The Role of Thermal Analysis Techniques in the In-Situ Combustion Process

Abstract

Summary Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) studies were conducted on two Lloydminster region, Canada, heavy-oil cores, extracted oil, and mineral matter. Fig. 1 is a general schematic of the thermal process, such as evaporation, distillation, thermolysis, low-temperature oxidation (LTO), thermal cracking, combustion, coking, polymerization, and thermal alteration of mineral matter. Four core samples and their extracted oils were examined for propensity of coke formation under helium atmosphere. A two- or three-fold propensity of coke formation under helium atmosphere. A two- or three-fold increase in the fuel content was observed in the cores compared with the extracted oil. This indicates a selective and significant contribution of the minerals to the coke-forming mechanism. The data from DSC and TGA experiments were used to calculate enthalpy values and ignition temperatures. These data suggest that heat generated by LTO reactions is significant during in-situ combustion. With the exception of Sand D, the thermal alteration of the mineral matter at 600 and 900C [1,112 and 1,652F] was significant. In Sands A and B, the percentage of fine particles, less than 2 mu m, doubled at 600 and 900C [1,112 and 1,652F] compared with that at 100C [212F]. Although kaolinite constituted between 15 and 75% of the fine particles in all cases, it was not detected in appreciable concentrations when the sand was heated at 600 or 900C [1, 1 12 or 1,652F]. These results indicate that potential problems during oil production could arise from the migration of fine particles. The mineral-analysis data obtained from these core samples suggest that swelling of clays during wet combustion may not be sufficient to have a deleterious effect on air/water injectivity or oil production. The results from TGA/DSC experiments are complementary to production. The results from TGA/DSC experiments are complementary to those from combustion-tube tests and provide the kinetic, thermal, and mineralogical data required for numerical simulation, planning, and design of an in-situ combustion project. Introduction Heavy-oil reservoirs of Lloydminster are characterized by thin net pay, high permeability, high oil saturation, bottomwater, and very viscous oil. 1 Recovery of heavy oil from these reservoirs is less than 9% of initial oil in place (IOIP) under primary and secondary methods. For place (IOIP) under primary and secondary methods. For the development and use of these resources, application of thermal processes appears to be the most suitable EOR technique. 24 In the thermal process, heat is transferred to the reservoirs by either injection of steam/hot water or in-situ combustion. The oil displacement results from (1) viscosity reduction caused primarily by heat and secondarily by dissolution of in-situ generated CO2 in oil; (2) thermal expansion of the oil resulting in increased relative permeability; (3) distillation and thermal cracking of oil; permeability; (3) distillation and thermal cracking of oil; (4) a solution-gas drive from produced gas that facilitates the flow of fluids within the reservoir toward the production wells; and (5) increased pressure gradient imposed production wells; and (5) increased pressure gradient imposed by the injected air and generated gases. The in-situ combustion process consists of injecting air/oxygen/water into an oil reservoir to establish a flow path for the movement of fluids, igniting the crude oil, path for the movement of fluids, igniting the crude oil, and propagating the combustion front by continued air/oxygen injection. Knowledge of the fuel (coke) content of the oil being burned, the volume of air/oxygen required to sustain combustion, the efficiency of oxygen use, and oxidation and combustion reactions is imperative for predicting in-situ combustion performance. predicting in-situ combustion performance. Numerical simulation of the in-situ combustion process usually requires reservoir description, reservoir fluid process usually requires reservoir description, reservoir fluid properties, thermodynamics, kinetics of chemical properties, thermodynamics, kinetics of chemical reactions, and well data. Experimental data for thermodynamic properties and reaction kinetics in the area of low-temperature oxidation, cracking, combustion, and coking that are required to make a meaningful prediction are lacking in the literature. These data are not provided by combustion-tube experiments. To this end, we used thermal analytical techniques, TGA, and DSC to generate required data for chemical reactions, such as energy of activation, preexponential factor, rate constant, and heat of reaction. Thermal techniques can also provide information on the nature of the oil, the minimum ignition temperature of crude oil to sustain combustion, the fuel content of the core, fluid/rock interaction, the decomposition of the mineral matter present in the core, and the residue left after heating. SPERE P. 329