SAGD Optimization With Air Injection

Abstract

Abstract This paper discusses the recent patent application filed by EnCana Corporation on the use of air injection to improve the performance of steam assisted gravity drainage (SAGD). EnCana's SAGD/Air Injection process employs standard SAGD well-pair infrastructure. It optimizes between the ability of steam to preheat the reservoir during SAGD and the superior (follow-up) oil displacement efficiency of in-situ combustion. Operationally, air injection is initiated after thermal communication has been established between well-pairs with steam. One interesting feature of this operating strategy is that down-hole bulk separation of oil and gas occurs which facilitates (a) efficient monitoring and control of the combustion, (b) design of surface facilities, and (c) corrosion mitigation. Laboratory combustion tube tests are presented that confirm the ability to initiate and sustain combustion, as well as mobilize residual oil saturation to steam, within a SAGD chamber. These experiments were initialized at oil saturations and conditions representative of those in a steam chamber. The residual oil saturations were determined from a full-hole core taken in the vicinity of a mature SAGD well-pair at Foster Creek. Numerical simulations of post-SAGD air injection are presented that suggest the ability to displace and produce oil banks between well-pairs and that recovery factor can be increased up to 8% of the original oil-in-place over conventional SAGD. The simulations show oil production rates and recovery factor are expected to increase with higher air injection rates. However, instantaneous air-oil ratios, which are indicative of operating costs, also increase. Thus there is an optimum continuous air injection rate that maximizes profitability. Simulations further indicate that it is possible to recycle flue gases in the injection stream without affecting oil recovery. Introduction Steam injection, to date, has been the most successful 'in situ heavy oil and bitumen recovery method. In 2004 there were several commercial cyclic-steam stimulation (CSS) projects in Alberta, Canada, producing a combined average of 27.7 103 m3/d bitumen1. In the same year, Alberta's bitumen production by steam-assisted gravity drainage (SAGD) averaged 11.1 103 m3/d. Bitumen production attributable to both processes has grown substantially since then. These achievements are due to technological innovations that have overcome several geological and reservoir challenges associated with steam injection. And, it is expected that in situ bitumen producers will continue to seek enhancements with the goal of reducing steam-oil ratio (SOR). Two historical developments suggested the SAGD optimization process proposed in this paper. First: From 1979 to 1984, BP Canada tested air injection as a follow-up process to fracture-assisted cyclic steam stimulation in the Clearwater formation at Wolf Lake2. It was expected that only 17% of the bitumen could be recovered using CSS and that combustion as a follow-up process would increase the recovery factor. Bitumen recovery factor at the pilot increased from 15% with CSS to a cumulative of almost 30% with in-situ combustion. The equivalent SOR was reduced from approximately 6.2 to 2.3. It should be noted that air injection was successful in this application because the bitumen was first preheated and mobilized with steam. Second: During SAGD operations, there comes a point where the cumulative SOR begins to increase indicating that it is no longer economic to continue steam injection. Injecting a non-condensable gas at this stage, to utilize the existing heat energy in place, can prolong oil production3. This significantly reduces the operating costs compared to continued steam injection. This paper describes the potential for implementing air injection as a follow-up to SAGD. It is shown that under controlled conditions, air injection promotes superior volumetric sweep efficiency and recovery factor in comparison with continuous steaming or with using methane as blow-down gas.