When Is It Important to Consider Geomechanics in SAGD Operations?

Abstract

Abstract This paper generalizes the typical reservoir conditions for which SAGD is being implemented or considered in order to parametrically analyze the influence of geomechanical factors on the startup and production phases of SAGD projects. Numerical simulation of the SAGD process using a thermal reservoir program and a geomechanical program is used to assess the relative influence geomechanics may have on SAGD operations. While variations to the initial dual well SAGD process are becoming numerous, this study presents analysis results for only dual well SAGD geometries. A primary focus of this research is to clearly define the role of pore volume change (compressibility or shear-induced) on the basis of fundamental geomechanical parameters and correct an ongoing misconception that formation dilation can be simulated based on injection pressure alone. Dilation is a complicated process controlled by significantly more parameters than just injection pressure. Clear, definable guidelines are presented to aid SAGD project designers in determining the relative importance of the geomechanical response of their particular reservoir. The major geomechanical/ reservoir factors studied include:initial in situ effective stress state;initial pore pressure;steam injection pressure and temperature; and,process geometry variables, such as well spacing and wellpair spacing. Introduction The geomechanical response of an oil sands/heavy oil reservoir is complex, reacting to both near and far field temperatures and pore pressures. To aid in elucidating fundamental geomechanical principles affecting the steam assisted gravity drainage (SAGD) rocess and to gain insight into a reservoir's response to thermal loading and pore pressure change, a parametric analysis of the SAGD process within three separate but similar reservoir settings was completed. Since the objective of this paper is to highlight how the geomechanical response affects the SAGD process, twodimensional analyses of reservoir cross sections with basic treatment of the inherent complex geology have been conducted. Thenalysis results presented herein are not intended to portray a history match of any particular SAGD operation. Fully coupled thermal-stress-fluid flow analyses are extremely difficult to conduct. While fully coupled mathematical formulations exist(1–3), the computational effort in their solution is onerous and continues to be an area of active research. Coupled solutions that consider single phase flow only have become common(4, 5) and are routinely utilized in both advanced reservoir simulations and geotechnical/hydrogeological simulations. Consequently, a decoupled approach was adopted for the analyses presented herein. The decoupled approach consisted of conducting a reservoir simulation of SAGD using STARS (an advanced process and hermal reservoir simulator developed by CMG in Calgary) and utilizing the temperatures and pore pressures as input to an effective stress geomechanical simulation of the formation response to SAGD. In agreement with Tortike(3), that while the removal of "feedback" to the fluid flow model would not allow conclusions to be drawn regarding the fluid solution, this decoupled approach would permit conclusions to be drawn and inferences to be made concerning the likely response of the formation to the SAGD process.