Quadrature Demodulation for Resistivity Logging
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Solution Overview
Problem
Current resistivity logging methods require high resolution measurements of amplitude and phase to accurately estimate formation resistivity, especially when using Oil Based Mud, which is inefficient and power-intensive.
Innovation Solution
The method involves introducing an electrical signal, sensing the current, and using quadrature demodulation to produce an in-phase demodulated signal, extracting a low frequency portion, and converting it into a digital measurement, reducing the need for high resolution and power consumption, with options including burst mode operation and specific demodulation techniques like Hadamard coherent demodulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If direct measurement of amplitude and phase is used to calculate formation resistivity, then measurement accuracy is improved, but measurement precision requirements and power consumption increase
Solution Approach 1:
The patent introduces an intermediary processing stage (quadrature demodulation) between the raw impedance measurement and the final resistivity calculation. This demodulation process extracts the in-phase component as an intermediate variable that linearly relates to formation resistivity, avoiding the need for high-resolution direct phase and amplitude measurements while maintaining measurement accuracy
2Reliability
If high frequency driving source is used to excite the formation through Oil Based Mud standoff, then formation resistivity can be measured, but power dissipation increases
Solution Approach 1:
The patent employs periodic excitation signals (sinusoidal or square waves at frequencies like 10 MHz, 100 MHz, or 1 GHz) to drive the formation through the OBM standoff. This periodic action enables efficient signal transmission through the capacitive standoff while allowing for demodulation-based detection that reduces continuous power requirements compared to direct measurement methods
3Measurement precision
If direct impedance measurement method is used, then formation resistivity can be estimated, but device complexity and resolution requirements increase
Solution Approach 1:
The patent replaces the complex direct measurement system (requiring high-resolution amplitude and phase detectors) with a demodulation-based system. The quadrature demodulator converts the high-frequency impedance measurement into a baseband in-phase signal that linearly represents formation resistivity, simplifying the overall measurement system while maintaining or improving accuracy
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 accurate estimation of formation resistivity with reduced power dissipation and resolution requirements, improving the efficiency and accuracy of resistivity logging.
Implementation Method 1
comparing the introduced electrical signal with the formation measured signal in a quadrature demodulation device, whereby an in-phase demodulated signal is produced
Data Source
AI summary
A method and system for measuring formation resisitivity is achieved by introducing an electrical signal into the formation. The current of the introduced electrical signal is then sensed, producing an analog voltage signal as a measurement of the formation. The introduced electrical signal is compared with the measured signal of the formation. The comparison is made in a quadrature demodulation device, producing an in-phase demodulated signal. A quasi-direct-current signal is extracted from the in-phase demodulated signal, producing an analog measurement related to the resistivity of the formation.