Rationale For Low Injection Rates And Pressure Cycles For Firefloods In Heavy Oil Reservoirs
Authors: Tsang P.W. 
Published: March 2010

Abstract
A reservoir flow path model with low injection rates has been used to develop an alternate approach to fire floods in heavy oil reservoirs with initial oil mobility. Low injection rates are used to maintain and propagate the firefront in the reservoir. The pressures in the burn volume and the/low path are maintained lower than that in the unaffected reservoir, allowing oil from the unaffected reservoir to drain into the flow path and to increase production. The fireflood process also generates upgraded oil with lower viscosities to further enhance oil mobility in the flow path. Pressure cycles can be used to facilitate additional oil resaturation of the flow path to alleviate severe gas channelling ahead of the burn front.
Injection and producing well casing pressure histories and fluid analysis data from the Husky Tangleflags Fireflood Project are reviewed to illustrate the beneficial effects of low injection rates and pressure cycles.

Introduction
The in-situ fireflood recovery process is one of the thermal techniques used for enhanced recovery from heavy oil reservoirs(1) Quite often, the design of fireflood projects were based on displacement considerations(2, 3). Invariably, some non-radial distribution of the burn front had been observed(2, 4, 5). Reduced production due to increased casing pressures(5) and elevated downhole temperatures(1, 4) were experienced and were considered to be sources of major problems.
A reservoir flow path model is proposed for firefloods using low injection rates in heavy oil reservoirs having some initial oil mobility. The pressures in both the burn zone and the flow path are lower than the pressure in the unaffected reservoir, allowingoil drainage into the flow path to increase production. The oil mobility is further enhanced by the reduced viscosity of the upgraded oil. Pressure cycles can be used to transport additional oil from the unaffected reservoir into the flow path to alleviate severe gas channelling ahead of the burn front. Selected injection and producing well casing pressure histories together with fluid analysis data from the Husky Tangleflags Fireflood Project are reviewed to illustrate the beneficial effects of reservoir fluid drainage into the flow path and reduction in oil viscosities due to the fireflood upgraded oil.

Classical Fireflood Design
One basic design criteria for the fireflood process has been the correlation between oil recovery and burn volume. Before inalizing the design of a fireflood project, a burn tube test was run to determine the burn characteristics of that particular reservoir. From the burn tube data, oil displaced and air requirement values are determined to assist in the design and operation of the fireflood project(2). Based on the field test results from a fireflood project in the South Belridge Field and related laboratory studies, Gates and Ramey(3) provided a correlation between oil displaced and volume burned as a method for engineering fireflood projects.
These design parameters are probably good starting points for most fireflood projects. For a fireflood design in a specific reservoir, an economic oil production rate is assumed and used in conjunction with these design parameters to determine the injection rate necessary for an economic fireflood project.