The Wizard Lake Vertical Miscible Flood Solvent Bank Redesign Concept

Abstract

Abstract The Wizard Lake D-3A Pool has been undergoing a tertiary hydrocarbon miscible flood (HCMF) since 1983 after completing primary production (1951 - 1967) and a secondary HCMF (1969 - 1983). The operation is comprised of three solvent injectors, four push gas injectors, and 42 production wells. Fresh solvent injection (2,500 m3/day) was initiated in 1991 to make up for significant solvent coning and changed the flood design from a vertical flood controlled by longitudinal diffusion to a horizontal flood dominated by transverse diffusion. Recycling in this case also allows the old propane-plus bank to be replaced by a more economical ethane-plus blend. Under solvent recycling, the mixing zone would be stabilized (balanced injection and withdrawal) with its thickness increasing from zero at the injector to the maximum at the producer. The design problem is thus reduced to finding the solvent residence time at each producer and the corresponding minimum bank thickness needed to maintain miscibility. The 1D diffusion equation coupled with miscibility data was used to calculate the required bank thickness and a full field 2D streamline model was used to predict the solvent residence times in this study. New solvent bank thickness design curves were generated by merging the residence times and the minimum miscible bank thickness data. Operating strategies based on the new design curves can potentially reduce the 1994 solvent bank size of 5.5 million reservoir cubic metres (rm3) to 3.0 million rm3. The concept applied in Wizard Lake can be extended to other miscible floods that involve solvent recycling. TABLE 1: Wizard Lake solvent bank design history. Background The Wizard Lake D-3A Pool is a dolomitized carbonate reef with a current estimated original-oil-in-place of 62.4 million m3. Reservoir pressure was decreased from 15.65 to 12.95 MPa during primary production. Prior to the secondary miscible flood (1968), water was injected to repressurize the reservoir to 14.80 MPa. At this time, the water-oil contact rose to 1,201 m sub-sea (mSS) from its initial position of 1,229.6 mSS. A first-contact miscible (FCM) propane-plus solvent and a 95 ﹪ methane push gas were injected to sustain a vertical miscible flood from 1969 to 1983. The solventoil contact reached the 1,201 mSS depth by 1983 and a new approval was granted to extend the flood to capture the tertiary oil trapped between the 1,201 and 1,229.6 mSS levels. This stage is known as the tertiary extension phase and the solvent was changed to an ethane-plus blend. Figure 1 shows a well map of the Wizard Lake D-3A Pool. Table 1 chronicles the history of the solvent bank design for the Wizard Lake flood. Extensive solvent coning commenced in 1991 (∼ 3:1 solvent to oil ratio). The produced solvent was purified and reinjected in order to maintain solvent bank integrity. Updated geological data indicated that the targeted reservoir had a lower porosity than was estimated originally. Due to solvent re-injection, the reservoir flow became largely horizontal with solvent mixing controlled by transverse diffusion. These events prompted the questioning of the original design assumptions and led to a redesign of the flood in 1994.