The Size of the Steam Zone in Wet Combustion
Authors: Holst P.H. , Karra P.S. 
Published: December 2007

In the wet-combusion recovery process, the velocity of the heat front is normally greater than the velocity of the burning front because of the growth of a steam zone ahead of the burning front. This paper presents a method of calculating the size of this steam zone as a function of time. The equations describing the physical system were derived in a manner analogous to the Marx and Langenheim derivation for hot fluid injection. The main difference between formulations, namely, the consideration of the movement of the steam zone, had a significant effect on the size of the steam zone predicted. In a typical example, after 3 1/2 years of injection, the size of the steam zone was 50 percent of the size predicted via a direct application percent of the size predicted via a direct application of the Marx-Langenheim solution. The results of the investigation are presented as a function of dimensionless groups and cover most situations encountered in the field.

Introduction

A significant fraction of the known oil deposits are in the form of low-gravity, high-viscosity crude oils. These accumulations normally do not respond satisfactorily to conventional recovery processes, primarily because of the high oil viscosity. Since primarily because of the high oil viscosity. Since oil viscosity is highly temperature dependent, the performance of these reservoirs can be significantly performance of these reservoirs can be significantly improved by raising the reservoir temperature through the addition of heat. In-situ combustion is one of a number of processes designed to increase the oil recovery through the simultaneous application of heat and a displacement mechanism. During dry combustion, air is injected to burn a part of the in-place hydrocarbons. Combustion gas part of the in-place hydrocarbons. Combustion gas and a steam zone ahead of the combustion front displace oil to the production well. The heat is transported ahead of the combustion front by the combustion gases, vaporized connate water, and conduction. This transport is often insufficient to remove all of the available heat. As a result, a portion of the heat is left in the burned zone behind portion of the heat is left in the burned zone behind the burning front to be ultimately dissipated to the over- and underburden. A popular means of increasing the thermal effciency consists of injecting water with the air, denoted as wet combustion. The water, after being heated to the boiling point behind the burning front, flashes to steam near the burning front, passes through the burning front, and adds heat to the steam zone ahead of the burning front. If the size of the steam zone grows with time, the displacing front, which may be considered to be the steam front, moves at a higher velocity than the burning front. Improved performance results. As more water is added more steam is passed through the flame front. A limit is reached when the incremental water lowers the temperature of the flame front below the steam temperature. In this case, water will pass through the burning front and partially quenched combustion results. Optimal wet combustion is reached at the point of impending quenching of the combustion front. The heat that is left behind the burning front below the (saturated) steam temperature is utilized to heat the injected water to the boiling point and is lost to the over- and underburden. point and is lost to the over- and underburden. In instances of very high fuel deposits and low reservoir pressures, the heat left behind the burning front (at the steam temperature), plus any reverse conduction from the overburden/underburden, may be insufficient to heat the water (which is to be flashed to steam) to the boiling point. In this case, less heat will be available to support the steam zone than is assumed in this analysis. Primarily as a result of the faster growth of the steam zone, the (injected) air-(produced) oil ratio for wet-combustion projects is usually lower than for dry-combustion projects. The savings in air-injection costs are somewhat offset by the water-injection costs. However, when the reduced air-injection costs are added to side benefits such as a more efficient bum and lower fuel deposits, it is usually observed that wet combustion is the preferred process for the majority of combustion preferred process for the majority of combustion recovery projects.

SPEJ

P. 13