Achievements and Learnings from a Waterflood Project in Small Sized Heterogeneous Reservoir
Authors: Zafar Jahangeer , Noman Muhammad , Iqbal Muneez , Sadiq Omair , Awase Azar 
Published: March 2023

Waterflooding is a proven technique to improve recovery of depletion drive oil reservoirs. However, its feasibility becomes challenging with limited volumes and complicated reservoir characteristics. Therefore, smart and cost-effective solutions are the focus areas to optimally design and implement the waterflood project in those fields. Similar strategies were deployed at S Field located in Pakistan's Lower Indus to achieve incremental recovery along with learnings for other regional fields.

S Field had two major challenges: Drastic depletion pressure and heterogeneity. The pressure dropped by 72% after recovering only ~480 MBO in 2 years. S Field was penetrated by two wells, S-1 and S-2, which were 0.5 km apart but still exhibited hugely different reservoir performance endorsing sharp facies variation apparent on G&G correlations. Based on production decline analysis and material balance, natural recovery was estimated to be ~17% so optimization opportunities were deliberated for recovery improvement. Simulation Model was run for different sensitivities on waterflood options. Reservoir heterogeneity was captured in the Model through pressure transient analysis, Open-hole Logs Correlations, and seismic attribute analysis. The results showed that a direct-line-drive would be optimal waterflood technique. As discussed, the small sized field warranted cost efficiency to make the project economically feasible. Consequently, surface injection system resources were optimized, in-house inventory was utilized after fitness for service assurance, and a water source well was drilled in shallow formation to optimize drilling cost. In addition, pre-waterflood steps were undertaken to ensure optimal injection performance, such as injector well coring program and compatibility test.

Injector S-WIW was drilled at a sweet spot identified through G&G understanding and simulation model, and injection was commenced at an initial high rate of 2500 Bwpd. However, it could not sustain due to permeability impairment and injection rates dropped by 40%. The issue was successfully addressed through a rigorous optimization approach, including hydraulic frac and vigilant monitoring of water quality, especially solids, iron count and bacterial growth, considering the heterogenous rock nature. Post waterflood, field life extended by 6 years, enhancing the recovery factor to 28%.

Besides oil recovery, the project provided learning which would benefit the upcoming projects in the region marked by various small-scale oil fields. Firstly, the timeline of the waterflood could be improved; it was sensitized on the simulation model that earlier project kick-off, ideally before the bubble point, would generate higher recovery. Besides, to sustain desired injection rates at S-WIW, recurrent rigless jobs optimization jobs were carried out, and finally, a robust hydraulic fracturing was conducted for permanent improvement. It is learned that a frac in the early stages could have pre-empted injector downtimes and rigless expenditure.