The purpose of this work is to perform an improved method to optimize different CO2 Enhanced Oil Recovery (EOR) processes in unconventional liquid reservoirs, particularly in the volatile oil region of the Eagle Ford shale. Previous simulation studies of CO2 EOR in the unconventional liquid reservoirs were not done in full field-scale and were not history matched before applying CO2 EOR to the model. Without history matching step, the simulation might generate misleading results in CO2 EOR studies. The dual-porosity, structured grid model in this paper will be history-matched with actual data collected from the field to ensure the results of CO2 EOR study to be meaningful. In addition, we are implementing the simulation in the dual-porosity mode to account for the presence of natural fractures which have been observed on Eagle Ford outcrop.

First, all data will be gathered from public sources. These data include production reports, outcrop map, natural fracture, hydraulic fracture, geology, rock, and fluid. Multiple grid sizes and number of refinements for hydraulic fractures will be tested to ensure the accuracy of the simulation is preserved yet speed up computational time. Several sensitivity analyses will be conducted to investigate which parameters from the matrix system and the natural fracture system would have a significant impact on the incremental oil recovery. These parameters will be adjusted scientifically in the process of history matching to capture the primary depletion. Then, this history-matched model will be used to apply multiple CO2 EOR studies.

The history matched model suggests that matrix porosity in the volatile oil region of Eagle Ford shale might be overestimated in many previous investigations. Also, the sensitivity analyses show that the natural fracture permeability perpendicular to the direction of the horizontal well has a significant impact on oil rates in numerical simulation. Among different CO2 EOR methods tested in this research, huff-n-puff yields the most promising outcome as compared to continuous injection and WAG (Water-alternating-gas) in both oil production and economic performance in the volatile oil region of the Eagle Ford shale. In some circumstances, continuous injection method might yield great utilization efficiency but it contains high possibility of negative incremental oil recovery at the end of EOR practice due to pressure loss during CO2 injection time, incomplete miscible process between CO2 and residual oil, and viscous fingering.

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