Geochemical element mobilisation by interaction of Bowland shale with acidic fluids

Abstract

Hydraulic fracturing is widely used to exploit unconventional hydrocarbon sources, to enhance exploitation of geothermal energy and to aid in carbon sequestration through underground storage of captured CO 2 . The hydraulic fracturing fluids, which are commonly acidic, cause dissolution of minerals and desorption of elements which can lead to groundwater contamination. Batch reactor experiments were conducted to explore the interaction of simulated fracturing fluids with two end member compositions of basinal shales of the Bowland-Hodder unit (Carboniferous, UK) whereby the impact of temperature, fluid acidity, and rock/fluid ratio conditions were investigated. The results demonstrate that the fluid acidity is mainly controlled by the oxidative dissolution of pyrite and the dissolution of calcite, impacting mobilisation and fate of major and trace elements. The dissolution of calcite and pyrite significantly dominates the leaching of Sr and As, respectively. Generally, increased fluid acidity and temperature facilitate element mobilisation due to enhanced mineral dissolution and ion desorption, whereas higher rock/fluid ratio (higher mass of carbonate minerals) raises the buffering capacity and may promote the immobilisation of some metal ions by adsorption and precipitation (e.g. Ba, Pb, Fe, Al, and Mn). Moreover, the surface topography of different minerals in polished shale sample sections after fluid-rock interaction indicates that mineralogical compositions may play an important role in determining the pore structure. This research identifies chemical reaction pathways of geochemical elements (including contaminants) in fracturing fluids over a range of fluid chemistries and environmental conditions, and helps to evaluate element mobilisation from shale reservoirs with differing mineralogies.

Impact of Concurrent Solubilization and Fines Migration on Fracture Aperture Growth in Shales during Acidized Brine Injection

“…This stimulation, which is spearheaded by highly oxic acidic fluid, generates both conductive fractures and a more permeable matrix/fracture interface, allowing the release of hydrocarbons from the shale interior into the propped advective conduits. , A cascade of physicochemical interactions subsequently unfold between low-pH HFF and shale rock, creating coupled mineral dissolution and precipitation reactions, along with wettability alteration, shale softening, and fines migration . These effects ultimately dictate fracture conductivity and the lifespan of productivity for a given well. , Multiple studies have focused on the chemical reactions occurring at the shale/HFF interface, − with the general consensus that pyrite readily oxidizes − and carbonates are dissolved creating pore space on the fracture surface. − The reactions that occur are highly dependent on the shale mineralogy, specifically the carbonate and clay content, and the HFF composition. , Most of these experiments involve immersion-type batch reaction systems, ,,, where reactive brine is allowed to interact with a shale sample subject to diffusive solute transport in an enclosed chamber for an extended period of time, mimicking the shut-in period during the hydraulic fracturing operation. A limited number of studies have been conducted under advective flow conditions.…”

Coupled Transport, Reactivity, and Mechanics in Fractured Shale Caprocks

“…and temperature conditions (Bratcher et al, 2021;Cui et al, 2021;Dieterich et al, 2016;Goodman et al, 2020;Hakala et al, 2021;Harrison et al, 2017;Jew et al, 2017;Ji et al, 2021;Li et al, 2019Li et al, , 2020Lu et al, 2017;Marcon et al, 2017;Noël et al, 2020;Pearce et al, 2018;Pilewski et al, 2019;Sun et al, 2019;Wang et al, 2015). These studies collectively illustrate the rapid dissolution of carbonates (e.g., calcite, dolomite, and siderite) and pyrite.…”

Mineralogy, microstructures and geomechanics of rock salt for underground gas storage

“…Automated mineral identification systems linked to scanning electron microscopes include the Mineral Liberation Analyser (Ferrari et al, 2021; Ji et al, 2021) and QEMSCAN instruments (Ma et al, 2016; Zhao, Zhang, et al, 2022). The method is based on high‐resolution backscatter electron image analysis, energy dispersive X‐ray analysis and automated microscope operation and data acquisition, and is usually employed on thin sections or polished thick sections.…”