Casing Correction via Transmitter Impedance Measurement
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Solution Overview
Problem
Electromagnetic induction surveys in cased boreholes face significant attenuation and variability due to conductive casing, leading to challenges in accurately mapping geologic formations, as casing properties like conductivity, permeability, and thickness can cause large field variations that obscure formation resistivity changes.
Innovation Solution
A method using impedance measurements of the electromagnetic transducer to determine casing attenuation factors, based on relationships between casing conductivity, thickness, and frequency, allowing for compensation of casing effects in electromagnetic surveys, enabling accurate formation imaging despite casing variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If electromagnetic induction surveys are performed in cased boreholes, then the ability to survey between wells with metallic liners is improved, but the signal attenuation due to conductive casing severely degrades measurement quality
Solution Approach 1:
The patent introduces an intermediary correction model that mediates between the transmitted signal and the received signal. By measuring the impedance of the transmitter and using theoretical models to calculate casing attenuation factors, the system creates a correction term that compensates for the signal loss introduced by the conductive casing, allowing accurate formation imaging despite the presence of the casing.
Solution Approach 2:
The patent changes the operational parameters by measuring the transmitter impedance at multiple frequencies and using these frequency-dependent measurements to characterize the casing properties. By obtaining correction factors at different frequencies and interpolating to the survey frequency, the system adapts the correction to match the actual operating conditions, improving accuracy across varying frequency regimes.
2Power
If multi-turn solenoid transmitters are operated at high current levels, then the transmitter strength is improved, but the magnetic permeability of the core material and casing is driven into a nonlinear regime causing current to be non-proportional to radiated field
Solution Approach 1:
The patent implements feedback by measuring the actual impedance of the transmitter in the cased borehole and using this measurement to calculate a correction factor. This correction factor accounts for the nonlinear effects and casing attenuation, allowing the system to compensate for the non-proportional relationship between current and radiated field, thereby maintaining accurate measurements even at high current levels.
Solution Approach 2:
The patent substitutes the direct mechanical/electromagnetic coupling relationship with an electrical measurement approach. Instead of relying on the proportional relationship between current and field, the system uses impedance measurements to indirectly characterize the system response and applies theoretical models to calculate corrections, replacing direct electromagnetic proportionality with an electrical measurement-based correction system.
3Measurement precision
If conventional induction logging uses multiple receivers and transmitters connected in series to cancel mutual signal in air, then the formation resistivity mapping ability is improved, but the system complexity and difficulty of detecting and measuring casing effects increases
Solution Approach 1:
The patent extracts the casing attenuation effect from the overall measurement system by measuring the transmitter impedance independently and using theoretical models to calculate the casing correction factors. This separation allows the casing effects to be removed as a distinct correction term, simplifying the interpretation of formation properties without requiring complex multi-receiver configurations.
Solution Approach 2:
The patent creates a theoretical copy of the casing attenuation effect through impedance measurements and theoretical modeling. By measuring the impedance and using these measurements to calculate correction factors that replicate the casing effect in a controlled manner, the system can then subtract this copied effect from the total measurement, isolating the formation response without needing complex physical configurations.
4Measurement precision
If the frequency is increased to improve resolution, then the ability to detect formation variations is improved, but the casing attenuation obliterates the formation response
Solution Approach 1:
The patent performs preliminary action by measuring the transmitter impedance at multiple frequencies before the actual survey. These preliminary measurements are used to calculate correction factors that are then applied to compensate for casing attenuation during the survey. This advance characterization of the casing effect allows the system to maintain high-frequency measurements without being overwhelmed by casing attenuation, as the corrections are pre-computed and applied.
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 allows for robust prediction and compensation of casing attenuation across various frequencies, improving the accuracy of electromagnetic induction surveys by separating casing effects from formation responses, even with non-linear transmitter operations and varying casing properties.
Implementation Method 1
A transmitter, usually a multi-turn coil of wire, carries an alternating current of frequency ω (radians/sec) when placed in a wellbore. The current in the coil creates a time varying magnetic field in the surrounding subterranean formation which in turn, by Faraday's law, induces an electromotive force (emf). The emf drives currents in the formation, which are proportional to the formation conductivity.
Implementation Method 2
The receiver measures the magnetic field arising from the transmitter and the secondary, or induced, currents in the formation.
Implementation Method 3
The signal from the transmitter to the receiver is severely attenuated in passing through the metallic casing because of its high conductivity and, usually, high magnetic permeability.
Data Source
AI summary
Methods and related systems are described for determining the casing attenuation factor for various frequencies from measurements of the impedance of the transmitting or receiving coil of wire of. The compensation is based on two relationships. The first relationship is between one or more measured impedance parameters and the product of casing conductivity, casing thickness and electromagnetic frequency. The second relationship is between the casing correction factor and the product of casing conductivity, casing thickness and electromagnetic frequency.