Abstract

The paper deals with the findings of the laboratory investigations and field test on spontaneous ignition technique as a means of initiation of in-situ combustion process in the heavy oil field of Balol in India. The ignition delaytime observed in the field is compared with the values obtained during laboratory investigations and by an analytical approach.

Introduction

Ignition is the first step of in-situ combustion process. It can be achieved by heating the formation around the well bore with burner or electrical heater and then injecting air or simply injecting air and allowing the reservoir to ignite spontaneously. Certain crude oils, when exposed to air, will oxidize rapidly even at low temperature. This oxidation reaction releases heat and if the reaction rate is high enough to generate heat in excess of the heat loss, spontaneous ignition occurs. The time of spontaneous ignition can vary from a few days to several months and it depends on the initial formation temperature and the reactivity of the crude oil. In order to estimate the ignition time for a given reservoir, it is necessary to have a laboratory determined specific reaction rate.

Laboratory investigations were carried out in Institute of Reservoir Studies(IRS) wayback in 1980's which ascertained the spontaneous ignition characteristics of Balol oil. In order to ascertain the viability of the process in the field, the test was initiated in Balol field in well IC#29 on March 10, 1995 and was continued till June 15, 1995. The main objective of the test was to ascertain the feasibility of this technique under field conditions and to find out the difference in ignition delay time between predicted and as observed in the field.

Laboratory Investigations

Laboratory investigations were carried out on Balol oil in early 80's at IRS to study the reaction kinetics and to find out ignition delay time. Experiments were carried out in a high pressure cell containing a small known quantity of sand oil mixture. The temperature of the cell was raised to reservoir temperature in an inert atmosphere to prevent premature oxidation. There after the inert gas was replaced by air at high pressure. After allowing sufficient time, depending on the temperature, the percentage of O2 utilized inside the cell was determined. The specific oxidation reaction rate i.e. the mass of oxygen consumed per unit time per unit mass of oil is related to the kinetic parameters -E/R and ko.pO2n.

Kinetic parameters evaluated from the test are as follows: Activation energy term B i.e. (-E/R) = 15652.2 R (8695.65 K) Specific reaction rate at temperature (kopO2n) =7758.28 sec-1.

From the kinetic parameters and the physical properties of the oil, ignition delay time, which is defined as the time required for temperature to exceed 200C at a particular point, was estimated. For Balol oil it is found to be in the range of 1.46 – 2.72 days. The estimation accounts for the change in porosity and does not account for the heat losses.

Analytical Prediction

Analytical calculations were done to predict the distance at which ignition occurs and ignition delay time and the change in injected O2 level with distance & time during spontaneous ignition. The methodology takes into account the reaction kinetics of the oil and heat transfer by conduction and convection.

Arrhenius equation describing the reaction of air (oxygen) with oil or coke-like fuel is given by

Oxygen depletion is then predicted by taking into account the reaction rate by following equation:

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