Duncan Mardon

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.