Laboratory and Field Evaluation of an Inelastic Neutron Scattering and Capture Gamma Ray Spectrometry Tool
- R.C. Hertzog (Schlumberger-Doll Research Center)
- Document ID
- Society of Petroleum Engineers
- Society of Petroleum Engineers Journal
- Publication Date
- October 1980
- Document Type
- Journal Paper
- 327 - 340
- 1980. Society of Petroleum Engineers
- 5.8.7 Carbonate Reservoir, 4.1.5 Processing Equipment, 5.2 Reservoir Fluid Dynamics, 4.3.4 Scale, 5.2.1 Phase Behavior and PVT Measurements, 5.8.5 Oil Sand, Oil Shale, Bitumen, 5.2.2 Fluid Modeling, Equations of State, 1.14 Casing and Cementing, 5.6.1 Open hole/cased hole log analysis, 4.1.2 Separation and Treating
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An experimental system for gamma ray spectroscopy logging has been developed which uses prompt and capture gamma radiation induced in formations by 14-MeV neutrons from a pulsed-neutron generator to determine relative concentrations of various elements in the formation. The logging system uses computer-processing techniques based on spectral modeling that has been developed to analyze the inelastic and capture gamma ray data obtained with a borehole spectrometer.
The physics of the production of gamma rays from fast-neutron interactions with elemental nuclei in formations is discussed, leading to a simple but realistic interpretation model for the tool's response. This model is confirmed by laboratory and field tests.
The relative spectroscopic contributions from carbon, oxygen, silicon, calcium, iron, chlorine, and hydrogen are used for various cased-hole and openhole logging applications. Particular emphasis is placed on the carbon/oxygen ratio used to obtain oil saturation independent of formation-water salinity. Carbon/oxygen ratio determinations made in the laboratory are compared with values predicted on the basis of known lithologies, porosities, and oil-saturation changes.
In addition, the spectral contributions from iron, silicon, and calcium are used to interpret lithology; hydrogen, silicon, and calcium contributions are used to determine the effects of porosity; and chlorine and hydrogen contributions are used to investigate salinity changes.
Field-test log examples of these elemental determinations are shown.
A principal objective of the induced gamma ray tool (IGT™) is to make a more direct measurement of oil saturation So in cased-hole formations where conditions are not favorable for the use of the thermal neutron decay time (TDT™) log. These include formations with low water salinity (less than 50,000 ppm NaCl) and formations where the producing zone is being flooded with water of unknown or variable salinity.
Any measurement of the relative number of carbon and oxygen atoms in the formation will be sensitive to the changing formation-fluid composition. Fig. 1 shows the calculated carbon/oxygen atomic-abundance ratio vs. porosity f for various oil saturations in limestone, dolomite, and sandstone formations and provides a picture of the expected changes in this ratio for typical clean formations. For example, in moderate to high porosities (0.20 < f < 0.30), the atomic carbon/oxygen ratio in limestone changes by about a factor of two as the oil saturation changes from 0 to 1000/0. In sandstone the atomic carbon/oxygen ratio starts from zero and increases with oil saturation by about the same amount.
However, since there is considerable carbon in the matrix of a limestone or dolomite formation, an elemental analysis for carbon and oxygen by itself is not sufficient to interpret unambiguously oil saturation using a crossplot such as Fig. 1. An independent knowledge of lithology and porosity is required.
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