WAG Pilot Design and Observation Well Data Analysis for Hassi Berkine South Field

Abstract

The Hassi Berkine South field (HBNS) is an undersaturated, low-viscosity, moderate permeability oil field within the center of the Berkine Basin in Algeria (Fig. 1). Discovered in January 1995, the HBNS field had first oil production in 1998 from the Trias Argilo-Greseux Inferieur (TAGI) reservoir with a reservoir development plan (RDP) of crestal, miscible gas injection and peripheral waterflood. Full-field WAG, as a secondary recovery process, is currently under investigation for the potential increased oil recovery and higher oil rate. This paper describes the design of the miscible water alternating gas (MWAG) pilot test and presents novel techniques for correcting observation well log data to reflect the miscible displacement process. A simulator sector model for the pilot area was used to design the MWAG pilot parameters (e.g., solvent/water volume, WAG ratio, timing). Next, measurements from the first WAG cycle were used to calibrate key parameters in the sector model, and to define WAG parameters for subsequent cycles. For the initial pilot design analysis, the fine areal gridding and vertical layering were taken from the Prudhoe Bay pilot simulation experience1,2. The objective of the pilot design is to collect optimal data from the observation well in the shortest possible time. The initial simulation work defined solvent injection rates, slug sizes, cycle lengths, and WAG ratios to achieve these goals. Subsequently, reservoir calibration with surveillance measurements from the first WAG cycle allowed for fine-tuning of key MWAG parameters. This work required correcting hydrocarbon saturations from the surveillance tools to be representative of the miscible displacement process. A method has been developed to correct log-derived immiscible saturations to reflect the miscible process occurring in the reservoir and simulated in the sector model. Introduction An observation-well pilot test was conducted in the HBNS field (Fig. 2) to evaluate the benefit of MWAG as a secondary recovery process. This area of the field has had no water or gas encroachment, but the reservoir pressure has dropped about 75 bars from initial conditions. The current RDP of crestal gas miscible injection and peripheral waterflood allows for a simple transition to MWAG. This pilot is part of a staged development for full-field WAG in the HBNS field. The key advantage of MWAG3,4 over the current RDP is improved utilization of the solvent (contacts more oil) with less available solvent supply leading to improved ultimate oil recovery at a higher oil rate. Careful design in the WAG pilot program is critical to ensure that data collected in the observation well can be used successfully in tuning the simulator5. Three parameters (solvent slug size, cycle length, and WAG ratio) are controlled in the WAG pilot design. The WAG parameters are scaled to ensure the oil saturation change can be measured over a defined period of time at the observation well. In addition, dynamic and gravity forces in the pilot area must be representative of what is occurring between the wells during full-field WAG implementation. Injection rates too high will displace the oil faster than can be measured. Injection rates too low will allow for gravity segregation to occur too quickly and may change the vertical proportion of injected fluid. The usefulness of the WAG pilot diminishes as the moveable oil between the injector and the observation well decreases. Reservoir simulation was used to determine the initial WAG parameters and to history match the first cycle of the WAG. The simulator sector model was defined for the pilot area with a detailed geocellular geologic model. Boundary conditions for the sector model were taken from a history-matched, full-field reservoir model. Data from Reservoir Saturation Tool (RST) and Cased Hole Formation Resistivity Tool (CHFR) were measured regularly at the observation well to monitor water and gas saturations vs. depth as first solvent and then alternate water and solvent slugs were injected.