The Upper Devonian Grosmont Formation is a bitumensaturated, carbonate unit located in northern Alberta. It is considered to be one of the world's next largest unconventional oil resource plays. Since early 2006, there has been an increased interest in Grosmont resources exhibited by a range of companies including super-majors. Several in-situ pilot tests were conducted in the central portion of this area in the 1970s and 1980s utilizing steam and in-situ combustion processes. Similar to field tests in the McMurray Formation oil sands prior to invention of the steam assisted gravity drainage (SAGD) process, none of the early recovery technologies tested were proven to be economic. As the "gravity" drainage process has been proven successful in commercial development of the McMurray Formation oil sands since mid to late 1990s, the recovery potential for the Grosmont Formation should be re-evaluated based on improved recovery techniques. Results from cyclic steam stimulation (CSS) field tests are compared and analyzed to understand the similarity and fundamental differences in reservoir properties between the McMurray Formation oil sands and the Grosmont Formation carbonate rocks. A preliminary interpretation is provided for laboratory test results for solvent processes applied to Grosmont carbonate cores. The scaling considerations from the laboratory results to field expectations are discussed. The paper also provides a direction for future studies and optimization opportunities for reservoir recovery leading to the commercial development of Grosmont carbonate reservoirs.
Starting in the 1970s, Union Oil Company of Canada (Unocal), in partnership with Canadian Superior and the Alberta Government, conducted a series of exploratory field tests in the bitumen-saturated carbonate rocks of the Grosmont Formation. These tests applied thermal recovery technologies, including steam drive and cyclic steam stimulation (CSS), that were in their early stages of development. Significant amounts of production and observational data were obtained. Although some results were encouraging, activities in Grosmont were eventually stalled in the mid 1980s as economic attention was shifted to Cold Lake and Athabasca Cretaceous siliclastic reservoirs. Since then, in situ bitumen recovery, 3D seismic, horizontal well and surface processing technologies have matured significantly. In light of the enormous resources (406 billion barrels) hosted within the Grosmont Formation, it is pertinent to ask whether those new technologies are applicable for carbonate reservoir development.To answer this question, we studied data from the Unocal pilots conducted in the Grosmont C and augmented it with recent laboratory tests on newly acquired Grosmont C cores. The previous pilot CSS results were encouraging, with the cycle steam-tooil ratio as low as 3.65 and a peak rate of 440 bbls/d from a single vertical well. With subsequent cycles the ratio of the produced fluid to the injected fluid increased, signifying the injected energy was retained and more effective in later cycles. The operation strategy of the Unocal pilots and its implementation were not optimal and we believe that this could be improved with modern techniques.Based on our new understanding of the Grosmont Formation and specifically the Grosmont C, a numerical model was created and verified with production data. Model results indicate that the application of SAGD will be a commercially viable recovery process for Grosmont carbonate reservoirs and that low pressure injection (below 3500kPa) would be desirable. The laboratory tests not only support these conclusions but also suggest that performance of the applicable thermal processes can be enhanced with the addition of solvent. A SAGD/solvent pilot test is planned to start up in late 2010. This pilot will be critical to the development of exploitation strategies applicable to Grosmont carbonate bitumen resources.
Cyclic Steam Stimulation (CSS) has been a commercial recovery process since the mid 1980's in the Cold Lake area in northeast Alberta. The current bitumen production is over 220,000 bbl/d using CSS from this area. To achieve desired injectivity in the bitumen saturated reservoir, steam is usually injected at a pressure above or close to the fracture pressure of the formation. A relatively high pressure drawdown is created between the wellbore and formation during the production phase, particularly in the early stage of the production cycle where formation compaction and solution gas drive are the two most important recovery mechanisms. The CSS process has limited application in reservoirs with thick bottom water or in reservoirs with fine grain sands. The Steam Assisted Gravity Drainage (SAGD) process has been field tested and commercially expanded in the Lower Grand Rapids and Clearwater Formations in the Cold Lake area. In contrast to CSS, SAGD is a continuous steam injection process that relies on "gravity" and requires a minimum pressure drawdown to drive the reservoir fluids to the wellbore. This provides a significant advantage for SAGD as an option for the reservoirs with bottom water, top gas or with formations with fine grain sands. Several SAGD projects are in operation in different types of reservoirs in the Cold Lake and Lloydminster areas; some with thick bottom water zones. A performance review is conducted based on the available data for various CSS and SAGD projects in the Cold Lake area. The selection criteria between CSS and SAGD technologies for Clearwater and Lower Grand Rapids are discussed. Reservoir modeling results are presented concerning the impact of well placement, reservoir heterogeneity and operating parameters on SAGD performance, based on Osum's Lower Grand Rapids and Clearwater geology in the Cold Lake area.
We delineated a bitumen-rich paleokarsted carbonate reservoir of the Upper Devonian (Frasnian) Grosmont Formation with a high-resolution 3D seismic survey tied to core and petrophysical log data from 35 wells within a 34.2 km 2 study area in northern Alberta, Canada. There were two laterally continuous karst facies: a solutionenhanced vuggy dolostone that resulted from the carbonate dissolution of body fossils and a stratiform breccia that resulted from the dissolution of interbedded evaporites. Three laterally discontinuous karst facies were identified: sinkhole fills, collapsed paleocaves, and solution valley fills. We measured 368 subcircular features (sinkholes and collapsed paleocaves) having a median circle-equivalent diameter of 69 m and representing 5.5% of the total study area. Sinkhole fills include Cretaceous-aged sandstone, mudstone, and coal. Collapsed paleocaves were filled with matrix-supported breccia that had clasts of disoriented blocks of dolomite and a matrix of disaggregated dolomite and Cretaceous-aged mudstone. The paleocaves and sinkholes formed in the solution-enhanced karst facies of the Grosmont C at the interface of an interpreted ancient vadose-phreatic mixing zone. The marine deepwater deposition of the Clearwater Formation during the Albian filled the depressions created by the mechanical collapse of the paleocaves and provided a seal for thermal operations. The fracture density inferred from seismic amplitude variation with angle and azimuth analysis and corroborated by well data showed that fractures are ubiquitous and were enhanced during meteoric karst. The high-vertical permeability resulting from solution-enhanced fractures, the laterally predictable flow units, and a competent seal make this an ideal reservoir for thermal bitumen recovery.
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