HLD-Based Formulation Prediction for Degassed Oil with Equivalent Minimum Miscibility Pressure

Abstract

Shale cores, with their ultralow permeability, often require pressurization for saturation with target fluids, as flowthrough saturation is inefficient. This saturation method, when applied by using live oil, leads to fluid loss during core transfer and system vacuuming in subsequent experimental steps. As a result, most shale-related experiments utilize degassed oils, typically dead oil or kerosene. However, researchers account for only density or viscosity similarities in degassed oil preparation, missing key aspects of the gas injection mechanism. This study focuses on the representativeness of degassed oil and develops a procedure to prepare degassed surrogate oil that accurately replicates the live oil's minimum miscibility pressure (MMP). We propose a predictive oil formulation procedure based on thermodynamic similarities between oil/gas miscible systems and oil/water middle-phase microemulsion systems under the hydrophilic−lipophilic deviation (HLD) theory. This procedure, validated with real reservoir oil samples, uses liquid light hydrocarbons to reduce the equivalent alkane carbon number (EACN) of dead oil for surrogate oil preparation. EACN for dead, live, and surrogate oils was determined through slim tube and microemulsion phase behavior experiments. Light hydrocarbon addition quantities were estimated by linear EACN mixing. The surrogate oil's MMP, tested against CO 2 in slim tube experiments and verified through salinity scanning phase behavior experiments, was 27.83 MPa, closely matching the live oil's MMP of 28.15 MPa. These results validate the predictivity of our procedure, eliminating trial and error in adjusting light hydrocarbon volumes. The surrogate oil's lack of dissolved gases addresses fluid loss issues during rock core transfers, solving saturation and transfer challenges in tight rock core experiments.