Steam-Solvent Coinjection under Reservoir Heterogeneity: Should ES-SAGD be Implemented for Highly Heterogeneous Reservoirs?

Abstract

Expanding-solvent steam-assisted gravity drainage (ES-SAGD) is a widely-investigated alternative to SAGD, considering its potential to reduce thermal losses while enhancing bitumen recovery. However, most prior studies on ES-SAGD were limited to homogeneous reservoirs. This research presents a mechanistic analysis of ES-SAGD in heterogeneous reservoirs in terms of cumulative steam-oil ratio (SOR) as a function of steam-chamber size. Simulation case studies for SAGD and ES-SAGD with normal hexane are conducted for geostatistical realizations of two types of heterogeneous Athabasca-bitumen reservoirs. For the first type, shale barriers are oriented horizontally relative to the top and basal planes of the reservoir. For the second type, they are inclined and more representative of the middle McMurray member. The solubility of water in the oleic phase at elevated temperatures is properly modeled to ensure reliable comparison between SAGD and ES-SAGD. Results show that ES-SAGD is less sensitive to heterogeneity than SAGD in terms of cumulative SOR for simulations at 35 bars and 2 mol% solvent-injection concentration for a reservoir thickness of 20 m. On average, the reduction in SOR due to steam-solvent coinjection is simulated to be greater under heterogeneity. The margin of SOR reduction is greater in reservoirs with inclined shale barriers than in those with horizontal shale barriers. Analysis of simulation results indicates that the injected solvent tends to accumulate more significantly under heterogeneity, which enhances the mechanisms of ES-SAGD, such as dilution of bitumen by solvent and reduced thermal losses to the overburden. Tortuous hydraulic paths and slower gravity drainage under heterogeneity enhance the mixing between solvent and bitumen in the transverse direction along the edge of a steam chamber. Then, a larger amount of the accumulated solvent tends to facilitate lower temperatures near the chamber edge. Lower chamber-edge temperatures combined with restricted access to the overburden under heterogeneity alter the chamber geometry such that the contact area for overburden heat losses is further reduced.

Mechanisms and Operational Strategies of Multi-Lateral Steam-Assisted Gravity Drainage (SAGD) for Heterogeneous Reservoirs

“…The feasibility of adding solvent into steam has also been validated. Based on these studies, the suitable reservoir condition was investigated [14,15], and the impact of shale barriers on SAGD performance and the hybrid optimization techniques were used to design the solvent-assisted SAGD process [16,17]. Although exponentially reducing the oil viscosity, the solvent is less effective in diffusing into shale barriers and speeds up the steam chamber evolution.…”

Bitumen Recovery Performance of SAGD and Butane- and Hexane-Aided SAGD in the Presence of Shale Barriers

“…Various studies have been done to demonstrate the impact of shale barriers on SAGD performance. − Kumar and Hassanzadeh have presented a detailed literature review on the impact of shale barriers on SAGD and ES-SAGD . However, very few studies are available regarding the impact of shales on ES-SAGD performance. − A major industry standard to assess the performance of SAGD and ES-SAGD is the steam oil ratio (SOR). The conventional range of the SOR for an efficient SAGD process in a homogeneous reservoir tends to fall between 2 and 3.…”

“…The conventional range of the SOR for an efficient SAGD process in a homogeneous reservoir tends to fall between 2 and 3. Studies based on ES-SAGD performance evaluation in the presence of shale barriers were based on the use of heptane and hexane as solvents. , All the studies reported an improved performance by ES-SAGD over SAGD in the presence of shale barriers. This paper proposes a further detailed study on comparing SAGD and ES-SAGD in heterogeneous reservoirs over a wide range of shale densities.…”