Laboratory Testing and Simulation Results for High Pressure Air Injection in a Waterflooded North Sea Oil Reservoir
Authors: Fraim M.L. , Moffitt P.D. , Yannimaras D.V. 
Published: December 2007

The Maureen field, a light oil reservoir in the North Sea which has achieved waterflood oil recovery close to 53 percent of the OOIP is nearing the end of its producing life under waterflooding operations. This field was evaluated as to the feasibility of improved oil recovery through high pressure air injection as an inexpensive substitute for other unavailable or costly gases. Six accelerating rate calorimeter (ARC) tests and five combustion tube tests were conducted to determine the oxidation characteristics of Maureen crude oil while injecting air in the presence of reservoir rock and brine. These tests showed that Maureen oil will reliably autoignite, generate flue gas (85 % N2 and 15 % CO2) and propagate a stable combustion front. In addition with air enrichment, a first contact miscible displacement process can be maintained. High pressure air injection was then modeled as a miscible process using the history matched Maureen waterflood model: the results showed incremental oil recovery due to air injection would range from 17.8 to 26.3 MM STB (4.5 to 6.6% OOIP) depending on the relative location of the air injection wells (flank or crestal).

Introduction

The Maureen field in the UK sector of the North Sea has achieved water flood oil recovery close to 53 % of the original oil-in-place (OOIP), but is nearing the end of its productive life under water- flooding operations. The water flood has achieved a volumetric sweep efficiency in the range of 90 percent and most of the reservoir hydrocarbon pore volume has been reduced to the residual oil saturation to water of 23 percent, but if abandoned in its present state, Maureen will leave 175 MM STB in place as unrecoverable oil.

The next phase of economic oil recovery will require a displacement fluid that is near first-contact miscible, can be recycled, and is available offshore for less than $1.5 US/reservoir bbl. One way to achieve these objectives is through high pressure air injection. High pressure air injection can reduce the residual oil saturation through formation of a miscible gas bank which will displace the remaining oil to the producing wells. This was confirmed by phase behavior modeling of Maureen oil and combustion product gases.

Scoping model runs show that high pressure enriched air injection (30 percent oxygen) in a waterflooded reservoir can generate a first-contact miscible fluid in-situ for less than $1.5 US/reservoir bbl. Laboratory combustion tube tests with Maureen oil and core material show that oxygen will oxidize about 3 to 5 saturation units of the oil to create carbon dioxide, generate heat for the steam, and upgrade the oil by 2 to 4 API units. The miscible gas mixture is expected to consist of 34% steam, 16% CO2 and 50% N2. As the extracted oil and steam cool, the foamy oil and water will form a temporary emulsion until carbon dioxide separates from the liquid phases. This temporary emulsion prevents oxygen bypassing the oxidation front and improves sweep efficiency by decreasing the mobility ratio to less than 0.5.

Based on Permian Basin experience of enhanced oil recovery from waterflooded reservoirs in west Texas, a carbon dioxide rich gas mixture would be the ideal fluid for increasing oil recovery from the Maureen reservoir. A carbon dioxide gas mixture will recover an additional 8 to 15 percent of the original oil-in-place in the contacted reservoir volume over that achievable by water-flooding. As the reservoir temperature increases due to depth of burial or due to combustion, less carbon dioxide is required in the gas mixture to effectively extract most of the medium to light gravity oil. For temperatures over 200 C, water (in the steam phase) and carbon dioxide can be injected to create a first-contact miscible gas mixture.

Experience learned from Amoco's West Hackberry project and Koch's Medicine Pole Hills project shows that it requires about two-thirds of a pore volume of injected air to sweep the reservoir. Laboratory tests show the Maureen oil will autoignite at reservoir temperature, therefore only a single well huff-and-puff test will be required to prove the Maureen oil will oxidize at field conditions.

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