Present Status of Nuclear Magnetism Logging

Abstract

Abstract The Nuclear Magnetism Log (NML), which responds only to formation fluids, is described. Basic principles on which this new development rests are reviewed. Field examples are given for a broad spectrum of operating conditions. These include large variations in mud composition, different geological provinces, various reservoir rock conditions, a wide range of formations fluid contents and composition, and well depth, temperature and pressure. Two distinct NML measurements are made. The Free Fluid Index (FFI) is a formation permeability and productivity, and is an excellent correlation index. Thermal relaxation data (T1) distinguish oil or gas and water zones with good reliability and provide information about reservoir permeability. Introduction The purpose of well logging is to obtain quantitative information about the presence and amount of reservoir fluids and the characterization and percentage of their chemical species namely, hydrocarbon or water. Other useful information desired is formation fluid for production. In addition, it is useful to have a logging technique which is a good correlation between well sand is universal in application irrespective of hole geometry, mud composition, reservoir rock composition, and physical properties, etc. Interpretation of existing commercial logging techniques is predicated upon empirical correlations derived from the measurement of gross reservoir properties. They are in the main properties such as resistivity, transmission of sound, natural radioactivity, induced radio-activity, and spontaneous potential which are not simple functions of the diverse reservoir rocks and fluid compositions encountered in oil exploration. The use of other than a primary property of the reservoir fluid frequently gives rise to ambiguous interpretation of well logging data. For a long time a direct means has been sought of measuring some physical or chemical property of solely the reservoir fluids and not the rock matrix to yield a relatively unambiguous result. Measurement of the nuclear magnetic properties of protons offers a unique means of meeting the objectives of well logging. Under operating conditions in the borehole, nuclear magnetism logging is responsive only to hydrogenous fluids in the reservoir. The magnitude of the signal is quantitatively related to the geometrical configurations of the tool and borehole and the thermodynamic and nuclear magnetic properties of the formation fluids.