Field Experience in Measuring Oil Content, Lithology and Porosity With a High-Energy Neutron-Induced Spectral Logging System

Abstract

Abstract A complete logging system, consisting of a pulsed high-energy neutron generator, a gated sodium iodide crystal, photomultiplier gamma-ray spectra is described. Typical results from 27 field tests and interpretation of the data are discussed. Since gamma rays induced by the pulses of 14-Mev neutrons have energies characteristic of the elements bombarded, analysis of a formation's spectrum furnishes direct identification of the formation lithology from the responses of key elements (silicon in sandstones, magnesium in dolomites, and sulfur in anhydrites), and porosity from the moderation of neutrons. Formation shaliness, water salinity, hole and casing sizes, and type of fluid in the well do not seriously affect responses of the logging system. Introduction With conventional logging tools now available, a series of surveys is necessary to obtain logging data from which formation lithology and hydrocarbon saturation can be deduced. Formation evaluation from these surveys is not based on direct measurements of the information, but must be deduced from the characteristics of certain physical properties (resistivity, density. acoustic velocity, etc.). Because deductive interpretation is indirect and often fails to produce the desired information, we need a logging method which measures directly formation lithology and hydrocarbon content. A new logging system based on a nuclear means of identifying elements has been in development for several years. Construction and preliminary performance of this logging system have been described previously. It consists of a pulsed, particle-accelerator type neutron generator, a gated gamma-ray detector, and associated surface equipment for obtaining gamma-ray spectra. The gamma-ray spectra result from prompt high-energy neutron reactions in such key elements as carbon, oxygen, silicon, magnesium and sulfur. This information on the elements comprising the subsurface strata and their contained fluids provides data for the direct identification of formation lithology and hydrocarbon content. Nuclear radiation has been used for a number of years to gain formation evaluation information." Commercial logging systems provide a record of the natural gamma-ray activity of formations, the gamma-ray activity induced by neutron capture reactions, or the scattering (by the formation) of neutrons or gamma rays. One system preferentially records gamma rays induced in chlorine; a more recent technique obtains a chlorine response by measuring the life of neutrons in a formation. In all radio-active systems commercially available, formation properties are deduced from a change in the detected activity. The new logging system differs from current commercial radioactive logging systems in both the means of obtaining data and the type of reactions involved. By pulsed operation of the generator and simultaneous gating of the detector, only those prompt gamma rays resulting from high-energy neutron reactions are detected. The detector remains "off" while the neutrons are being slowed sufficiently by formation elements for capture reactions to occur. In practice, the detector remains "on" about 5 microsec and "off" about 995 microsec. The prompt gamma rays are detected and collected to form gamma-ray energy spectra for analysis of elements present in the formation. One objective of the current research was to construct a logging tool and to use it in field operations to demonstrate the feasibility of directly detecting elements in formations by nuclear means. Another objective was to develop preliminary techniques of interpreting the logged data to identify formation lithology and to measure oil content and porosity. The purposes of this paper are to summarize field results, to disclose data interpretation, and to describe briefly important performance characteristics of the new logging system. Summary of Field Experience Twenty-seven field tests have been made with the spectral logger, and all but three have been in Texas. For practical purposes, tests have been made in all typical areas and under all logging conditions that might normally he encountered in field operations. For example, several of the first field tests were conducted in the Gulf Coast area where sandstones, ranging from fairly clean to shaly, were logged. Subsequently, nine wells were logged in limestone or evaporite formations. Wells in three fields penetrated limestone, dolomite and anhydrite. JPT P. 801ˆ