Geochemical reactions and alteration of pore architecture in saturated shale after injection of stimulation fluid

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

“…An immediate increase of aqueous magnesium was universally observed in experimental studies of shale-water interactions, but subsequent behavior varied. Aqueous magnesium continued to increase in most experiments, ,,,,, but the rate of increase slowed to steady state concentrations in several. ,,,, Concentrations of aqueous magnesium declined after initially peaking in two studies. , …”

“…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 .…”

“…Previously we showed that the reaction of reservoir rock with hydraulic fracturing fluid formulated from a natural water (river water with ionic strength 0.05 mol/kg) dissolves feldspar as well as carbonate minerals and precipitates an Mg-aluminosilicate. − Feldspar also began to dissolve in experiments that reacted mudstone with hydraulic fracturing fluid formulated from river water ionic strength 0.005 mol/kg and mixed with saline formation water ionic strength 2 mol/kg . In contrast, the reaction of mudstone with hydraulic fracturing fluid formulated from both a natural spring water (ionic strength 3.1 × 10 –4 mol/kg) and a produced water (ionic strength 0.5 mol/kg) yielded no evidence of silicate reaction. , Five other studies reacted mudstone with hydraulic fracturing fluid formulated from a natural water; one provided temporal water chemistry needed to evaluate fluid-mineral processes and four did not. ,,, Thus, the importance of silicate–fluid equilibria relative to carbonate–fluid equilibria is not known for reactions between reservoir rock and hydraulic fracturing fluid formulated from a natural water.…”

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

“…In the past several years, laboratory experiments have been performed at ambient and reservoir conditions to evaluate geochemical and mineralogic interactions between unconventional reservoirs and stimulation fluids used in hydraulic fracturing (Supplemental Table 1, attached spreadsheet). Most studies document mineral dissolution and precipitation as a consequence of these interactions [1][2][3][4][5][6][7][8][9][10][11][12][13][14] .…”

“…Powder River Basin, Wyoming, USA 3 ; the Utica and Point Pleasant Shales of the Appalachian Basin, USA 17 ; and the Baxter Shale of the Green River Basin, USA 8,24 . Although not an unconventional reservoir, experiments have evaluated the Green River Shale of the Green River Basin, USA as a comparator to unconventional shale reservoirs 2,7,22 .…”