DOI: 10.25549/usctheses-c3-159597
title: Optimization of in-situ combustion: maximizing production and reducing CO₂ footprint

description: The in-situ combustion (ISC) process is an enhanced oil recovery (EOR) method that utilizes fuel in place to upgrade and displace the hydrocarbons in heavy oil reservoirs. In ISC processes, air is injected into a heated section of the reservoir. Upon reaching a threshold temperature, oxygen from the injected air reacts with the oil in place to generate heat, steam and reaction products such as CO₂. This process drives oil towards production wells, but at the same time produces large amounts of CO₂. ❧ In this research project we investigate the potential of recycling produced CO₂ back into the reservoir simultaneously with the ISC process. We attempt to maintain the advantages of the ISC process while at the same time reduce emissions of CO₂. Numerical simulations have been performed using CMG's STARS, a commercial thermal recovery simulator. To validate the model and the software we first simulated a lab-scale combustion tube experiment, following Belgrave et al. (1990) who previously modeled combustion tube experiments done by Hayashitani (1978).  Following initial model validation, we then investigated the effect of CO₂ recycle in both adiabatic and non-adiabatic settings. Our simulations show that net CO₂ productions can be reduced significantly, but this reduction, however, comes at the cost of lower oil production rates, higher injection rates and/or both. In the most optimal adiabatic setting, CO₂ production is reduced by 62% while the oil production is reduced by 10.5%. In the most optimal non-adiabatic setting, CO₂ production is reduced by 42% while oil production remains the same. However, in the non-adiabatic case, an additional 60% increase in total injection rate is necessary. ❧ Leveraging on our findings from the initial modeling/simulation work, a field scale pseudo 2-D model of the hybrid ISC process was developed. Using this model we showed that recycling of the CO₂ back into the reservoir, to replace some of the air that would be otherwise injected, can be beneficial for both reducing the CO₂ production as well as for increasing the oil production. We observed that the CO₂ that is recycled back into the system dissolves readily into the oil phase thus resulting in lower oil viscosity and in improved production rates. The results of our numerical calculations provide justification for the proposed combined ISC/CO₂-flood process. In this process the ISC itself provides on-site the gas that is needed for the CO₂ flood, and a substantial fraction of the total CO₂ produced can be permanently sequestered in the subsurface. ❧ The pseudo 2-D model was utilized to study the process for a wide range dimensionless parameters (e.g., Peclet and Damköhler numbers), and initial saturations. Favorable trends induced by CO₂ recycling are observed over a broad range of the dimensionless parameters. Under a set of realistic conditions, an increase in the oil recovery of ~33% for a fixed time of operation was observed when comparing the ISC process with CO₂ recycle to the more conventional ISC process. In addition, at the time when 80% of the total oil in place had been produced, the simulations showed that the CO₂ emissions were consistently lower by 18-22% for the case when CO₂ was recycled back into the formation. ❧ Following our2-D investigations, we initialed oil displacement simulations of full 3-D systems. We have studied two different cases, an inverted 9-spot and a line-drive configuration. Simulations in these 3-D settings were on a much larger scale and were performed with a shared-memory parallel machine available through HPCC at USC. The results of our numerical calculations show that, due to the naturally-occurring heterogeneity in the 3-D reservoir, the displacement behavior resembles an array of channels that are very similar to those observed in the simulation of the 1-D and pseudo 2-D models. Large ""well-to-well"" distances, as in the case of the inverted 9-spot configuration lead to poor overall sweep and performance (e.g., only 11% overall recovery). Recycling of CO₂ under these circumstances widens the ""transport"" channels and improves the oil sweep and recovery to a value of ~20%. However, due to the still low sweep efficiency, CO₂ emissions savings were observed to be negligible. ❧ In the line-drive scenario, with a relatively shorter ""well-to-well"" distance, the performance of the standard ISC process was noticeably improved with an overall oil recovery of 32%. Recycling of the CO₂ was not as effective as in the 9-spot configuration in increasing oil production, largely because flow channels were well developed, leaving little room for further improvement. However, because much more oil was produced relative to the 9-spot case, a large amount of void volume was available for CO₂ storage. With recycling, up to 30% reduction in CO₂ emissions was observed. ❧ In summary, this research has demonstrated the potential for reducing CO₂ emissions from ISC processes via efficient recycling strategies. The proposed mode of operation is a first step towards application of ISC in areas with strict emission regulations. Additional research, at various scales, is warranted in order to further test and validate the ideas developed in this work.