Near-Wellbore and Reservoir Effects in In-Situ Combustion

Abstract

Summary To operate fields under in-situ combustion (ISC), the near-wellbore dynamics and far-field conditions have to be considered. In the near-wellbore region of vertical injection wells, the flow advancement of the combustion front is characterized by high velocities. Farther away from the injection wells, the advancement rate of the combustion front is much smaller. For a line-drive configuration, the advancement of the front slows down from several meters per day near the wellbore to several centimeters per day in the far-field region. To investigate the effects in the near-wellbore region and far-field conditions, laboratory experiments and simulations were performed and compared with the behavior of a Central European field produced by using ISC. The laboratory experiments covered the kinetics in the near-wellbore region as well as the far-field region by applying various heating rates and by preheating a kinetic cell before injecting air. The dynamic effects were investigated with a combustion tube. Mechanistic numerical simulation was based on the kinetics derived from the kinetic-cell experiments and applied to the combustion tube. The same set of conditions was then used to simulate the near-wellbore conditions in the Central European field and far-field conditions. The results show that, in the near-wellbore region, the advancement of the combustion front is fast compared with heat conduction ahead of the front. Hence, low-temperature-oxidation (LTO) reactions and high-temperature-oxidation (HTO) reactions, as derived from the kinetic-cell experiments, are occurring in different distances from the injection well. In the far field, heat conduction ahead of the front and the flow of hot combustion gases preheat the reservoir before oxygen arriving at the combustion front. For these conditions, LTO and HTO reactions are occurring at the same location. In the Central European field produced with ISC, the various operating conditions are shown at an example well. Four different phases of production can be seen: (1) oil production with cyclic steam stimulation (CSS), (2) shut-in of the well to stabilize the combustion front that is approaching, (3) oil-production response of the combustion front, and (4) conversion of the well for air injection. The air-injection rate is slowly increased to avoid too-high temperatures in the early-injection phase (faster advancement of the heat front than heat conduction). The distance of the wells is approximately 70 m to allow sufficient oil recovery per well and speed of the combustion front.