Feasibility of In-situ Combustion of Tar from a Tarmat Reservoir

Combustion-tube tests were conducted on a tar of physical and chemical characteristics similar to a natural reservoir tar. Ottawa sand of 20-30 mesh size was used to prepare tar-sand mixtures of about 37% porosity, 19 to 25% water saturation and 21 to 32% tar saturation. In runs with distilled water, a combustion front was successfully initiated and moved into the sand pack. Later, however, the front’s temperature dropped below 500 °C causing the front to stagnate and become extinguished a short distance away from the tube’s inlet. When ferric nitrate was added to the water, sustained combustion was achieved with front temperatures maintained above 500 °C and high oxygen utilization. It appears that a minimum iron concentration in the water of about 2700 ppm is required for stable combustion. Concentrations above 4200 ppm did not enhance combustion performance. INTRODUCTION Tarmat is a loosely defined term ascribed to a layer of very viscous tar or bitumen that exists around the periphery of some oil reservoirs, usually at the oil/water contact. Problems posed by this phenomenon and techniques proposed to breach the tar layer were discussed in a previous article (Abu-Khamsin, 2001). In that article, oxidation kinetics of tar were studied as a preliminary step before feasibility of in-situ combustion as a tar-dissipation technique could be established. The main finding of that study was that high-temperature oxidation (HTO) of tar peaked at temperatures above 500 °C and the reaction’s activation energy was about 100 kJ/mol. Since such a temperature is difficult to mainatain under ordinary combustion-drive conditions, the tar appeared incapable of supporting sustained in-situ combustion. To confirm this hypothesis, it was decided to run combustion-tube tests on the tar. Two factors play crucial roles in the performance of in-situ combustion; these are availability and reactivity of the fuel. Insufficient fuel deposition ahead of the combustion front deprives the front of the heat necessary to sustain the process leading to premature extinction. Since medium and heavy crudes usually satisfy the fuel requirement (Alexander et al., 1962), this factor is not expected to be of concern with the tar. Fuel reactivity, on the other hand, controls the rate at which combustion heat is released. If HTO of the fuel is fast, convective and conductive heat losses from the combustion zone become small compared with the rate of heat release. This leads to fast accumulation of heat and, hence, rapid heating of cooler reservoir rock ahead of the front propelling this rock to combustion temperatures. As specified by reaction kinetics, the rate of HTO is computed by: RHTO = kHTO PO Cf (1) Where RHTO is the rate of HTO of fuel (g per cm of rock per s), kHTO is HTO reaction rate constant (consistent units), PO is partial pressure of oxygen (Pa), Cf is fuel concentration (g per cm of rock), m is reaction order with respect to oxygen, and n is reaction order with respect to fuel. kHTO is dictated chiefly by the reaction’s activation energy (EHTO, kJ/mol) and temperature (T, °K) as governed by Arrhenius law: kHTO = A exp[-EHTO /(RT)] (2) Where A is a frequency factor (consistent units) and R is the universal gas constant. If ample fuel and oxygen are available, rate of HTO would be set by kHTO. Should the nature of the fuel necessitate a large EHTO, a higher temperature would be required for the reaction to proceed at the rate needed to generate enough heat to maintain such temperature. Achieving this thermal-kinetic balance is the prerequisite for a sustained combustion front, which, given the large EHTO of the tar, appears rather difficult. EXPERIMENTAL WORK Materials Due to the scarcity of reservoir tar samples, the tar used in this study was prepared by distilling Ghawwar Arab-D crude to yield a residue of physical and chemical properties similar to the reservoir tar. Justification for this approach as well as properties of the prepared and reservoir tars are found elsewhere (AbuKhamsin, 2001). Some properties of the prepared tar are repeated in Table 1 for reference. Table 1: Physical and Chemical Properties of Prepared Tar. Density @ 15 C, g/ml 0.922 Viscosity @ 15 C, cP 10322 Atomic Hydrogen/Carbon ratio 1.73 Saturates, wt% 32.8 Aromatics, wt% 32.4 Resins, wt% 9.0 Asphaltenes, wt% 25.8 Ottawa sand of 20-30 mesh size was used. The sand was mixed with about 5 wt% mortar clay to enhance combustion. Setup Figure 1 shows a schematic of the apparatus, which consisted of gas supply and control, combustion tube and pressure jacket, effluent gas and liquid separation and metering, effluent-gas analysis instruments, sand-pack temperature monitoring, and data acquisition system. The combustion tube (Fig. 2) was made of a thin-walled, stainless-steel tube measuring 99 cm (length) x 7.6 cm (diameter) x 0.4 mm (wall thickness). Two stainless steel thermowells were fitted along the axis of the tube. The larger thermowell (3.2 mm OD) housed a bundle of thermocouples whose tips were spaced at 10 cm intervals. These provided temperature readings at fixed locations within the tube. The smaller thermowell (1.6 mm OD) housed a thermocouple that could be moved along the length of the tube for tracking the Legend 1. Pressure regulator 6. Pressure transducer 11. Condenser 16. Oxygen analyzer 2. Valve 7. Thermocouples 12. Acid scrubber 17. Carbon monoxide analyzer 3. Two-way valve 8. Combustion tube 13. Gas flowmeter 18. Carbon dioxide analyzer 4. Drying bed 9. Pressure jacket 14. Back-pressure regulator 19. Data acquisition system 5. Gas flow controller 10. Separator 15. Ball flowmeter 3 1 1