Leakage dynamics of fault zones: experimental and analytical study with application to CO₂ storage

Abstract

Fault zones have the potential to act as leakage pathways through low permeability structural seals in geological reservoirs. Faults may facilitate migration of groundwater contaminants and stored anthropogenic carbon dioxide (CO $_2$ ), where the waste fluids would otherwise remain securely trapped. We present an analytical model that describes the dynamics of leakage through a fault zone cutting multiple aquifers and seals. Current analytical models for a buoyant plume in a semi-infinite porous media are combined with models for a leaking gravity current and a new model motivated by experimental observation, to account for increased pressure gradients within the fault due to an increase in Darcy velocity directly above the fault. In contrast to previous analytical fault models, we verify our results using a series of analogous porous medium tank experiments, with good matching of observed leakage rates and fluid distribution. We demonstrate the utility of the model for the assessment of CO $_2$ storage security, by application to a naturally occurring CO $_2$ reservoir, showing the dependence of the leakage rates and fluid distribution on the fault/aquifer permeability contrast. The framework developed within this study can be used for quick assessment of fluid leakage through fault zones, given a set of input parameters relating to properties of the fault, aquifer and fluids, and can be incorporated into basin-scale models to improve computational efficiency. The results show the utility of using analytical methods and reduced-order modelling in complex geological systems, as well as the value of laboratory porous medium experiments to verify results.

Computation of vertical fluid mobility of CO$$_{2}$$, methane, hydrogen and hydrocarbons through sandstones and carbonates

“…Gilmore et al 35 evaluate the case of a fault cutting through three vertically stacked aquifers and seals based on data from a naturally CO -charged aquifer at Green River, Utah. CO has been escaping at the site along fault zones for several hundred thousand years.…”

The near-field shape and stability of a porous plume

“…In the case where the inlet velocity is close to the buoyancy velocity (), the solution to (2.16)–(2.21) was calculated by Gilmore et al. (2022) using the method of separation of variables, in which case where . Likewise, the approximate plume shape is given by the solution to This is obtained from the linearised version of (2.20) (i.e.…”

“…Examples of steady plumes from the study of Gilmore et al. (2022) are shown in experimental photographs in figures 3( a , b ). The plume width at the inlet for each case is cm, respectively.…”

“…However, it is important to understand the characteristics of the near-field plume due to the effects that it can have on the pressure near the injection point and consequent flow rates (Gilmore et al. 2022).…”

“…Comparisons are also made with the porous media tank experiments of Gilmore et al. (2022). Section 3 treats the stability of these steady plume shapes using a linear perturbation analysis.…”