Performance of the SAGD Process In the Presence of a Water Sand - A Preliminary Investigation

Abstract Many heavy oil and oil sand reservoirs are in communication with a gas cap and - in some cases - with a water sand. Depending on the density (° API gravity) of oil, the water sand could lie above or below the oil zone. Conventional thermal recovery methods - using vertical wells - in these thin reservoirs (5–10 m of pay) have been unsuccessful. The use of optimally placed horizontal wells in steam injection has proven economically feasible due to reduction of heat loss to the cap rock, and bottom water layer, as well as better overall heat distribution in the reservoir. Steamflooding a heavy oil/oil sand reservoirs with a contiguous water sand (water which may lie below or above the oil-bearing zone) is risky due to the possibility of short circuiting the steam chamber. The success of the Steam Assisted Gravity Drainage (SAGD) Process at the Underground Test Facility (UTF) in Fort McMurray, Alberta has lent its application to these reservoirs. The success of subsequent projects depends on 1) accurate reservoir description, 2) efficient utilization of heat injected into the reservoir, 3) understanding of geomechanics (the interaction between the fluids and the reservoir at elevated temperatures and pressure), and 4) operational constraints. This paper looks at how the SAGD process is affected by the presence of a water sand, and determines how heat is distributed in these reservoirs. Results of this numerical simulation study show a relationship between ultimate recovery, heat accumulated in the reservoir and the thickness of the water sand (bottom or overlying water). Oil recovery in the SAGD process is hindered when water sand is present. However, the presence of a bottom water layer has lesser impact on recovery than the case where an overlying water layer is present. As the thickness of the water layer increases, the recovery efficiency decreases. Increasing the areal coverage of the bottom water layer resulted in slightly reduced recovery as compared with the confined bottom water layer. On the other hand, increasing the areal coverage of the overlying water layer severely reduced the recovery efficiency of the process as heat is diverted (or channeled) into the thief zone. The oil steam ratio (OSR) in this run was below 0.15 m3/ m3 (CWE) after 400 days of injection. When a bottom water layer is present, the BHFP (bottom hole flowing pressure) of the horizontal producer could be operated at or above the pressure of the aquifer to prevent water coning and hence only effecting the heat source slightly. Introduction The successful application of the SAGD Process at the Underground Test Facility (UTF) has demonstrated a commercially viable recovery method to exploit heavy oil and tar sand/bitumen reservoirs. The application of the SAGD Process to oil sands that are in communication with water sand is investigated in this paper along with the effect of confined and unconfined water bearing zones. Heat balance calculations are also performed to establish relationships between heat accumulated in the reservoir to the thickness of the water sand and the recovery efficiency.