EFFECT OF IONIC STRENGTH (SALINITY) AND PH (ACIDITY) ON GEOCHEMICAL WATER-ROCK INTERACTIONS DURING HYDRAULIC FRACTURING IN THE FRONTIER FORMATION OF THE POWDER RIVER BASIN, WYOMING

Current hydraulic fracturing techniques rely on fresh water use for unconventional reservoir stimulation. Water transportation expenditures along with produced and flow-back water treatment can become quite costly but the ability to replace fresh water in a hydraulic fracturing operations with saline water could potentially mitigate these costs. Reuse of wastewaters in hydraulic fracturing has been researched in the past but it is our belief that fluid-rock interactions play a significant role in the fluid chemistry evolution and has generally been overlooked.

This study assesses the effects of pH and ionic strength on fluid-rock interactions associated with saline hydraulic fracturing fluids. Frontier Fm. core samples (consisting of interbedded shales and sandstones) used in experiments were collected from the Hornbuckle 1-11H well within the Powder River Basin of Wyoming. A simplified fracturing fluid was constructed based on information retrieved from the Hornbuckle 1-11H completion report and includes HCl, methanol, a clay stabilizer, and an iron chelating agent. The saline water used as the fracturing fluid’s mixing water was geochemically modeled to represent fm. waters that naturally exist in the Frontier Fm.

Experiments react core samples and hydraulic fracturing fluids at ionic strengths of ~0.015, ~0.15, and ~1.5 molal as well as neutral and low pH at 115°C (~240°F) and 350 bar (~5000 psi) for 28 days to replicate in-situ reservoir conditions. Results show significant changes in the release of Ca²⁺, K⁺, Mg²⁺, Sr²⁺, Li⁺, and SiO₂ (aq) from the Frontier Fm. Low starting pH as well as high starting ionic strength dissolves the most carbonates and feldspars. K⁺ release from Illite clays is enhanced with higher initial ionic strengths and shows no effect from starting pH. Mg²⁺ trends are similar to K⁺, however significant removal from solution occurs in neutral pH conditions. SiO₂ (aq) release is fastest in acidic pH conditions and is unaffected by initial ionic strength. By combining experimental data with geochemical models, insight into mineral reactivity and fluid chemistry development due to pH and ionic strength variation is possible and coupling these findings to already existing research has the potential to optimize well production in the future.