2D Surface Roughness Quantification for Enhanced Petrophysical Applications

Summary Surface roughness is an essential rock parameter affecting petrophysical properties that are surface sensitive such as characterization of pore structure and wettability. For instance, Wenzel’s contact angle formula for rough surfaces requires knowledge of the surface roughness, and surface roughness is expected to speed up the aging of cores in crude oil for wettability restoration. In addition, proper quantification of surface roughness is critical for obtaining representative, roughness-independent, pore sizes for applications such as prediction of permeability and interpretation of capillary pressure curves. Intuitively, a surface is better characterized in 2D than in 1D. This 2D study is a continuation and enhancement of the previous 1D work, recently published in the SPE Journal (Ma et al. 2021). In this current paper, a comprehensive investigation of 1D vs. 2D surface roughness measurements is conducted to evaluate and cross validate the two approaches. In this study, surface roughness is measured on 26 carbonate rock samples by laser scanning confocal microscopy (LSCM), where both the 1D absolute increment surface roughness, Sr, and the 2D interfacial area ratio of surface roughness, Sdr, are reported. As expected, the results indicate that surface roughness characterized by 2D Sdr has a greater dynamic range than the 1D Sr measurement, i.e., the 2D Sdr provides a more representative characterization of surface roughness. A detailed account of methodologies, assumptions, limitations, validation, and applications of the 1D and 2D surface roughness characterization is documented in this paper. To extract the roughness features present on rock grain surfaces, effects of de-spiking and filter length, used to eliminate pore-size effects, are investigated. For specific applications of surface roughness corrected pore-size estimation from nuclear magnetic resonance (NMR) measurements, differences in length scales of surface roughness are compared between LSCM measurement and that derived from NMR diffusion-T2 plus BET (Brunauer-Emmett-Teller) surface area. The surface roughness-corrected NMR pore-size distribution is also validated against the pore-size distribution obtained from the measurement of micro-computed tomography (CT) scanning.