Natural Gamma Ray Logging Correction for Mud Activation Bias
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Solution Overview
Problem
Natural gamma ray logging measurements in the oil and gas industry are biased by gamma rays emitted from drilling fluids activated by neutron sources, which interfere with the accurate determination of potassium, uranium, and thorium concentrations in subterranean formations.
Innovation Solution
A method that deploys a logging string with a neutron source and a natural gamma ray sensor, emitting neutrons and acquiring a gamma ray spectrum, which is then processed in combination with elemental standard spectra and standard mud activation spectra to compute corrected measurements, accounting for both downward and upward drilling fluid activation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a neutron source is used in the logging string to enable spectral gamma ray measurements, then the ability to determine potassium, uranium, and thorium concentrations is improved, but mud activation occurs which emits gamma rays and biases the natural gamma ray logging measurements
Solution Approach 1:
The measured gamma ray spectrum is segmented into distinct components: formation natural gamma rays, mud activation gamma rays, and tool-induced gamma rays. By separating these contributions mathematically, the harmful mud activation signal is isolated and can be subtracted from the total measurement, leaving the pure formation signal for accurate concentration determination.
Solution Approach 2:
Standard mud activation spectra are used as intermediary references to characterize the mud activation response. These standard spectra, obtained from controlled measurements with known mud compositions, serve as templates that can be applied to field measurements to identify and quantify the mud activation component without directly measuring it during formation logging.
2Reliability
If drilling fluid flows both downward and upward in the borehole, then complete circulation is achieved, but activation occurs in both flow directions creating complex gamma ray interference patterns
Solution Approach 1:
The method dynamically adapts to bidirectional flow conditions by using separate standard measurements for downflow and upflow activation. The system determines the relative contribution of each flow direction's activation based on actual logging conditions, allowing the correction algorithm to account for the dynamic nature of mud circulation patterns during drilling operations.
Solution Approach 2:
The correction methodology changes parameters by using distinct activation spectra for different flow directions (downflow vs. upflow). By measuring and storing separate standard spectra for each flow direction, the system can adjust the correction applied based on the actual flow conditions observed during logging, rather than using a single static correction factor.
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 method provides accurate corrections for mud activation-induced bias, enabling precise determination of potassium, uranium, and thorium concentrations in subterranean formations, effectively accounting for both downflow and upflow drilling fluid activation.
Implementation Method 1
drilling fluids (mud) in the borehole may be activated by the generated neutrons. Such activated mud is known to emit gamma rays
Implementation Method 2
a gamma ray spectrum is acquired by the gamma ray detector
Data Source
AI summary
A method for correcting natural gamma ray measurements includes processing an acquired gamma ray spectrum in combination with elemental standard spectra and at least one standard mud activation spectrum to compute corrected natural gamma ray measurements. The gamma ray spectrum is acquired using a logging string employing a neutron source and a natural gamma ray sensor.