Waterflooding in Giant Carbonate Reservoir; Successes and Challenges

Abstract

Objectives/Scope The Mishrif reservoir in the Rumaila field is one of the largest carbonate oil reservoir in the world. The reservoir exhibits heterogeneity from good grainstones to poor lagoonal mudstones, with the presence of thin high permeability layers containing vuggy porosity. Since 1973, the Mishrif Reservoir went through several production phases which indicated lack of pressure support. Line-drive waterflooding presented risks of bypassed oil, low sweep efficiency and high residual saturations due to reservoir heterogeneity leading to low recovery. Methods, Procedures, Process The reservoir development has been modified from a line-drive towards a progressive pattern-flood to mitigate key uncertainties and risks for improved oil recovery. After a field trial, large scale water injection in the North of the reservoir began in 2014. This is achieved by utilizing existing infrastructure, converting some existing Producers to Injectors and carefully reactivating existing well-stock following the water injection response. A drilling campaign is planned to complete the inverted 9 Spot pattern, combined with the gradual upgrading of existing facilities. This will be followed by a 5-Spot pattern depending upon drainage and sweep performance. Results, Observations, Conclusions The results to-date are promising with a successful ramp up of production from ~150mbd to +400mbd from +200 active wells while successfully maintaining reservoir pressure above saturation pressure with current water injection of over 400mbd. In order to optimise the full field development and effectively mitigate by-passed oil and poor sweep efficiency risks, the sweep patterns must be properly understood across the various reservoir layers and areas. As the production and injection ramp-up continues, water injection priority is being continuously optimized along with reservoir surveillance with specific focus on monitoring reservoir pressure and water-cut trends in the producers to understand sweep and drainage across this giant reservoir. This will need to be complemented with the well conformance through a well thought out completion strategy and effective water shut-offs. The development plan is estimated to achieve an aerial sweep efficiency of ~75% in the most developed water flood patterns by 2035. Novel/Additive Information In addition to the production ramp-up, progressing pattern-flood and waterflood management, future development options are being evaluated. This involves fully utilizing learnings from the reservoir performance to-date and further developing deeper understanding of this world-class carbonate reservoir using new technology to further enhance production and recovery in more challenging reservoir quality areas.

Effect of Well Orientation on Oil Recovery from Waterflooding in Shallow Green Reservoirs: A Case Study from Central Africa

“…Despite the volume of published literature [26][27][28][29][30][31][32][33][34][35][36][37][38] on the recovery efficiencies of different well types, to the best of our knowledge, there has been no research on the performance of highly deviated wells in the highly heterogeneous shallow reservoirs of the Central African Rift Valley region, which are significantly different from most moderate-depth to deep reservoirs in other parts of world. This is because of (i) the complications associated with drilling along a precise path within an inclined path, (ii) difficulties encountered in preventing sand influx, and (iii) challenges encountered during screen installation for sand production control, which render directional drilling more difficult and time-consuming for complex reservoirs.…”