Steam thief zones appear in many reservoirs. They occur in the form of permeable zones that have high gas or water content and are embedded in, on top or below oil zones.
This analysis is based on work done evaluating Husky Oil's Lease T87 about 70 km NE of Fort McMurray, Alberta. Thief zones in and above oil sand pay zones are examined. The analysis consists of a combination of analytical concepts backed by simulation results. This analysis produced the following results in a reservoir with moderate thief zones:
The steam oil ratio is predicted to decrease significantly as the number of well pairs is increased.
Horizontal well pairs are predicted to have much lower early to midlife SOR's than would an equivalent arrangement of vertical wells using the SAGD process.
Simulations indicate significant oil plugging takes place in thinner thief zones.
Simulations of thicker thief zones with high loss rates don't have very significant oil plugging but do exhibit an effective restriction due to high steam pressure gradients.
Mid formation water thief zones plug off entirely.
An analytical model was developed that agrees well with simulation results.
The paper presents the analytical analysis and simulation results supporting these findings along with a discussion of the physical reasons for the findings.
There are a number of heavy oil and bitumen leases that have gas and water zones imbedded in them. These zones can act as steam thief zones to SAGD or other steam stimulation processes where the injection pressure exceeds the reservoir pressure. One way to lessen the steam loss problem is to keep the steam injection pressure at or near the thief zone pressure thereby lowering the project SOR. However this is not always desirable because it can result in lower production rates and artificial lifting costs, both of which lower profitability. The optimum injection pressure is determined by considering all these effects.
This work was initiated by Husky Oil for the purpose of examining the effects of relatively thin potential thief zones on a potential SAGD project on Husky's Lease T87 near Fort McMurray, Alberta. This paper looks specifically at the case of injecting steam at 1800 kPa into a reservoir at 400-kPa pressure. This case was used because it was the initial case examined for Husky Oil. While this injection pressure would likely be too high for a major thief zone, it does provide a good range of loss rates for examining the effects of different size thief zones. The basic reservoir parameters and thief zone cases studied are presented in the tables of Figure 1.
