This paper presents an experimental study and comparison of the wettability of the organic-rich carbonate shale from Eagle Ford (EF) and Middle East (ME) formations. Rock samples used have the similar mineralogical compositions (mainly composed of calcite) and total organic carbon (TOC) contents. However, our experiments demonstrate that their wetting affinity of oil and water is completely different.

We conduct spontaneous imbibition experiments using 5% KCl brine and diesel to compare the wetting behavior of EF and ME samples. Plugs are drilled perpendicular or parallel to the beddings, and referred as the vertical and horizontal samples, respectively. Each plug is further cut into two smaller twin plugs. One of them imbibes the diesel and the other twin plug imbibes the brine for varied times in the regular imbibition tests. Later they are exchanged to imbibe the other fluid in their reversed imbibition. The rate and amount of fluid imbibed are determined from the nuclear magnetic resonance (NMR) T2 measurements.

Both EF and ME samples imbibe the brine and diesel, which indicates a mixed wettability for these organic-rich shale. The NMR T2 signature of the imbibed brine and diesel occurred mostly at relaxation times faster than their bulk relaxation time, respectively, indicating the dominance of surface relaxation. ME samples imbibe the diesel faster than the brine, while EF samples imbibes the brine faster than the diesel. For ME samples, the brine is observed to percolate continuously into large pores while the diesel invades into small pores. The fractures (or gaps) between thin layers of ME sample are interpreted as large pores in the NMR T2 spectra and are strongly hydrophilic-wet. For EF samples, the opposite imbibition behavior is observed: the brine imbibes continuously into small pores while the diesel percolates into large pores. In addition, EF horizontal samples imbibe both brine and diesel much faster than the vertical samples.

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