The paper presents experimental results on the impact of top water and gas caps on SAGD performance. The effect of the top thief zone on oil drainage rates and potential oil and steam loss into the top zone were measured. The study involved the use of a large scale high-pressure / high temperature experimental facility for injecting steam into an oil sand pack and measuring oil drainage rates and development of temperature ahead of the steam chamber. Numerical modeling was conducted to predict field scale performance using the CMG's STARS simulator.
An elemental experimental approach was used in the study to simulate a generic reservoir in the Athabasca region with a pay zone thickness of 50 m and an overlying thief zone thickness of 8 m. In this approach, a reservoir element was selected close to the oil / top thief zone interface. The element was located ahead of an advancing steam front. In order to set the initial conditions of the laboratory element to be similar to those in the field, field scale numerical simulation was conducted to determine the temperature distribution in the element. The field scale temperature profile was established in the laboratory elemental model to represent the element's initial temperature before the start of steam injection during the experiments.
The paper discusses the results from the study and highlights the potential implications of the top thief zone on SAGD applications. In addition, differences between gas cap and top water thief zones on impacting the thermal and recovery efficiency of the SAGD process are demonstrated.
There is a major concern by oil producers that the gas production from the gas cap overlying the oil sand deposit has a significant detrimental effect on oil recovery. A case in point is the Surmont oil sands lease, which Gulf operates a Steam Assisted Gravity Drainage (SAGD) pilot, and plans to follow quickly by commercial development. The lease has a top gas zone and a mobile water sand zone overlying the oil sand pay zone. An observation well indicates that the gas cap pressure at the pilot site has fallen from 1327 kPa to 858 kPa over 3 years due to production of the gas. It is estimated that the pressure may fall to less than 300 kPa when the gas wells are abandoned. Based on the geology and pressure measurements, there is communication between the gas cap and the oil sands. This indicates that the gas cap may be a thief zone at the pilot site. It is also believed that the mobile top water zone may extend the area of influence of the pressure-depleted gas caps.
As part of the ADOE/EUB Task Force studying this problem, numerical studies on SAGD processes in reservoirs with top water / gas cap zones were conducted by ADOE1,2 as well as the oil and gas producers.