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

Abstract

A detailed study on geochemical processes following hydraulic fracturing can provide important information on the origin of solutes and potential improvement of fracturing technology. However, this remains difficult due to the low resolution of flowback water and high salinity of formation water. To fill this knowledge gap, a shale-gas well was drilled and freshwater was used to fracture the shale. In parallel, laboratory water-rock interaction experiments were conducted. The intensive sampling for flowback water and the use of multiple isotopes provided novel and detailed insights into the water-rock interactions after hydraulic fracturing. The results showed that beyond mixing processes, cation exchange, adsorption/desorption, and barite precipitation were observed both in the laboratory and field studies. Although oxidation of pyrite was observed in most of the laboratory experiments, our findings demonstrate that this process was not evident in field flowback samples that were dominated by mixing of fracturing fluids and formation water. Plain Language Summary The hydraulic fracturing technologies have paved the way toward the development of unconventional shale gas from low-permeability shale reservoirs. The solutes in flowback water after hydraulic fracturing of shale gas reservoirs can originate from injected fracturing fluids and formation water, but may also be derived from solutes liberated by geochemical reactions. However, in most cases, the laboratory data do not match the field data from hydraulically fractured wells. To date, a further refined understanding of geochemical reactions after hydraulic fracturing occurred is critical for tracing the origin of solutes in flowback water (as the main potential pollutants for near-surface environments) and potentially improving the fracturing technology. Except for reported cation exchange and carbonate dissolution, this study showed several new findings, including nitrate adsorption/desorption and limited oxidation of pyrite in field flowback samples that were dominated by mixing of fracturing fluids and formation water, although oxidation of pyrite was observed in most of the laboratory experiments and expected to be comprehensive during field flowback after hydraulic fracturing. Therefore, the potential creation of matrix porosity induced by water-rock interactions of oxidation of pyrite and further induced dissolution of minerals may be limited.

Potential Impacts of Shale Gas Development on Inorganic Groundwater Chemistry: Implications for Environmental Baseline Assessment in Shallow Aquifers

“…Significant volumes of flowback and produced waters are generated during shale gas extraction (hundreds to tens of thousands of m 3 per well). , Flowback waters return to the surface a few days to a few weeks following hydraulic fracturing and prior to gas production, while produced water is coproduced continuously with the gas once the well is placed into production. , The distinction between flowback and produced waters is neither obvious nor clearly defined, and most studies have reported flowback/produced water data without distinction. , Flowback and produced waters generally consist of highly mineralized Na-(Ca)-Cl waters (thousands to hundreds of thousands ppm total dissolved solids or TDS) with high levels of alkaline earth elements (Ca, Sr, Ba, Ra), alkali elements (Na, K, Li), and halides (Cl, Br) and moderate to low concentrations of dissolved inorganic carbon and sulfate (Figure ). The salinity of flowback/produced water rapidly increases with time after hydraulic stimulation, which is attributed to an increasing proportion of natural formation brines. − Saline formation waters are commonly found at depth in sedimentary basins and likely originated as evaporated seawater further modified by water–rock interactions and dilution processes in the shale gas reservoir itself or in adjacent units. − …”

“…Although there is no WHO guideline for Mn and Sr, concentrations as high as tens of ppm Mn and thousands of ppm of Sr may pose a risk to human health , (Figure ). Elevated concentrations of the alkaline earth elements Sr, Ba, and Ra in flowback/produced waters have been attributed to (1) the limited precipitation of sulfate minerals (e.g., barite, celestite) due to strongly reducing conditions and elevated temperatures in the reservoir and (2) the increased competition for sorption sites on clay minerals and organic matter due to high ionic strength of the fluids. ,, There is still a debate whether Ba and Ra in flowback/produced water are derived from formation brines or are released from black shales during hydraulic fracturing. − Elevated B concentrations are thought to result either from carbonate dissolution or desorption from clays. , To our knowledge, no information is currently available regarding the mobilization of F and Mn in flowback/produced waters.…”

“…Flowback/early produced waters have been found to contain elevated concentrations of trace metals/metalloids such as As, Cd, Ni, Pb, Sb, and Se (Figure ). The release of inorganic constituents during shale–fluid interaction as a result of the hydraulic fracturing process has been mainly investigated through laboratory experiments ,,,,− and recently by exploring the geochemical and isotopic composition of flowback/early produced water (δ 34 S–SO 4 , δ 18 O–SO 4 , δ 2 H-δ 18 O of water, δ 15 N–NO 3 , δ 18 O–NO 3 , δ 15 N–NH 4 , δ 13 C-DIC, 14 C-DIC, δ 7 Li, δ 11 B, 87 Sr/ 86 Sr). ,, Most of the experimental studies have used geochemical and/or mineralogical characterization combined with batch experiments on core samples to assess major controls on element mobility including shale mineralogy, solution pH, redox conditions, ionic strength, solid to liquid ratio, and presence of specific chemical additives. Batch leachate and flowback water chemistries suggested that the oxidation of pyrite and organic matter may release trace metals/metalloids in the early stage of hydraulic fracturing ,,,,,, (Figure ).…”

“…62−64 Elevated B concentrations are thought to result either from carbonate dissolution 65 or desorption from clays. 53,66 To our knowledge, no information is currently available regarding the mobilization of F and Mn in flowback/produced waters.…”

Soil Column Experimental Study on the Effect of Soil Structure Disturbance on Water Chemistry

“…However, it has been proved that soil disturbance can significantly affect the major ions in the water environment [ 13 ]. Previous studies have analyzed the influence of disturbed soil on the SO 4 content in groundwater through water–rock interactions, such as SO 4 -containing evaporite dissolution and pyrite oxidation [ 16 , 27 ]. However, the detailed causes and mechanisms of water–rock interaction have not been discussed.…”