Reservoir Characterization Using NMR Data in Absence of Core - Breakthrough in NMR Data Utilization

摘要

Considering the modern oil price environment, oil companies are more pressured than ever to reduce costs. This need affects tools used for reservoir characterization. Coring is important but expensive and is usually not available for the entire length of the well. A novel methodology is presented to perform reservoir characterization from wireline nuclear magnetic resonance (NMR) data, in the absence of any core, in offshore gas-bearing wells. This includes computing T2 cutoff, hydrocarbon saturation, permeability, and poro-fluid sand facies determination. NMR is a shallow measurement and using wireline NMR measurements is even more challenging due to higher time after bit and increased mud filtrate invasion. Consequently, its use is restricted to quantifying porosity, and even the basic assessment of bound/free fluid require correct T2 cutoff to be determined from cores. Traditional formation evaluation methods use various equations like Archie’s, dual water, Waxman Smits, etc. to determine hydrocarbon saturation, all of which have many variables which, again, must be determined from cores. This makes it imperative to have core measurements to get precise results. In this paper, we present the results of successful implementation of the proposed methodology, which functions without core data. It employs NMR data along with modern processing techniques like factor analysis and fluid substitution, and integrates density data to evaluate reservoir by 1) minimizing the mud signal, 2) using the virgin zone data to extract dominant peaks and repeated patterns on T2 distribution to divide the entire reservoir section into different poro-fluid types, 3) obtaining the T2 cutoffs for various poro-fluids facies, 4) calculating density magnetic resonance porosity (DMRP) and adopting it to drive fluid substitution, 5) obtaining original water equivalent porosity which is divided by DMRP to get water saturation, 6) employing the fluid substituted (water-only equivalent) T2 distribution along with the T2 cutoff determined by factor analysis to calculate permeability using the Timur-Coates equation.