Abstract

A new analytical model of the combustion zone in combustion tube experiments has been developed. In the combustion zone model, the amount of fuel is based on reactions kinetics; the fuel concentration and produced gas composition are based on combustion stochiometry; and the amount of heat generated is based on a heat balance.

Six experimental runs were performed using a 3-inch diameter, 40-inch long steel combustion tube in which was placed a sand mix containing 10 °API Jobo crude oil from the Orinoco Belt, Venezuela. Air containing 21, 30, and 40 mole% oxygen were used. Air injection rate and combustion tube oulet presssure were kept constant at 3 L/min and 300 psig respectively in all runs.

Calculated combustion zone temperatures and temperature profiles are in good agreement with the experimental data. The use of oxygen-enriched air increased the combustion front temperature and velocity from 450 °C and 13.4 cm/hr (21mole% O2) to 475 °C and 24.7 cm/hr (40 mole% O2), reducing the start of oil production from 3.3 hours (21 mole% O2) to 1.8 hours (40 mole% O2). The new analytical model helps reasearchers better understand the process of oxygen-enriched in-situ combustion.

Introduction

In-situ combustion was probably the first thermal EOR process to be developed(1). In-situ combustion tube laboratory experiments were conducted as early as 1947(2,3), and important field tests were performed by 1958(4–6). The first commercial in-situ combustion operation began in 1959(7).

The most common form of in-situ combustion is dry, forward combustion. In this process, air is injected into an oil reservoir, the oil is ignited, and the resulting combustion front travels away from the injection well towards the production well(s). The heat generated at the combustion front propagates through the reservoir, by conduction and convection, reducing the oil viscosity and thereby increasing the oil production rate nd recovery.

Initial models to describe the in-situ combustion process were analytical heat transfer models(8–14). Subsequent models have included the kinetics of lumped reactions: a steady-state model(15) and a model for simulation of combustion tube experiments, which incorporates thermal cracking and lowtemperature oxidation(16,17). Numerical simulation models have been developed in which the physical and chemical reactions are described by basic kinetic relationships(18–20).

Recently, the term air injection has been used to describe the process whereby air is injected into mainly deep reservoirs to either create miscibility of nitrogen with the oil or to create spontaneous ignition. Greaves et al. (21) conducted kinetic and combustion tube experiments in different types of oils and concluded that the air injection under low temperature oxidation process can be considered for application to all light oils with sufficiently high reactivity. Turta and Singhal(22) proposed classifying air injection as immiscible airflooding with high temperature oxidation.

Main parameters required in the design of an in-situ combustion project are(23): fuel concentration per unit reservoir volume burned, the composition of the fuel, the amount of air required to burn the fuel, the volume of reservoir swept by the combustion zone, the required air-injection rates and pressures, the oil production rate and recovery, the investment, and operating costs.

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