Assay for bacteria in porous media by diffusion-weighted NMR.

In this work, an NMR technique capable of detecting bacterial cells and measuring the cell density in suspension and in porous media has been developed. It is based on the pulsed-field-gradient technique and relies on the fact that extracellular water diffuses freely while intracellular water is completely restricted by the relatively impermeable cell wall of the bacterium. At high wave vectors, the signal from extracellular water is completely suppressed while the signal from intracellular water is comparatively unaffected. This technique has been applied to the mapping of bacterial distributions in porous media. This method is presented as a non-destructive, real-time technique for biomass characterization within laboratory column and flow cell experiments, and possibly for monitoring in situ bioremediation.

Utility of NMR T2 distributions, connection with capillary pressure, clay effect, and determination of the surface relaxivity parameter rho 2.

Nuclear magnetic resonance magnetization decays are now used routinely to determine reservoir properties of economic significance to the oil and gas industry. Although individual features of relaxation time distributions are subject to degradation in the presence of measurement noise, integrated and average measures of those distributions are remarkably stable. Properties such as porosity, producible porosity, hydraulic permeability, and the capillary pressure curve can be measured rapidly and continuously. Laboratory comparisons of NMR T2 distributions with two different capillary pressure measurements enable the surface relaxivity parameter rho 2 to be determined for sandstones. Comparison with relaxivities of synthetic materials having well-defined surface areas strongly suggests that the NMR-effective internal surface area of rocks is closely connected with the hydraulic radius, and, surprisingly, is not greatly affected by the presence of clay, which dominates the pore space surface are as measured by gas adsorption.

Pore size distributions, pore coupling, and transverse relaxation spectra of porous rocks.

The properties of porous rocks make NMR measurements difficult to interpret. Broad distributions of pore sizes give rise to magnetization decays characterized by broad distributions of relaxation times. The pore space, which is well coupled with respect to steady state fluid flow and low frequency electrical conductivity, appears disjoint on time scales probed by NMR. Pulsed field gradient NMR measurements show that pores are elongated and that relaxation times measure the minimum dimension. Diffusion in magnetic field gradients affects the transverse relaxation spectrum in complicated ways, but in the surface dominated regime there is a simple relationship between T1 and T2.