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Abstract

A field test of wet in-situ oxygen combustion has been carried out in the Esperson Dome field in southeast Texas. The objectives of the pilot test were to evaluate the potential of oxygen combustion technology, to assess its advantages over air fireflooding, and to gain experience in the safe handling and downhole injection of high purity oxygen in an oil field environment.

The Miocene sandstone chosen for the pilot is relatively deep (2700 feet), thin (20 feet), and had been substantially watered-out by a strong natural water drive. At the beginning of the combustion pilot an estimated 850 bbl/acre-ft (36% saturation) of 90 cp (21 degrees pilot an estimated 850 bbl/acre-ft (36% saturation) of 90 cp (21 degrees API) oil remained in place. The natural water drive dominated the combustion process and assisted in displacing the oil. At project termination, 200 MMscf of oxygen and 55 MMscf of nitrogen had been injected, 90,000 barrels of oil recovered, four existing older wells failed, four new wells drilled, and three wells burned out. Determination of incremental oil is uncertain due to these operational changes.

The objectives of the project were met, and improvements in the project design and operation are expected to lead to an economic project design and operation are expected to lead to an economic process for further applications. process for further applications

Introduction

In-situ combustion has been used since the early 1940's to thermally stimulate oil reservoirs. Traditionally, compressed air has been injected into the formation at a flux great enough to drive a combustion front across the reservoir. Gates and Ramey reported results of the first successful field test of air fireflooding in South Belridge in the mid 1950's. A review, of all reported field tests is given by Chu.

The oxygen-driven in-situ combustion process is considered to be more efficient than air, due to the absence of nitrogen. Advantages include:

  1. a lower gas flux mitigates sanding, erosion, and pumping problems at the production well and allows a higher effective pumping problems at the production well and allows a higher effective permeability to oil in the reservoir;

  2. a higher CO2 concentration permeability to oil in the reservoir;

  3. a higher CO2 concentration in the flue gas allows more CO2 dissolution in the oil thereby reducing its viscosity; and

  4. a higher net oxygen injection rate will result in an accelerated oil recovery and possibly permit a wider well spacing.

The process can be applied to reservoirs where combustion would not be sustained with air. In the wet combustion mode, higher water injection rates are possible, thereby improving the sweep efficiency. With recent developments in cryogenic technology, oxygen prices are competitive with air compression costs and may be lower at injection pressures higher than 1000 psi.

This emerging technology is being applied to recover heavy and medium gravity oils and shows promise. For example, the process has been applied to heavy oils in a secondary mode and to medium and light oils in a tertiary mode.

To test the viability of this technology, Mobil designed and implemented a pilot test in the Esperson Dome field. This inverted seven-spot pilot was designed in 1983 based on laboratory and simulation results. The injector was a new well drilled low in the structure, with existing older wells as producers. The location of the injection well took advantage of the water influx to constitute a wet in-situ oxygen fireflood.

This paper describes the design and performance of the pilot test. The laboratory efforts associated with the pilot design are briefly presented. presented.

THE WET IN-SITU OXYGEN COMBUSTION PROCESS

The oxygen combustion process with its associated zones and temperatures is shown in Figure 1. The process involves injecting oxygen or oxygen enriched air into an oil bearing formation through an injector well. After ignition occurs, continued oxidant injection causes the combustion front to move through the formation away from the injection well. The heat generated by combustion lowers the viscosity of the oil in place, thus enhances its mobility. Combustion or flue gases are absorbed by the reservoir oil or produced with the other fluids.

Heat generated at the combustion front, where peak temperatures may range from 400 to 2000 degrees F, will vaporize formation water to create a steam zone. Most of the oil ahead of the combustion front is displaced by this advancing steam zone. The oil left behind, which is often the heavy hydrocarbons of the crude, is used as the fuel to sustain the combustion.

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