Connection Between Wettability and NMR-Detected Isotherms

Reservoir wettability is a parameter of crucial importance for oil recovery1. However, its definition and its measurement are quite complex. For a flat surface, wettability is directly related to the contact angle of a water droplet on a surface controlled by the hydrophilicity of the surface. For an arbitrary surface, the contact angle, however, does not only depend on the hydrophilicity of the surface but also depends on the surface roughness at various length scales. Therefore, wetting is a macroscopic property that is only directly related to the hydrophilicity of the surface if the surface is flat and smooth. The hydrophilicity of the surface is a microscopic property that determined the surface interactions with water and hydrocarbons and plays a crucial role in oil production and recovery. The unknown surface roughness of internal surfaces of rock porous media makes wettability definition and measurement of such internal surfaces extremely challenging. Identifying a wettability index of porous media surfaces with broad applicability is a primary objective in oil and gas industry. Here we demonstrate an NMR-detected isotherm technique for measuring surface wettability of porous media. This technique is not only related directly to the traditional measure of wettability using macroscopic contact angles, it is also directly related to the microscopic surface property of hydrophilicity. It is shown that NMR-detected isotherms of both water and isopropanol (IPA) are needed to obtain the wettability index. Through systematic studies of quartz glass beads and quartz slides that are hydrophilic- or hydrophobic-modified, we established a quantitative relationship between the NMR isotherm-based wettability index and the traditional measure of wettability using contact angle. Therefore, the proposed wettability index derived from NMR isotherms provides a clear link to both macroscopic and microscopic properties of internal surfaces of porous media and can be a reliable measure of interactions between water and hydrocarbon with internal surfaces of rock porous media, as evidenced by preliminary studies of rock samples.