Laboratory Study of a Combination of Forward Combustion and Waterflooding The COFCAW Process
Authors: Parrish David R. , Craig F.F. 
Published: December 2007

The major benefit of a combination of forward combustion and waterflooding is a marked reduction in the air required to recover a barrel of oil. Another advantage is that less oil is burned as fuel, so that more of it is potentially recoverable.

Introduction

In some respects, forward combustion (alias: fire-flooding, underground combustion, in-situ combustion, dry combustion) is an efficient oil recovery method. For example, its displacement efficiency (as much as 95 percent in reservoirs with high porosity and high oil saturation) is good. In its use of heat, however, forward combustion is grossly inefficient.

Most of the beat generated during forward combustion is wasted. About one-fifth of the beat is picked up by the incoming air and re-used (regenerated); the remainder is left behind the combustion zone and ultimately is lost. Compressed air is expensive, and heat generated from compressed air via underground combustion is too valuable to be thrown away. Improvement is needed.

Since the early 1950's, improvements in forward combustion by addition of water injection have been discussed, particularly in the patent literature, with increasing frequency. These concepts are based on the fact that relatively inexpensive water can be used to pick up much of the heat that is ordinarily wasted during forward combustion, and transport it forward to a more useful place, thereby reducing the amount of air required to move a heat front through the reservoir. Several approaches have been proposed.

Merriam and Squires taught that a combustible (fuel) gas should be injected along with the air and water. This, however, would give at least a tendency toward reverse combustion instead of forward combustion, and would reduce the velocity of the heat bank, defeating the primary purpose of the water injection.

Pelzer described primarily a batchwise method of forward combustion and waterflooding. He advocated burning part way through a reservoir with forward combustion, then switching from air to water injection alone to push the heat toward the producing wells. Though possible, this method has serious disadvantages, the major one being that, by the time water injection is started, much of the heat has been irretrievably lost to the overlying and underlying formations.

We have described what is considered a practical Combination Of Forward Combustion And Waterflooding (COFCAW). Air and water are injected simultaneously (or alternately) after a small heat bank has been formed by forward combustion. Some of the injected fluid fills up the region behind the heat bank; the rest enters the heat bank. Upon encountering heated rock, the water is converted into steam, which then flows ahead of the combustion zone and displaces much of the oil in place. Only enough air is injected to provide heat for vaporization of the water and to provide heat for vaporization of the water and to compensate for reservoir heat losses. Since the size of the heat bank is minimized, heat losses also are minimized. Because of more efficient oil displacement ahead of the combustion zone, fuel consumption is reduced.

JPT

P. 753