Field D is located in an average water depth of about 115ft in the Balingian province, about 35 km northwest offshore of Bintulu, Sarawak. The field was discovered in 1983 by drilling D-1 exploration well, and subsequently delineated by seven (7) appraisal wells. The field recovery mechanism is predominantly solution gas drive with limited aquifer support in the North and North-east of the Cycle II Sheet reservoir. Within three years of production, oil rate has rapidly declined to around 10,000 stb/d with gas oil ratio (GOR) increasing from 800 scf/stb to around 3,000 scf/stb. Gas re-injection scheme was introduced in 2000 with Gi/Gp performance was the main control measure to optimize the injection efficiency. This paper presents the performance of two major reservoirs in D Field with gas re-injection schemes.

The first part of this paper discusses the performance of Reservoir C3-3A. Wells performance during pre-injection period was studied. Basic performance plots such as Qo vs Time and GOR vs Time were utilized to understand the well behavior and decline curve analysis (DCA) was used to calculate the NFA reserves for this reservoir. The performance of this reservoir after the gas re- injection was then further analyzed. Similar performance plots are utilized (Qo vs t, GOR vs t, DCA) with the addition of injection wells performance plot. The oil producers performance were plotted to compare their reaction toward different injection rates. The Gi/Gp control measure was applied as part of the reservoir management target. In order to quantify the incremental reserves from the gas re-injection, oil DCA plots were established. Similar approach was applied for the second reservoir, C3-4A.

In overall, the gas re-injection in D Field has successfully arrested the annual field decline rate from 45% to 20% and increased the ultimate recovery factor to 35% after 19 years of re-injection. However, further scrutiny towards each well showed that some inefficiencies did occur in the re- injection schemes. Due to poor location of injector wells, some oil producers produced excessive GOR due to gas recycling if the gas injection rate was maintained as per Gi/Gp target. This resulted in lower oil recovery and lower revenue from gas export. Furthermore, some injectors were also not injecting as per target due to poor reservoir quality and mechanical issues. Lessons captured from 19 years of re-injection history will be the reference to set the operating strategy of existing and future injection wells at field D.

The information and lessons learnt from this paper will be applicable for future field development involving gas re-injection by setting an example on how to design optimum gas injection strategy.

You can access this article if you purchase or spend a download.