Depletion Studies of Two Contrasting D-2 Reefs

Abstract

Introduction The Nisku Band G pools are two West Pembina D-2 poolswith contrasting reservoir properties. Average porosity, permeability and maximum thickness are 5%, 130 mD and 95 metres, and 16.4%, 7100 mD and 19 metres respectively. Theresults of the depletion model studies of waterflooding andmiscible flooding and some of the problems that occured duringthese studies will be reviewed. Introduction The West Pembina play has resulted in the discovery in the past three years, of a series of reservoirs located some 120 km west-southwest of the city of Edmonton. A recent tally shows that some 40 pinnacle reef pools have been discovered with oil-in-place varying from 100,000 m3 to 5.75 × 106 m3. Many of the pools have shown very prolific production rates, with potentials of up to several hundred cubic metres per day. Amoco Canada has participated in a number of these discoveries. The two pools that will be described in this paper are of interest in that they illustrate the wide range of reservoir properties that have been encountered in these pools and the different approaches required in developing depletion methods suitable for them. The two pools are the Bigoray Nisku Band G pools, located as shown in Figure 1. Reservoir and Fluid Properties Some data on the two pools are summarized in Table 1. As shown, the oil-in-place is 1.5 × 106 m3 and 0.75 × 106 m3 for the B and G pools, respectively. However, they exhibit a very wide range in reservoir properties. The thickness for the B pool is 95 m and that for the G pool is 19 m. The permeability shows an even greater contrast. The average permeability of the G pool is 7,100 mD, with some individual values in the 20- to 40-Darcy range. The average permeability of the B pool is 130 mD, but this does not reflect the streakiness of the pay. The rock is generally in the 1-10-mD range, with vugular sections that raise the average. In addition, the 13–9 well to the north is limestone, whereas the 8-8 well to the south has been dolomitized. Figures 2 and 3 show core samples from the two pools. The fluid properties of the two pools are given in Figures 4 and 5. The bubble points are 9,067 kPa and 14,451 kPa and the solution gas-oil ratios are 51.4 m3/m3at 58°C and 104.4 m3/m3at 74°C for the Band G pools respectively. Selection of Modes of Depletion The thick section present in the B pool invited investigations of a vertical miscible flood, although the poor quality of the pay was of concern and prompted a bottom water-flood study as well. There was no known water underlying this pool. Although the G pool is of high quality, the low thickness and oil-in-place were insufficient to warrant anything except water-flooding. A preliminary solvent slug sizing calculation using linear diffusion indicated that a 90070 P. V. slug would be necessary assuming a 20-year life.