Improve Waterflood Vertical Conformance for a Heavy Oil Multi-Layer Sandstone Reservoir. A Case Study from South of Oman Field

Abstract One of the challenging Waterflood field in south of Oman was suffering significantly from poor waterflood vertical conformance. The field lost 60% of its oil production rate within 4 years, despite drilling 40 additional oil producers at the same time, which is corresponding to 50% increase in number of wells. The field is a heavy oil, multi-layer sandstone oil reservoir. Each layer has its own depositional environment, oil viscosity and static properties. Moreover, the extensive faulting of the field leading to compartmentalization adds more complexity to the field remedy plans. The original development strategy was based on commingled production/Injection from all layers. It was found that one layer (L4) has heavily dominated production and injection in the field, which only has 25% of the total STOIIP, leaving the remaining 75% oil unswept. This paper will highlight and showcase the mitigation plans that were implemented that lead to significant improvement to the WF vertical conformance which led to stable field production recently. The first and the most important step toward fixing the production crisis was to improve the understanding of the communication paths in the reservoirs by integrating production data, pressure data, and an improved understanding of the sand distribution in the reservoirs. The key surveillance data that helped was the Production Logging Tools (PLTs), tracers and the Reservoir Pressure. Considering all limitations and risks, it was found that the best development scenario was implementing a double inverted five spot waterflood pattern in the field (Fig 5). These two patterns are called: the A-pattern and the B-pattern. The aim of this development is to establish dedicated production from remaining layers units other than the L4, as the L4 is interpreted to have heavily dominated production and injection in the field. The B-pattern will first be completed on the L5 & L6 units (both deeper than the L4). The A-pattern wells will be gradually converted to dedicated wells on the L3 and L1&2 (if present). Check Fig 2 In 2023, 95% of the recovery plan has already been implemented which led to a sustainable oil production with 0% decline rate in the last 5 years. Also, it is clear from the PLTs that the vertical conformance has improved significantly. There are clear evidences that all others units are now contributing to both production and injection. In some patters, it was observed that the WF vertical conformance has improved up to 50%. Also, from economic point of view, the project has achieved its promised NPV with very attractive UTC of less than 15 /bbl and more than 20 mln bbl of additional reserve.