The present work refers to an experimental study on oil recovery by in-situ combustion. A specific apparatus holding a combustion tube of 100 cm in length and 6.9 cm internal diameter was designed and constructed for the study. Experimental tests were performed with a heavy oil of 12.8ºAPI from a Brazilian onshore field. Plain air was injected at a constant rate of 3 SLPM, while the production pressure was set at 10 bar.
The main purpose of this study was to survey the influence of clay content in the reservoir rock with initial oil saturations ranging from 25 to 50%.
The results indicate that the in-situ combustion method is technically applicable to the rock-fluid system tested. Moreover, the tests were useful in providing the proper range of parameters for the oxidation reactions to occur. Clay has proved to play a key role on fuel deposition and, consequently, on propagation of the combustion front. In a clean sandpack medium, the combustion front was not self-sustained, while in presence of clay, in amount ranging from 4.5 to 10.0% mass fraction, sustainable combustion reactions were achieved. Front peak temperatures were recorded in the range of 457 - 501 ºC, and for oil recovery factors were greater than 84%. From the set of data collected during the tests, results show combustion front velocities to span between 14.1 to 18.3 cm/h. Worth mentioning, upgradings of 3.2º to 8.4º API were observed in the produced oil.
The basic combustion parameters - fuel consumption, air requirement, air-fuel ratio, atomic H/C ratio, oxygen utilization - obtained during the experiments are favorable to the implementation of in-situ combustion and shall be used as a guide to the pilot project planned for the field.
Introduction
Heavy oil and tar sands occur in many countries, with a total resource on the order of 8 trillion barrels in-place. The largest accumulations are in Canada (3 trillion bbls) and Venezuela (2 trillion bbls) (Farouq Ali, 2002). Due to known difficulties to produce, heavy and viscous oil reservoirs are usually recommended to undergo thermal recovery methods. These methods help to reduce oil viscosity, so increasing its mobility. Among the thermal methods, in-situ combustion is the oldest one and has shown to provide the best recovery efficiency (Sarathi, 1999; Castanier, 1992). It consists in burning a fraction of the oil in place to create a burning front that propagates through the reservoir.
The advantages of in-situ combustion over other thermal recovery methods lay primarily in that the heat is generated within the reservoir, thus no heat losses occur at the wellbore. Furthermore, such characteristic allows the application of this method at depths that are greater than those feasible for steam or hot water, with heat generated at the surface (Rodriguez, 2004).
In-situ combustion presents a high thermal efficiency. He still presents better oil recovery and less environmental impact, as reported in the literature (Xia and Greaves, 2001).
Laboratory experiments are fundamental to analyze the in-situ combustion process. These experiments are faster and have a lower cost, compared with field experiments. However, the interpretation and the scalling of the phenomena are not totally dominated yet. Literature shows that such experiments are fundamental, before conducting projects in the field (Moore, 1994). The studies carried out in combustion tubes are important to evaluate and the influence and determine the effects of variables on the behavior of the process.
