Christian Straley

Magnetic resonance, digital image analysis, and permeability of porous media

The results of an experimental and theoretical study of consolidated, acid‐cleaned fused glass beads are presented. Measurements of the spin‐lattice lifetime, permeability, and capillary pressure curves in conjunction with digital analysis of scanning electron microscope images and theoretical modeling lead to a description of this porous material consistent with the fast diffusion picture of nuclear magnetic relaxation.

Nuclear magnetic resonance pulse sequences for determining bound fluid volume

An NMR pulse sequence for use in a borehole logging tool includes a series of CPMG pulses according to: T.sub.r -90°.sub.±x -(t.sub.cp -180°.sub.y -t.sub.cp -echoj) where j is the index of CPMG echoes gathered, Tr is wait time, tcp is the Carr-Purcell spacing. This pulse sequence is used to determine Bound Fluid Volume (BFV) which is subtracted from total porosity to yield Unbound Fluid Volume (UFV) of a formation surrounding the borehole. Measuring the BVF, the amount of rapidly relaxing fluid (less than 50 ms), is more efficient than measuring UFV (up to 2 secs), and is insensitive to motion of the logging tool.

Transverse relaxation in random bead packs: Comparison of experimental data and numerical simulations

Inverse Laplace transform techniques are commonly used to convert nuclear magnetic resonance (NMR) relaxation data in porous media into lifetime distributions. In this connection, we present a direct comparison of T2 measurements made on monosized packings of sintered glass beads and numerical simulations based on the grain consolidation model. Three systems, with porosities of 0.38, 0.22, and 0.14, were studied, and in each case we found that the position and width of the measured T2 distribution was well represented by the random walk simulations. These systems are generally in the fast diffusion (weak surface relaxation) regime and have relatively narrow T2 distributions. For this reason, we have found that inverse Laplace transform techniques are particularly sensitive to random noise in the measured and simulated decay spectra.

T1‐permeability correlations

We begin by presenting evidence which indicates a strong correlation exists between single fluid Darcy permeability and the T1 parameter characterizing the relaxation of nuclear magnetism within sandstones and carbonates. After defining terms, we discuss why NMR decay of water in rocks might reveal information about rock microgeometry. Finally, we discuss efforts being made to understand how to relate the length scale measured in NMR with that involved in permeability.

Chemical shift imaging of particle filtration in sandstone cores

Recent developments have led to increased interest in the application of borehole nuclear magnetic resonance (NMR) as a probe of petrophysical properties. Of particular importance in this connection is the measurement of the longitudinal relaxation time, T1. As T1 is controlled by the pore surface area, its value may be strongly influenced by the invasion of submicron-sized clay particles found in drilling muds. We have studied this effect by the application of phase encode magnetic resonance imaging (MRI) techniques. The extent to which T1 values are affected by particulate invasion is found to depend strongly on the mud characteristics. With thinned spud muds there is a region deep within the core where T1 values are significantly reduced due to an initial spurt of clay particles. In better formulated muds this effect is greatly reduced.

Application of single species chemical shift imaging to sandstone cores

Abstract Sandstone cores typically contain appreciable concentrations of paramagnetic impurities. These impurities enhance the susceptibility contrast between the predominantly quartz matrix and the pore fluid. In an applied magnetic field, the internal gradients generated by these susceptibility differences act to limit the resolution of readout gradient frequency selection imaging techniques. We show that this problem can be overcome by the application of a chemical shift phase encode imaging sequence. To demonstrate the utility of this technique, we have made high resolution axial profiles of the longitudinal relaxation time, T1, on Berea sandstone cores.