IMPROVING THE ACCURACY OF NMR RELAXATION DISTRIBUTION ANALYSIS IN CLAY- RICH RESERVOIRS AND CORE SAMPLES

摘要

Deriving accurate T2 relaxation spectra from nuclear magnetic resonance (NMR) CPMG measurements is critical to the determination of total and effective porosities, irreducible water saturations, and permeability from NMR logs and core measurements. Accurate spectra are also essential to estimate coefficients for the film model or Spectral Bulk Volume Irreducible (SBVI) model. This paper analyzes the effects of noise, inadequate sampling rate, and CBW on the estimation of T2 distribution and effective porosity. Synthetic data were used so that it is easy to compare the results with underlying model inputs. A new method is described which utilizes the partial porosity distributions obtained from two echo trains acquired with different TEs. High signal-to-noise ratio echo data with sampling time TE of 0.6 ms are used to obtain the CBW T2 distribution. These data are then used to reconstruct CBW contributions to the time domain early echoes of the conventional effective porosity echo data (TE = 1.2ms). This residual signal is then subtracted from the original echoes. The effective porosity distribution is estimated from the reconstructed echo train.