Time-Domain Correction Of Mril Logs Contaminated By Borehole Signal

MRIL logs acquired in elliptical or washed-out boreholes can be adversely affected by borehole NMR signal contamination. Washout effect is observed if the sensitive volume of the tool intersects the borehole and is characterized by excessive porosity readings and a distorted T2 spectrum that is not representative of the formation. This paper outlines a procedure for correcting MRIL data that is partially contaminated by a mud signal from the borehole. The correction operates in the time-domain by subtracting a computed estimate of the borehole signal from the measured echo data (formation + borehole) to obtain the signal due to formation fluid(s) only. The amplitude of the modeled borehole signal is estimated by comparing the uncorrected MRIL porosity to independent log-derived porosity. T2 decay parameters of the mud signal are derived from laboratory measurements or MRIL data itself. The corrected echo data are processed by standard methods to obtain T2 spectra and petrophysical parameters there from that are representative of the formation. The proposed method has been validated by three case studies. In the first example, borehole-corrected MRTL irreducible water saturation is in excellent agreement with the results of a resistivity-based saturation model, The second example illustrates that the borehole signal can be effectively extracted from log data in low porosity shale sections. The third example shows that the correction can be successfully applied even with relatively poor definition of the borehole signal characteristics. A numerical modeling study illustrates sensitivities of the correction method to uncertainties in input parameters including reference porosity and mud T2 spectrum characteristics. The results illustrate the validity of the method, even at relatively high borehole signal levels, and lead to guidelines for proper application of the correction.