Insights from mixing calculations and geochemical modeling of Montney Formation post hydraulic fracturing flowback water chemistry

The major ion chemistry of the flowback water from 24 wells completed in the Montney Formation in northeastern British Columbia and northwestern Alberta provides insights into the mixing of formation water and fracturing fluid as well as fluid-fluid and fluid-rock interactions during well completions. We use chloride concentrations and oxygen and hydrogen isotopes as conservative tracers to calculate the proportion of formation water throughout the flowback period. During the flowback period prior to the wells being put on production, between 5% and 33% of the equivalent volume of injected fluid is recovered. Based on conserved element mixing calculations only a fraction of the flowed back volume is the injected completion fluid, the remainder being formation water. The proportion of formation water in the flowback fluid at the commencement of the flowback period for most of the study wells ranges between 10 and 35% and increases to approximately 40-60% at the end of the flowback period. Such results indicate that, although the majority of the fracturing fluid is retained in the reservoir, due to imbibition, at the same time, significant connate formation water is produced from the same reservoir. These results provide unique insights into the complex reservoir dynamics of multistage hydraulically fractured wells. The extent to which simple mixing between connate formation water and fracturing fluid accounts for the concentrations of major ions (Na, Ca, K, Mg, Sr and SO4), in the flowback fluids can be assessed by comparing the ion concentrations to the mixing lines derived from the conservative tracers in each well. In wells where the measured ion concentrations are not in agreement with concentrations predicted from simple mixing calculations, different geochemical processes were sequentially evaluated in an attempt to match and interpret the measured major ion concentrations. Na and K concentrations in the flowback water is primarily due to simply mixing, whereas Ca, Mg, and Sr are impacted by ion exchange in addition to mixing. Calcite dissolution or precipitation has a minor effect on Ca concentrations. Sulfate concentrations are less explicable and are likely related to pyrite oxidation and sulfate reduction due to the introduction of oxygenated fluids during drilling and completion. Overall all mixing of connate formation water and fracturing fluid is the dominant determinant of flowback water chemistry but fluid-fluid and fluid-rock geochemical reactions occurring in the subsurface are also variably important.