Manfred G. Prammer

Measurements of Clay-Bound Water and Total Porosity by Magnetic Resonance Logging

Pulsed nuclear magnetic resonance (NMR) logging has until now been limited to measurements of capillary bound water and of free fluids, the sum of which is considered the effective porosity of rock. Clay-bound water and fluids trapped in micropores generally exhibit NMR relaxation times too fast to be detected, given the echo sampling rates and sensitivity limitations of current state-of-the-art NMR logging tools. Core studies performed on representative clay samples confirm a linear relationship between the transverse relaxation time T 2 and the water content. At 1 MHz, clays with the largest specific surface areas (smectites) exhibit T 2 s in the sub-millisecond range; illites have characteristic T 2 s of one millisecond, and kaolinites, having the smallest specific surface areas, relax with T 2 s in the range of ten milliseconds. A new MRIL application was implemented based on the industry-standard MRIL logging tool. By incorporating twice the standard sampling rate and an acquisition scheme designed to boost the signal-to-noise ratio of very fast decay modes, the tool is sensitive to transverse decay components as short as 0.5 ms. During a field test campaign, the tool demonstrated the feasibility of simultaneous acquisition of effective porosity and total porosity. Neither porosity measurement requires prior knowledge of rock matrix properties. In shaly sands, the difference between MRIL total porosity and effective porosity can be interpreted as the clay-bound water volume, relevant as the clay correction term for resistivity analysis.

Lithology-Independent Gas Detection by Gradient-NMR Logging

It has been shown that the Magnetic Resonance Imaging Log (MRIL{reg_sign}) can detect hydrocarbon gases present in the near wellbore zone (approximately 10 cm from the borehole wall). By exploiting the T{sub 1}- and diffusion-weighting capabilities of this tool, mineralogy-independent water and hydrocarbon saturations can be determined in this zone. Gas and oil saturations are used to correct the reduction in apparent NMR porosity, an effect due to reductions in hydrogen index and incomplete recovery of magnetization polarization. The proposed method exploits the predictability of density, hydrogen index, T{sub 1} and diffusivity of the non-wetting phase from its bulk properties. Gas is always and oil is often non-wetting. The MRIL{reg_sign}/C`s multi-frequency capabilities are used to integrate changes in recovery time and/or echo spacing into a single logging pass. The resultant data streams are processed in the complex time-domain to extract signal components characteristic for hydrocarbons.