Oil Re-injection to Mitigate Solvent Coning

Abstract

Abstract 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. At this advanced stage of depletion, the vertical floods have a low reserve life index and, hence, major investments cannot be justified to extend the producing life of these floods. Imperial Oil Resources Limited, in its search for new low cost techniques to increase reserves while maintaining/increasing oil production, began to examine the concept of oil reinjection in its Pembina Nisku reservoirs. Exploratory laboratory tests and numerical simulation work demonstrates that a dual completion to re-inject part of the produced oil back into the formation, a few metres above the producing perforations, could lead to incremental recoveries. Simulation results specifically showed a 2 – 3% of the original-oil-in-place (OOIP) increase for the model studied. 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 reservoirs. Introduction The Pembina Nisku area is located approximately 120 km west of the city of Edmonton, Alberta, Canada (Figure 1). The Pembina Nisku discoveries were the result of a seismic exploration play initiated by Chevron in the mid 1970's(1). Imperial Oil operates seven vertical miscible floods in this area, These pools are relatively small in areal extent (130 ha - 325 ha) with OOIP ranging from a low of 1,700 km3 to a high of 5,000 km3. Vertical miscible floods were initiated in these pools in the 1982 to 1987 time frame. Figures 2 and 3 show the oil production and GOR performance of the Imperial Oil operated miscible floods. Oil production from these pools peaked in 1987 at approximately 5,100 m3/day. The gas-oil ratios (GOR) remained at the base level of 150 m3/m3 through 1987 but then, as a result of solvent breakthrough, started increasing rapidly to the present level of280 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 1,900 m3/day, and this represents a decline of more than 60% from their peak rates. The Nisku pools have now recovered approximately 88% of their reserves with individual pool recoveries approaching 70% of the OOIP. The low remaining reserves combined with high individual pool recoveries make it difficult to justify major capital FIGURE 1: Nisku regional geology and pools, Pembina area, Alberta, Canada. FIGURE 2: Imperial Oil operated Pembina Nisku miscible flood-oil rate. investments to extend the producing life of these pools.