Fluid And Heat Movements During In-Situ Combustion In A Channelled Reservoir
Authors: Galas C.M.F. , Ejiogu G.C. , Donnelly J.K. 
Published: March 2010

BP Resources Canada Ltd., in association with partners, has been developing the patented Pressure-up/Blowdown in-situ combustion process as a follow-up to cyclic steam stimulation in a pilot on the Wolf Lake lease in the Cold Lake area of east-central Alberta, Canada.

During the pressure-up phase, oxygen and water are injected into the reservoir, while the surrounding producers are controlled to distribute the heat and to increase the reservoir pressure. Oxygen injection is terminated to initiate the blowdown phase, and the producers are operated without restriction to de-pressure the reservoir. The Pressure-up/Blowdown sequence is then repeated a number of times.

The reservoir was preheated by cyclic steam and it was necessary to fracture the reservoir to achieve acceptable injection rates. As a result, during combustion, the fluids and heat fronts move along the paths healed and depleted during the cyclic steam phase. This paper describes these movements and the associated changes in the fluid properties and shows how these observations can be used to interpret the processes in the reservoir.

This paper also discusses gas composition and movement, oil bank characteristics, steam front profiles and velocities, combustion front temperature profile and associated phenomena (such as the pH of produced water and sulphate production) and finally the effect of the "surface" operations such as the switch from oxygen to water injection.

Introduction

The early laboratory and field work in in-situ combustion, especially in the 1950s, led to a description of the process which is still generally used today (Tadema(1), Nelson and McNie(2). One of the major assumptions in the early work was that the combustion front propagates in a uniform manner and results in high areal sweep. In many projects, this is not the case due to channeling of one form or another. In some projects, e.g. the Suplacu de Bareau project in Romania(3), production wells are converted to injectors when the combustion from approaches them.

A number of recent and current projects have acknowledged channeling as a fact of life and have attempted to use the channeling to their advantage, usually by cycling the pressure, BP Canada's project Oxygen Wolf Lake (OWL), in the Cold Lake region of east central Alberta, is one which utilizes the parented Pressure-up/Blowdown combustion process where the reservoir is pressured-up by oxygen in cycles; after each injection phase, the production wells are operated without restriction. A full description of the process has been provided by Hallam and Donnelly(4). Other projects using cyclic injection of oxygen or air are Husky Oil's Tangleflags Combustion Project(5) and Amoco Canada's Morgan fireflood(6).

Mechanisms of In-Situ Combustion

The simplified sketch of the process in the reservoir as presented by Nelson and McNiel(2) is reproduced as Figure 1. Briefly, the injected oxygen passes through the burned zone until it reaches the combustion front where the fuel is consumed by either the high or low-temperature oxidation reactions. The fuel used during high temperature oxidation is coke and the reaction products are mainly carbon oxides and water, whereas, during low-temperature oxidation, the fuel is the bitumen itself and the products are coke and oxygenated compounds(7).