SPECTRAL GAMMA RAY MEASUREMENT WHILE DRILLING

摘要

With the significant growth in the volume ofunconventional wells over the past decade there is aneed to an economical and reliable method forformation evaluation in horizontal wells. To lowerdrilling program costs, the majority of these wells aredrilled with only measurement-while-drilling (MWD)gamma ray logs. This limits available data to makecritical well placement decisions while drilling or forpost-drilling well evaluation.A low-cost spectral gamma ray measurement hasrecently been developed for use in logging-whiledrilling(LWD) applications. Applications rangefrom geosteering and bed boundary detection toformation evaluation for clay typing and forempirical correlations to estimate total organiccarbon (TOC). The spectral gamma ray measurementis incorporated into an MWD collar, thereby avoidingthe need for additional collars in the bottom-holeassembly(BHA). This improves system reliabilityand reduces distance behind bit in a shorter BHA.The measurement quality, which is designed to matchthat of the industry-leading wireline spectral gammaray tool, is ensured by employing two large NaIdetectors that provide high counting rates andspectral definition, thereby enabling a measurementwith very good statistical precision. Highly accuratecharacterization formations for potassium (K),uranium (U), and thorium (Th) were constructed.Tool responses acquired in these standard formations,as well as in the spectral gamma spectroscopy pits atthe University of Houston, allowed a highly accuratecharacterization of the measurement. Advancedfeatures include robust sourceless gain regulation,real-time full spectrum analysis, and automaticborehole potassium correction. To further enhancegeosteering, the design may also be configured with asingle azimuthal total gamma ray detector and asingle spectral gamma ray detector. Thiscombination provides bedding layers andcomposition data in real time.This novel system, available in 4 ¾-in. or 6 ¾-in.MWD collar sizes, makes accurate measurements offormation potassium, uranium, and thoriumconcentrations in an economical and reliable manner.Log examples, laboratory data, computer modelingresults, and comparisons with wireline spectralgamma ray data are presented.