Study impact of sample treatment and insitu fluids on shale wettability measurement using NMR

Abstract

Nuclear Magnetic Resonance (NMR) is a good alternative to conventional Amott-Harvey imbibition test or US Bureau of Mines (USBM) test, to measure wettability in shales. The conventional methods do not work primarily because of the long time it takes to flow through the nano pores in the shales. Contact angle measurements typically used in shales do not account for pore connectivity. Hence, NMR is becoming a standard reliable method to measure wettability in shales.This study focuses on two aspects of wettability measurement in shales. First, Looyestijn and Hofman (2006) proposed an equation to calculate wettability using NMR data based on their experiments on carbonate rocks. Sulucarnain et al. (2012) applied this method to shales. In their method, the residual fluids in the plugs are neglected because it is difficult to ascertain what proportion of residual fluids is hydrocarbon and what proportion is water. However, in liquid-rich shales like Eagle Ford, there could be significant amount of residual fluids which must be accounted in the wettability calculation. This study integrates Source Rock Analysis (SRA), NMR and helium porosity data to determine the nature of the residual fluids and account for them while doing wettability calculations. Thus, an improved equation for wettability is proposed by modifying the original equation.Second, in this study, wettability measurements on preserved, dried and cleaned sample states were carried out. The comparison of the three states suggest that drying and cleaning the samples, changes the original wettability of the system and make the rocks appear more oil wet. The effects of drying are more pronounced than cleaning. The above argument is supported both by SRA data and Scanning Electron Microscope (SEM) images. The SRA data shows an increase in S1 and a corresponding decrease in S2 peak which suggests that heating the sample at 100 °C may crack the heavy hydrocarbons in the sample. The SEM image analysis also show the possible movement and resolidification of bitumen and other heavy hydrocarbons. These mobile fractions are believed to flow into the water-wet pores and turn them oil-wet.