Magnus WAG Pattern Optimization Through Data Integration

Abstract

Abstract This paper focuses on studies carried out to explore possibilities to improve both areal and vertical sweep efficiency in mature WAG patterns in the Magnus oil field. Magnus tertiary miscible gas injection started in 2002 through a WAG scheme taking the recovery factor close to 56% to date at a very high net efficiency of 3.5 mscf/stb. Pore scale sweep efficiency is very good with 8% residual oil to miscible gas flood (Sorm) based on core flood data. However, areal and vertical gas sweep efficiency seems to be sub-optimal based on 4D seismic and PLT data. The Magnus Sandstone Member (MSM) consists of a number of stacked turbidite sand lobes separated by intra formational shales with varying thickness and lateral extent. The WAG scheme has been operated and managed by considering the MSM as a single reservoir unit. As the patterns have matured, it has become apparent that the scheme could be optimized by further vertical separation of the reservoir units. Key surveillance data such as 4D seismic monitoring gas movement, PLTs from both producers and injectors, well performance and open hole saturation logs have been coupled with simulation to study options for vertical and areal sweep improvement. These options involve changing sweep direction and intervals by means of adding new perforations and shutting off zones, complete reversal of the patterns – i.e. converting producers to injectors and vice versa, and use of chemicals for flood diversion. Evaluation of multiple options resulted in a phased WAG optimization programme of which the first phase in the Central Panel is being proposed for execution in 2014. This phase involves changing perforation intervals in both producers and injectors. Later phases consist of further changes in perforation intervals, potentially pattern reversals and use of chemicals for diversion. A behind flood front core is planned in 2015, which is expected to help calibrate the optimization programme by quantifying Sorm and the degree of vertical sweep achieved in the field. The study required a detailed review of the surveillance data from various sources including 4D seismic and PLT data which were crucial in understanding the sweep efficiency. Relatively low cost options were identified to improve the sweep as opposed to drilling new wells. Integration with the operations team was the key in creating a business case for pattern optimization on a 30-yrs old, bed space constrained platform. The workflow of this study and the learnings are applicable to mature patterns in any WAG EOR scheme. Optimizing the WAG patterns enables more efficient use of the available gas, which (given that the cost of gas is a significant component of any gas injection project) makes this more commercially viable and cost effective tertiary recovery option.