Pulsed Eddy Current Logging for Individual Nested Casing Corrosion
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Solution Overview
Problem
Conventional electromagnetic (EM) techniques for corrosion evaluation in multiple casing strings in oil and gas wells struggle to accurately differentiate between individual casing thicknesses due to their sensitivity to total metal thickness, leading to significant errors in corrosion assessment.
Innovation Solution
A downhole logging tool with a transmitter and multiple receivers spaced at distinct distances, combined with a processing system, records and processes broadband excitation signals to derive information on individual casing thicknesses by modeling receiver responses to ideal step excitations, accounting for non-ideal transmitter signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional electromagnetic techniques are used for corrosion evaluation in multiple casing strings, then the measurement can be performed, but the ability to accurately differentiate between individual casing thicknesses deteriorates due to sensitivity to total metal thickness
Solution Approach 1:
The patent segments the measurement process by using multiple receivers at different distances from the transmitter, where each receiver is sensitive to different casings based on their spatial separation. This allows the total response to be divided into individual casing contributions, enabling accurate differentiation of each casing's thickness independently.
Solution Approach 2:
The patent introduces a spatial dimension by positioning receivers at variable distances from the transmitter. This dimensional arrangement creates distinct measurement zones where each receiver primarily detects a specific casing, transforming the single-point measurement limitation into a multi-dimensional measurement capability that resolves individual thicknesses.
2Measurement precision
If multiple receivers at variable distances are used to differentiate individual casings, then individual thickness measurement improves, but device complexity increases
Solution Approach 1:
The patent makes the receiver system universal by enabling each receiver to serve multiple purposes: they detect signals from different casings depending on their position, and the same receiver array can measure both individual casing thicknesses and overall formation characteristics. This multi-functionality justifies the added complexity by providing versatile measurement capabilities.
Solution Approach 2:
The patent introduces dynamic measurement capabilities through broadband excitation signals that can be analyzed in both time-domain and frequency-domain. The system can adaptively select measurement modes and process signals dynamically to extract individual casing information, making the fixed hardware configuration flexible and adaptable to different measurement requirements.
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
Enables accurate determination of individual casing thickness variations, reducing errors in corrosion assessment by separating and analyzing the contributions of each casing, even when they are not concentric.
Implementation Method 1
The transmitter sends a current at a selected frequency into the formation that creates an alternating-current (AC) EM field in a permanent regime, which induces eddy current in the surrounding environment
Implementation Method 2
An EM signal is induced by the eddy current in the receiver coils
Implementation Method 3
For a time-domain measurement, a square pulse signal is fed to the transmitter. Such signal also induces eddy current in the environment that, in turn, induces transient voltage signals which are measured by the receivers
Data Source
AI summary
Methods and systems pertaining to a processing system and a downhole logging tool for investigating nested tubulars in a borehole. The downhole logging tool includes a transmitter coil for emitting a broadband excitation signal including one or more step excitations. The downhole logging tool also includes receiver coils including at least two receiver coils spaced apart from the transmitter coil at respective first and second distances from the transmitter, the first and second distances being distinct from each other, and each receiver coil being able to receive a response signal resulting from the excitation signal. The downhole logging tool also includes an acquisition system for recording the excitation signal and the response signal of each receiver coil over time. The processing system derives information indicative of variations in individual thicknesses of the nested casing tubulars based on variations of the response signals over time.