Geochemistry in an Unconventional Oil Reservoir: Experimental Insights Into Effects of Fluid-Rock Interactions on Pore-Lining Minerals

Zandanel, Amber

doi:10.1130/abs/2016am-285490

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“…One hydrothermal experiment reacted this hydraulic fracturing fluid with samples of core from the Wall Creek at reservoir conditions (115 °C and 35 MPa, Table ); this experiment is designated “Intermediate I” (Table and Figure ). Hydraulic fracturing fluid and rock were added together to the reaction vessel at the outset of the experiment, before heating and pressurizing to reservoir conditions, an approach commonly used to investigate geochemical interactions between hydraulic fracturing fluids and rocks. ,,,− ,, An alternative approach is to inject hydraulic fracturing fluid into an ongoing experiment containing formation water and rock at reservoir conditions. , These two experimental approaches represent a continuum between dry formations and formations that produce significant amounts of water. A water–rock ratio of approximately 20:1 was used to represent conditions present in a large open fracture created during hydraulic fracturing .…”

Section: Methodsmentioning

confidence: 99%

Ionic Strength and pH Effects on Water–Rock Interaction in an Unconventional Siliceous Reservoir: On the Use of Formation Water in Hydraulic Fracturing

et al. 2021

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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.

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