Influence of Rock Mineralogy on Reactive Fracture Evolution in Carbonate-Rich Caprocks

Abstract

Fractures present environmental risks for subsurface engineering activities, such as geologic storage of greenhouse gases, because of the possibility of unwanted upward fluid migration. The risks of fluid leakage may be exacerbated if fractures are subjected to physical and chemical perturbations that alter their geometry. This study investigated this by constructing a 2D fracture model to numerically simulate fluid flow, acid-driven reactions, and mechanical deformation. Three rock mineralogies were simulated: a limestone with 100% calcite, a limestone with 68% calcite, and a banded shale with 34% calcite. One might expect transmissivity to increase fastest for rocks with more calcite due to its high solubility and fast reaction rate. Yet, results show that initially transmissivity increases fastest for rocks with less calcite because of their ability to deliver unbuffered-acid downstream faster. Moreover, less reactive minerals become persistent asperities that sustain mechanical support within the fracture. However, later in the simulations, the spatial pattern of less reactive mineral, not abundance, controls transmissivity evolution. Results show that a banded mineral pattern creates persistent bottlenecks, prevents channelization, and stabilizes transmissivity. For sites for geologic storage of CO that have carbonate caprocks, banded mineral variation may limit reactive evolution of fracture transmissivity and increase storage reliability.

Mineral Fabric as a Hidden Variable in Fracture Formation in Layered Media

“…Additive manufacturing can create "geo-architected" rock with repeatable physical and chemical properties to test the hypothesis that in-layer mineral fabric orientation affects fracture surface roughness and macro-scale volumetric flow rates. Samples with different orientations of bassanite (calcium sulfate hemi-hydrate, 2CaSO 4 ⋅ H 2 O) layers relative to gypsum (CaSO 4 ⋅ 2H 2 O) mineral fabric were printed ( Fig. 2) to examine the effect of fabric direction relative to layer direction on tensile fracture growth and on the geometric properties of the induced fracture surfaces.…”

Modeling Reactive Transport Processes in Fractures

“…Therefore, a modeling capability that resolves coupled mechanical and geochemical processes in fractures is needed. Some groundbreaking efforts have been taken in this direction, and highlighted that geomechnical processes can mitigate fracture opening caused by dissolution reactions (Yasuhara et al 2004;Ameli et al 2014;Walsh et al 2014b;Kim et al 2015;Spokas et al 2018). A mechanistic understanding of mechanical weakening of near fracture regions (such as the detachment of the altered layer) and modeling of microscopic processes are still lacking.…”

Saline Brine Reaction with Fractured Wellbore Cement and Changes in Hardness and Hydraulic Properties

“…Acidic brines, such as those associated with CO 2 sequestration applications, induce reaction zones in cement that alter its mechanical properties (Kutchko et al, 2007;Huerta et al, 2012;Walsh et al, 2014). Acidic brines can cause plastic deformation of asperities, deterioration of mechanical properties of cement (e.g., elastic moduli and hardness) (Huerta et al, 2011;Spokas et al, 2018), and increase the dissolution rates of portlandite (Baur et al, 2004;Marty et al, 2009), which increase the porosity of cement (Kutchko et al, 2007;Huerta et al, 2011;Mason et al, 2013).…”