Abstract
In this study, we review the concepts of capillary pressure and total suction. We propose that for shales and siltstones, capillary pressure (matric suction) is not the only dominant source of suction. Therefore, total suction, which is the sum of matric and osmotic pressure (electrostatic forces), provides a better description of the physical process behind water imbibition. We investigate the application of a dew-point psychrometer to measure total suction and compare that to mercury injection capillary pressure (MICP) measurements. The MICP method, traditionally used in energy industry, does not capture the impact of pores smaller than 3 nm. It is also limited to measuring only matric suction, ignoring electrostatic forces. Our results indicate that the suction values typically assumed in the literature from MICP tests significantly underestimate the total suction in shales and siltstones. Total suction for samples from the Montney Formation in Canada can be as high as 250 MPa at water saturations between 1 and 5%. The magnitude of suction measured in this work explains the significant water uptake by this class of reservoirs during hydraulic fracturing. Total suction in the present experiments correlate well to mineralogy and porosity of the samples. The results of this paper suggest that measurements of capillary pressure in unconventional reservoirs are generally inappropriate to describe the behavior of rocks. The order of magnitude of suction reported here has major implications in modelling basin-wide fluid migration, water imbibition into tight rocks, and describing rock's mechanical response after hydraulic fracturing.
Published Version
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