Geomechanical Response of Heavy Oil Reservoirs to the Steam Assisted Gravity Drainage Process

Abstract

Abstract In the steam assisted gravity drainage (SAGD) process, a special form of steam flooding, the movement of oil to the production well is caused by gravity forces and is approximately parallel to the interface which forms the boundary of a growing, steam saturated zone known as the "steam chamber". SAGD results in a complex interaction of geomechanics and multiphase thermal flow in cohesion less porous media. Changes in fluid pressure and temperature result in changes in stress and deformations that affect formation shearing and absolute permeability. For thermal recovery projects undertaken in Alberta oil sand deposits, the most intense geomechanical monitoring program was completed during the Phase A SAGD trials at an Underground Test Facility constructed northwest of Fort McMurray, Alberta, Canada. As part of the geomechanics program of the Phase A trials, an extensive laboratory testing program was conducted to examine the thermomechanical properties of oil sands and its intraformational shales and the underlying limestone. The testing program comprised thermal expansion, thermal conductivity, compressibility, stress-strain, strength and gas evolution tests. Selected results of this testing program are presented and discussed. Particular attention is paid to the relevance of these thermomechanical properties to the SAGD process. To illustrate the impact of formation property values on the SAGD process, the results of numerical parametric analyses are presented and discussed. These results, linked back to the laboratory determined properties and compared with field measured formation response, provide substantial insight into the behavior of unconsolidated heavy oil deposits for SAGD thermal recovery projects. In particular, increases in absolute permeability of 30% to 50% may occur in "cold" regions of the reservoir in "front" of a growing steam chamber. Introduction A special form of steam flooding, known as steam assisted gravity drainage (SAGD), has been developed to provide a process which could recover more oil than is possible by conventional steam flooding processes. In the steam assisted gravity drainage process, the movement of oil to the production well is caused by gravity forces and the geometry is such that the oil moves approximately parallel to the interface which forms the boundary of a growing, steam-saturated zone known as the steam chamber. The SAGD process results in a complex interaction of geomechanics and multiphase thermal flow in cohesionless porous media. The geomechanical response of an oil sands reservoir to fluid pressure changes or to temperature changes results in stress and deformations that affect formation shearing, hydraulic properties such as absolute permeability and the stability of underground openings (uncased wells, tunnels and shafts). A temperature increase causes thermal expansion of the sand grains and sand structure. A pore pressure increase during steam injection decreases the effective confining stress. For an anisotropic in situ stress state in the reservoir, pore pressure injection will also generate shear stresses and shear strains in the sand structure. These processes combine to result in a net change in reservoir pore volume and permeability. Most reservoir models used in simulating the steam assisted gravity drainage process do not account for the geomechanical responses described above. Several important parameters which affect process performance are however, directly impacted by the geomechanical response of the reservoir. Porosity is directly proportional to the bitumen production yet porosity can be altered due to shear induced volume changes or changes in effective confining stress. Absolute permeability of the reservoir affects the drainage of fluids from the steam front and therefore the frontal advance rate and the production rate; it is one the most important parameters in the simulation of the SAGD process. Stress change induced volume changes within the reservoir, especially within non-heated zones, will result in changes in absolute permeability. P. 485^