EFFECT OF IONIC STRENGTH 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. However, the cost of large volumes of fresh water is increasing due to the lengthening of laterals and fresh water transportation expenditures. The ability to replace fresh water with a more saline water could potentially decrease these costs. Research on scaling potential and fluid additive effectiveness exists, but little research addresses fundamentals of water-rock interactions that occur in these systems as a function of ionic strength.

The goal of this study is to assess the effects of varying ionic strength on fluid-rock interactions associated with saline hydraulic fracturing fluids. Frontier Formation core samples used in experiments were collected from the Hornbuckle 1-11H well within the Powder River Basin of Wyoming. In this locality, the Frontier Formation consists of interbedded shales and sandstones. A simplified fracturing fluid was constructed based on information retrieved from the Hornbuckle 1-11H completion report and includes HCl (15%), methanol, a clay stabilizer, and an iron chelating agent. Certain chemicals were omitted due to experimental limitations, laboratory safety, and component accessibility. The saline water (I = 0.15) used as the fracturing fluid’s mixing water was modeled to mimic formation waters that naturally exist in the Frontier Formation.

Initial experiments react rock samples and hydraulic fracturing fluids with varying salinities in flexible reaction cells. Experiments are conducted at 115°C and 350 bar for 28 days to replicate in-situ conditions. Fluid samples collected from the reaction cells during the 28 days were analyzed for total dissolved carbon by coulometric titration, anions by IC, and major, minor, and trace cations by ICP-OES. Once experiments reached completion, reacted rock samples were collected for mineral analysis by SEM-EDS, XRD and XRF. A combination of experimental data and geochemical models provide insight into mineral reactivity and fluid chemistry development due to ionic strength variation.