Effect of total organic carbon (TOC) content on shale wettability at high pressure and high temperature conditions

Abstract

Rock wettability is a key parameter determining fluids distributions and migrations in hydrocarbon reservoirs. In shale rocks, the widespread existence of organic matter and the significant variation of total organic carbon (TOC) content lead to huge uncertainty on shale wettability. Therefore, this work systematically examines the dependence of shale wettability on TOC content (ranging from 1.2 wt% to 20.05 wt%). Multiple relations were probed experimentally, namely: 1) the effect of TOC content on n-dodecane (n-C12) contact angles for shale-methane (CH4)-n-C12 systems at 5 MPa and 50 °C; 2) the effect of TOC content on n-C12 contact angles for shale-carbon dioxide (CO2)-n-C12 systems at 5 MPa and 50 °C; 3) the effect of TOC content on brine contact angles for shale-brine-n-C12 systems at 0.1 MPa and 50 °C; 4) the effect of pressure (0.1 MPa–25 MPa), temperature (25 °C and 50 °C) and TOC content on brine contact angles for shale-CH4-brine systems. The results demonstrate that for shale-CH4-n-C12, shale-CO2-n-C12 and shale-brine-n-C12 systems, the affinity of n-C12 to shale increases with increasing TOC content. However, for shale-CH4-brine systems, the affinity of CH4 to shale increases firstly, then flatten out and afterwards increases again; while shale wettability shifted from water-wet to CH4-wet with TOC content increase. In addition, a larger pressure renders shale less hydrophilic while a higher temperature causes shale more hydrophilic. The temperature influence is reduced with TOC content increase. The results can aid in better understanding of shale wettability at reservoir conditions.