The hydrocarbon resources in the Grosmont carbonate deposits in Alberta are over 400 billion barrels. Pilots have been carried out in Grosmont since 1974, but hitherto a commercial recovery scheme remains elusive. This study addresses oil recovery techniques from the Grosmont carbonate deposits, using numerical simulation for several steam-based recovery schemes. Selected simulation results were further analyzed and compared with those from the classical analytical model.
Given the extremely heterogeneous nature of the deposits (karsted, fractured, and high variability in formation properties) and high oil viscosity (2-11 million cp at 20 °C), a typical geological model of Grosmont, with representative petrophysical and geological characteristics, was used to examine several steam injection schemes, using a dual porosity/dual permeability model in CMG STARS. The schemes tested included: single vertical well cyclic, single horizontal well cyclic, steam assisted gravity drainage and steamfloods involving five-spot, nine-spot, seven-spot patterns and the vertical well injector-horizontal well producer arrangement.
The dominant feature of all steam injection schemes was a high steam-oil ratio, although the oil recovery was high in most cases. Experiments showed that the steam-oil ratio of steamfloods can be improved by reducing the steam injection rate and closing certain perforation intervals at critical points in the production life of the well. As a result of optimization, a pattern steamflood case achieved a steam-oil ratio of 8 m3/m3 and recovery factor of 41%. The best horizontal well CSS case, for which the well location is chosen to avoid steam channelling into the top water, gave a good response with a steam-oil ratio as low as 7 m3/m3 and a recovery factor of 18%. The recovery factors of the two processes are comparable considering the fact that the wells in the former were perforated into Grosmont C and D members of the model while the latter targets only the Grsomont C member. SAGD cases have similar steam-oil ratio but smaller oil recovery.
Simulation results were analyzed in the light of dynamic fluid flow, thermal efficiency and drive mechanism and were compared with results from classic analytical methods. Affected by the high fracture vertical permeability and severe heterogeneous formation properties, steam override and oil gravity flow occur in all processes. Uneven steam zone and limited drainage area make it difficult to achieve optimal performance of processes including SAGD, seven-spot pattern steamflood and horizontal well CSS. Early steam breakthrough happened in the pattern steamfloods. A combination of vertical steam injectors and a horizontal producer provided the most uniform formation heating and production of the mobilized oil by both gravity flow and viscous drive, thus achieved the same recovery performance faster than horizontal well CSS and delayed steam breakthrough.