A Multiscale Study on Shale Wettability: Spontaneous Imbibition Versus Contact Angle

Abstract

Environmental and technical issues associated with hydraulic fracturing fluid leak-off and its low recovery in clay-rich shale reservoirs has challenged the energy sector. One of the main hypothesized mechanisms for the uptake of fracturing fluid is fluid imbibition due to capillary forces that are a strong function of the rock's wettability. However, shale wettability estimated by contact angle and spontaneous imbibition experiments are highly inconsistent throughout the literature. It is therefore critical to shed light on the possible reasons for such discrepancies and identify the superior technique. In order to characterize the physical processes from microscale to macroscale, the contact angles and spontaneous imbibition characteristics of shale samples were measured using aqueous ionic solutions (NaCl, KCl, CaCl 2 , and MgCl 2 ) at different concentrations (0, 0.1, and 0.5 M) and oil (Soltrol-130). The internal structure of the samples exposed to deionized water was imaged by three-dimensional X-ray micro-computed tomography before and after spontaneous imbibition at both unconfined and confined stress states. Additionally, the structure of all samples exposed to each ionic solution was analyzed via environmental scanning electron microscopy. Experimental observations were further substantiated by combining digital image correlation with Poisson-Boltzmann analysis to assess the hydration-induced strains. It was shown that while spontaneous imbibition is a more realistic measurement for liquids, results obtained under unconfined condition can be highly biased due to additional forces acting (e.g., osmotic or hydration forces) and also potential microstructural alterations. Thus, spontaneous imbibition measured under reservoir stress condition is strongly recommended for shale wettability estimation.

Direct Evidence of Salinity and pH Effects on the Interfacial Interactions of Asphaltene-Brine-Silica Systems

“…It is important to explore the wettability alteration mechanism in improving oil recovery, especially to find an effective method to improve the wetting property of reservoir surfaces. In previous reports, a series of contact angles on shale samples were measured with aqueous ionic solutions at different concentrations, indicating that the shale had a higher affinity for more diluted solutions [16]. Some other experimental results show that the presence of divalent cations can significantly increase surface wettability compared to monovalent cations [2,17].…”

A Comparison of Gas and Water Permeability in Clay‐Bearing Fault and Reservoir Rocks: Experimental Results and Evolution Mechanisms

“…Due to the preferred orientation of clay minerals in sedimentary beddings, the swelling strain is anisotropic; that is, the strain in the direction perpendicular to the bedding planes should be much larger than that in the parallel direction (Chenevert, 1970). The bedding together with local heterogeneities of formation rocks (e.g., in both mineralogy and internal texture) will generate anisotropic swelling stresses, which can potentially create local microfractures provided that the swelling stress is comparable to the external confining pressure (e.g., Siddiqui et al, 2019). In other scenarios, the swelling stress may work against the surface of hydraulic fractures (depending on the relative location and distance between the expansion formation and fractures) and therefore prevent fracturing and hinder the gas well performance (Figure 15).…”

Spontaneous imbibition in shale: A review of recent advances

“…Chakraborty et al (2017) used micro-CT imaging to observe shale microcracks introduced by liquid imbibition. Siddiqui et al (2019) used micro-CT imaging to scan the internal structure of samples before and after spontaneous imbibition at unconfined and confined stress states. There have been other similar studies published in recent years, but almost no experimental tecnique can directly observe the spontaneous imbibition process in shale nanopores.…”