Rejuvenation Through Integration: Maximising Production from a Complex Clastic Reservoir with a Bio-Degraded Oil Rim Under Waterflood in the Sultanate of Oman

Abstract

Optimising the late-life development of heavy oil reservoirs due to biodegradation in a fresh water aquifer remains a challenge in the industry. When the variation of viscosity with depth is coupled with a significant degree of compartmentalisation due to structural complexities, the identification of a technically and economically viable development requires an integrated approach in field development studies. This paper presents a case study for such a complex field, a Gharif reservoir situated in the Eastern Flank of the South Oman Salt Basin. The integration between various data sets from across disciplines of varying fidelity and by adopting a decision-based planning approach has achieved two outcomes. Firstly, the highest field production since coming on stream; and secondly, the delivery of an updated Field Development Plan (FDP) that unlocks remaining hydrocarbon potential in a phased approach to mitigate key risks. On stream since 1981, the heavily compartmentalized Marmul Gharif South Rim Field has evolved from a primary depletion to a mature waterflood by flank injection. The sands, distributed in a rim setting with a steep dip tend to be vertically discontinuous in the wells, so that direct observation of fluid contacts is very rare and most wells yield only a Water Up To (WUT) or Oil Down To (ODT). In addition, the poor contrast of heavy oil density against fresh formation water makes it difficult to obtain accurate pressure gradients. The field can be subdivided into a number of compartments with varying degrees of communication from complete hydraulic independence to weak/moderate pressure communication. Over the course of 2016-17, a study was carried out by a multi-disciplinary team to deliver a FDP. By integrating existing data, the team created a new structural framework. This involved integrating faults based on Bore Hole Images (BHI) together with seismic re-interpretation; analyzing production and pressure data for connectivity mapping; updating the OWC assessment which considered oil biodegradation as a function of height above free water level. This was followed by combining the new insights into a fit-for-purpose dynamic modelling approach which led to the identification of new infill/appraisal targets and formed the basis of the redevelopment plan. The increased understanding of the field has allowed early WRFM activities which contributed to increase production by the order of 20%. The effort has materialized into an improved field understanding and delivered a rejuvenation plan with an immediate impact of unlocking reserves with the drilling of 5 drilling & appraisal targets in 2017. This is followed by a phased development with 30 development and 3 appraisal wells in Phase 1; and additional 65 development and 1 appraisal well in Phase 2, to increase the field recovery factor by 5%.