Multiphysics Investigation of Geochemical Alterations in Marcellus Shale Using Reactive Core-Floods

Abstract

The interaction of reactive fracture fluid with host shale and formation water plays an important role on fractured reservoir productivity. This study explores the prominent impacts of shale−fluid reactions on flow properties using representative core-flood experiments under confining stress. Alteration of shale is monitored using time-lapse X-ray computed tomography (CT), microCT (μCT) of samples preand post-reaction, and scanning electron microscopy (SEM). The imaging approach is multiscale from nm's to cm's. The samples are clayrich and partially fractured Marcellus outcrop and carbonate-rich MSEEL (Marcellus Shale Energy and Environmental Laboratory) downhole endmembers. Both samples have distinct microcracks for probing reactive transport in fractures communicating with matrices. A reduction in krypton-accessible CT porosity and liquid permeability was observed for both samples after fracture fluid exposure. Based on SEM-EDS surface analysis, an iron-bearing precipitate formed on and near fracture openings and in the shale matrix of the Marcellus outcrop indicating partial dissolution of pyrite and/or ferruginous dolomite followed by precipitation of iron (hydro)oxide. The compiled images reveal fracture filling with migrated and/or precipitated fine particles. Significant barite scale growth was detected on the reacted MSEEL surfaces together with halite and other (hydro)oxide precipitates resulting from geochemical reactions between the basin-specific injectants and shale minerals. The MSEEL sample experienced substantial calcite dissolution and a corresponding decrease in its bulk density and microcrack openings. Experimental results presented here indicate the significance of fracture fluid composition optimization based on intrinsic shale and resident brine chemistries.

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.…”

“…For example, Xiong et al injected brine through fractured Wolfcamp shale and reported evidence of a more pronounced dissolution in carbonate-rich shale, compared to the clay-rich shale sample on the fracture surfaces. Similarly, Gundogar et al saw carbonate-rich Marcellus samples exhibit evidence of washout and etching on the rock surface, which was absent for the low-carbonate Marcellus samples. Both of these experiments were conducted over long flow durations (>1 week), at slow flow rates (>20 μL/min), and at temperatures and pressures representative of subsurface conditions.…”

“…For example, flow promotes the generation of fines and the structural evolution of fractures, as a function of the injected fluid composition, velocity, and shale mineralogy. , Fines, which can be generated through a combination of processes including matrix dissolution, clay dispersion, and fracture surface spalling, , increase fracture aperture sizes and improve well productivity. On the other hand, fines can potentially reduce permeability and well productivity by depositing in narrow pore throats and fracture constrictions. ,,− …”

Fe Oxidation and Species Distribution at the Rock–Fluid Interface of Marcellus Shale Reacted with Hydraulic Fracturing Fluid

“…Prior static batch experiments revealed that the dissolution of pyrite and carbonate minerals occurred rapidly, releasing metal contaminants and generating porosity . The chemically altered reaction front extends from the shale–fluid interface into the rock matrix on a micrometer to millimeter scale. ,, The thickness of the reaction front and permeability of the shale cores can provide key insights for fluid penetration into the shale matrix …”

Impact of Acid–Base Stimulation Sequence on Mineral Stability for Tight/Impermeable Unconventional Carbonate-Rich Rocks: A Delaware Basin Case Study

“…36,37 Pyrite, in particular, readily oxidizes to Fe(III) and, depending on the solution pH, rapidly precipitates as Fe (oxyhydr)oxides. 6,10,14,38 Because solution pH has a strong influence on the rate of Fe(II) oxidation, the carbonate content of the rock can also influence this process. Jew et al 39 observed that in Marcellus shales with a low carbonate content (12% calcite, 1% dolomite), dissolved Fe(II) remains in solution for a significant amount of time, allowing transport to occur prior to oxidation and precipitation.…”