NMR response of non-reservoir fluids in sandstone and chalk

Transverse (T2) NMR relaxation time at 2 MHz proton resonance frequency was measured on core plug samples from two different lithologies, sandstone an d chalk, before and after exposure to selected drilling fluids. The results show that NMR signal response was significantly altered after d isplacing 50% of the original pore fluids, crude oil and water, by drilling fluid filtrate. Relaxation spectra of the rock samples invaded by water-ba sed filtrate shift to significantly shorter T2-Values. This shift yields an underestimation of the free-fluid Volumes when selecting cut-off values of 33 ms and 100 ms for sandstone and chalk, respectively. In opposite, rock samples affected by oil-based filtrate respond with a signal indicating sig nificantly larger free-fluid volumes than present before exposure. NMR-perm eability calculated based on the Timur-Coates Free Fluid model altered in s ome cases by one order of magnitude. Inc. All rights reserved. (C) 2001 Els evier Science Inc. All rights reserved.