Quantification of the impact of acidified brine on fracture-matrix transport in a naturally fractured shale using in situ imaging and modeling

Abstract

Understanding flow, transport, chemical reactions, and hydro-mechanical processes in fractured geologic materials is key for optimizing a range of subsurface processes including carbon dioxide and hydrogen storage, unconventional energy resource extraction, and geothermal energy recovery. Flow and transport processes in naturally fractured shale rocks have been challenging to characterize due to experimental complexity and the multiscale nature of quantifying exchange between micrometer-scale fractures and nanometer-scale pores. In this study, we use positron emission tomography (PET) to image the transport of a conservative tracer in a naturally fractured Wolfcamp shale core before and after exposure of the core to low pH brine conditions. Image-based experimental observations are interpreted by fitting an analytical transport model to every fracture-containing voxel in the core. Results of this analysis indicate subtle increases in matrix diffusivity and a strong reduction in fracture dispersivity following exposure to low pH conditions. These observations are supported by a multi-component reactive transport model that indicates the capacity for a 10% increase in porosity at the fracture-matrix interface over the duration of the low pH brine injection experiment. This porosity enhancement is the result of exposure of carbonate minerals in the shale matrix to low pH conditions. This workflow represents a new direct approach for quantifying fracture-matrix transport processes and provides a foundation for future work to better understand the role of coupled transport, reaction, and mechanical processes in naturally fractured rocks.

Coupled Transport, Reactivity, and Mechanics in Fractured Shale Caprocks

“…A reactive transport simulation conducted on this sample by Zahasky et al. (2023) showed fast dissolution of carbonate (within 6 d) and slow dissolution of feldpars and clays during acidic brine injection. In contrast, minimal to depleting concentrations of Ca, Mg, K, and Na species were observed in the effluent compared to the originally injected brine.…”

“…Further, Zahasky et al. (2023) reported an increased fracture‐to‐matrix diffusion coefficient, indicative of secondary porosity at near‐fracture matrix surfaces. Porosity decreased predominantly in the matrix near the inlet and along fractures.…”

“…(2023) and Zahasky et al. (2023). The total time for each measurement depends on instrument resolution, equilibrium time for fluid saturation and imbibition, and flow‐through duration.…”

“…(2023) discusses imbibition dynamics of the naturally fractured shale sample as imaged using CT. Zahasky et al. (2023) captures the alterations in advection, dispersion, and diffusion mechanisms of the sample after exposure to reactive fluid using positron emission tomography (PET).…”

“…The core was previously vacuumed (confining pressure = 1,720 kPa/250 psi), CO 2 ‐saturated (injection pressure = 2,068 kPa/300 psi, confining pressure = 3,792 kPa/550 psi), and brine‐saturated (injection pressure = 3,650 kPa/530 psi, backpressure = 3,170 kPa/460 psi, confining pressure = 4,826 kPa/700 psi) with and without [ 1 8F]‐fluorodeoxyglucose (FDG) (Kurotori et al., 2023; Zahasky et al., 2023). The core was received in a depressurized state.…”