Leveraging Strategic Lessons Learned from a Challenging Marginal Field Water Flood Development
Authors: Al Menhali Helal , Samantray Ajay Kumar , Alneaimi Ahmed , Elbaramawi Mohamed Lotfy , Loobari Sultan , Mukhtar Muhammad , Bejaoui Riadh , Bell Ferran , Mishra Anoop 
Published: October 2023

Abstract
The case study relates to a marginal green field located in the shallow waters, offshore Abu Dhabi. The target reservoir consists of layered and heterogeneous Jurassic carbonates encountered at about 10,000ft subsea. The field is a four-way structural closure with about 26km2 area and 340ft structural relief, with a weak edge aquifer. The Development strategy was two-fold: 1. Early Production Scheme (EPS) with primary depletion, followed by Full Field Development (FFD) with water flood. Production commenced in March 2018 with natural depletion drive. As of March 2020, with just about 3% cumulative recovery, the field was already experiencing differential depletion and rapid pressure decline at about 2 psi per day.
Based on the dynamic simulation model, five horizontal water injectors were designed for flank injection to restore the reservoir energy. However, these peripheral injectors were placed on the field outline defined by the structural spill due to lack of adequate flank appraisal and unknown free water level (FWL). Reservoir guidelines strictly required that pressure was maintained above the bubble point, and no oil was flushed behind the injector. However, flank well placement became challenging due to structural uncertainties, extended reach, unknown FWL, and deteriorated aquifer properties.
The field is under water flood for the past 6 to 12 months. It is currently in the voidage filling stage; downhole gauges supporting a correction of the decline rate. However, full restoration of reservoir energy is expected to take longer.
Some of the key strategic lessons learnt that would be useful for new marginal field developments in the concession area with similar geologic settings are:Although EPS allows early monetization, it is advisable to avoid EPS phase and commence water flood development right away in small Marginal Fields due to limited size, fast depletion and little aquifer support.Limited or deferred flank appraisal can be counterproductive. Upfront appraisal is necessary to reduce flank structural uncertainties, ascertain rock-fluid properties and nature of fluid contacts. This would help better estimation of resource volumes and optimize well placements.Peripheral injection and injection into the water leg should be avoided. This offers suboptimal energy support due to i) deteriorated aquifer properties, and ii) well trajectories leaving behind oil, or requiring injected water to travel long distance, causing delay in pressure support. Better to implement a hybrid strategy early on, including in-field injection into the oil leg to mitigate the effect of areal and vertical heterogeneity.Injection performance was seen to deteriorate over time (ca. months) due to dynamic skin. Periodic stimulation with recipe consisting of retarders and diverters helped improve injectivity, however overall injection performance remains variable.The injectors were completed with ICD and some with PPL. We found that the injection performance was independent of the completion type. Also increasing the maximum injection pressure from 3300 to 3500 psi had limited impact on improving the well injection rates.