Critical Elements Extraction from Flowback and Produced Water: From Lab to Field

Abstract

Flowback and produced water (FPW) from multi-fractured horizontal wells (MFHWs) are possible sources of critical elements (CE) from unconventional hydrocarbon reservoirs. The objective of this study is to compare temporal variations of CE (Li, Mg) concentrations in FPW at lab- and field-scales, with examples from prominent Canadian unconventional hydrocarbon plays. A secondary objective was to evaluate whether CE could be extracted (i.e. ‘leached’) from reservoir rocks by FPW. Quantifying elemental leaching is important for reserves evaluation and identifying the relative importance of mechanisms contributing to CE enrichment in FPW (e.g., fluid mixing vs. fluid-rock interaction). High-temperature (150 °C), high-pressure (2200 psi) fluid-rock interaction experiments were conducted on three crushed-rock Montney (siltstones/sandstones) and Duvernay (organic/clay-rich shales) samples with variable composition, fabric, and reservoir quality. Time-lapsed fluid analysis (+30 days), using spectroscopy and ion chromatography (ICP-OES/IC) enabled observations of Li and Mg concentration profiles at the lab-scale. Lab-scale Li and Mg concentration profiles were then compared to post-fracture Li and Mg concentration profiles from multiple MFHWs completed in the Montney and Duvernay formations (public data). At the lab-scale, maximum measured Li concentrations for the Montney and Duvernay samples were 0.27 mg/L and 0.53 mg/L, respectively. Maximum lab-scale Li recoveries were significantly (about two orders of magnitude) smaller than those measured in the field (28-72 mg/L for the Montney wells, 26-54 mg/L for the Duvernay wells). This could be attributed to the 1) dominance of the fluid mixing mechanism in the field, relative to fluid-rock interaction, 2) variable rock-water mass ratios at lab and field scales, and/or limited (initial) content of Li in the analyzed samples, amongst other factors. Lab-scale Li and Mg concentration profiles exhibited similarities to and discrepancies with those observed in the field. Notably, larger Li concentrations (up to twice) were associated with lower pH, in agreement with field observations. Interestingly, lab-scale Li and Sr concentrations appear to co-vary for the Duvernay FPW, in agreement with field observations, suggesting the possibility of using Sr as a ‘proxy element’ for predicting Li anomalies in the Duvernay FPW. Quantifying temporal evolution of CE concentrations in FPW is essential for evaluating the feasibility of CE recovery from MFHWs and the selection of optimal Li extraction technologies over the well lifetime. This study provides the first-time comparison between lab- and field-scale temporal variations of CE concentrations in FPW for the purpose of evaluating CE extraction from unconventional hydrocarbon reservoirs.