Abstract

In the past 20 years, there has been a string of exploration successes in newly discovered oil-bearing carbonate reservoirs within the South Oman Ara salt basin. These oil reservoirs are deep (3000–5000 m), encased by salt, and have high reservoir pressure far above the hydrostatic gradient. The crude is light (30–45 API gravity) and contains 5–17% CO2 and 2–6% H2S. Reservoir properties are relatively poor: average porosity varies from 5 to 10%, and permeability varies from one to several tens millidarcy. The primary recovery mechanism is pressure depletion, with low recovery in the range of 4–8%.

Enhanced oil recovery (EOR) by means of immiscible gas injection (GI) was implemented in Field-B in 1993, and miscible gas injection (MGI) was started in Field-Z in 2012. The MGI program is expected to increase the ultimate recovery up to 45%. The main subsurface risks to the performance of the gas injection projects include vertical and areal conformance, gas breakthrough, and reservoir pressure decline. The key objective of the MGI surveillance is to establish residual oil saturation in gas-flooded rock. The key issues and challenges for the surveillance include low porosity, complex drilling operations, well design, and completion.

The well and reservoir surveillance management (WRM) activities included the drilling of a dedicated monitoring well, collection of various pressure measurements, injection of tracers, and acquisition of advanced openhole and casedhole data, which included nuclear magnetic resonance (NMR), pulsed neutron logging (PNC), and production logging (PL). The primary focus of this active WRM plan is to monitor the performance of the GI and MGI. The plan has been successfully implemented.

We present examples of reservoir surveillance data, such as the molecular diffusion-sensitive NMR and enhanced PNC data acquired in Field-B and Field-Z. The objectives of these measurements are to monitor the current oil saturation and detect and evaluate gas breakthrough. The static downhole pressure survey and formation pressure measurements were collected in newly-drilled infill wells. The PL and tracer survey data effectively provided information about gas breakthrough intervals and enhanced the understanding of the impact of vertical and lateral heterogeneity. This data is also useful for planning gas shutoff when the breakthrough is detected.

The applied surveillance strategy supports Field-Z and Field-B gas injection performance. The strategy can potentially benefit other analogue fields undergoing similar development options to reduce subsurface risks, proactively recognize problems, and devise effective mitigation actions.

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