Maximizing the Oil Recovery through Immiscible Water Alternating Gas (IWAG) in Mature Offshore Field

This paper discusses an enhanced oil recovery study of a multilayered mature offshore oil field in Baram Delta. Multiple reservoirs of the field have varying rock and fluid properties.The shallower accumulations contain oil of low API gravity of 19º with very low solution gas-oil ratios, while API gravity increases with depth to 24º in middle formations and 37º in the deepest sands. Upper sands are poorly consolidated with resultant high porosity and permeability. Most of the reservoirs are undersaturated. Initial field development comprised primary depletion with gas lift through vertical and directional wells - mostly dually completed. Field development strategy implemented measures like gravel packing wells to meet the challenges and possible problems during the course of field exploitation.

Field performance indicates that the current development strategy and practice may lead to moderate recovery lower than average recovery in Baram Delta fields through primary production. Detailed reservoir characterization has been carried out to capture the reservoir heterogeneity and multiple realizations have been made to understand the major uncertainties of the field. Screening studies for maximizing the recovery from the field suggests gas based EOR techniques may find application. The EOR method of Water Alternating Gas injection (WAG), both immiscible and miscible, is gaining significant interest for improving the oil recovery in offshore fields. The improvement in recovery due to WAG injection is attributed to contact of the upswept zones and modification of residual oil saturations and targeting the attic oil. The advantage of the process is that combination of water and gas injection provides the improved displacement efficiency of gas flooding with improved macroscopic sweep efficiency due to water injection. The result is improvement in recovery compared to separate gas or water injection. Also three phase flow (oil, water and gas) is better in displacing the residual oil compared to two phase flow water and oil or gas and oil.

Parameters which are critical for impacting the WAG enhanced oil recovery process are studied thoroughly. These parameters include WAG ratio, injection rates, cycle, well spacing, sweep efficiency and conformance control. WAG is planned for AB, C, D, and EF reservoirs which have low to moderate recovery. The study suggests that improving the well spacing and proper injection volume provides improvement in recovery factor of the order of 8-10%. The paper also discusses the mechanism of the process through mechanistic modeling and laboratory studies for injection water compatibility, rock- fluid interaction, gas mobility control using Nanofluid/Foam which is expected to provide the better sweep and upside potential.