Pore-Scale Comparisons of Primary Drainage Techniques on Nonwater-Wet Reservoir Rocks

Initial water saturation (Swi) is a critical parameter that significantly influences multiphase flow properties, wettability, and the prediction of reservoir dynamics during fossil fuel energy recovery, non-aqueous phase liquid (NAPL) depollution, or CO2 storage campaigns. In a recent study (Nono et al., 2022), we investigated the differences between the commonly used initialization methods and the impact of various laboratory protocols on achieving Swi at the pore scale. The study primarily focused on the primary drainage of water-wet Bentheimer sandstones, with some preliminary observations made on initial nonwater-wet Bentheimer samples. The study revealed several unexpected artifacts. Building upon the previous work, the present study employs three-dimensional (3D) X-ray microtomography to extend the investigations to reservoir rocks that are not water-wet at the beginning of primary drainage. Specifically, the porous plate (PP) and oilflooding (OF) techniques are the focus. The study examines Swi average values and profiles, pore occupancies, and effective permeabilities. Several key observations are made. (i) The PP technique always leads to lower Swi values than the OF technique, even when both techniques are performed with the same inlet pressures. It is argued that these differences may depend on both the pressure buildup until the saturation front, which is higher for the PP technique compared to the OF technique, or the reversal flow, which may re-inject trapped fluids from the diffusers’ dead volumes into the sample. The latter is mainly a function of the rock’s wettability regarding the invading fluid (Nono et al., 2019). (ii) The study highlights the differences in pore occupancies when comparing different initiation methods on oil-wet reservoir rocks. Important parameters to consider here are how wettability is initially distributed in the sample and the connectivity of the macropores. The findings highlight the not-so-well-expected complexities associated with Swi initialization, pore occupancies, and flow behavior. Understanding these aspects is crucial for accurate reservoir characterization and improved decision making in oil and gas exploration and other related applications, and the digital rock physics (DRP) tool can provide insights to predict these behaviors.