Fully coupled thermo-hydro-mechanical model with strain-dependent properties for numerical simulation of heavy oil production

Abstract

Strain-dependency of permeability is a key feature for coupled multiphysics phenomena during heavy oil production under steam stimulation. This feature is also the basis for accurate estimation of steam needed to properly stimulate a given target heavy oil block. In this study, a fully coupled thermal-hydro-mechanical model is established and applied to the numerical simulation of steam injection in a block of western China Xinjiang region's heavy oil field. Strain-dependent relationships have been established for hydraulic conductivity as well as Young's modulus of reservoir formations. The distribution of reservoir temperature field and pore pressure field is obtained, and a quantitative evaluation of steam injection effectiveness is provided. The principal contents include: 1) A simplified fully coupled thermal-hydro-mechanical model for heavy oil reservoir under steam injection is presented. The model incorporates strain-dependent characteristics for rock's permeability/hydraulic conductivity and elastic modulus. 2) The simplified fully coupled model is implemented via the user subroutines of Abaqus software, and 3D finite element simulations are conducted for the steam stimulation operation and oil production processes in the heavy oil block. The temperature field and pore pressure field of the reservoir are obtained under given production history and steam injection conditions. These numerical results have provided an accurate evaluation of steam injection effectiveness. These numerical results indicate that: 1) The original design of steam injection volume and soak time is insufficient to connect the temperature rise regions around wells A1 and A2 in the block in order to get ideal production rate. The temperature rise data obtained from monitoring wells verify the accuracy of the calculation results, demonstrating the accuracy and practicality of the proposed theoretical model. 2) It is found that the maximum local pore pressure induced by steam injection may exceed the bottomhole pressure of injection wells. This is due to the thermal expansion of both solids and pore fluids caused by rapid temperature rise. 3) For the target reservoir, numerical results indicate that further steam injection for 17 days on top of the original design will connect the temperature rise regions around wells A1 and A2, and the minimum temperature at various points between wells will exceed 40 °C.