Assessment of a Vertical Hydrocarbon Miscible Flood in the Westpem Nisku D Reef
- W.J. Da Sle (Chevron Canada Resources Ltd.) | D.S. Guo (Chevron Oil Field Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1990
- Document Type
- Journal Paper
- 147 - 154
- 1990. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.3.4 Reduction of Residual Oil Saturation, 5.6.2 Core Analysis, 5.4.9 Miscible Methods, 5.5.2 Core Analysis, 5.2.1 Phase Behavior and PVT Measurements, 4.1.9 Tanks and storage systems, 4.6 Natural Gas, 2.2.2 Perforating, 1.6.9 Coring, Fishing, 5.1 Reservoir Characterisation, 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating, 1.6 Drilling Operations, 5.7.2 Recovery Factors, 5.4.2 Gas Injection Methods, 5.4.1 Waterflooding
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In 1981, a vertical hydrocarbon miscible flood was implemented in the Westpem Nisku D pool, one of 40 isolated carbonate pinnacle reefs in a trend located west of Edmonton, Alta, Canada. A solvent composed of methane and 15% ethane-plus components was injected into the reservoir, which is maintained at a pressure of about 4,800 psig [33 MPa]. Positive indications of the location of the solvent/oil interface have been obtained from interface logging [pulsed-neutron-capture (PNC) logs] in a producing wellbore and from special analysis of core from an infill well recently drilled into the pool. Two types of chromatographic analysis of residual oil and a conventional core residual oil determination were done. The results show that the flood is gravity stable and has excellent volumetric conformance. The special core analysis provided information on the quantity and composition of residual oil saturation (ROS) in the miscibly swept zone. On the basis of these flood performance data, an estimate of ultimate recovery has been prepared.
The Westpem Nisku D pool is one of a series of isolated pinnacle reefs discovered in the Pembina area 100 miles [161 km] southwest of Edmonton. The original discovery well (Well 7-12-49-13 W5) was drilled in 1978, and the pool was produced in a primary-depletion mode until May 1981. A vertical hydrocarbon miscible flood implemented in May 1981 included injection of a lean gas composed of 80% methane and 20% ethane-plus into Well 7-12. A second well (Well 6-12-49-13 W5) was drilled as an oil producer. Pool pressure initially was maintained at 4,500 psig [31 MPa]. In mid-1983, improved miscibility predictions prompted a change in the solvent composition and reservoir operating pressure. Pool pressure was increased to 4,800 psig [33 MPa] and solvent enrichment was reduced to 85% methane and 15% ethane-plus. A third well was drilled into the pool in March 1987. The addition of a new well 6 years after the start of the miscible flood, coupled with successes in obtaining interface logs results, provided an opportunity to assess the miscible flood performance in this pool. Two aspects of flood performance are considered. The first is the stability of the vertical flood. The results show that the solvent/oil interface is consistently flat across the reef. The second aspect of flood performance is displacement efficiency. Core analysis results show that ROS to the miscible flood is on the order of 5%. Dumore et al. presented an analytical model for condensing and vaporizing gasdrives that is used here to provide a conceptual description of vertical hydrocarbon miscible flood behavior in this pool.
Table 1 presents a summary of basic reservoir data. Fig. 1 shows a cross section through the reservoir with porosity traces for each well. Because the reservoir is about 292 ft [89 m] thick and has no underlying water leg, it was considered a good candidate for a vertical miscible flood scheme. This reef is elliptical, with the gas injector (Well 7-12) at its center. The producing well (Well 6-12) was drilled about 1,300 ft [400 m] southwest of the injector in 1980 and Well 10-12, drilled in 1987, is located directly opposite Well 6-12 about 1,300 ft [400 m] northwest of the injection well. A complete geologic description of the Nisku pinnacle reefs can be found elsewhere. It is important to note the existence of two zones of different quality in this reservoir. The main reef (Zeta Lake member) is fully dolomitized and has abundant vuggy and pinpoint vuggy porosity as well as brecciation fracturing. A tidal flat stratum (Wolf Lake equivalent) exists at the top of the reef, which is of distinctly poorer quality than the main reef stratum. The tidal flat stratum varies in thickness from about 6.5 ft [2 m] in Well 10-12 in the north to 43 ft [13 m] in Well 6-12 in the southern half of the pool. Average porosity in the main reef is 12%, and horizontal permeability is about 1,050 md. In the tidal flat stratum, porosity averages only 8% and horizontal permeability only 30 md. Arithmetically averaged vertical permeability is about 110 md in the main reef and 2.5 md in the tidal flat stratum. Table 2 lists reservoir fluid properties. This is a volatile oil with a field GOR of about 1,800 scf/bbl [324 std m3/m3] and a stocktank oil gravity of 45 degrees API [0.8 g/cm3]. The reservoir pressure has never dropped below the reservoir fluid bubblepoint pressure of 3,966 psig [27.3 MPa]. Material-balance calculations were used to estimate original oil in place (OOIP). Cumulative primary production from Well 7-12 totaled 530,000 bbl [84 260 m3] of stock-tank oil and caused a drop in reservoir pressure from 5,911.5 to 4,693 psig [40.76 to 32.36 MPa]. From this information, OOIP is calculated to be 15.7 million bbl [2.5 x 10(6) m3] of stock-tank oil.
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