Volume And Permeability Changes Associated With Steam Stimulation In an Oil Sands Reservoir

Abstract

Abstract Field performance and previous research have indicated that the stresses and deformations caused by steam stimulation processes in oil sands formations alter the oil sands structure. The resulting pore volume changes (that is, pore volume and pore interconnectivity) affect the reservoir permeability and water mobility. The relevant geomechanical and fluid flow properties can be measured in the laboratory under the appropriate temperature, stress and fluid pressure conditions which simulate field conditions. Reservoir models which incorporate the geomechanical effects accompanying steam stimulation in oil sands formations can then incorporate these measured properties. The volume and permeability changes are the results of three effects: a change in the mean principal effective stress, a change in the shear stress and a change in temperature. Testing equipment and experimental procedures to measure these material characteristics are described. Initial test results on Cold Lake oil sands are compared to research published in the literature on unconsolidated sands and on sandstones. The stress and material properties which affect such permeability changes are discussed. Introduction Cyclic steam stimulation is the most important enhanced oil recovery method in Alberta for in-situ production of oil sands and heavy oil reservoirs. Hydraulic fracturing through the injection of steam above fracture pressure is an integral part of the cyclic steam process. Recent history matching of reservoir performance has confirmed laboratory research which suggested that significant pore volume changes are associated with the mechanical response of an oil sands reservoir to steam stimulation and fracturing(1). The resulting changes, in volume and pore interconnectivity, alter the reservoir permeability. The effects of steam stimulation on the oil sands pore volume can be considered in terms of the increases in the reservoir's. temperature, pore pressure and shear stress. Temperature increase causes thermal expansion of the sand grains and sand structure. Pore pressure increase during steam injection decreases the effective confining stress and causes an unloading of the reservoir. For an isotropic in-situ stress state in the oil sands reservoir, pore pressure injection will also generate shear stresses and shear strains in the sand structure. These three processes will combine to result in a net change in the reservoir pore volume and permeability. To isolate and determine the magnitudes of permeability change from these three effects, different laboratory testing procedures have been developed. All tests are described in terms of effective stress. The stress unloading of the reservoir by fluid injection is modeled by testing core under isotropic stress conditions. A pore pressure injection test is performed with the three principal stresses (σ1, (σ2, (σ3) kept equal during stress reduction. The effect of shear stress can be determined by a test which follows a constant mean principal stress (Pm) path where Equation (Available in full paper). However, this stress path does not model reservoir stress paths and a pore pressure injection test is used instead with the specimen under initial an isotropic stresses similar to the initial in-situ stresses. Therefore this test gives volume and permeability changes which incorporate effects from both a reduction in Pm and an increase in shear stress.