Abstract

This paper presents a strategy where two commercial simulators: CMG-STARS and FLAC2D are linked together and jointly executed for thermal-stress-fluid analyses during the steam-assisted gravity drainage process. These simulation results are subsequently used to compute the safety factor against tensile failure within the caprock. A novel technique is described and implemented to also compute the safety factor against shear failure. The results clearly show that the safety factors against both tensile and shear failure will decrease as the steam chambers develop within the reservoir. Selection of material strength properties of the caprock is a critical factor in the defensible selection of an operating factor of safety at a given maximum steam injection pressure. Injection pressures, well pair spacing and caprock thickness also have remarkable impact on caprock safety. The methodology presented in this paper can be used to rationalize tensile/shear failure factors of safety for caprock integrity and be used as a criterion for the control of steam injection pressure to prevent caprock failure and steam leakage.

1. INTRODUCTION

According to the Alberta Department of Energy, there are 167.2 billion barrels of remaining established reserves in the oil sands deposits of Northern Alberta. Approximately 80% of oil sands are recoverable through in-situ production, with only 20% recoverable by mining. Recovery rate percentages vary depending on the method of extraction but over the past several decades, cyclic steam stimulation (CSS) and steam assisted gravity drainage (SAGD) have been the commercial in situ recovery technologies of choice. For the shallower oil sands deposits (e.g. depths less than 300 m), SAGD methods have been the in situ recovery method of choice. For SAGD recovery process, pair of vertically adjacent wells is drilled horizontally within the oil sands reservoir; the upper well serves as the steam injection well and the lower well serves as the production well. Ideally, they lie directly above one another separated by approximately 5m. Once heated by steam, the lower viscous bitumen flows to the production well as a result of gravity forces.

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