Increased surface area of reservoir rock due to the presence of clays and the catalytic impact of clays are known to enhance the in-situ combustion (ISC) performance. But the basics behind these mechanisms are still not known. In this study, we investigated the role of clays on ISC in microscopic scale.
Six one-dimensional combustion tube experiments were conducted on three different crude oil samples. The combustion performance of each crude oil was evaluated with two combustion runs; reservoir rock prepared with sand-oil and with sand-clay-oil mixtures. Each combustion tube test was evaluated in terms of cumulative oil production, combustion front propagation, and characterization of the produced oil samples. Activation energy and heat of combustion were calculated empirically. Quality of the produced oil samples was determined through viscosity measurements. Saturates, aromatics, resins, and asphaltenes (SARA) fractions of initial and produced oil samples were compared. To better understand the fuel formation mechanism, asphaltenes surfaces were visualized by a Scanning Electron Microscope (SEM) and SARA fractions with Fourier Transform Infrared (FTIR).
Combustion tube experimental results highlight that crude oil type affects the process performance the most. Clay presence in the rock expedited the combustion front velocity by increasing the oxygen utilization rate. Activation energy was reduced drastically with the presence of clays, however, the heat of combustion has not changed. Thus, the generated heat has been consumed more effectively with the presence of clays. Produced oil quality has been increased significantly in terms of viscosity, more viscosity reduction was observed with the presence of clays. Since saturates acts like an ignitor during ISC, the amount of saturates fraction was decreased in produced oil when compared to initial oil. While the amount of aromatics fraction was increased significantly, the asphaltenes fraction is decreased with the presence of clays when compared to the aromatics and asphaltenes fractions of the initial oil. The reduction in viscosity is mainly due to increased aromatics content of produced oil with high solvent power. With the SEM images taken on asphaltenes surface, the role of clays has been observed clearly on fuel formation. With the presence of clays, the asphaltenes surface have created cribriform structures. Without clays, asphaltenes surfaces were observed as smooth surface. Those holes should increase the surface area on asphaltenes surfaces and increase the effective transformation of asphaltenes into fuel.