Natural Gamma-Ray Mud Activation Correction
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Solution Overview
Problem
Existing gamma-ray logging methods in boreholes face inaccuracies due to gamma-rays emitted by activated mud, which are not accurately corrected for, especially when the inside and outside mud activation spectra differ, leading to discrepancies in natural gamma-ray measurements.
Innovation Solution
A method and system that determine separate calibration ratios for gamma-rays generated inside and outside the borehole annulus, allowing for precise correction of natural gamma-ray measurements by subtracting specific count rates based on these ratios, accounting for the varying contributions of activated mud from different locations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single calibration ratio is used for gamma-ray correction, then the correction process is simple, but the measurement precision deteriorates when inside and outside mud activation spectra differ
Solution Approach 1:
The patent divides the mud activation correction into two separate components: inside mud activation (within the BHA) and outside mud activation (in the borehole annulus). Each component has its own calibration ratio determined and applied separately. This segmentation allows the complex spectral differences between inside and outside mud to be handled individually, improving measurement precision while maintaining manageable correction complexity through systematic separation of the correction tasks.
2Measurement precision
If separate calibration ratios for inside and outside mud activation are determined, then the measurement precision improves, but the device complexity increases
Solution Approach 1:
The patent applies local quality by determining calibration ratios that are specific to local conditions: inside mud activation characteristics (within the BHA geometry and flow conditions) and outside mud activation characteristics (in the borehole annulus with different flow and geometric conditions). Each calibration ratio is tailored to its specific location and activation conditions, allowing precise correction for each source while the overall process remains systematic and manageable through this localized approach.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach provides an accurate correction for mud activation in natural gamma-ray measurements, even when neutron source activation is significant, minimizing errors caused by differing inside and outside mud activation spectra.
Implementation Method 1
The drilling mud contains elements, in particular oxygen, that may be activated as explained above. In particular, oxygen-16 nuclei may be converted into nitrogen-16 nuclei
Implementation Method 2
the radioactive nitrogen atoms decay quickly by beta decay mostly to an excited state of oxygen, which in turn decays by emitting gamma-rays. A majority of the emitted gamma-rays have an energy around 6.1 MeV
Implementation Method 3
A detector in a spectral mode, i.e. a detector that is sensitive to the energy of the gamma-rays, may provide a gamma-ray spectrum, i.e. a gamma-ray count rate as a function of energy
Data Source
AI summary
The disclosure relates to a method for correcting a downhole natural gamma-ray measurement performed in a wellbore. A gamma-ray measurement including at least a gamma-ray count rate is obtained by a gamma-ray detector disposed in a bottom hole assembly having a mud channel inside of the assembly, such that mud flows downwards in the mud channel and upwards outside of the assembly and a neutron source situated above the gamma-ray detector and activating the mud. The method includes:Determining from the gamma-ray measurement an interval count rate corresponding to a count rate of gamma-rays having an energy within a predetermined correction interval;Computing an outside and an inside calibration ratio (ratio of a gamma ray count rate in the correction interval to a gamma-ray count rate outside of the correction interval) representative of gamma-rays generated by an activation of mud flowing respectively outside of the assembly and inside of the assembly,Based on the outside calibration ratio and the interval count rate, determining a first correction count rate,Based on at least the inside and the outside calibration ratios, determining a second correction count rate,Subtracting from the total count rate the first and second correction count rates in order to get a natural gamma-ray measurement corrected for mud activation.