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

摘要

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 degrees from its axis. The test was performed at 40 MPa of constant hydrostatic confining pressure and similar to 11 MPa of pore pressure, at room temperature (similar to 19.5 degrees 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 similar to 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.