Transport properties of saline CO₂ storage reservoirs with unconnected fractures from brine-CO₂ flow-through tests

Abstract

CO₂ storage in fractured reservoirs may lead to fast CO₂ flow through interconnected fracture networks; but the role of isolated fractures on brine-CO₂ multiphase flow systems remains unclear. We present the results of a brine-CO₂ flow-through experiment in which we assess the change in transport properties of a synthetic sandstone plug (a surrogate of a saline siliciclastic CO₂ reservoir) containing non-connected fractures aligned 45° from its axis. The test was performed at 40 MPa of constant hydrostatic confining pressure and ~11 MPa of pore pressure, at room temperature (~19.5 °C), using pure liquid-CO₂ and 35 g L⁻¹ NaCl salt solution. The injected CO₂-brine volume fraction was increased from 0 to 1 in 0.2 units-steps (drainage). Upon achievement of the maximum CO₂ saturation (S_CO2 ~0.6), the plug was flushed-back with the original brine (imbibition). During the test, we monitored simultaneously pore pressure, temperature, axial and radial strains, and bulk electrical resistivity. The fractured sample showed lower values of cross- and end-points in the relative permeability curves to CO₂ compared to non-fractured samples, from comparable experiments performed at similar pressure and brine salinity conditions, but different temperature. Our results suggest that a non-connected fracture network affects the mobility of the individual phases, favouring the trapping of CO₂ in the porous medium and improving the storage efficiency of the reservoir. These evidences show the need of a better understanding of fracture connectivity prior to discard fractured reservoirs as unsuitable geological formations for CO₂ storage.

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ORCID for Héctor Marín-Moreno: orcid.org/0000-0002-3412-1359

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