Geomechanical Modelling Of Phase A At The Aostra Underground Test Facility

Abstract The Alberta Oil Sands Technology and Research Authority (AOSTRA) and its participants have successfully completed the Phase A pilot operation at the Underground Test Facility (UTF). As part of the Phase A project, a numerical modelling program has been underway to study the geomechanical response of the reservoir during the stream assisted gravity drainage (SAGO) process. One component of the modelling program involved a geomechanical analysis of an idealized Phase A cross-section. This paper presents and discusses the results of this geomechanical analysis. The geomechanical analyses were completed using a decoupled approach between a reservoir simulator, ISCOM and a finite element program, CONOIL-IJ. The major emphasis of the geomechanical analyses was to conduct parametric analyses in order to examine the relative influence of material parameters on reservoir behavior. Several geomechanical parameters such as initial stress ration (Ko) and material stiffness and several reservoir parameters such as kh I kv and ka were examined as part of the parametric analyses. The relative influence of each parameter is discussed, The results of the geomechanical analyses are used to illustrate the complex relationship between temperature (strain due to temperature change) and pore pressure (strains due to effective stress changes) and their concomitant effect on reservoir behavior. The analyses also reveal the potential for the development of zones of shear induced porosity enhancement occurring between pairs of wells rather than at an injection or production well. Depending on the magnitude of this porosity enhancement, the impact upon reservoir simulations, which generally do not incorporate shear induced porosity changes, may be significant. Introduction The Alberta Oil Sands Technology and Research Authority (AOSTRA) and its participants have recently completed the Underground Test Facility (UTF) Phase A pilot. A numerical modelling program, to study the geomechanical response of the reservoir during the steam assisted gravity drainage (SAGO) process, is being performed as part of the Phase A project. The identification of important mechanisms of behavior and the influence particular geomechanical properties have on reservoir behavior form the major objectives of the geomechanical modelling program. The ultimate goal will be a geomechanical history match analysis of Phase A. This will involve the synthesis of the field instrumentation results, the laboratory geomechanical test results and the numerical modelling results. Recognizing the complexity of a thermal-geomechanical- fluid flow analysis, an analysis of an idealized Phase A cross-section was conducted. This analysis was comprised of a reservoir simulation and a geomechanical analysis. The geomechanical analysis was subdivided into two stages. Stage one involved a mechanistic analysis which examined the effects of geomechanical principles on the stresses and pore pressures developed within the reservoir. Stage two involved a parametric analysis where selected properties of the oil sands reservoir were varied systematically. By comparing the results of each analysis, the magnitude of influence that anyone property has on the formation response can be determined. The major thrust of the parametric analyses is to provide a framework for the evaluation of laboratory geomechanical test results and the selection of material parameters during a geomechanical history match analysis of the Phase A reservoir performance.