Multiple-Quenched Fireflood Process Boosts Efficiency
Authors: Bousaid Sam 
Published: December 2007

A study of dry, wet, and quenched combustion allowed in-depth analysis of the mechanisms of quenching and reignition of oil in sandpacks. Reignition of crude oil after quenching an in-situ combustion front was confirmed in a 5-ft [1.5-m] -long thin-wall tube.

Laboratory results indicated that the cycle of quenching and reignition can be repeated to yield a fast-moving front with an increase in oil rates. Wet combustion was more efficient than dry combustion, but quenching of the burn at high water/air ratios (WAR's) resulted in minimum fuel and air requirements. Multiple quenching of the burn produced an improved oil viscosity up to 40 times less than the original. The results provided criteria for optimizing the WAR, the limits of quenched combustion, and the potential of reignition.

Introduction

In-situ combustion is a potential thermal recovery process. Laboratory linear floods show that most of the in-place oil is banked ahead of the combustion front, with no oil left after a high-temperature bum. In field tests, however, the bum is limited to a small portion of the reservoir because of low sweep efficiency.

Wet combustion, the injection of water and air, offers a better sweep and requires less air and fuel than dry combustion. This improvement increases at higher WAR'S. Combustion-tube runs were performed at increasing WAR to optimize its value. The tests included quenching the bum to define the region of quenched combustion. Earlier studies of dry and wet combustion made in a 2-ft [0.6-m] -long tube gave unstable combustion fronts at high WAR. Although the results were useful for estimating an optimum WAR for a field test, quenching of the burning zone could not be observed.

A longer thin-wall tube was built with newly automated equipment for monitoring the progress of the bum front. The 5-ft [1.5-m]-long combustion cell was capable of tracking the peak temperature and other regions ahead of and behind the front. The cell was useful in identifying quenching of the burning zone and establishing a maximum WAR. Although wet combustion becomes more efficient at high WAR, the process is limited by a WAR value below that of quenching. Excessive water injection quenches the bum and reduces the high-temperature front to a hot-water or steam zone.

The initial objective of this study was to extend the limits of wet combustion by replacing cold-water injection with hot-water or steam injection. These results were encouraging, but the economics of coinjecting hot water or steam with air became less attractive. Because of this, the idea of quenching the bum by using excessive WAR and then reigniting the hot oil for continued combustion became the main objective of this work.

Quenching the burn front at flood-out stage is not a new idea in the field. This step is commonly used to replace air with water and to maintain reservoir pressure while scavenging residual heat and oil near the end of a project. However, the process of reigniting the oil after deliberate quenching is a new approach for firefloods.

This paper presents laboratory results that support a combustion process where reignition after quenching the burning front is a feasible recovery method. Furthermore, when a high-temperature front was restarted, the cycle of quenching and reignition was repeatable for even lower fuel and air requirements. In addition to increased oil production rates, in-situ combustion by this multiple-quenching process offers improved economics over conventional fireflooding.