After solvent breakthrough, vertical miscible floods are characterized by rapid increases in producing gas-oil ratios which in turn are accompanied by rapid declines in oil production. Al this advanced stage of depletion, the vertical floods have a low reserve life index and hence, major investments cannot be justified to attend the producing life of these floods.
Imperial Oil Resources Ltd. in its search for new low cost techniques to increase reserves while maintaining increasing oil production began to examine concept of oil re-injection in its Pembina Nisku reservoirs. Exploratory laboratory tests and numerical simulation work demonstrated that a dual completion to re-inject part of the produced oil back into the formation, a few meters above the producing perforations could lead to incremental recoveries of two to three percent of the original-oil-in-place (OOIP).
A field test to evaluate the benefits of the oil re-injection process is presently being conducted in the Pembina Nisku "Q" pool Initial results indicate that oil re-injection is successful in controlling solvent production while maintaining oil rates. As a consequence, additional piloting to expand the applicability of the oil, re-injection process is being planned for other Nisku and Imperial Oil Resources Ltd. reservoirs.
The Pembina Nisku area is located approximately 120 km west of th city of Edmonton, Alberta. Canada (Figure 1). The Pembina Nisku discoveries were me result of a seismic exploration play initiated by Chevron in me mid 1970's1.
Imperial Oil Resources Ltd. operates seven vertical miscible floods in this area. These pools are relatively small in areal extent (130 ha - 325 hal) with OOIP ranging from a low of 1700 km3 10 a high of 5000 km3. Vertical miscible floods were initiated in these pools in th 1982 to 1987 time frame.
Figures 2 and 3 show me oil production and GOR performance of the Imperial Oil operated miscible floods. Oil production from these pools peaked in 1987 at approximately 5100 m3/day. The gas-oil ratio's (GOR) remained at the base level of 150 m3/m3 through 1987 but then, as a result of solvent breakthrough, started increasing rapidly 10 the present level of 280 m3/m3. As is the case with all vertical miscible floods, the increase in the GOR was accompanied by a rapid decline in the oil production rates. The pools presently produce approximately 1900 m3/day and this represents a decline of mare than sixty percent from their peak rates.
The Nisku pools have now recovered approximately 88 percent of their reserves with individual pool recoveries approaching 70 percent of the OOIP. The low remaining reserves combined with high individual pool recoveries make it difficult to justify major capital investments to extend the producing life of these pools.
In the past, various techniques to reduce coning (gas and water) have been proposed and tested. The conventional options have ranged from relocating perforations, managing production rates and periodically shutting in wells to allow the cone to heal. The non-conventional methods included wellbore modifications such as applying gravel packs and the placement of barriers created by the injection of diverting and blocking agents.
