Summary This paper introduces a new method to screen crude oils for applicability of the air-injection/in-situ combustion process. Testing is performed at reservoir conditions, up to 41.4 MPa, with a specially modified accelerating-rate calorimeter (ARCTM). ARC results are shown for four medium- and high-gravity oils, combustion-tube data is presented, and air-injection field data are discussed and compared. We interpret that the continuity of the ARC trace ties in kinetics, combustion-tube, and field air-injection results. Thus, a method is available to delimit the envelope of applicability of air-injection/in-situ combustion to those oil reservoirs where the probability of technical and economic success is greatest. Introduction Air Injection for Crude-Oil Recovery. Air injection can offer unique economic and technical opportunities for improved oil recovery in many candidate reservoirs. Air injection is an efficient oil recovery process because only a small amount of the in-place oil is consumed while the rest is displaced, banked, and eventually produced. It has often been applied to heavy oils (for viscosity reduction owing to heat release) and sometimes to light oils. In the economically advantaged class of light-oil reservoirs, potential process benefits include the following.Excellent displacement efficiency and mobilization of extra, combustion oil.Reservoir pressurization.Flue gas stripping of the reservoir oil.Oil swelling.Injection-gas substitution. For air injection into high-pressure, hot reservoirs, the following additional benefits may accrue: spontaneous oil ignition and complete oxygen utilization, operation above the critical point of water with possible superextraction benefits, and near-miscibility of the generated flue gas and the oil.
Summary In deep, high-pressure light-oil reservoirs, air injection provides several advantages compared with other improved recovery injection processes. The combination of rapid pressurization, spontaneous ignition, complete oxygen utilization, stripping of the light hydrocarbons [natural gas liquids (NGL's)], and near miscibility of the in-situ-generated combustion gases with the reservoir oil results in improved recovery. The availability of the injectant should allow wide application of air injection. Estimation of the recovery factor for this process is subject to uncertainties and requires history matching. To date, computer models have shown limited predictive capability owing to the complexity of the process. Also, an estimate of the fuel laydown at high pressure is not available. In this paper, a method is proposed for estimating the recovery factor on the basis of the producing gas/oil ratio (GOR) of the field. The field results of two ongoing air-injection projects, Medicine Pole Hills Unit (MPHU) in North Dakota and Buffalo Red River Unit (BRRU) in South Dakota, are used to illustrate this technique. MPHU and BRRU are unique applications because they use high-pressure air injection into deep, high-temperature, low-permeability carbonate reservoirs producing light oils. The produced-gas analysis from MPHU is used to arrive at the fuel laydown and the H/C ratio of the fuel. This is the first time the fuel laydown has been estimated for a high-pressure air-injection project in a light-oil reservoir. Finally, the results of the produced-gas analysis are used to arrive at the NGL content of the effluent stream. The combined oil recovery and NGL production provide the basis for estimating the total recovery factor for this field. Such an estimate can be used by other operators to assess the economic viability of this process in their fields. Introduction Air Injection and Light Oils Air injection, and the resulting in-situ combustion process, uses an inexpensive injectant, air, 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 can offer unique economic and technical opportunities for improved oil recovery in many candidate reservoirs.
Several papers have been recently published1,2,3,4,5 discussing the process of air injection for light oil recovery and describing the criteria for a successful project. The performance of some light oil air injection field projects has also been discussed in these papers.2,5 However, the economics of this process have never been fully addressed before. This paper discusses the economics of a successful on-going project, the Medicine Pole Hills Unit (MPHU, ND), and a new project underway at West Hackberry, LA. The economics of air injection in low pressure fault blocks for repositioning and producing the oil rim are discussed as well.
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. P. 655^
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