Determination of Equivalent Elastic Moduli for Coupled Geomechanical-Flow Simulation of SAGD

Abstract When characterizing stress sensitive reservoirs for reservoir performance prediction, considering flow simulation alone is insufficient; geomechanical and flow responses should be assessed concurrently. Geostatistical techniques are well developed and can be used to build heterogeneous models of reservoir properties as input to subsequent transfer functions such as flow and geomechanical simulators. In the case of conventional flow simulation, each geological realization consists of a structural model, facies model and petrophysical property models (porosity, permeability, saturation, etc.), which are used to solve the appropriate fluid flow equations. Rock mechanical properties play a similar role in geomechanical simulation as petrophysical properties play in fluid flow. Impact of heterogeneity consideration for geomechanical properties on coupled geomechanical flow simulation of SAGD process has been discussed by Khajeh et al. (2011) and they showed that to assess more accurate uncertainty analysis on flow and geomechanical responses of the SAGD process, considering realizations of the rock mechanical properties are required. The well documented Mechanical Earth Model (MEM) is a comprehensive geological model which can be used for coupled geomechanical-flow simulation of the SAGD process; however, homogenous rock mechanical properties are typically considered instead of stochastic models for computational reasons. The geomechanical response of a reservoir is sensitive to the values selected for geomechanical properties and it is practical to consider an equivalent homogeneous continuum such that geomechanical responses obtained from the homogenized model matches the geomechanical response of the truth (heterogeneous) model. Different analytical homogenization techniques are developed to determine equivalent elastic moduli (EEM), but the majority of these techniques consider specific configurations of facies and do not work well for complex spatial configurations such as the sand/shale sequences typical of the McMurray formation of Alberta-Canada. Considering sand geomechanical properties to be representative of the reservoir is a common approach. Moreover, a mixing rule averaging approach could also be used for determination of EEM. In this work, the accuracy of these various EEM methodologies are compared to EEM values obtained numerically by optimizing the geomechanical response of the reservoir. By knowing the vertical displacement profile (VDP) at the top of the reservoir for the fine scale (truth) model and the VDP’s obtained from the listed techniques, the accuracy of considered EEM values is assessed. Sensitivity of change in VDP with respect to different operating conditions, type of elastic deformation and also spatial distribution of facies is examined.

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