Steamflooding Wabasca Tar Sand Through the Bottomwater Zone--Scaled Model Tests

Steam flooding a tar sand with a communicating bottom-water zone was investigated in a three-dimensional, scaled, laboratory model. Scaling is discussed and equipment and procedures are described. We studied the process mechanisms and the influence of steam rate and initial oil saturation, and compared performance of single and multiple patterns. The bottom water was found to counteract gravity segregation of the steam. If the steam rate is high enough, no gravity override occurs, and much of the formation is swept layer by layer, resulting in high recovery of oil. But oil/steam ratios are rather low, particularly at low initial oil saturation and in the single pattern pilot where oil bypasses the production wells. Introduction In a world of decreasing conventional oil reserves, a number of alternative sources of hydrocarbons are being evaluated. One of the major alternatives, because of its size, is tar sands. Deposits are located primarily in Canada and Venezuela; each has an estimated reserve of nearly 1 trillion bbl (159 × 10(9) m3). At present, only 10% of these tar sands are shallow enough to be recovered economically by surface mining, pointing to the need for methods of in-situ recovery for the remaining 90%. Steam injection is highly effective in delivering heat and work to a tar sand. Heat raises the temperature of the tar, which greatly reduces its viscosity, thus increasing the reservoir fluid flow potential. Work provides the drive to recover the mobilized oil from tar sands that normally have no primary production. Steam drives exhibit good sweep and displacement efficiencies, even in heavy oil and tar reservoirs, because of the favorable effects of steam condensation. The principal problem in tar sands is the lack of injectivity caused by the very low mobility of the highly viscous tar, even though permeability of the sand is high. Steam could be injected above fracturing pressure or into a naturally permeable channel like a bottom water zone. But control and prediction of the direction, the orientation, and the extent of fractures in tar sands are uncertain. By contrast, a bottom water zone is already in the right location, linking wells, and allowing high steam injection rates. A disadvantage is that the water zone may be too thick, soaking up heat and oil. Gulf Canada Resources Inc. (GCRI) holds leases in Wabasca, Alta., which contain a very viscous, 6 degrees API (1.029-g/cm3) tar [5 million cp at the reservoir temperature of 55 degrees F (5 kPas at 13 degrees C)] in an unconsolidated sand. The deposit is located at a depth of about 800 ft (244 m) and consists of a 32- to 36-ft (10-m) oil zone overlying an 8- to 9-ft (2.5-m) bottom water zone, thought to be at least in partial communication. Permeability averages 400 md vertically and 1,000 md horizontally. The tar is high in asphaltenes and shows no significant distillation at anticipated steam temperatures. Various laboratory studies have been reported in the literature. Pursley steam flooded Cold Lake crude in a 500-psi (3.5-MPa) scaled model and concluded that steaming through a bottom water recovers more oil than steaming through a gas cap. Bursell and Pittman studied the behavior of Kern River crude in a 300-psi (2.1-MPa) model. SPEJ P. 92^