Optimization of CPMG sequences to measure NMR transverse relaxation time 2 in borehole applications

Abstract

Nuclear Magnetic Resonance (NMR) can provide key information such as porosity and permeability for hydrological characterization of geological material. In particular the NMR transverse relaxation time 2 is used to estimate permeability since it reflects a pore-size dependent relaxation process. The measurement sequence (CPMG) usually consists of several thousands of electromagnetic pulses to densely record the relaxation process and to avoid relaxation processes that are due to diffusion. These pulses are equidistantly spaced by a time constant E. In NMR borehole applications the use of CPMG sequences for measuring the transverse relaxation time 2 is limited due to requirements on energy consumption. For measuring 2, it is state-of-the-art to conduct at least two sequences with different echo spacings (E) for recording fast and slow relaxing processes that correspond to different pore-sizes. We focus on conducting only a single CPMG sequence and reducing the amount of energy while obtaining both slow and fast decaying components and minimizing the influence of relaxation due to diffusion. Therefore, we tested the usage of CPMG sequences with an increasing E and a decreasing number of pulses. A synthetic study as well as laboratory measurements on samples of glass beads and granulate material of different grain size spectra were conducted to evaluate the effects of an increasing E. We show that 2 distributions are broadened if the number of pulses is decreasing and the mean grain size is increasing, which is mostly an effect of a significantly shortened acquisition time. The shift of 2 distributions to small decay times as a function of E and the mean grain size distribution is observed. We found that it is possible to conduct CPMG sequences with an increased E. According to the acquisition time and increasing influence of relaxation due to diffusion, the sequence parameters need to be chosen carefully to avoid misinterpretations.