Page 280 - Enhanced Oil Recovery in Shale and Tight Reservoirs

EOR mechanisms of wettability alteration and its comparison with IFT 257

data are resampled onto a plane perpendicular to the contact line, and the
contact angle is measured manually by tracing vectors tangential to the solid
surface and the scCO 2 ebrine interface. Although this technology requires
detailed imaging and sophisticated experimental methods, it can image
multiphase fluid configurations at reservoir conditions. The images can be
taken at many locations and a statistical analysis can be conducted. This
direct approach has been used successfully to predict fluid configurations
and multiphase properties using contact angles measured using a combina-
tion of micro-CT scanning, high-resolution SEM images and imaging to
determine the chemical composition of the rock surface (Idowu et al.,
2015). However, for nanopore systems, the resolution is an issue. Kumar
et al. (2008) used atomic force microscope to study wettability alteration
by surfactants.
Akbarabadi et al. (2017) used nano-CT to directly study fluid occupancy
inside nanopores of ultratight reservoir rock samples, and to investigate
spontaneous imbibition and pore-scale wettability.

9.8.5 Nuclear magnetic resonance (NMR) method
Nuclear magnetic resonance (NMR) method is a fast and nondestructive
method to study wettability. NMR occurs in a nuclear system. In a porous
medium, the amplitude of NMR signal is proportional to the number of
hydrogen atoms in the hydrogenous fluid. Thus, this technology can be
used to study the hydrocarbon and water distribution in the porous medium.
Thereare two kinds of NMR relaxations during the dipole moment time
evolution: longitudinal relaxations (T 1 ), transverse relaxations (T 2 ). T 2 spec-
trum is more widely used than T 1 because it requires less measured time and
can provide the same pore information. At the solid-liquid interface, molec-
ular motion is slower than that in the bulk liquid (Brown and Fatt, 1956).
The solid surface slows down the molecular spin; longer relaxation time is
needed to adjust to a new magnetic field. The relaxation rate is reflected
on the transverse relaxation time T 2 , with higher rate (v) corresponding to
longer T 2 (v proportional to exp( t/T 2 ). The magnitude of this effect
depends on the solid area covered by the liquid which is related to the wetta-
bility characteristics of the solid with respect to the liquid. The wettability of
the surface can reduce the relaxation time. Oil-wet surfaces cause a smaller
reduction in relaxation time than water-wet surfaces. In other words, if the
rock is more water-wet, T 2 will be smaller.