Micro-Scale Investigation of Wettability and Surface Roughness

Abstract

Carbonate rocks are host to enormous volumes of crude oil and are potential candidates for enhanced oil recovery. However, the rock/fluid interactions in carbonates are complex and vary over a range of scales. One of the crucial parameters that significantly controls fluid distribution and fluid flow in porous media is the wettability of rock/fluid systems. Rock/fluid interactions due to injected fluid or in the natural conditions can lead to a corresponding change in wettability of the system. Thus, it is crucial to measure representative wettability of the surface and evaluate the factors influencing it. Contact angle has been known as a popular method in the oil industry to quantify wettability at the macro or core scale. However, the macro-scale observations may not represent the features at the pore scale. Indeed, recent literature has reported that wettability distribution based on macro contact angle can lead to significant errors in deriving other reservoir parameters such as capillary pressures and relative permeabilities. Researchers related these variations to mineral heterogeneity and surface roughness. In order to better characterize the heterogenous carbonate reservoirs, solid understanding of rock-fluid interaction at the pore scale is required in addition to rock and fluid characteristics.

Over the last decade, there has been significant progress in micro-scale imaging techniques e.g., Scanning Electron Microcopy (SEM), Transmission Electron Microscopy (TEM), and Micro-CT imaging. These techniques have made it possible to acquire high-resolution imaging of the pore structure (microstructure) of the rocks. Specifically, the E-SEM (Environmental SEM) can allow generation of micro-scale droplets within the thin section of rock surface and thus facilitate micro-scale contact angle measurements. Interestingly, there is a significant variation between the measured contact angles for the same system at different scales. Surface roughness and mineral heterogeneity are thought to be responsible for this discrepancy.

In this context, many research studies have reported contact angle measurements for a variety of rock/fluid systems under different conditions of pressure, temperature, and salinity. However, it has been observed that there is a significant discrepancy in the reported wetting behavior of a particular rock/fluid system under the same conditions. One of the reasons for such deviations is the surface roughness of the rock, which is often neglected during contact angle measurements. Additionally, the effect of surface roughness has not been thoroughly investigated for a wide range of conditions.

To address this gap, we conducted an analysis of the wetting behavior of eight carbonate rocks and pure calcite and quartz minerals at micro- and macro-scale to evaluate the multiscale variation of wetting behavior. Furthermore, the wettability of artificially modified carbonate rock surfaces that exhibit a range of surface roughness values, was also evaluated. We used commercially available abrasives of various sizes (from liquid SiC to 240-micron grits SiC sandpaper and 600-micron grits SiC sandpaper) to alter the surface roughness and examine the effect of polish-abrasive size and type on both roughness and wettability. Moreover, surface wettability is affected by surface cleavage, however, this important factor has not received much attention. Geometrically, pure calcite mineral has multiple cleavage planes, which depicts different wettability behavior, nevertheless the wettability of the freshly cleaved surfaces is found to be different from the wettability of the exposed surfaces. These analyses help in determining the rock wettability effectively which in turns helps in better understanding of the impact of heterogeneity on carbonate wettability and the associated fluid distribution.

Our results indicate a considerable variation in wettability at micro- and macro scales owing to the intrinsic surface roughness and surface chemical heterogeneities at the micro-scale. Furthermore, it has been found that the discrepancy in the contact angles measurements for pure minerals like quartz and calcite is small compared to the observed variations for the complex rocks. Moreover, the effect of different polishing types on contact angles and on surface roughness has been investigated. The results show that the contact angles and surface roughness are proved to be dependent on the method of preparation of rock surface and that ultra-smooth rock surfaces could depict significantly different behavior than their rougher versions. Lastly, the contact angles of freshly cleaved calcite and freshly fractured quartz are considerably lower compared to the contact angles of their exposed surfaces. This suggests that freshly cleaved surfaces are more likely to be hydrophilic than their exposed counterparts. Therefore, when working with calcite and quartz, it’s essential to exercise caution because the duration of exposure could significantly impact wettability determination.

The finding of this research assures the variation in wettability behavior at different scales for the same rock/fluid systems, and in turn will be broadly applicable to improved reservoir characterization and selection of enhanced oil recovery techniques.

Date of Award	Apr 2023
Original language	American English
Supervisor	Muhammad Arif (Supervisor)