Non-Collimated Gamma Logging Tool for Formation Density
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Solution Overview
Problem
Current well logging technologies face limitations in accurately measuring apparent formation bulk density due to collimation issues, which result in shallow measurements, inability to correct for mudcake and cement thickness, and variations in casing and annular fill material, leading to inaccurate density readings, especially in cased-hole environments.
Innovation Solution
A non-collimated gamma-ray source and detectors are used to measure scattered gamma-rays from a centralized logging tool, allowing for the determination of formation density, cement thickness, and casing characteristics without direct contact with the borehole wall, using multiple detectors and calibration methods to account for various borehole parameters and conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If collimated gamma-ray sources and detectors are used to measure formation density, then the measurement depth is increased, but the ability to correct for mudcake and cement thickness is lost
Solution Approach 1:
The tool uses multiple detectors at different spacings from the source (short-spacing, mid-spacing, and long-spacing detectors) to segment the measurement depths. This allows each detector to probe different formation depths and materials (casing, cement, mudcake, formation), enabling comprehensive correction capabilities while maintaining deep measurement capability through the long-spacing detector
Solution Approach 2:
The invention transitions from traditional single-depth collimated measurements to multi-dimensional measurements by spacing detectors at multiple distances from the source. This creates a depth-dimension matrix where each detector provides information from a different radial distance, enabling both deep formation penetration and detailed near-wall correction measurements simultaneously
2Measurement precision
If collimated tools are used for density measurement, then the gamma-ray beam is focused, but the measurements become shallow and sensitive to tool standoff
Solution Approach 1:
The invention employs detectors with different spacing configurations that provide dynamic measurement capabilities. The short-spacing, mid-spacing, and long-spacing detectors create a dynamic measurement system that can adapt to different borehole conditions, maintaining measurement depth while providing focus through the multi-level detector architecture rather than simple collimation
3Measurement precision
If multiple detectors at different spacings are used, then correction for mudcake and cement is enabled, but the device complexity increases
Solution Approach 1:
The multi-detector tool configuration serves multiple functions simultaneously: the long-spacing detector provides deep formation density measurement, mid-spacing detectors measure cement and mudcake properties, and short-spacing detectors measure near-wall conditions. This universal design allows a single tool to perform comprehensive borehole environment characterization without requiring multiple separate tools
Solution Approach 2:
The tool uses its own multi-detector measurements to self-correct for borehole environment effects. By measuring the gamma-ray attenuation through different materials (casing, cement, mudcake) using detectors at various spacings, the tool automatically calculates and applies corrections to the formation density measurement, eliminating the need for external correction procedures
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 enables more accurate and comprehensive measurements of formation bulk density and related parameters, reducing the need for corrections like 'Spine and Ribs' and improving repeatability and sensitivity, while accommodating variable wellbore conditions and geometries.
Implementation Method 1
The 137Cs source and the gamma-ray scintillation detectors are collimated toward the borehole wall... The use of two or more gamma-ray detectors spaced apart at different distances from the gamma-ray source is known to those skilled in the art as a method used to produce a formation bulk density log
Data Source
AI summary
Apparatus for wellbore logging and method of use are provided for measurement of the annular materials interposed between the logging tool and formation and the apparent formation bulk density in open-hole, cased-hole or other conduit. The logging tool can be centralized in the borehole. Non-collimated gamma radiation is emitted into the surrounding media and the amount of radiation which returns to the tool is measured by multiple omni-directional detectors spaced a pre-set distances from the source to provide multiple radial depths of investigation. The annular thickness and density of materials interposed between the logging tool and the formation may be determined from the detector responses and be used to derive the apparent formation bulk density.