Improving Steam-Assisted Gravity Drainage performance in oil sands with a top water zone using polymer injection and the fishbone well pattern

Abstract

In western Canada, a significant number of oil sands reserves have little or no cap rock with a top water zone. Due to huge heat loss to the top water zone, the conventional Steam-Assisted Gravity Drainage (SAGD) process is uneconomical when applied directly in this type of reservoir.In this study, it is proposed that a high temperature polymer solution can be injected into the bottom of the top water zone to establish a stable high viscosity layer that will prevent steam from leaking into the top water zone. In order to select a suitable polymer that has stable viscosity under high temperature, the viscosities of different polymer solutions at different temperatures were measured and the concentration of the selected polymer solution was optimized. Furthermore, in order to extend the connection area between the oil sands and the steam chamber, the fishbone well pattern was applied instead of the single well pair pattern.Numerical simulations were performed to evaluate the feasibility of using the selected polymer in the fishbone well pattern to improve SAGD performance in oil sands with a top water zone. The numerical simulation model was based on a typical Athabasca oil sands reservoir. In this study, the effects of steam injection pressure, polymer solution injection time, steam injection rate, and different fishbone well patterns on the performance of the SAGD process were studied and optimized.The numerical simulation results suggest that the fishbone well pattern could extend the steam distribution and that polymer injection is able to prevent heat from leaking into the top water zone. Compared to the conventional SAGD process in an oil sands reservoir with a top water zone, the optimal case using a one-fishbone well pattern and polymer injection could enhance the oil production significantly. Under these conditions, the oil recovery factor in this study increased from 10.58% to 59.02%; the cumulative steam oil ratio decreased from 10.44m3/m3 to 3.85m3/m3; and the cumulative injected energy oil ratio decreased from 24.00GJ/m3 to 8.87GJ/m3. This indicates that the SAGD process with the one-fishbone well pattern and polymer injection is able to improve SAGD performance in oil sands with a top water zone.