Golden Spike Enhanced Recovery By Gas Cycling
Authors: Biberdorf O.C. 
Published: March 2010

Abstract
This paper describes the studies which have been completed to evaluate the benefits of gas cycling as a recovery scheme in the Golden Spike D-3A reservoir. This reservoir contains a secondary gas cap with a dispersed hydrocarbon miscible bank. The cycling of dry gas not only accelerates the Cs+ recovery from the rich gas, but also recovers additional volatile light ends from the immobile residual oil through vapourization. Prior studies of the Golden Spike D-3A reservoir defined a complex reservoir description and fluid distribution. This description was incorporated into a numerical cross-sectional model of the reservoir which analyzed and predicted the flow and distribution of fluids within the reservoir. A one-dimensional, multi-component reservoir simulator, utilizing a modified version of the Bennedict-Webb-Rubin equation of state, determined the phase behaviour of injected and in-place hydrocarbons. The combined results of these models provided total pool recovery performance and the basis for the Golden Spike gas cycling depletion strategy.

Introduction
The Golden Spike D-3A Pool, located eighteen miles southwest of Edmonton in central Alberta, was discovered in 1949 (Fig. 1). The reservoir is a Leduc-age Woodbend limestone reef which has an oil zone 550 feet thick within the 1000-acre crestal area. Early pool performance confirmed that the reservoir was a closed system, containing 319 MMSTB of oil with no gas cap or water leg present.
Initial production by fluid expansion, resulted in a rapid decline in reservoir pressure. Therefore, a pressure maintenance scheme by gas injection was installed in 1953 (Fig. 2). In 1964, a miscible flood was started with the injection of a solvent bank between the gas and oil zones to reduce the residual oil saturation and thus improve over-all recovery. Solvent was obtained by stripping the LPG fraction from the produced crude. During the period 1973 and 1974, the miscible bank encountered a large barrier to vertical flow, covering the interior of the reservoir. This barrier dramatically influenced the flow path of the solvent bank, causing it to disperse into the gas cap(l,2). As a result, the recovery mechanism reverted to gas displacement.
With the dispersion of the solvent bank, the reservoir contains a high inventory of LPG products. This high inventory of LPG prompted the investigation of a gas cycling scheme with the objective of maximizing the recovery of these products.
The Golden Spike crude oil producing wells (Fig. 11) exhibit high productivities such that the desired production can be achieved with low drawdown. Therefore, it was decided to evaluate the feasibility of concurrent gas production with gas cycling, since lowering the reservoir pressure would not have a significant effect on oil productivity or recovery.
At the end of 1977, oil sales totalled 155 MMSTB. To the end of the same period, a total of 32 MMRB of solvent and 124 BCF of gas had been injected (Fig. 2).

Study Procedure
The gas cycling recovery process (Fig. 3) involves dry residue gas displacing wet gas as well as vapourizing volatile light ends from the residual oil.