Abstract
Abstract
Air injection is an Enhanced Oil Recovery (EOR) technique with limited exposure in the Asia-Pacific region and no previous application in Australia. Analogy with successful air injection projects in the USA, suggests that it could be a suitable EOR process for onshore light oil fields in Australia; no evaluation has been conducted to date.
Using open file data, high level screening criteria are used in this study to identify prospective petroleum basins, and an individual candidate reservoir is examined through a simulation study. Key issues in the application of the technique are discussed, as are directions for implementation in Australia.
Air injection involves the continuous injection of high-pressure air into the reservoir. The oxygen in the air reacts with the reservoir crude, consuming 5–10 % of the Original-Oil-In-Place (OOIP) and generating flue gases in-situ. This creates a gas drive process and acts to re-pressurize the reservoir. The process does not require water as a mobility control agent; a significant advantage in water-scarce Australia. It could also replace hydrocarbon (HC) miscible floods, freeing cleaner HC gases for energy use. Ideally the process is suited to deep, high-temperature, light oil reservoirs, and is applicable to both secondary and tertiary recovery.
The Cooper-Eromanga Basin, Carnarvon Basin (Barrow Island) and the Surat-Bowen Basin were identified as the most prospective. The simulation study conducted for ‘Reservoir A’ in the Cooper Basin indicated the potential for spontaneous ignition and propagation of a stable combustion front within the reservoir; hence it is a potentially good candidate for EOR by air injection.
Given the ‘high’ oil price and maturity of Australia's oil provinces, significant value is associated with EOR. Air injection is potentially suitable for Australian onshore application. The process warrants further evaluation and consideration as an alternative to accepted EOR techniques.
Introduction
Air injection was first introduced as a secondary recovery technique in 19791 (Buffalo Field) to improve upon primary recovery in deep, high temperature, low relief, low permeability reservoirs. Since its first application, air injection has been applied successfully, technically and economically as both a secondary and tertiary (Double Displacement Process, DDP, West Hackberry2,3) EOR process, over a variety of reservoir scenarios, in both vertical and horizontal flooding modes. Full-field air injection projects have been developed as recently as 19964,5 (Horse Creek), and a range of field pilots and evaluation studies are underway or have been proposed in recent times; these have tended to focus on the use of the process for tertiary recovery, including the potential of the process for use offshore in the North Sea6,7.
One example of the successful application of air injection for secondary EOR is the Medicine Pole Hills (MPH) field1,8. This field is indicative of those projects which have been undertaken in the Williston Basin in the Dakotas. The Williston Basin has been the focus of air injection for secondary recovery for over three decades. The reservoirs in the Williston Basin (Buffalo, MPH, Horse Creek) are low permeability non-fractured carbonate reservoirs. There is little structural relief, and the primary drive mechanism of liquid and rock expansion results in low primary oil recoveries due to a lack of pressure support. This results in rapidly declining production rates, and a high residual oil saturation due to capillarity. The air injection technique allowed for rapid reservoir re-pressurisation, to provide energy, and helped mobilise trapped oil, through improved sweep efficiency.