Numerical Modeling of Fluid injection in unconsolidated formations using the FEM with zero-thickness interfaces at the micro-scale

Numerous applications in hydrogeology, geo-environmental and geo-energy engineering require the injection of a fluid into the formation. The interaction between the soil or rock matrix and the selected fluids is pivotal for each application, and relevant parameters in the design must be adjusted accordingly. Such designs must also prioritize safety, operational effectiveness, and the minimization of potential environmental impacts and infrastructure failure risks. Given that these weak materials have different hydraulic and mechanical properties compared to competent rocks, their infiltration and fracturing response remains largely unknown.

This study focuses on modelling the results of a series of experimental tests that were carried out on artificially cemented porous media which were generated via microbially induced carbonate precipitation (MICP), a process that results in the cementation of silica particles. The cores were subjected to an anisotropic triaxial stress state, and a prescribed fluid injection along a central small-diameter axial perforation. Depending on the flow rate, degree of cementation and stress intensity and anisotropy, the fluid caused radial fractures of various configurations in the samples.

The modelling has been undertaken with code DRAC, based on the FEM with multi-phase multi-physics features and zero-thickness interface elements (also known as cohesive elements). These elements are inserted in between standard continuum elements to represent the effect of existing or potential fractures, not only in the mechanical behavior but also flow or diffusion-wise. From the mechanical viewpoint, these elements are equipped with constitutive laws incorporating fracture mechanics principles including fracturing parameters Gf.

For the numerical simulation of the lab tests, the sample geometry includes the representation of individual grains generated randomly by Voronoi Tesselation, separated from each other by interface elements. The anisotropic confinement stresses are applied on the outer boundary while the injection is applied on a central circle (experimental perforation). The fracture parameters are adjusted to the cementation level, and the fluid flow along inter-grain interfaces follows the cubic law. The paper includes a sample of the results obtained, exhibiting realistic fracture values and fracture patterns as compared to experiments. The conclusion is that the approach used seems to be a valid approach to model the fracture of weakly consolidated sandstone samples subjected to fluid injection.

The developed micro-scale model can be used in applications where fluid flow in porous media serves as the underlying mechanism. Such applications include the generation of hydraulic barriers, fluid injection for groundwater contamination remediation, water decontamination, leaching-induced flow to groundwater, artificial ground freezing for groundwater containment, CO₂ sequestration, managed aquifer recharge (MAR), and subsurface transportation of various fluids.