Effects of Dissolved Oxygen on Water Imbibition in Gas Shales

Abstract

Understanding the water uptake of gas shales is critical for designing and optimizing hydraulic fracturing operations during which a large volume of fracturing water containing dissolved oxygen is injected into tight reservoirs. Recent studies show that the dissolved oxygen may promote oxidation reactions which can affect salinity and pH value of flowback water; however, the effects of dissolved oxygen and oxidation reactions on water imbibition into the shale matrix and on the concentration of individual ions in flowback water are still poorly understood. In this study, we conduct water imbibition experiments under degassed and oxic conditions, and measure the imbibed water mass and concentrations of different ions in water. The results show that the initial rate and final amount of water imbibition are higher under degassed conditions compared with that under oxic conditions. These differences are mainly due to the enhanced dissolution of air in the shale pore network into the imbibing water under degassed conditions and the consequent increase in relative permeability of water. The results also suggest that oxidation of pyrite by dissolved oxygen produces sulfate and iron ions. The abundance of pores in the vicinity of pyrite minerals as observed in the SEM/EDS images provides a pyrite, water, and oxygen rich environment which favors pyrite oxidation.

Identification of Geochemical Processes During Hydraulic Fracturing of a Shale Gas Reservoir: A Controlled Field and Laboratory Water‐Rock Interaction Experiment

“…Because O 2 is present in FRW and atmospheric O 2 is entrained in the injected FRW (Xu et al, 2018; Zolfaghari et al, 2016) and other oxidizers (such as a persulfate breaker or H 2 O 2 ) are added to fracturing fluids at shale gas sites (Osselin et al, 2019; Phan et al, 2020), and because there are always pyrite minerals in shales (Heller & Zoback, 2014), oxidation of pyrites after hydraulic fracturing and during initial flowback is very likely (Osselin et al, 2019; Paukert Vankeuren et al, 2017; Xu et al, 2018; Zolfaghari et al, 2016). Laboratory experiments have revealed evidence for oxidation of pyrites (Pearce et al, 2018; Xu et al, 2018; d and e in Figure 4), which would contribute additional SO 4 to the FBW. However, in our field experiment, even an additional 10 mg/L of SO 4 derived from the oxidation of pyrite in the shale at the initial stage of flowback (158 + 10 mg/L) would deviate from the mixing line (g in Figure 4), suggesting that the data did not provide evidence for pyrite oxidation.…”

“…However, FBW samples do not plot perfectly on a Rayleigh fractionation trend for sulfate reduction (process a, b, and c in Figure 4) with S isotope enrichment factors ranging from −60‰ to −9‰ (Krouse & Mayer, 2000; Mayer et al, 2007). Because O 2 is present in FRW and atmospheric O 2 is entrained in the injected FRW (Xu et al, 2018; Zolfaghari et al, 2016) and other oxidizers (such as a persulfate breaker or H 2 O 2 ) are added to fracturing fluids at shale gas sites (Osselin et al, 2019; Phan et al, 2020), and because there are always pyrite minerals in shales (Heller & Zoback, 2014), oxidation of pyrites after hydraulic fracturing and during initial flowback is very likely (Osselin et al, 2019; Paukert Vankeuren et al, 2017; Xu et al, 2018; Zolfaghari et al, 2016). Laboratory experiments have revealed evidence for oxidation of pyrites (Pearce et al, 2018; Xu et al, 2018; d and e in Figure 4), which would contribute additional SO 4 to the FBW.…”

“…The reactive minerals (e.g., pyrite, carbonates, organic matter, and clay) are widely distributed in most shale units (Heller & Zoback, 2014; Phan et al, 2020). A series of laboratory water and rock interaction experiments were conducted in the past to illustrate the geochemical processes following hydraulic fracturing (e.g., Harrison et al, 2017; Xu et al, 2018; Zeng et al, 2020). Geochemical processes observed in the laboratory included the oxidation of pyrite, dissolution of carbonates, and precipitation of specific minerals, including aluminosilicate (Herz‐Thyhsen et al, 2019), Fe oxyhydroxide (Lu et al, 2017; Zolfaghari et al, 2016), and barite (Li et al, 2019; Paukert Vankeuren et al, 2017).…”

Pore Structure Alteration Characteristics of Different Mineralogical Composition Shale during Shale-Fracturing Fluid Physical-Chemical Interactions

“…Previously, studies showed that the dissolved oxygen in the fracturing fluid could create an oxic condition in the shale reservoirs [19,33]. Pyrite (FeS 2 ) is a reductive sulfide with iron (II) which is stable only at very low oxidation states, and the pyrite in shale can be easily oxidized under the atmospheric oxygen and water environment [24,34].…”

“…Although experiments of water-rock interactions at the laboratory scale offered valuable preliminary data on physical-chemical interactions between shale and fracturing fluid, most of these imbibition experiments were performed at low temperature and low pressure [15][16][17]. Considering that the reservoir usually has high-temperature and high-pressure conditions [18], the experiments performed at reservoir temperature and pressure are more approximate conditions in the field [19].…”