ROBUST MACROPOROUS DOLOMITE PORE TYPING AND QUANTIFICATION WITH LWD T_1 OR WIRELINE T_(2,INTRINSIC) LOGS

摘要

Nuclear magnetic resonance (NMR) logging has been frequently used in situ to detect and classify pore size for carbonate reservoirs by partitioning the T_2 distribution spectra. However, there are a few challenges for measuring dolomite pore size with current carbonate NMR interpretation techniques. For wireline NMR logging, its T_2 distributions are significantly shortened by the tool gradients; for logging while drilling (LWD) NMR logging, its T_2 distributions can also be distorted because of the drilling-induced lateral vibration.This paper discusses two solutions for identifying and quantifying macroporous dolomites for wireline and LWD NMR logging. For wireline NMR, an inversion-forward modeling-inversion (IFMI) technique computes both T_(2,int) and T_(2,app) distributions for any tool gradient and any inter-echo time. For LWD NMR, the best solution is to acquire a T_1 log with a broadband saturation pulse followed by narrow-band excitation and refocusing pulses.Both NMR carobonate intepretation methods are tested in a well from a complex carbonate reservoir where both wireline and LWD NMR logs were run back-to-back. Compared to a standard T_(2,app) log, both methods increased sensitivity to macroporosity in dolomitic intervals.The methods presented in this paper successfully reduced fluid diffusion effects to accentuate pore size variations particularly in macroporous dolomitic intervals. The diffusion-free NMR spectra will help pore typing and rock quality evaluation in carbonate reservoirs where large macropores or vugs are present.