An experimental investigation of in situ combustion in fractured heavy oil systems

In situ combustion (ISC) is one of the thermal oil recovery processes, which has been applied successfully in a variety of reservoirs. The energy is generated within the reservoir by series of multistep reactions that provide heat to reduce oil viscosity, increase reservoir pressure and upgrade heavy oil in situ by reducing the heaviest fractions of crude oil. Within the scope of this study, in situ combustion in fractured systems is investigated by both conducting kinetic reactor and combustion tube tests using Batı Raman crude oil. The combustion characteristics of the crude were analyzed first to understand reaction kinetics of the oil. Thus, ramped temperature oxidation (RTO) experiments with effluent gas analysis (EGA) were conducted with different sand particle sizes and heating rates. 50 kinetic reactor tests were conducted throughout the study and 25 of them were used in the analyses. The results were analyzed using isoconversional kinetic approach based on the produced effluent gas and temperature data obtained from experiments. A reaction model was proposed for each pair and effective activation energy, atomic hydrogen – carbon (H/C), combustion reaction stoichiometric coefficients and ignition temperature were estimated for the sample with different sand particle sizes. Kinetic reactor tests were used to determine the combustion characteristics of the oil sample and define the particle size that would later be used as fractures for tube tests with sand. In addition to RTO tests, 35 combustion tube tests were performed. During the tests, the temperature was measured along the tube and the produced effluent gas composition was recorded to be used for the analyses. Initially, 12 combustion tube tests were conducted with homogeneous sandpacks to understand the effect of the sand particle size on combustion behavior. Later, 2 tests were performed with heterogeneous sandpacks to investigate the combustion characteristics in the transition region of different porous media with various particle size. Later, 9 fracture tests with sandpack were conducted to obtain a better understanding of the effect of horizontal (perpendicular to air flow), vertical (parallel to air flow), and combined (both vertical and horizontal) fractures on combustion front behavior and determine the conditions that are required for a self-sustained front propagation. Finally, 12 fracture tests with core were performed to investigate the effect of direction, width, spacing, and network of fractures on the combustion front behavior. In 5 tests with sandpack and core, dry air (21% oxygen) was used as the feed gas whereas oxygen – enriched air (39% oxygen) was used in 9 tests with core samples to investigate the effect of oxygen concentration on the front behavior. After the tests, density and the amount of the produced oil were measured for comparison. The temperature and the produced effluent gas data were used to estimate the average front temperature and velocity, H/C ratio, air/fuel ratio, and oxygen utilization. It was concluded that the oxygen diffusion into the matrix and oil availability in the matrix during the process are the main factors for self-sustained front propagation in fractured systems. The failure of the unfractured core tests was related to heat losses, the mass of the core sample and energy generated during combustion reactions. Furthermore, vertical fractures may help to sustain the combustion front in heavy oil system due to the improvement of the available oxygen for the reactions. The horizontal fractures perpendicular to the flow direction have almost no effect on the sustainability of the combustion front whereas the vertical fractures have a significant impact on the front propagation since injected air almost exclusively flows through the vertical fractures. Two different fronts were observed regarding the vertical fracture dominated systems. The first front propagates through the matrix and it is continuous similar to homogeneous packs. The second front is observed in the fracture and it is not continuous; depending on the oil availability the front is intermittent; dies out and rejuvenates. The front in the matrix feeds the front in the fracture by displacing oil into the fracture and oil goes through the combustion process multiple times. The front in the fracture thermally feeds the front in the matrix by providing additional energy to sustain the combustion. It was observed that the duration of the test increases for vertical fracture dominated systems due to the low oxygen diffusion into the matrix and higher amount of oil combusted during these tests. The recovery factor was lower for the vertical fracture dominated systems since the oil combusted more than one time due to the displacement of oil from the matrix into the fracture and because of that the API gravity of the produced oil increased for these tests. An increase in effluent gas methane concentration was observed while the front was moving from higher surface area to lower one. Furthermore, more methane released for the horizontal and combined fractured systems compared to homogeneous packs. The oxygen – enriched air promoted the combustion characteristics in porous media.