Numerical assessment of the maximum operating pressure for SAGD projects considering the effects of anisotropy and natural fractures

Abstract

There has been an increasing awareness of the importance of caprock integrity during Steam Assisted Gravity Drainage (SAGD) operations. However, mathematical tools that have been developed for caprock integrity studies have not incorporated an important characteristic of cap shales, which is the intrinsic anisotropy and structural anisotropy caused by the natural fractures (NFs) in the cap shale. This paper addresses the effects of the intrinsic anisotropy of the shales and also the existence of NFs on the Maximum Operating Pressure (MOP) in SAGD projects.In this study, a new constitutive model is developed and incorporated to consider the effect of NFs and intrinsic anisotropy in the cap shale. A coupled numerical tool was utilized to determine the MOP for different case scenarios in the number of fracture sets, fracture density and dip angle, as well as the height. Results of all cases were analyzed to evaluate their effect on the MOP.Results of the coupled model indicate continued integrity of the caprock for the case study project for the current operational program for almost all the considered case scenarios, except for an extreme case. Therefore, higher injection pressures than the current pressure were investigated to assess the MOP. Results indicate that the MOP was highly sensitive to the fracture density, direction, and height. Results also display a minor impact for the horizontal fractures on the MOP for the case study while fractures with the dip angle between 25° and 65° show a significant impact on the caprock integrity, hence, the MOP. Moreover, results indicate an overestimation of the MOP by up to 23% when neglecting the NFs. Comparing the results of different case scenarios with those of conventional isotropic models highlights the requirement to include the effect of NFs in caprock integrity studies.