Comparative Analysis of Enhanced Oil Recovery Across Lithologies in Confining Reservoirs

Thesis Abstract

This study investigates how lithology influences the effectiveness of enhanced oil recovery (EOR) strategies in confining reservoirs, addressing the persistent production disparities observed across sandstones, carbonates, and shales under similar pressure-temperature regimes and confinement conditions. The aim is to quantify the differential performance of chemical, thermal, and gas-CO2 EOR methods across lithologies and to identify the dominant mechanisms driving recovery variance in constrained pore networks. Specific objectives include (i) evaluating baseline reservoir properties (porosity, permeability, pore throat geometry) across lithologies using core plug analyses and petrographic thin sections; (ii) comparing EOR performance under polymer flooding, low-salinity waterflooding, steam-assisted gravity drainage, and CO2-enhanced gas expansion in controlled core-scale experiments; (iii) developing predictive regression models and a multi-criteria decision framework to assess lithology-EOR compatibility; and (iv) formulating guidelines for field-scale implementation in confining reservoirs. The methodology adopts an exploratory mixed-methods design combining quantitative experiments with qualitative process interpretation. The population comprises core samples from three lithologies—tight sandstone, fractured carbonate, and shale-rich sandstone—from three representative fields with confining boundaries. A stratified random sample of 60 full-diameter core plugs (20 per lithology) will be used for laboratory EOR testing, complemented by 15 carbonate core plugs subjected to high-pressure CO2 experiments to capture confinement effects. Data collection employs standardized core flooding tests, NMR porosity and T2 relaxation analyses, X-ray micro-CT imaging for pore-scale characterization, and petrographic analysis for mineralogy and fracture density. Instrumentation includes high-temperature/high-pressure core flood rigs, a CO2 injection system with solubility calibration, and imaging facilities for pore-network reconstruction. Validity and reliability are ensured through repeated measures (n=3 per test condition per lithology), calibration runs, and inter-lab cross-validation with established reference cores. Data analysis integrates a combination of engineering and statistical techniques (i) analysis of variance (ANOVA) and multivariate regression to quantify lithology-by-EOR method interactions on recovery factors; (ii) response surface methodology (RSM) to model optimization of operating conditions within confinement constraints; (iii) non-parametric tests for small-sample subsets where normality assumptions fail; (iv) pore-scale modeling using percolation theory to interpret transport pathways and sweep efficiency; and (v) a logistic regression framework to classify lithology-EOR feasibility. The study anticipates key findings (1) distinct recovery differentials among lithologies, with carbonate-dominated confining reservoirs showing superior response to CO2-ENHANCED gas expansion and steam-assisted methods due to fracture networks and lithofacial heterogeneity; (2) polymer and low-salinity waterflooding yielding moderate gains in sandstone with intermediate pore connectivity, but limited uplift in shale-rich matrices; (3) confinement-induced capillary pressure and tortuosity strongly mediating sweep efficiency, especially in tight sandstone and shale-rich lithologies; and (4) predictive models with acceptable goodness-of-fit (R2 > 0.75) enabling lithology-specific EOR decision guidelines. The contribution to knowledge lies in providing a rigorous, quantitatively robust cross-l lithology framework for selecting and tuning EOR methods under confinement, integrating pore-scale insights with macroscopic performance. The study informs field deployment by delivering a decision-support tool that links lithology, confinement parameters, and EOR mechanism to expected incremental oil recovery, alongside a set of recommended operating envelopes for each lithology. The main conclusion is that lithology and confinement jointly dictate EOR efficacy, necessitating lithology-tailored strategies rather than uniform application of EOR methods. Recommendations include incorporating lithology-aware screening in project development, prioritizing CO2-enhanced processes in carbonate-dominated confined reservoirs and combining fracture-aware thermal methods in carbonate and sandstone systems with substantial connectivity, while reserving conservative chemical EOR approaches for shale-rich, low-permeability zones. Further research is suggested to extend the framework to field-scale uncertainty quantification and economic optimization under fluctuating injection costs and reservoir pressure support.