Modelling of long-term along-fault flow of CO₂ from a natural reservoir

As part of the European ACT-sponsored research consortium, DETECT, we developed an integrated characterisation and risk assessment toolkit for natural fault/fracture pathways. In this paper, we describe the DETECT experimental-modelling workflow, which aims to be predictive for fault-related leakage quantification, and its application to a field case example for validation. The workflow combines laboratory experiments to obtain single-fracture stress-dependent permeabilities; single-fracture modelling for stress-dependent relative permeabilities and capillary pressures; fracture network characterisation and modelling for the caprock(s); upscaling of properties and constitutive functions in fracture networks; and full compositional flow modelling at field scale.

Here we focus on the application of the workflow to the Green River site in Utah. This is a rare case of leakage from a shallow natural CO2 reservoir, where CO2 (dissolved or gaseous) migrates along two fault zones to the surface. This site provides a unique opportunity to understand CO2 leakage mechanisms and volumes along faults, because of its extensive characterisation including a large dataset of present-day CO2 surface flux measurements as well as historical records of CO2 leakage in the form of travertine mounds. When applied to this site, our methodology predicts leakage locations accurately and, within an order of magnitude, leakage rates correctly without extensive history matching. Subsequent history matching achieves accurate leak rate matches within a-priori uncertainty ranges for model input parameters.