Environmental Corrections and System Calibration for a New Pulsed-Neutron Mineralogy Instrument

摘要

A new openhole logging service has been developedwhich provides lithological and quantitativemineralogical information for improved formationevaluation. The system uses data collected from bothpulsed-neutron and natural gamma ray logginginstruments. Whereas the natural gamma ray toolmeasures the energy of gamma rays from naturallyoccurring sources, the pulsed-neutron device inducesgamma ray activity from elements in the immediatesurroundings, mainly through neutron inelastic scatteringand capture interactions. Before reaching atoms withinthe formation, the 14-MeV neutrons must first penetratematerials within both the tool and surrounding borehole.The emitted neutrons lose energy and quickly slow down,or become thermalized, as a result of elastic and inelasticscattering interactions. The thermal neutrons areeventually absorbed via the capture interaction. Thesecapture events, in conjunction with the inelasticinteractions, account for almost all characteristic gammarays detected by the tool.The measured spectrum is a superposition of all gammarays that reach the detector. Accordingly, it must beseparated into its fundamental elemental components.The magnitude of each spectral component is referred toas its elemental yield. Algorithms convert these yieldsinto elemental concentrations, which, in turn, provideinput into geoscience models for determination offormation lithology and mineralogy. The yield-to-weightconversion algorithms are derived by using data fromlaboratory and subsurface in-situ formations whosechemical compositions are well understood.One of the problems encountered in any such system isthat the neutrons interact not only with the formation, butwith elements in the tool and borehole as well.Consequently the detected gamma ray response isaffected by factors such as wellbore conditions, boreholesize, mud weight, mud type, mud additives, and salinity.Three steps are required to obtain meaningful results.First, one must correctly decompose the spectrum intoelemental yields; second, the contributions from eachelement must be corrected for environmental conditions;and third, the elemental yields must be converted intoelemental concentrations. During this last step, somepetrophysical parameters such as porosity are taken intoaccount.This paper focuses on the environmental correctionsrequired to calculate accurate concentrations of theelements within the formation. Required correctionsinclude those for borehole size, mud weight, mud type,mud additives, and salinity. The neutron transport andone-group neutron diffusion equations are examined todemonstrate that borehole size affects neutron flux. Asthe actual case is much more complex, a completeunderstanding can best be obtained from Monte Carlomethods. The derived system of corrections for theelements in the natural, capture, and inelastic spectra arebased upon a combination of experimental data andMonte Carlo computer simulations.