Biomedical subjects

G A LaTorraca

Publications and source records attributed to G A LaTorraca.

4 recordsLinked to original sources

PubMed1999

The inversion of NMR log data sets with different measurement errors.

We present a composite-data processing method which simultaneously processes two or more data sets with different measurement errors. We examine the role of the noise level of the data in the singular value decomposition inversion process, the criteria for a proper cutoff, and its effect on the uncertainty of the solution. Examples of processed logs using the composite-data processing method are presented and discussed. The possible usefulness of the apparent T(1)/T(2) ratio extracted from the logs is illustrated.

Algorithms↗

PubMed1996

Magnetic field nonuniformities and NMR of protons diffusing in a porous medium.

Magnetic field inhomogeneity can arise either because of an externally applied field gradient or because of spatial variations in magnetic susceptibility. The latter are most important when the solid matrix includes paramagnetic substances and when the uniform applied field, and, consequently, also the Larmor precession frequency are very large. Both types of field inhomogeneity add extra phase shifts to the precessing spins. These phase shifts vary with time and position in a complex and random fashion as a result of the diffusive motion of the spins. We have studied these effects by performing detailed calculations for the case of a fluid filled porous medium with a periodic microstructure. Special attention was devoted to the question of whether the statistical distribution of the phase shifts encountered in a Hahn spin echo experiment or in a Carr-Purcell-Meiboom-Gill (CPMG) spin-echo train can be approximated as a Gaussian. The mean square phase shift is measured in such experiments as an enhanced relaxation rate of the precessing transverse magnetization. We determine this mean square phase shift for periodic composites from the diffusion eigenstates, which were calculated using a previously developed Fourier expansion method. The enhanced relaxation rate depends on the echo spacing time tau in a way that can be correlated with important length scales of the porous microstructure. Those correlations can be extended also to disordered microstructures, like the ones that are found in natural rocks. We compare these theoretically predicted correlations with CPMG measurements performed on protons in laboratory samples of brine saturated sandstone.

Diffusion↗

PubMedDate not supplied

Low-field NMR determinations of the properties of heavy oils and water-in-oil emulsions.

Low-field (< 50 mT) nuclear magnetic resonance (NMR) well-logging measurements are beginning to be used to obtain estimates of oil viscosity in situ. To build an interpretive capability, we made laboratory T1 and T2 relaxation measurements on a suite of high-density, high-viscosity crude oils. These measurements were also used to estimate oil viscosity and water fraction from T1 and T2 measurements on stable, water-in-oil emulsions. High-density, high-viscosity oils have components that relax faster than can be measured by nuclear magnetic resonance logging tools. This requires corrections to T2 logging measurements for accurate estimates of oil saturation and porosity.

Emulsions↗

PubMedDate not supplied

Permeability relation for periodic structures.

The permeability relation for periodic porous media is studied with respect to other petrophysical parameters such as formation factor, porosity, surface-to-volume ratio, and nuclear magnetic resonance (NMR) relaxation time. All these quantities were computed for periodic structures of simple, body-centered, and face-centered cubic arrays of touching and overlapping spheres. The formation factors were calculated by using a method which is based on a Fourier-space representation of an integral equation for the electric potential in a two-component composite. The nuclear magnetic resonance relaxation time for the case where surface-enchanced relaxation plays a dominant role is known to be V P/rho S (VP is the pore volume, S is the pore surface, is the surface relaxation strength) when rho is not too large. Previously calculated permeabilities for these structures from the literature were used for correlation studies with other petrophysical parameters. Various correlation schemes among these quantities, such as k = aTbFc, and k = aTb phi c, were investigated, where k is permeability, T is the NMR relaxation time, phi is the porosity, and F is the formation factor.

Geologic Sediments↗