Investigation of mechanisms of supercritical CO2 trapping in deep saline reservoirs using surrogate fluids at ambient laboratory conditions

Abstract

Geological storage of carbon dioxide relies on the effectiveness of immobilizing CO2 in the pore space of deep geological formations through a number of trapping mechanisms that include capillary, dissolution, and mineral trapping. Improved fundamental understanding of these processes is expected to contribute toward better conceptual models, improved numerical models, more accurate assessment of storage capacities, and optimized placement strategies. However, studying these processes at a fundamental level is not feasible in field settings because fully characterizing the geologic variability at all relevant scales and making observations on the spatial and temporal distribution of the migration and trapping of supercritical CO2 (scCO2) is not practical. The specific goal of this study is to develop and implement an experimental method in intermediate scale test tanks under ambient laboratory conditions to make observations and generate data to improve the understanding of capillary trapping affected by fluid and formation properties. Since it is challenging to visualize multiphase flow processes occurring at high pressure conditions at the meter scale, a testing method was developed based on the use of surrogate test fluids to replace the scCO2 and formation saline water. To set a foundation for extrapolating experimental results to the field, we chose a set of dimensionless groups that define the relative contributions of buoyancy, viscous, and capillary forces to the displacement behavior of immiscible fluids. The experiments were designed with the goal of understanding and accurately quantifying the immobilization of the scCO2 analog in a homogeneous formation confined by a slightly dipping structural barrier. A set of three displacement experiments through unconsolidated sands with variable permeability was conducted in a quasi-two-dimensional flow cell to gain insight into the influence of buoyancy forces on the propagation of the displacing phase. This work takes advantage of laboratory experiments at the intermediate scale to investigate gravitational and hysteresis effects on entrapment of scCO2 currents in brine-saturated reservoirs. Understanding these phenomena at a fundamental level represents a critical step to improve injection strategies and to enhance capillary trapping mechanisms.