The Upper Devonian Grosmont formation, located in the West Athabasca Oil Sands Deposit, contains an estimated 318 billion barrels of bitumen. The reservoir is a heavily karsted and fractured, bitumen-saturated dolomite with up to 38% porosity and permeabilities commonly measured in the Darcies. These properties make the Grosmont an excellent candidate for Steam Assisted Gravity Drainage (SAGD). Grosmont SAGD development will be exposed to similar operating conditions as McMurray SAGD projects, and will have similar logistics, well construction and materials challenges. However, there are many additional challenges, including severe loss circulation, that further complicate well design. This paper presents the challenges experienced by Laricina Energy, drilling Grosmont wells at Saleski and Burnt Lakes over the last three seasons. It will also address additional areas for future research and development in the drilling of vertical and horizontal wells.
The Athabasca Deposit is recognized for its oil sands, which have received a great deal of attention because of their enormous bitumen reserves. Most of the focus has been put on exploiting the McMurray sands; however a large resource of bitumen is also present within the Devonian carbonate. The Grosmont formation alone is estimated to contain 318 billion barrels of bitumen and significant additional bitumen resources exist in the overlying Ireton and Winterburn formations. High porosities, permeabilities, oil saturations, and pay thickness make the Grosmont an attractive reservoir. The Grosmont has been sub-divided into four units, of which the upper two, the Grosmont C and D are bitumen bearing on Laricina's leases at Saleski(1). The bitumen bearing interval is approximately 50m thick and the net to gross pay ratio commonly exceeds 90%. The Grosmont Formation has been subjected to an intense period of karsting and carbonate dissolution that has resulted in extensive leaching and porosity development on a massive scale. The reservoir rock is highly fractured, and consists of interbedded porous carbonate sandstones, vuggy dolomites and dolomite breccias. Breccia intervals have exceptionally high porosity and permeability. Examination of core from many brecciated intervals reveals the presence of angular dolomite rock fragments that appear to float in a matrix of fine dolomite sediment and bitumen. Porosities are typically 20–35% (up to 38%) and permeabilities are commonly measured in the Darcies. The Grosmont is slightly under-pressured, at approximately 1400 kPa.
Most of the techniques used for the production of bitumen rely on heating the formation to temperatures typically in excess of 200 °C to reduce bitumen viscosity, although solvent processes may be an effective way to reduce this temperature. To date, Steam Assisted Gravity Drainage (SAGD) has emerged as the most popular recovery technique and it is being implemented at several projects. These techniques should also be applicable to the carbonate formations. Laricina plans to use SAGD technology in combination with solvents to recover bitumen from the Grosmont.
Through the 1970s and 1980s, several thermal pilots were conducted on the bitumen deposits of the Grosmont formation, including Unocal's Buffalo Creek (2).
