Page 128 - Reservoir Formation Damage

110 Reservoir Formation Damage

being optically isotropic, amorphous substances can be distinguished from
the majority of the crystalline matter, except for the optically isotropic
halides (Braun and Boles, 1982). The polarized light microscopy is based
on distinguishing between various substances by the difference in their
refractive indices. Braun and Boles (1982) recommend supporting the
PLM method by at least another method, such as the scanning electron
microscopy combined with the energy dispersive X-ray spectrometry
(SEM-EDS) and the X-ray diffraction (XRD) method.

Nuclear Magnetic Resonance Spectroscopy (NMR)

The nuclear magnetic resonance spectropy is a nondestructive tech-
nique, which measures the spin-lattice and spin-spin relaxation times by
means of the radio-frequency resonance of protons in a magnetic field
to infer for the petrophysical parameters, including porosity, permeability,
and free and bound fluids using specially derived correlations (Unalmiser
and Funk, 1998; Rueslatten et al., 1998). Because fines mobilization,
migration, and retention in porous media causes porosity variation,
the NMR can also be used for examination of core plugs during fines
invasion. For example, Fordham et al. (1993) examined the invasion of
clay particles within natural sedimentary rocks by injection of suspension
of clay particles using the NMR imaging technique. Fordham et al. (1993)
show that the proton spin-lattice relaxation time profiles measured at
different times indeed indicate the effect of clay fines invasion into core
plugs. This information can be used to determine the penetration depth
of the clay fines and the effect of fines invasion to permeability. Xiao et
al. (1999) state that:

The NMR (nuclear magnetic resonance) techniques, namely NMRI
(nuclear magnetic resonance imaging) and NMRR (nuclear magnetic
resonance relaxation), can support the observations obtained with
the return permeability tests, helping in the identification and
comprehension of the formation damage mechanisms caused by
solids and filtrate invasion in the pores of a reservoir rock.

However, the NMR techniques are expensive and time consuming, and
better suited for in depth studies (Xiao et al., 1999). Xiao et al. (1999)
show typical NMR images and relaxation time curves on invasion of
a typical bentonite/mixed metal hydroxide (MMH)/sized carbonate
mud system into a core plug. The core plug images provided visual
inspections for the core initially saturated with a 3% NH 4Cl brine, then
contaminated by mud invasion, and finally back flushed with brine for
mud removal, respectively.