The Influence of Oil Composition, Rock Mineralogy, Aging Time, and Brine Pre-soak on Shale Wettability

Abstract

Experimental and field studies have indicated that surfactants enhance oil recovery (EOR) in unconventional reservoirs. Rock surface wettability plays an important role in determining the efficacy of this EOR method. In these reservoirs, the initial wettability of the rock surface is especially important due to the extremely low porosity, permeability, and resulting proximity of fluids to the solid surface. This study is designed to investigate the effect of oil components, rock mineralogy, and brine salinity on rock surface wettability in unconventional shale oil/brine/rock systems. Six crude oils, seven reservoir rocks, and seven reservoir brine samples were studied. These oil samples were obtained from various shale reservoirs (light Eagle Ford, heavy Eagle Ford, Wolfcamp, Middle Bakken, and Three Forks) in the US. SARA (saturates, aromatics, resins and asphaltenes) analysis was conducted for each of the crude oil samples. Additionally, this study also aims to provide a guideline to standardize the rock sample aging protocol for surfactant-related laboratory experiments on shale reservoir samples. The included shale reservoir systems were all found to be oil-wet. Oil composition and brine salinity showed a greater effect on wettability as compared to rock mineralogy. Oil with a greater amount of aromatic and resin components and higher salinity rendered the surface more oil-wet. Rock samples with a higher quartz content were also observed to increase the oil-wetness. The combination of aromatic/resin and the quartz interaction resulted in an even more oil-wet system. These observations are explained by a mutual solubility/polarity concept. The minimum aging time required to achieve a statistically stable wettability state was 35 days according to Tukey’s analysis performed on more than 1100 contact angle measurements. Pre-wetting the surface with its corresponding brine was observed to render the rock surface more oil-wet.

A Guide for Selection of Aging Time and Temperature for Wettability Alteration in Various Rock-Oil Systems

“…For instance, they indicated that it takes from 14 to 30 days based on the lithology and temperature to change wettability from initial water-wet to oil-wet. 37,38,40,45 Other authors such as Al-Mahrooqi et al 1 reported 100 days for such wettability alteration. Nonetheless, the result analysis led to a conclusion that changing the wettability of a strongly water-wet sample takes a relatively longer time than that of initially weakly wet or neutral wet.…”

“…Lara Orozco et al reported that to alter the wettability of the rock sample from water-wet to oil-wet using asphaltene content of 14 wt%, it took 21 days at 95 °C temperature. The sample needed 35 days to alter its wettability using crude oil with 5% asphaltene at reservoir temperature conditions (it was aged for 56 days, but there is no change in wettability after 35 days) . Additionally, using crude oil with 11 wt% asphaltene to age the core sample for 7 days at 80 °C changed its wettability from strongly water wet to intermediate wet .…”

Rock–Fluid Interactions of Alkyl Ether Carboxylate Surfactants with Carbonates: Wettability Alteration, ζ-Potential, and Adsorption

“…Das et al 34 soaked Iceland Spar plates in oil at 120 °C for 2−3 days. Sharma et al 4 aged calcite mineral plates in oil 78 performed a statistical analysis of core samples with different mineral compositions soaked in various oils. The authors observed no significant changes in contact angle values after 35 days of aging.…”

Dissolving Nonionic Surfactants in CO2 to Improve Oil Recovery in Unconventional Reservoirs via Wettability Alteration

“…Because the IFT between CO 2 and oil is already low at reservoir temperatures and pressures − and CO 2 –oil foams are very difficult to generate, , we anticipated that wettability alteration of shale from oil-wet to water-wet (or CO 2 -wet) would be the key mechanism by which CO 2 -dissolved surfactants would improve oil recovery. Shale is oil-wet because of the presence of oil-wetting deposits on the shale mineral surface (Figure A). − These deposits cause oil to adhere to the shale, making it more difficult to produce. The oil-wetting deposits are polar, charged, or polyaromatic compounds that are insoluble in CO 2 .…”