Effects of Gas Production on Pressure in the Upper McMurray Water Sands Overlying Bitumen Reservoirs in the Athabasca Area of Alberta

Abstract The pressure depletion in producing gas pools in the Upper McMurray sands transmits into the adjacent water sands and may propagate over significant distances, depending on aquifer properties and production characteristics. Any pressure decrease in the already low-pressure, shallow Upper McMurray sands has a negative impact on bitumen production from the underlying Middle McMurray bitumen reservoirs. The sensitivity analysis performed using site-specific and regionally-characteristic values shows that the most important parameters affecting pressure transmission in the Upper McMurray water sands are aquifer diffusivity, which depends on the interplay between permeability and compressibility, and gas production rate. Depending on aquifer and gas production characteristics, the pressure regime in the Upper McMurray water sands may be affected to >10 km away from a producing gas pool, with potentially adverse effects on bitumen production. Introduction Alberta's major oil sands deposits (Figure 1a), with an estimated 1.7 trillion bbls of bitumen in place(1), account for 40% of the world's bitumen resource. Recoverable bitumen reserves in Alberta, estimated at 300 billion bbls(2), overwhelm the remaining established reserves of conventional crude oil by a factor exceeding 150. Future oil production in Alberta will be increasingly dependent on the oil sands and will require the satisfactory resolution of numerous issues specific to oil sands development, among them gas and bitumen production from the McMurray Formation. South of Fort McMurray, the Athabasca oil sands deposit occurs in the McMurray Formation at depths that are suitable for in-situ bitumen extraction using horizontal wells drilled from the surface and steamassisted gravity drainage (SAGD) (3–6). Steam at high pressure and temperature (≥1500 kPa and ≥200 °C) is injected through a horizontal well into the bitumen reservoir. This leads to a drop in bitumen viscosity from more than 2,500 Pa?s (2,500,000 cp) at natural in-situ conditions to less than 0.01 Pa?s (10 cp). Consequently, the bitumen flows towards a producing well located a few metres below the injector well. Steam thief zones, defined as permeable zones that have high gas or water content and that occur in and/or adjacent to the bitumen reservoir, have a negative impact on SAGD operations because of the possibility of short circuiting the pressure in the steam chamber and because of heat losses(7, 8). The heterogeneity and complexity of McMurray oil sands leads to naturally-occurring steam thief zones in the area. In addition to oil sands resource development, gas is produced in the area from shallow reservoirs located mostly in the McMurray and Clearwater formations. The gas and bitumen production, and water pumping and injection from overlying and underlying aquifers, have the potential for mutual interference. This situation would be similar to the production interference through the underlying Cooking Lake aquifer that was identified previously between Leduc oil pools in Alberta(9, 10). The pressure interference has the potential to enhance the negative impact of the steam thief zones by further lowering their already low pressure. Thus, the sustainable development of gas and bitumen resources in the Athabasca area depends to a large extent on the pressure depletion, transmission and recovery in the McMurray Formation.

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