Abstract

Low pressure air injection/in situ combustion in waterflooded light oil reservoirs has been investigated in a 3-D combustion cell using a horizontal producer well in line drive. This well arrangement was chosen because, for heavy oil experiments, it was observed to confer stability on the combustion front. The main aim of the investigation was to determine if a light oil was capable of sustaining in situ combustion under these conditions, in view of the fact that a light oil is considered to be 'fuel deficient' and may not be capable of sustaining stable combustion. The particular experiments are an exacting test since:

  • at low pressure, the light ends are easily vaporised leading to desaturation ahead of the combustion front and reduced fuel availability;

  • possible adverse effects of reduced oil saturation and increased water saturation following waterflood and

  • more realistic three-dimensional physical model conditions where the combustion front is not constrained as it would be in a combustion tube.

Self-propagating combustion was achieved in the 3-D experiments, with stable combustion occurring at around 500 C. The level of CO2 in the produced gas was typically in the region of 8%, or less, and necessarily the oxygen utilisation was on the low side. This did not appear to cause any significant low temperature oxidation (LTO) of the oil downstream. Gas gravity override was greatly stabilised by the 'draw-down' effect of the horizontal producer well, leading to improved sweep efficiency. This was reflected in the high oil recoveries achieved, in the range 63.9 to 85% OOIP. Waterflooded light oil reservoirs which are particularly good candidates for in situ combustion, benefit from the in situ generated steam. This is capable of producing a very extensive steamflood, requiring only a partial burn of the reservoir.

Introduction

The success of horizontal well technology has been extensively reviewed by the US Department of Energy. The most common applications in the USA were cited as intersecting fractures (53% of all fields) and delaying water and/or gas coning (33% of all fields). The least used applications were for water drive (9%), EOR - enhanced oil recovery (9%) and to avoid surface restrictions above the target formation. About 90% of the horizontal wells in the USA were drilled in carbonate formations and only 54% of horizontal well projects were reported to be successful compared to a 95% technical success rate. Very high horizontal well activity exists in Canada, with more than half drilled in Saskatchewan, particularly in heavy oil reservoirs. In heavy oil reservoirs, 41% of applications were for EOR. Greaves et al. have reported on an advanced EOR horizontal well process for heavy oil recovery using air injection/in situ combustion. There are a number of advantages of using a horizontal producer well in line drive, allowing stabilisation of the moving combustion front by draw-down action of fluids into the horizontal well. The limited oil mobility ahead of the combustion front with this well arrangement causes oil and other fluids to flow directly into the horizontal well, leading to early production and avoidance of downstream plugging, communication and emulsion problems.

Improved recovery of light oil is a major area for horizontal well applications, since many reservoirs that are considered 'depleted' still contain as much as 50% or more of the OOIP. Three major areas of enhanced oil recovery (EOR the area of improved oil recovery (IOR) focusing on reservoir processes) are recognised: thermal, chemical and miscible. Thermal EOR includes steam stimulation, in situ combustion (high temperature oxidation - HTO - at temperatures above about 350 C, otherwise the process is limited to low temperature oxidation - LTO) and steam flooding. P. 951

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