Applying NMR Technologies to FTI in the Complex Low-Porosity Reservoirs in China
Authors: Lu Dawei , Yang Weiying , He Qinyuan , Qiutao Gao 
Published: December 2007

With the laboratory NMR data of more than two hundred ddifferent core samples and the study of NMR logs from nearly forty wells, NMR log signal preprocessing methods are compared and selected for different NMR tools (CMRT or MRIL) under different logging modes. Constrained least square fit is used to study the T2 spectrum, and a NMR log interpretationmodelsuitable for Complex Low - Porosity Reservoirs is presented. The T2 spectrum and T2 cutoff value are classified according the lithology and grain size of core sample, Three methods for the I rreducible watersaturation evaluation are determined.

Methods used for hydrocarbon detection and its adaptation to different geological application are investigated according to the NMR log original signal intensity and its noise distribution. Effective fluid type identification (FTI) are conducted in the shaly low porosity sands and dolomatic carbonate formations in eastern China, and conglomeratic low porosity sands in western China, the FTI results are favorable comparing with the well tests. For example, both hydrocarbon detection interpretation from MRIL logs of Well XX1 in eastern oilfield and CMRT logs of Well XX2 in western oilfield give a considerable hydrocarbon shows in low-porosity shaly sand formation and conglomeratic sand formation, a industrial oil stream is obtained after the well tests. It provides a reliable evidence for the hydrocarbon evaluation in these regions.

In addition, NMR logs can also be used for the study of the reservoir structure and productivity evaluation.

Introduction

NMR logging techniques were introduced into China in the early of 1990's, NMR uses and applications also began in the period. In locating gas-bearing formations, identifyingthin-bed intervals, and evaluating complex reservoirs, NMR has shown favorable geological effects.

In many regions of China, because of the complexity in the lithology, type and pore structure in reservoirs, geological application of NMR techniques, especially NMR hydrocarbon detection in FTI inlowporosity andcomplexlithology formation, arechallengingbythosecomplexgeological conditions.

During the application research of NMR logging technique, and the processing and interpretation of real NMR logs, results from the laboratory NMR core data analysis are used in order to determine suitable models, signal processing and parameter selectionforNMRloginterpretationunderdifferent geologicalenvironments. These results further lead to the hydrocarbon detection technique verification.

Main technologies used for hydrocarbon detection with NMR methodology are Differential Spectrum Method (DSM), Enhanced Diffusion Method (EDM), Shifting Spectrum Method (SSM), Echo ratio and Amplitude methods etc. SSM and EDM have a primary good application in the sandstone formation evaluation in the Golf of Mexico. Amplitude method gives a primary application in the shaly sand evaluation in the North Sea.

Interpretation Model and Processing Tecnique of NMR Log Data and Core Data Analysis

Parameters of core porosity, permeability, irreducible water saturation, characterized data of T2 spectrum, obtained from Laboratory NMR data, are classified according to the statistics of lithology and size of grains.