Steam Assisted Gravity Drainage (SAGD) is a commercially successful recovery process to produce heavy oils and bitumen. The method ensures both a stable displacement front of steam and economical rates by using gravity as the driving force with a pair of horizontal wells for injection/production. Although several ways of improving the performance have been discussed in the literature, the well configuration employed in the process has remained the same as originally proposed by Butler. A systematic attempt to improve the performance by using radically different well configurations has not been reported.
This paper presents a study aimed at examining the applicability of a new well configuration to SAGD process in Athabasca and Cold Lake reservoirs in central and northern Alberta, Canada. The fully implicit thermal reservoir simulator, CMG's STARS 2007, with fully coupled wellbores was used to account for frictional pressure drop and heat losses along the wellbore. 3-dimensional numerical simulation models were set up and sensitivity analyses were conducted to injection pressure. After optimization of the injection pressure an investigation of new well configurations was conducted using these models. The result of this work shows that the SAGD process performance in Athabasca and Cold Lake reservoirs can be significantly improved by changing the well configuration.
Development of horizontal well technology and subsequent improvements in the drilling techniques have made SAGD a reliable recovery method for immobile bitumen recovery [1]. The well configuration employed in vast majority of the field project has been two parallel horizontal wells in the same vertical plane separated by a vertical distance of five meters. The combination of horizontal and vertical wellbores instead of the standard two horizontal wells has also been tested and appears to be a viable option.
Joshi compared the SAGD performance for combination of vertical and horizontal injectors-producers, and found that the maximum recovery was obtained from the horizontal well pair [2]. Chung and Butler's [3] experimental observations showed that the rate of bitumen recovery is higher when the steam is injected near the top of net pay, i.e. through vertical injector. Liebe and Butler [4] investigated the effect of well configuration with two types of reservoirs, cold lake and Lloydminster. Chan and Fong [5] examined the offset configuration for heavy oil type of reservoir. Their result showed an additional 5–15% of oil recovery in the case of offsetting or staggered injector.
The investigation of effects of well spacing was continued by Ehlig-Economides et al. [6] through thermal reservoir simulation. Sasaki et al [7] found that in a SAGD process, longer well spacing will improve oil production rate and expansion rate of the chamber area. Stalder [8] introduced XSAGD configuration for low pressure reservoirs. It was found that an increase in steam pressure would make this configuration's thermal efficiency worse than that of the standard SAGD configuration. Gates et al. [9] presented the JAGD idea as a new well configuration for reservoirs with vertical viscosity gradient. A higher thermal efficiency than that in the standard SAGD configuration was reported.
