Brazil’s National Operator, Petrobras has been experimenting with advanced Water-Alternating-Gas injection in a number of offshore plays and recently at the Rio O&G 2014, Petrobras’ Pre-salt manager confirmed that the super-major will be using the technology at most, if not all of its pre-salt plays. The WAG injection process aims to recover more oil from a reservoir and also to recover oil more efficiently.
The technology was originally intended to improve sweep efficiency during gas flooding, with intermittent slugs of water and gas designed by and large to follow the same route through the reservoir. Variants include injecting gas as a supplement to water or vice versa, primarily to reach other parts of the reservoir. In the case of supplementary water injection, it also saves on valuable injection gas. A distinction can often be drawn between miscible and immiscible WAG injection, and the water and gas can be injected simultaneously (SWAG) rather than intermittently. Adding a foaming agent to the injection water can also improve the gas sweep. The typical improved oil recovery (IOR) potential for WAG injection when compared with water injection is quoted in the literature at 5-10%.
Gas represents a large fraction of the total cost, making WAG injection a somewhat expensive method except in cases where a gas surplus is available, such as in large deepwater pre-salt plays like Lula and Libra in Brazil. It is known that the remaining (residual) oil in the flooded rock may be lowest when three phases (oil, water and gas) have been achieved in this volume. Water injection alone tends to sweep the lower parts of a reservoir, while gas injected alone sweeps more of the upper parts of a reservoir owing to gravitational forces. Three-phase oil, water and gas flow is better at displacing residual oil in the pore system than two-phase flow. WAG and SWAG thus improve the efficiency of both microscopic and macroscopic displacement. One of the main challenges though, is to achieve sufficient sweep in the reservoirs.