil sands, it shows the following characteristics: absence of cohesion, highly quartzose mineralogy, high strength, steeply curved failure envelopes, low porosities, lack of interstitial cement, brittle behaviour, and exceptionally large dilation rates at failure(3). These properties are the basis of large variations of reservoir parameters and processes in the SAGD process. In the SAGD process, continuous steam injection and fluid flow can change reservoir pore pressure and temperature, which can increase or decrease the effective stress in the reservoir. Thus, deformations can occur in some regions. Likewise, the deformations of the oil sand material (skeleton and pores) changes the fluid flow-related reservoir parameters(4). This is obviously a coupled problem. Therefore, coupled reservoir geomechanical simulations are required. In this paper, two existing well-established simulator codes, EXOTHERM and FLAC, are explicitly coupled to characterize both the fluid flow and geomechanical behaviors in the SAGD process.