Coupled Geochemical-Geomechanical Alterations in Shale Fracture Systems
Authors: Menefee A. H. , Frash L. P. , Hicks W. , Carey J. W. 
Published: September 2022

ABSTRACT:
Shales are typified by low permeability, where geochemically-induced permeability alterations to shales serving as energy sources or waste repositories can hinder or shut down operations. In this work, we explored the influence of key physical and chemical controls (temperature, fracture geometry, fluid saturation) on barite precipitation in shale under representative subsurface conditions, as well as the effects of induced barite precipitation on fracture permeability. A series of triaxial direct shear experiments was designed to promote barite precipitation in fractured shale cores, where continuous x-ray radiography and periodic x-ray computed tomography was applied to monitor the onset and development of barite precipitation resulting from mixing of incompatible fluids. The results confirm that precipitation becomes more favorable at increasing temperatures and barite saturation indices, and particularly with elevated [Ba2+] relative to [(Equation)]. No significant permeability reductions were observed, due primarily to the fact that precipitates preferentially filled offshoot fractures and coated fracture walls, leaving primary flow paths uninhibited. The results have direct implications for minimizing and preventing barite scaling in shale oil and gas reservoirs, as well as important implications for predicting fluid transport in tight subsurface systems that are susceptible to geochemical alterations.

1. INTRODUCTION
Tight shale formations serve as a critical energy source for unconventional oil and gas production, and as secure caprocks for subsurface disposal of emission and waste streams. Shales are characterized by low permeability, where fluid transport is primarily confined to existing or stimulated fracture networks. Consequently, they are particularly susceptible to permeability alterations that occur in the presence of geochemical reactions. In particular, mineral precipitation resulting from fluid-rock interactions or from the mixing of incompatible fluids can lead to scaling that partially or completely blocks flow through wellbores, tubing, or the formation itself.
Barite (BaSO4) is a notorious scaling agent in hydrocarbon and geothermal energy reservoirs. In subsurface injection and/or extraction operations, barite most often forms from the mixing of incompatible fluids. For example, produced waters from Marcellus shale gas production are often enriched with Ba2+; these waters are typically recycled as fracking fluids, where they are often blended with base fluids from surface waters that are enriched in SO42- (Xiong, Lopano, et al., 2020). This mixing can induce BaSO4 precipitation in equipment or tubing at the surface, and/or within the reservoir when it is injected to stimulate fractures. While fluid transport through low-permeability shale is typically confined to these stimulated fractures, previous studies have shown that BaSO4 scaling can also occur within the shale matrix (Li et al., 2018). Barite precipitation is particularly problematic in low-permeability shales, where it can obstruct pores or coat surfaces, preventing gas transport from the shale matrix to the fractures serving as production pathways. Batch experiments exposing Eagle Ford and Barnett shale samples to synthetic hydraulic fracturing fluids observed heavy barite and celestite (SrSO4) precipitation on shale surfaces, which was estimated to reduce pore space by over 50% (Sanguinito et al., 2021). Although antiscalants are typically added to frac fluids, previous experiments have shown barite can form even in the presence of antiscalants under reservoir conditions (Paukert Vankeuren et al., 2017). Prevention is critical to engineering operations, as barite removal is challenging due to its low solubility and recalcitrance against acid flushing (Xiong, Gill, et al., 2020).