Application of isotopic and geochemical signals in unconventional oil and gas reservoir produced waters toward characterizing in situ geochemical fluid-shale reactions

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

“…There was an increasing trend in B concentrations beyond mixing for FBW (Figure 2l), which was caused by the desorption from clays (Phan et al, 2020; Warner et al, 2014). In the WRI experiments, B concentrations showed an increasing trend with increasing WRI time (Tables S3 and S4).…”

“…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.…”

“…Although the mixing process between fracturing water (FRW) and FOW appears to be dominant for the water chemistry change of FBW, WRIs between water and shale may be expected to be significant in the early stage of flowback (e.g., Phan et al, 2018; Zolfaghari et al, 2016). 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).…”

“…Also, previous research suggested that most WRI may occur at the very early stage of flowback and that mixing becomes increasingly dominant in controlling the chemical and isotopic composition in flowback at the late stage (Kondash et al, 2017). Therefore, daily sampling at the initial stage after hydraulic fracturing (Osselin et al, 2019; Phan et al, 2020) may not be sufficient for characterizing geochemical processes in detail. In addition, when the water chemistry of FBW is mainly controlled by the mixing process between FRW and FOW with very high salinity (tens to hundreds of g/L), the geochemical processes like cation exchanges and adsorption/desorption occurring at the very early stage of flowback may be masked.…”

“…In addition, when the water chemistry of FBW is mainly controlled by the mixing process between FRW and FOW with very high salinity (tens to hundreds of g/L), the geochemical processes like cation exchanges and adsorption/desorption occurring at the very early stage of flowback may be masked. To date, a further refined understanding of geochemical reactions after hydraulic fracturing occurred is critical for tracing the origin of solutes in FBW (as the main potential pollutants for near‐surface environments), improving the fracturing technology (Phan et al, 2020; Rosenblum et al, 2017), and extending knowledge of hydrogeochemical processes in deep low‐permeability reservoirs.…”