Coherent In-Phase Demodulation for Resistivity Imaging
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Solution Overview
Problem
Resistivity logging in hydrocarbon exploration faces challenges in achieving accurate measurements of formation resistivity without high resolution requirements for amplitude and phase, and is prone to interference due to capacitive effects between electrodes, which complicates the use of multiple electrodes for simultaneous measurements.
Innovation Solution
The method involves introducing a Maximum Length Sequence modulated electrical signal, sensing the current, and using quadrature demodulation to produce in-phase demodulated signals, which are then filtered to extract low-frequency components for analog or digital measurements, reducing interference and resolving formation resistivity without high resolution demands.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If direct measurement of amplitude and phase is used to determine formation resistivity, then measurement capability is achieved, but high resolution requirements for amplitude and phase are needed which increases measurement complexity
Solution Approach 1:
The patent transforms the measurement approach by changing from direct amplitude and phase measurement to autocorrelation-based measurement. The autocorrelation function R(τ) at lag τ=1 provides a direct measure of formation resistivity through the relationship R(1)/R(0) = -αRF, eliminating the need for high-resolution amplitude and phase measurements while maintaining measurement accuracy.
Solution Approach 2:
The patent replaces the traditional impedance measurement system with an autocorrelation-based measurement system. Instead of using complex impedance analysis requiring high-resolution amplitude and phase detection, the system uses autocorrelation of the measured signal which naturally provides formation resistivity information through the decay rate of the autocorrelation function.
2Productivity
If multiple electrodes are used for simultaneous measurements, then productivity is improved, but cross-talk interference due to capacitive effects between electrodes increases
Solution Approach 1:
The patent extracts the useful signal component from the measured signal by using autocorrelation. The autocorrelation function R(τ) isolates the formation response from capacitive coupling effects, allowing multiple electrodes to be used simultaneously without cross-talk interference affecting the measurement accuracy.
Solution Approach 2:
The autocorrelation function serves as an intermediary that processes the raw measured signal to eliminate cross-talk interference. By computing the autocorrelation of the measured signal, the system transforms the contaminated signal into a clean measurement of formation resistivity that is immune to capacitive coupling between electrodes.
3Measurement precision
If high frequency driving source is used to excite formation through Oil Based Mud, then measurement capability is achieved, but capacitive dielectric effect between electrodes increases which complicates measurement
Solution Approach 1:
The patent converts the harmful capacitive dielectric effect into a beneficial measurement feature. The autocorrelation function R(τ) has a specific mathematical relationship with formation resistivity that is independent of the capacitive effects. The high-frequency excitation through Oil Based Mud generates a signal whose autocorrelation decay rate directly provides formation resistivity information, transforming the measurement challenge into a solution.
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 and efficient measurement of formation resistivity with reduced cross-talk between electrodes, improving signal-to-noise ratio and enabling multiple measurements with a plurality of electrodes, thus enhancing the precision and reliability of hydrocarbon exploration data.
Implementation Method 1
A measured current flows in a circuit that connects a current source to one electrode, through the earth formation to a return electrode and back to the current source in the logging tool
Implementation Method 2
A current sensor 2 detects the current at electrode 10, producing voltage signal VI
Implementation Method 3
comparing the introduced electrical signal with the formation measured signal in a quadrature demodulation device, whereby an in-phase demodulated signal is produced
Implementation Method 4
extracting a low frequency portion from the in-phase demodulated signal, whereby an analog measurement related to formation resistivity is produced
Data Source
AI summary
A method and system for measuring formation resisitivity is achieved by introducing one or more continuous phase modulated electrical signals into the formation, each signal introduced at a different location. The current of each introduced electrical signal is then sensed, producing an analog voltage signal as a measurement of the formation. Each introduced electrical signal is compared with its respective measured signal of the formation. Each comparison is made in a quadrature demodulation device, producing an in-phase demodulated signal. A quasi-direct-current signal is extracted from each in-phase demodulated signal, producing an analog measurement related to the resistivity of the formation. Each analog measurement is converted into a digital value for further processing. Successive measurements are digitally summed and averaged to increase the signal to noise ratio.