T85 Advances in Characterization of Groundwater Flow Processes

Monday, 10 October 2011
8:00 AM - 12:00 PM
Room M101AB

COMPARISON OF NUCLEAR MAGNETIC RESONANCE (NMR) LOGS IN WELLS COMPLETED IN GLACIAL SEDIMENTS IN THE NORTHEASTERN UNITED STATES

Paper number: T85-15
Program: Technical Sessions
Time: 11:35 AM – 11:50 AM

Authors

Abstract

Borehole nuclear magnetic resonance (NMR) logs provide in situ estimates of porosity, saturation, mobile and immobile water fraction, and estimates of permeability, which are critical aquifer properties important for groundwater-resource and contamination investigations. NMR tools have been available for many years and are widely used in the oil and gas industry, but only recently have been designed for smaller boreholes more widely used in groundwater investigations. From June 2010 to July 2011, the U.S. Geological Survey, in cooperation with Vista Clara, Inc., tested three small-diameter borehole NMR logging tools. Two of the tools have tool diameters of 44 mm, vertical resolutions of 1 m, and focused radial depths of investigation (DOI) with thickness of about 2 mm and a radius of either 101 or 127 mm. The third tool has a diameter of 89 mm, a vertical resolution of 0.5 m, and a focused radial DOI of 190 mm over a 2 mm thickness.

NMR logging was conducted in four PVC-cased boreholes completed in glacial materials with interbedded layers of sand, silt, and gravel. The boreholes were completed using rotary, auger, roto-sonic, or drive-and-wash drilling methods, which have different zones of disturbance around the borehole. Bound water, free water, and total water content were estimated from the distribution of transverse relaxation times at discrete intervals. NMR-derived water-content data and estimates of aquifer properties compared favorably to drilling, geologic, natural gamma, and electromagnetic-induction logs. Log sections where NMR data indicated lower total water content also showed lower fractions of mobile water and correspondingly higher percentages of bound water. Zones of low total and high immobile water content corresponded to silt layers identified in drilling logs. Results from the three tools show similar distributions of porosity and permeability. Differences in estimates of porosity and permeability among the tools appear to be caused by differences in the radial DOI and well-construction methods, which highlights the importance of tool selection such that the radial DOI is outside the annular zone of disturbance.