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

Abstract

Current hydraulic fracturing techniques rely on fresh water use for unconventional reservoir stimulation. The length of horizontal wells is increasing which has led to larger volumes of fresh water needed to be transported to drilling sites. 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. This study assesses the effects of pH and ionic strength on fluid-rock interactions associated with saline hydraulic fracturing fluids. Frontier Formation (consisting of interbedded shales and sandstones) core samples 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 formation waters that naturally exist in the Frontier Formation. 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 (239°F) and 350 bar (5075 psi) for 28 days to replicate in-situ reservoir 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 pH and ionic strength variation.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016