Shifting Reservoir Development Focus to IWAG for a Matured Water Flood Case
Authors: Shrivastava Sanjiv Kumar , Rahim Mas B A , Cheng Ho Arthur Ng , Sedaralit M Faizal , Rasid Ruziean B A 
Published: March 2015

Abstract
The subject oil reservoir in offshore Sarawak, Malaysia, has oil accumulation in an anticlinal structure, bounded by two growth faults running ENE-WSW direction. It comprises of coarsening upward sequences in the shore face environment. Initial development, purely under depletion using conventional wells, has been associated with rising GOR trend and declining reservoir pressure, raising concerns about depletion strategy. Second phase of development wells has further aggravated the pressure scenario, and water injection has been contemplated as the field development strategy. Strict regulation on gas production has been imposed until commencement of water injection.
Starting oil production since 1972 under depletion, over last 40 years adapting to new information and technology the strategy has progressively advanced to reservoir pressure maintenance by water injection, then to horizontal wells, and has now focused more into reservoir heterogeneity and layer wise recovery. The current study further probes the EOR development options focusing on micro displacement of oil.
Once the peripheral water injection has been fully operational, oil production rate has steadily been built with horizontal infill wells. Water injection has successfully been able to enhance the reservoir pressure close to its initial value in a span of 5 years.
Thereafter, with rise in water production, the reservoir oil production has substantially declined and focus has now been more on understanding reservoir heterogeneity and tracking the oil saturation. Detailed laboratory studies to evaluate gas miscibility options for EOR and to enhance the understanding on rock fluid dynamics and reservoir heterogeneity has been recently completed. A detailed reservoir simulation model identifying different reservoir facies embedded with new saturation height function has been thus built. Detailed investigations have been made using the new dynamic model to evaluate the best development option. The current study further probes the EOR development options focusing on reservoir heterogeneity, layer wise recovery and micro displacement of oil. One dimensional and compositional model has also been used to understand applicability of different Gas EOR processes. Though Miscible CO2 gas injection process gives a higher recovery; after rigorous evaluation, in line with the overall field EOR strategy, iWAG has been identified as the new EOR technology to enhance the field recovery over water injection.

Introduction
XamXung (name changed) field is located offshore Sarawak in the south western part of the Baram Delta Province (Figure-1), at a water depth of 250 ft. The field was discovered in July 1967 with Well XN-01. Since then a total of 69 wells and 11 sidetracks have been drilled from 5 platforms. Oil production started in 1972 through two isolated appraisal wells completed subsea.
The main accumulation is located at the crest of the structure, bounded to the north and south by two north-hading growth faults that run almost parallel in an ENE-WSW direction (Figure-2). A total of 25 hydrocarbon bearing reservoirs are encountered in the XamXung Field (Figure-3). No faults are present within the main structure. Hydrocarbons are trapped in stacked sand bodies separated by sealing shales. The hydrocarbon bearing zone occurs between 3100 ft TVDSS and 10350 ft TVDSS with the primary oil producing zones at 5,400 to 8,000 ft TVDSS. The main oil-producing reservoirs are AS1, AS2 and AS4, with additional production from the Minor reservoirs (Figure-3). These oil-bearing reservoirs are sandwiched between the shallow gas-bearing reservoirs and the deeper gas/condensate-bearing reservoirs.