Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
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Light Oil Recovery via Air Injection Technology

Demetrios Yannimaras of BP Exploration evaluates air injection technology and investigates its potential to increase recovery from North Sea light oil reservoirs (co author Charles Christopher)

Introduction - Air injection, and the resulting in-situ combustion process, utilises an inexpensive injectant, air, in order to accelerate oil recovery and increase reserves. Under appropriate conditions, when air is injected into the reservoir, a small amount of the in-place oil is consumed while the rest is displaced, banked and eventually produced. Air injection at high pressure can offer unique economic and technical opportunities for improved oil recovery in many candidate reservoirs. Successful field applications have been documented for oils at elevated reservoir pressures and temperatures, in both clastic and carbonate reservoirs.

Air has been used in heavy oil reservoirs to generate heat and steam to mobilise in-place oil. For light oil applications however, the in-situ generated flue gas (85%N2 + 15%CO2) is the main driving force for oil displacement. The flue gas pressures up the reservoir, mobilises, strips, and swells the in-place oil, and at sufficient pressure can miscibly and near-miscibly displace the oil. For deeper reservoirs, the generated supercritical steam (above 3,206 psi and 706°F) also efficiently extracts and displaces crude oil components in-situ.

Air is an inexpensive injectant available everywhere, while other gaseous injectants are often unavailable or too costly. Even in an environment of low oil and gas prices, it is clear that some form of gas injection may be desirable for those large, light oil reservoirs that are in their plateauing or declining phases.

Although all current air injection commercial projects are situated onshore, offshore air injection is feasible with today's technology. Joint-industry and internal studies have shown that there are no insurmountable problems for offshore air injection concerning facilities, or environmental issues (using flue gas reinjection).

Reactions and Autoignition - Air injection for crude oil recovery involves injection of compressed air into a reservoir. For hot enough (>170°F) reservoirs, spontaneous ignition occurs and an in-situ combustion front is established. This travelling high-temperature front consumes a small percentage of the in-place oil, while displacing and producing the rest. It is a very efficient method of enhanced oil production, on par with miscible displacement. Approximately 12,000 BOPD incremental production due to air injection is obtained from N. American fields (examples in Figures 1 and 2).

(Click for a larger image (image1)) (Click for a larger image (image2))

Crude oil in-situ combustion is made up of two overall reactions. The LTO (Low Temperature Oxidation) extends from the ignition point to approximately 300-350°C. The HTO (High Temperature Oxidation) reaction follows LTO and extends up to 500-600°C for light oils. A large temperature increase under LTO, or a smooth, continuous LTO-to-HTO transition, is indicative of successful in-situ combustion operation. Operation in the LTO temperature regime is sufficient for light and medium gravity crudes. Operation in the HTO regime is necessary in heavy oil applications, where viscosity reduction via heat release is the primary objective.

Spontaneous ignition, after air injection into an oil reservoir, simplifies process initiation and substantially improves process stability. It is determined in the laboratory with an Accelerating Rate Calorimeter (ARC) test, described below. Laboratory autoignition, or a temperature increase when the system is held at reservoir conditions, is taken as indicative of field autoignition.

Laboratory Screening of Candidate Reservoirs - Recent advances in process understanding have resulted in an efficient laboratory-based method of identifying those reservoir and oils most suitable to enhanced oil recovery via air injection.

The Accelerating Rate Calorimeter (ARC, Figure 3) is an instrument that has been developed in recent years and is capable of operating up to high pressures (6000 psi or more) and studying exothermic reactions in the range 30-500°C. The ARC screening of oils for suitability to air injection is based on examining the continuity of the exotherm trace obtained in an ARC experiment, over the temperature interval of interest. The method has been found to predict combustion tube, and even field, performance of oils under air injection. Moreover, reliable data on the likelihood of autoignition can be derived from isothermal ageing experiments in the presence of air, at reservoir temperature and pressure.

The Combustion Tube (CT, Figure 4) test is a dynamic, complex experiment which mimics the actual in-situ combustion process. It provides information on the stability of the burn front, peak temperatures generated, oxygen utilisation and amount of air needed to produce an incremental unit of oil.

(Click for a larger image (image3)) (Click for a larger image (image4))

ARC and CT systems capable of operating to 6,000 psig with sophisticated automation and data gathering system and state-of-the art analysis and monitoring equipment have been developed and are available. Two premier sites for laboratory testing of air injection candidate reservoirs are the University of Calgary, Canada (Profs. G. Moore and R. Mehta), and the University of Bath, UK (Prof. M. Greaves).

Recent BP Activity and the Road Forward - BP personnel evaluated BP North Sea light oil reservoirs for suitability to air injection in 1999-2000 and identified six suitable candidates, three of which underwent laboratory testing. The results from ARC and CT tests were evaluated and interpreted and two reservoir oil and rock systems were declared highly prospective for air injection. Current work for these two fields aims to scale the reaction kinetics appropriately for field simulation and run such simulations to quantify the incremental recovery benefit. In 2002, similar evaluations are likely to be undertaken for Norwegian light oil reservoirs.

There are reservoirs suitable for air injection in the N. Sea, Gulf of Mexico, Alaska, S. America, Eurasia and, significantly, in the Middle East. If reasonable oil prices prevail and as personnel experienced in EOR applications diffuse through the industry's technical positions, success with gas injection in general - and air injection in particular - should prove beneficial to both technically forward-looking companies and our industry as a whole.

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