Modelling the Reservoir Mechanisms for Gas Cap Blowdown of the Virginia Hills Belloy Reservoir

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Abstract Concurrent gas and oil production from the Virginia Hills Belloy Shunda Unit #1 was considered. A full field simulation study was used to evaluate the impact of initiating early blowdown of the gas cap. An excellent match of the production, pressures, and two-phase interface movements was achieved. The model predicts that early blowdown and delayed blowdown cases achieve comparable ultimate oil and gas recoveries. The role of the well established water fence between the gas cap and oil leg, unique reservoir characteristics and reservoir management strategy, which contribute to this result, are discussed. Research and analytical work was a prerequisite to the simulation study to establish many of the model input parameters. Conformance plots are discussed and used to determine:how efficient reservoir depletion has been to date anda residual oil saturation. Results of the simulation study also confirm the input relative permeability curves and trapped gas saturation obtained from referenced studies. Introduction The Virginia Hills Belloy field is located approximately 165 km northwest of Edmonton, Alberta in Townships 63 and 64, Range 13 and 14, W5M (see Figure 1). The field was discovered in 1970 during the development of the nearby Virginia Hills field and was initially developed as a gas pool. Up dip movement of the oil column towards the gas cap led to the discovery of the oil leg in 1976. By 1978, the oil column was delineated with 21 oil wells. The gas cap was shut-in by 1980 after having produced approximately 800 E6m3. A waterflood was implemented as an up dip line drive while also creating a water fence between the gas cap and oil reserves in 1981 (Figure 1). Down dip water injection began in 1991. Since 1994, a total of seven horizontal (five successful, two wet) wells have been drilled to recover bypassed oil. One of the five successful horizontal wells was drilled on the basis of this simulation study. With the field maturing, a simulation study was commissioned to determine the ultimate recovery and whether oil and gas recovery would be impacted by early blowdown of the gas cap. With the existing gas plant approaching turndown and economic limits, decommissioning the plant in 1999 would be followed by capital expenditures to re-inject the sour gas and eventual blowdown of the gas cap. Early blowdown would conserve capital and make efficient use of existing resources. In addition, concurrent production (early blowdown) would result in a reduction of the operating life by 12 years, and thereby also reduce associated expenses. FIGURE 1: Location of producers and injectors in Belloy Field. (Available in full paper) From a regulatory perspective, approval for concurrent production is normally not considered unless the pool is in the final stages of depletion (greater than 90% of ultimate recoverable oil) and the ultimate recovery is not impacted significantly. Depending on the recovery estimate used, current recovery from Belloy is between 80 – 90% of ultimate oil recovery. This study will show that concurrent production of oil and gas from the Belloy pool does not adversely affect hydrocarbon recovery.