Application of Reservoir Geology to Enhanced Oil Recovery from Upper Devonian Nisku Reefs, Alberta, Canada

Abstract

The Upper Devonian West Pembina reef trend of west-central Alberta contains recoverable reserves of over 79 × 106 m3 (500 million bbl) of oil and 1.4 × 1010 m3 (500 billion ft3) of gas within approximately 50 pinnacle reefs in the Nisku Formation. Although the oil is saturated with gas at original reservoir pressure, primary depletion would soon lower the reservoir pressure below the bubble point, decreasing recovery. Thus, pressure maintenance is applied early in the producing life of the pools through waterflood or miscible flood schemes. Selection of the appropriate enhanced recovery scheme depends upon the internal flow-unit geometry of the reefs. The Bigoray Nisku C pool and the Pembina Nisku L pool for end members of the reservoir spectrum. They can be used as flow-unit models in the geological input for reservoir simulation studies. The Bigoray Nisku C pool is dominantly limestone. The primary textures, well preserved in this reef, provide the key to interpreting the relict textures in fully dolomitized reefs. A well-displayed ecological zonation of rugose corals passes vertically upward into laminar stromatoporoids, which, in turn, pass into solenoporacean algae. The coral and stromatoporoid zones each contain two generic flow-unit types, depending upon the presence or absence of dolomitization. The incipient reef facies and the algal cap with associated tidal-flat sediments represent two additional flow units. Due to the presence of horizontal permeability barriers associated with the limestone lithology, the pool is developed with a waterflood displacement scheme. Ultimate recovery is estimated to be on the or er of 0.55 × 106 m3 (3.5 million bbl) or 46% of original oil in place (OOIP). The Pembina Nisku L pool is a completely dolomitized reef, representative of the majority of the reefs in the Nisku fairway. In contrast to the Bigoray Nisku C pool, the complete dolomitization reduces the number of generic reservoir flow units observed in the L pool reef from six to three. Due to the excellent reservoir quality and absence of horizontal permeability barriers, it is being exploited by a vertical miscible flood. The Nisku L pool is one of the largest pinnacle reefs discovered in the Nisku reef fairway and contains an estimated 5 × 106 m3 (31 million bbl) OOIP. Ultimate recovery is estimated to be approximately 4.1 × 106 m3 (25.8 million bbl) or 82% of OOIP.