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Hydraulic fracturing employs a mixture of freshwater and chemical additives. Substituting produced formation water for freshwater conserves freshwater resources, but formation water possessing a high ionic strength may damage formations and inhibit production. Hydrothermal experiments were conducted at reservoir conditions (115 °C, 35 MPa) to evaluate geochemical interactions between reservoir rock and hydraulic fracturing fluid synthesized using formation water. The specific hypotheses tested are (1) progressively greater ionic strength and acidity enhance the solubility of carbonate and aluminosilicate minerals and (2) acidity is more important than ionic strength for carbonate solubility but ionic strength is more important for aluminosilicate mineral solubility. Experiments replicated a shut-in well in the Wall Creek Member of the Cretaceous Frontier Formation, Powder River Basin, Wyoming , an important unconventional reservoir composed of low permeability sandstones interbedded with organic-rich mudstones. The core was reacted for ∼650 h (27 days) with (1) acidic hydraulic fracturing fluid (pH ∼ 2.3) mixed from formation water spanning two orders of magnitude ionic strength (0.016, 0.16, and 1.13 mol/kg) and (2) hydraulic fracturing fluid (ionic strength 0.11 mol/kg) of circumneutral pH (7.3). Reaction with hydraulic fracturing fluid of progressively greater ionic strength increased calcite solubility, but acidity was more important to calcite solubility than ionic strength. Trends of aqueous silica, potassium, and magnesium are consistent with the dissolution of feldspar and/or quartz and the precipitation of clay. Ionic strength was more important than acidity for feldspar–clay equilibrium, the predominant aluminosilicate mineral reaction. Hydraulic fracturing fluid with a progressively greater ionic strength contained larger amounts of aqueous silica, enhancing the potential importance of secondary clay mineralization. Mineralogic evidence for dissolution was limited to calcite and feldspar; no secondary clay was observed. Formation water may be used for hydraulic fracturing fluids, provided that the potential benefits and deleterious effects of calcite and feldspar dissolution and secondary clay mineralization are assessed on a case-by-case basis.

Hydraulic fracturing of low-permeability rocks significantly enhances hydrocarbon production from unconventional reservoirs. However, fluid transport through low-permeability rocks and the influence of geochemical transformations on pore networks are poorly constrained. Mineral reactivity during interactions with injected water may alter the physical nature of the rock, which may affect hydrocarbon mobility. To assess alterations to the rock, we have previously conducted a hydrothermal experiment that reacted cubed rock samples (1 cm3) with synthetic hydraulic fracturing fluid (HFF) to simulate physicochemical reactivity during hydraulic fracturing. Here, we analyze unreacted and reacted rocks by small-angle neutron scattering and high-pressure mercury intrusion to determine how the pore networks of unconventional reservoir rocks are influenced by the reaction with hydraulic fracturing injectates. Our results suggest that fluid–rock interactions exhibit a two-fold influence on hydrocarbon recovery, promoting both hydrocarbon mobilization and transport. Pore–matrix interfaces smooth via the removal of clay mineral surface asperities, reducing the available surface area for hydrocarbon adsorption by 12–75%. Additionally, HFF-induced dissolution creates new pores with diameters ranging from 800–1400 nm, increasing the permeability of the rocks by a factor of 5–10. These two consequences of mineral dissolution likely act in concert to release hydrocarbons from the host rock and facilitate transport through the rock during unconventional reservoir production.

The dynamics and reactivity of nanoconfined fluids play critical roles across a wide range of environmental and technological systems, though reaction mechanisms and kinetics are not well understood. The carbonation kinetics of forsterite (Mg2SiO4) exposed to 90 atm supercritical carbon dioxide at 35–65 °C and 85–100% relative humidity (RH) was monitored with in situX-ray diffraction, and partner molecular dynamics simulations were used to describe the free energy landscape of Mg2+adsorption and diffusion on forsterite surfaces covered in water films 3–10 monolayers thick. The collective findings reveal how decreasing the water film thickness by ∼1.4 monolayers, from ∼0.92 to ∼0.64 nm, inhibited reaction rates by up to 97%, promoted anhydrous Mg-carbonate (magnesite, MgCO3) precipitation, and more than doubled the apparent activation energy of carbonation. The transport simulations suggest that four monolayers are required to enable sufficiently facile Mg2+diffusion, helping explain previously observed water film thickness-dependent reactivity thresholds.

Hydrothermal experiments were conducted to evaluate mineral dissolution and precipitation that may occur during hydraulic fracturing of unconventional reservoirs. The B Bench of the Niobrara Formation and the Wall Creek Member of the Frontier Formation (Powder River Basin, WY) were reacted with a hydraulic fracturing fluid (pH = 2.5, ionic strength = 0.05) at 115 °C and 35 MPa for one month. Data collected from both experiments indicate that calcite began to dissolve in less than 50 h, Al-bearing solids began to dissolve immediately upon contact with hydraulic fracturing fluid, and an aluminosilicate precipitate began to form in less than 27 h. Data from the Wall Creek experiment suggest potential dissolution of chert. We estimate that calcite dissolution increased porosity of the B Bench from 1.7% to 2.8% and of the Wall Creek from 3.5% to 4.8%. Dissolution of calcite increases porosity and storage space for injected fluids. However, removal of calcite may weaken the mechanical integrity of the rock, leading to proppant embedment and fracture closure. Precipitation of an Al-bearing phase may decrease rock and fracture permeability. Aqueous Al in our experiments behaves similarly to aqueous Al collected during flowback from hydraulically fractured wells, suggesting that Al-bearing phases precipitate during time scales of hydraulic fracturing.

Laboratory measurements including gas (N2) porosity and permeability, time‐domain nuclear magnetic resonance, thin section, and scanning electron microscopy analysis were conducted to obtain petrographical and petrophysical descriptions of the Weber Sandstone and Madison Limestone at the Rock Spring Uplift, a potential carbon dioxide storage site in Southwestern Wyoming. The relationships between pore structures, such as pore geometry, pore‐size distribution, pore network, and porosity/permeability are investigated. First, using thin sections combined with scanning electron microscopy for pore structures description, all samples are described in detail from the geological, petrographysical, and diagenetic viewpoint. Results show that within the Madison Limestone, pore types include intercrystalline, vuggy, moldic, or mixed (combination of all other pore types). Both moldic and vuggy pore types are associated with samples of high porosity and permeability. Nuclear magnetic resonance relaxation time distributions show either bimodal or multimodal distributions. Large relaxation time components are associated with samples with large pores, whereas small components are dominated by small pores. The T2geometric mean correlates well with gas permeability. Additionally, short‐time diffusion coefficients (D) were measured by pulsed field gradient method using a series of gradient strengths. We found that diffusion coefficient distributions correlate with the corresponding T2distributions for macropores. By comparing the dominant peak position of T2distributions and their corresponding diffusion coefficient distributions, we predicted the surface relaxivity of different rock types. We found that surface relaxivities of Weber Sandstone samples can be well predicted, while for Madison Limestone samples, surface relaxivities are overestimated due to diffusive pore coupling effect. Time‐domain NMR characterization of CO2storage site was conductedInvestigation of petrophysical characterization and pore structure using restricted diffusion and T2distributions yielded physically meaningful relationshipsDetermination of variability of surface relaxivity by comparing diffusion distributions and T2distributions was successfully obtained

We explored the influence of a model organic ligand on mineral carbonation in nanoscale interfacial water films by conducting five time-resolved in situ X-ray diffraction (XRD) experiments at 50 °C. Forsterite was exposed to water-saturated supercritical carbon dioxide (90 bar) that had been equilibrated with 0–0.5 mcitrate (C6H5O7–3) solutions. The experimental results demonstrated that greater concentrations of citrate in the nanoscale interfacial water film promoted the precipitation of magnesite (MgCO3) relative to nesquehonite (MgCO3·3H2O). At the highest concentrations tested, magnesite nucleation and growth were inhibited, lowering the carbonation rate constant from 9.1 × 10–6to 3.6 × 10–6s–1. These impacts of citrate were due to partial dehydration of Mg2+(aq) and the adsorption of citrate onto nuclei and magnesite surfaces. This type of information may be used to predict and tailor subsurface mineralization rates and pathways.

Fluid-rock interactions may differ in mesopores (2–50 ​nm diameter) compared to macropores (>50 ​nm diameter). This study evaluates the influence of nano-confinement and pore geometry on point of zero charge (PZC) and hence surface charge and adsorption. PZC was determined for a suite of synthetic mesoporous amorphous silica materials and compared to the PZC of macroporous and non-porous amorphous silica. Reactive molecular dynamics simulations using ReaxFF determined surface equilibrium constants (pK) and clarified the large range of PZC values reported for macroporous and non-porous amorphous silica. Potentiometric titrations measured PZC and pKvalues of three mesoporous amorphous silica materials (SBA-15, SBA-16, and MCM-41) possessing a range of pore diameters (∼4–13 ​nm) and distinct pore geometries (cylindrical pores in a hexagonal lattice, spherical pores in a body-centered-cubic lattice, and hexagonal pores arranged in a hexagonal lattice); fluids were pre-saturated with silica to inhibit reaction with pore walls. Results are integrated with data from more than 150 published experimental studies. Neither nano-confinement nor pore geometry affects the PZC of the mesoporous amorphous silica materials. These results have implications for adsorption chemistry under confinement in mesoporous siliceous natural systems such as shales.

Manganese (Mn) exists as Mn(II), Mn(III), or Mn(IV) in soils, and the Mn oxidation state controls the roles of Mn in numerous environmental processes. However, the variations of Mn oxidation states with climate remain unknown. We determined the Mn oxidation states in highly weathered bulk volcanic soils (primary minerals free) across two rainfall gradients covering mean annual precipitation (MAP) of 0.25–5 m in the Hawaiian Islands. With increasing MAP, the soil redox conditions generally shifted from oxic to suboxic and to anoxic despite fluctuating at each site; concurrently, the proportions of Mn(IV) and Mn(II) decreased and increased, respectively. Mn(III) was low at both low and high MAP, but accumulated substantially, up to 80% of total Mn, in soils with prevalent suboxic conditions at intermediate MAP. Mn(III) was likely hosted in Mn(III,IV) and iron(III) oxides or complexed with organic matter, and its distribution among these hosts varied with soil redox potentials and soil pH. Soil redox conditions and rainfall-driven leaching jointly controlled exchangeable Mn(II) in soils, with its concentration peaking at intermediate MAP. The Mn redox chemistry was at disequilibrium, with the oxidation states correlating with long-term average soil redox potentials better than with soil pH. The soil redox conditions likely fluctuated between oxic and anoxic conditions more frequently at intermediate than at low and high MAP, creating biogeochemical hot spots where Mn, Fe, and other redox-sensitive elements may be actively cycled.

Changes in pore (throat) size, surface roughness, and mineralogy induced by supercritical CO2‐water‐rock reactions impact petrophysical properties such as porosity, permeability, and especially wettability. Herein, we show that these changes directly impact relative permeability and capillary pressure curves, a fact rarely studied in the literature. In this work, we show that CO2contact angle changes emerge after Madison Limestone samples were soaked for 400 hr in CO2‐enriched brine. Coreflooding results show that the water production rate and cumulative water production increased after the rock was exposed to carbonic acid. Moreover, the mercury capillary pressure decreased in mesopores and macropores, indicating the increase of size in these pores due to reactions. This compounded wettability and pore network alteration can directly affect CO2injectivity, migration, and storage capacity. This fundamental insight into CO2geological storage processes should aid practitioners to reduce uncertainties in forecasting CO2distribution via injection simulation. Emitting a large amount of greenhouse gases into the atmosphere has disrupted the global carbon cycle. CO2captured from various sources could be transported to a close‐by site for injection into deep saline aquifers or oil/gas reservoirs for storage or/and enhanced oil recovery. Reactions between the rock and the carbonic acid can alter the rock fabric. These reactions can change rock properties, for example, pore structure, porosity, permeability, and wettability. These changes can affect capillary pressure and relative permeability. These rock alterations can significantly impact the fate of the injected CO2. Despite efforts made to determine changes in pore architecture or petrophysical properties, a few studies have investigated the impact of rock alteration on multiphase flow properties, namely, capillary pressure and relative permeability. Even less is understood on this subject when diffusion dominates over convective transport. Here we focused on changes in pore (throat) size distribution and wettability, and dynamic properties, as induced by geochemical reactions. We observed pore enlargement among the larger pores (mesopores to macropores) due to mineral dissolution, which contrasts with a decreased size in smaller pores (micropores), resulting from mineral precipitation. These processes lead to a more CO2surface wetting and thereby capillary pressure and relative permeability. CO2‐brine‐rock reactions at reservoir conditions improve CO2wettabilityGeochemical reactions alter the rock pore architecture and multiphase flow responseMultiphase flow alterations correlate well with changes in wettability and pore structure