Comparison of micro- and macro-wettability measurements and evaluation of micro-scale imbibition rates for unconventional reservoirs: Implications for modeling multi-phase flow at the micro-scale

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Significant advances have been made in the evaluation of micro- and nano-scale variations of pore structure in low-permeability (unconventional) reservoirs using imaging techniques. In parallel, digital rock physics (DRP) methods have been advanced so that pore structure information extracted from these images may be used, in combination with pore-scale modeling, to predict critical petrophysical rock properties such as porosity and permeability. Recent work using DRP applied to multi-phase flow of unconventional reservoirs has suggested that wettability variations at the micro-scale caused by mineralogical heterogeneity or aging can have a profound effect on simulated capillary pressure and relative permeability curves. However, wettability is typically measured at the macro-scale, with the resulting contact angles used for populating DRP models. Further, fluid imbibition rates are usually measured at the macro-scale to compare, for example, the impact of fracturing fluid additives on oil recovery.This study compares water contact angle measurements made at the micro-scale, using an environmental field emission scanning electron microscope (E-FESEM) combined with innovative procedures for contact-angle extraction, with a conventional macro-scale approach (sessile drop) for low-permeability samples obtained from the Montney Formation in Western Canada. For the first time, quantification of imbibition rates at the micro-scale is demonstrated. Two micro-wettability evaluation procedures developed previously are applied to these samples to evaluate water micro-contact angles: 1) imaging of condensation/evaporation experiments and 2) imaging of injected fluids using a micro-injection system. Micro contact angles were first estimated by extracting sessile droplet profiles (with user-guided software developed in-house) and then fitting a parameterized Young-Laplace equation to the droplet profile. For some of the micro-injection experiments, the geometry of the micro-droplet, as captured with the parameterized Young-Laplace equation, was used to compute the volume of the micro-droplet at different stages of imbibition, which in turn was used to evaluate imbibition rates at this scale. Macro contact angles were evaluated using the parameterized Young-Laplace equation and commercial software.This study suggests that laboratory-derived macro-droplet contact angles cannot be confidently and consistently applied at the micro-scale for use in DRP models for tight heterogeneous formations such as the Montney. Significant errors in simulating fluid displacement processes, fluid saturation distributions, capillary pressure and relative permeability curves using DRP methods will result if micro-scale variations in wettability are not taken into account. Finally, the study demonstrates that fluid imbibition measurements may be performed at the micro-scale, enabling fine-scale rock composition/pore structure controls on fluid imbibition to be explored.