An important part of any engineer's tool box is a set of simple equations that will allow them to quickly assess the performance potential of a given project. For many years, the model of Nelson and McNeil, as well as that of Gates and Ramey, has remained the standard for the preliminary design of in-situ combustion projects; however, advances have been made in our understanding of the fundamental mechanisms and reaction kinetics of the combustion process, both for heavy and light oils. Combining established procedures with an improved interpretation of the mechanisms forms the basis for this paper. The inclusion of the negative temperature gradient region, which affects the transition to the high temperature combustion mode, is one of the key elements of these advances.
The paper also addresses the sizing of air injection capacity and its importance, as well as the monitoring and analysis of gas-phase combustion products. In summary, the paper will provide the engineer with some realistic guidelines. for estimating the oil recovery performance of an in-situ combustion project.
In-situ combustion is an enhanced recovery process with tremendous theoretical potential, but it has failed to perform up to expectations in many of its field implementations. Two decades of research work at The University of Calgary on in-situ combustion has focused on the question "What makes in-situ combustion fail?". While many mechanisms have been identified, the most critical with regard to heavy oil is the temperature of the oxidation (combustion) reactions.
Figures 1 and 2 illustrate the importance of the oxidation zone temperature on the residual oil and oil production for Athabasca bitumen (8 °API). Figure 1 shows high residual hydrocarbon concentrations for temperatures of less than 300 °C, while Figure 2 shows that no tests exhibited significant levels of oil recovery for reaction temperatures of less than 350 °C. These data, which were previously reported by Moore et. al.1, were determined by heating pre-mixed Athabasca Oil Sands cores in a one-dimensional core holder at a rate of 40 °C/hour while flowing an oxygen-containing gas. Maximum oil recoveries for this ramped-temperature oxidation tests were significantly lower than those for combustion tube tests due to the oxidation of the oil during the period of heating when temperatures were less than 300 °C. However, the qualitative behavior was similar in both types of tests.
What these ramped-temperature oxidation data show is that the mobilization efficiency of air is low for oxidation temperatures of less than 300 °C. However, reaction temperatures in excess of 350 °C, while necessary, are not a sufficient condition to mobilize oil from the region swept by the oxidation zone. It was found that elevated levels of oil recovery were associated with a rapid transition of the leading edge temperatures to a value in excess of 350 °C. Oxygen flux (defined as the oxygen flow rate per unit area normal to the flow) was the prime operating variable controlling the ability of the oxidation reactions to rapidly attain this high temperature mode.