Reactive Transport Simulation of Fracture Channelization and Transmissivity Evolution

Abstract

Underground fractures serve as flow conduits, and they may produce unwanted migration of water and other fluids in the subsurface. An example is the migration and leakage of greenhouse gases in the context of geologic carbon sequestration. This study has generated new understanding about how acids erode carbonate fracture surfaces and the positive feedback between reaction and flow. A two-dimensional reactive transport model was developed and used to investigate the extent to which geochemical factors influence fracture permeability and transmissivity evolution in carbonate rocks. The only mineral modeled as reactive is calcite, a fast-reacting mineral that is abundant in subsurface formations. The X-ray computed tomography dataset from a previous experimental study of fractured cores exposed to carbonic acid served as a testbed to benchmark the model simulation results. The model was able to capture not only erosion of fracture surfaces but also the specific phenomenon of channelization, which produces accelerating transmissivity increase. Results corroborated experimental findings that higher reactivity of the influent solution leads to strong channelization without substantial mineral dissolution. Simulations using mineral maps of calcite in a specimen of Amherstburg limestone demonstrated that mineral heterogeneity can either facilitate or suppress the development of flow channels depending on the spatial patterns of reactive mineral. In these cases, fracture transmissivity may increase rapidly, increase slowly, or stay constant, and for all these possibilities, the calcite mineral continues to dissolve. Collectively, these results illustrate that fluid chemistry and mineral spatial patterns need to be considered in predictions of reaction-induced fracture alteration and risks of fluid migration.

Rapid Mineral Precipitation During Shear Fracturing of Carbonate‐Rich Shales

“…After injecting synthetic hydraulic fracturing fluid through a Marcellus shale core without acid, Vankeuren et al reported that the absence of acid-induced calcite dissolution resulted in net fracture volume reductions due to mixing-induced barite precipitation, which could either constrict flow or complement proppants to maintain fracture apertures over longer time frames (Vankeuren et al, 2017). Conversely, dissolution in excess of precipitation can lead to asperity removal and fracture compaction (Ellis et al, 2013;Elsworth & Yasuhara, 2010) or channelization and fracture opening (Deng & Peters, 2019;Luquot & Gouze, 2009). In addition to competing reactions, precipitates could be impacted by changes in environmental conditions such as temperature and pressure swings that were not explicitly evaluated in this study.…”

Effect of Cumulative Surface on Pore Development in Chalk

“…When the particles accumulate at pore throats, they can exert important control on the flow field, and thus on the dissolution localization. This issue can be circumvented by assuming an inert silicate matrix and thus capping the upper porosity of elemental units in a simulation (Deng et al, ; Deng & Peters, ). In a related paper, we discussed this issue in the context of how these mechanical impacts disrupt the positive feedback loop (Yang, Hakim, et al, ).…”

Role of Mineralogy in Controlling Fracture Formation

“…Beyond the general composition of fracture surfaces, the amount and distribution of reactive minerals on fracture surfaces plays an important role in controlling the reactive evolution of fractures 26,27 and is not well understood, often considered to be simply reflective of the bulk mineralogy. 25,26,41 In this work, we analyzed the role of mineralogy in controlling fracture formation by considering the abundance and distribution of minerals on fracture surfaces. Our goal was to improve our ability to predict reactive fracture evolution and understand its implications for subsurface CO 2 sequestration.…”