Pulsed Neutron Cement Void Detection in Boreholes
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Solution Overview
Problem
Current methods for detecting cement voids between a casing and a borehole wall are inefficient and inaccurate, as they struggle to differentiate between insignificant microannuli and significant voids, which can compromise the hydraulic seal and are often performed separately from well logging processes.
Innovation Solution
A pulsed-neutron logging tool is used to irradiate the earth formation with neutrons, detecting gamma rays with multiple detectors to calculate a ratio (RIN) that differentiates between cemented and uncemented conditions, allowing for the estimation of cement voids by normalizing the RIN value within a dynamic range representing the cement volume percentage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If acoustic signals are used to detect cement voids, then the detection process can be performed, but the accuracy and efficiency are insufficient to differentiate between insignificant microannuli and significant voids
Solution Approach 1:
The patent replaces acoustic signal-based detection with neutron interrogation technology. Neutrons are emitted into the borehole and interact with cement and formation materials, producing gamma rays that are detected to determine cement presence and void conditions. This substitution of physical detection mechanism enables superior differentiation between microannuli and significant voids while improving measurement precision and efficiency.
Solution Approach 2:
The patent utilizes changes in neutron interaction parameters (gamma ray count rates, inelastic scattering ratios) to detect cement voids. By measuring the ratio of inelastic to capture gamma rays and analyzing neutron capture cross-sections, the system determines cement saturation and void conditions. This parameter-based approach provides accurate differentiation between insignificant microannuli and significant voids.
2Reliability
If well logging is performed separately from void detection, then each process can be optimized independently, but the overall process time and complexity increase
Solution Approach 1:
The patent merges well logging and void detection into a single integrated neutron interrogation process. The same neutron source and detector system used for formation logging also detects cement voids by analyzing gamma ray interactions with cement and formation materials. This consolidation eliminates separate detection processes, reducing time loss while maintaining reliable assessment of both formation properties and cement integrity.
Solution Approach 2:
The neutron interrogation system performs multiple functions simultaneously: it conducts well logging measurements of formation properties while also detecting cement voids and assessing cement saturation. This multi-functional approach allows a single tool to evaluate both the earth formation and the cement annulus, eliminating the need for separate detection processes and reducing overall time requirements.
3Ease of operation
If traditional acoustic detection methods are used, then the basic void detection can be performed, but the ability to differentiate between insignificant microannuli and significant voids is compromised
Solution Approach 1:
The patent replaces acoustic detection with neutron interrogation, which provides superior differentiation capability. Neutrons interact differently with cement, water, and gas, allowing the system to distinguish between insignificant microannuli and significant voids based on gamma ray count rates and interaction patterns. This substitution maintains operational simplicity while dramatically improving measurement precision.
Solution Approach 2:
The patent utilizes neutron interaction parameters (gamma ray energy spectra, count rates, inelastic scattering ratios) to achieve precise differentiation. By analyzing the energy distribution and intensity of gamma rays produced during neutron-cement and neutron-formations interactions, the system can reliably differentiate between insignificant microannuli and significant voids, providing both ease of operation and high measurement precision.
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 a more accurate and efficient detection of cement voids by utilizing Monte Carlo modeling to interpret the RIN response, enabling precise determination of cement presence and extent, thereby maintaining the integrity of the borehole and casing.
Implementation Method 1
The earth formation is irradiated with neutrons from a neutron source disposed at the logging tool in the borehole. Radiation in the form of gamma rays is emitted from the formation due to interactions between the neutrons and materials in the formation.
Implementation Method 2
Radiation in the form of gamma rays is emitted from the formation due to interactions between the neutrons and materials in the formation.
Implementation Method 3
a ratio of inelastic gamma rays measured by the two detectors (referred to herein as 'RIN') is utilized to detect the presence, and in some embodiments, the extent, of voids in the cement.
Data Source
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AI summary
Estimating cement voids in between a casing and a borehole wall of a borehole penetrating the earth formation includes forming with a computing device an actual ratio of radiation induced by a neutron source and measured by a first detector and a second detector when the tool is at two or more different locations in the borehole. A relationship between the actual ratio and the porosity at each of the two or more different locations is used and compared to two different cases of estimates of the ratio. Based on the comparison, an estimate of the cement voids at the two or more locations.