Optimal Well Spacing within the Context of Decision Optionality Regarding the Long-Term Reservoir Recovery Process for a Complex Carbonate Offshore Oil Field

Abstract

Abstract Optimized well-spacing is fundamental to any field development. For the case presented here, an offshore, low permeability field in early development, the current well spacing is about 1000m. Ongoing analysis are focused on determination of the most optimum well spacing for the development of the reservoir understudy. Drilling an offshore well is expensive and the operator requires a smooth transition from waterflood to EOR operations and high profitability from the asset development plan, therefore significant effort goes into evaluation of the optimal well spacing, ensuring full life-cycle flexibility. We present a case study as a step-by-step workflow to evaluate optimal well spacing as well as EOR effects for a complex carbonate offshore oil field. The evaluation is completed within the context of decision optionality regarding the long-term reservoir recovery process (Waterflood vs WAG HC vs. WAG CO2). Several recovery options were evaluated using a sector model extracted from the fullfield model. Local grid refinement (LGR) was used to evaluate the impact of numerical dispersion on recovery and breakthrough time. A constant property model was used in all scenarios. The fluid was described by a tuned compositional formulation. Hysteresis saturation functions were used along with inert numerical tracers to track fluid movement. Spacing between horizontal injectors and producers was varied. The WAG cycle was adjusted to reflect the spacing distance. The key objective of the work was a simultaneous optimization of the production plateau duration, maximum oil recovery, and a minimum unit technical cost (UTC). The well spacing assessment was completed preserving optionality (final decision on EOR not yet sanctioned), having the following components/workflows: LGR was done to assess the optimal recovery methods with different WAG schemes (effects of numerical dispersion)Use of inert numerical water tracers (unique water tracer to each water injector) to differentiate and track the movement of injected water versus the connate water (evaluation of sweep efficiency), varied WAG cycle length according to the well spacing with shorter WAG frequency for smaller spacing size (evaluation of optimum injection periods)Sector model for a given representative geologic realization, mimicking rigorous element of symmetry workflows by adjustments to boundary and edge well connections.Recovery strategy (water injection, WAG-HC, WAG-CO2). Small scale effects were rapidly and efficiently evaluated. Waterflood recovery is very efficient (M<1) leaving a small remaining oil saturation (ROS) target for EOR. Incremental recovery factor for EOR was less than 10%, varying with the well spacing and injectant fluid. Faster breakthrough occurred as the spacing size decreased, particularly for gas injection, due to high permeability streaks or even when using constant property model. The optimal development scenario is a function of the distance between wells and the injectant, based on the recovery factor and UTC. This provides the operator options for field development, depending upon how the operator views the importance of different cost structure inputs in the future.