Downhole Transient Resistivity Measurements via Time Derivative Analysis
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Solution Overview
Problem
Transient electromagnetic measurement techniques used in geologic formation evaluation and geo-steering are hindered by the conductive drill string, which causes parasitic eddy currents that limit the resolution of formation parameter measurements, particularly in identifying boundaries like water-oil interfaces.
Innovation Solution
The method involves using the time derivative of the electromotive force measured in receivers during transient electromagnetic measurements, either by calculating it or selecting a rectangular pulse that approximates a step function, to reduce the parasitic effect of conductive downhole components and enhance the resolution of formation properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If transient electromagnetic measurements are performed using conventional methods, then formation electrical conductivity can be estimated, but the measured signal is severely affected by eddy currents in the conductive drill string, limiting measurement resolution
Solution Approach 1:
The patent changes the temporal parameter of signal measurement by using the time derivative of the electromotive force instead of the raw signal. This transformation emphasizes early-time formation responses while suppressing late-time drill string eddy current effects, thereby improving measurement resolution despite the presence of conductive drill string
Solution Approach 2:
The patent applies a preprocessing operation (calculating the time derivative) to the measured electromotive force signal before formation parameter estimation. This preliminary action removes or reduces the parasitic eddy current effects from the signal, allowing subsequent formation evaluation to proceed with improved accuracy
2Measurement precision
If the time derivative of electromotive force is used to reduce parasitic effects, then formation signal resolution is improved, but additional signal processing complexity is introduced
Solution Approach 1:
The patent replaces direct measurement of formation parameters with an indirect approach using time derivative analysis of the electromotive force signal. This substitution transforms a complex measurement problem into a more manageable signal processing task that can be implemented through standard computational methods
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 significantly improves the relative contribution of formation signals over parasitic signals, allowing for more accurate resistivity measurements and effective geo-steering even with a short MWD tool, by suppressing eddy currents and improving signal quality.
Implementation Method 1
transmitting transient pulses of current into a formation. The induced electromagnetic field and decay responses are measured
Implementation Method 2
The measured transient induction signal is severally affected by eddy currents that develop in the drill string
Data Source
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AI summary
A method of performing an electromagnetic measurement of an earth formation includes disposing a downhole tool in a borehole in an earth formation, the downhole tool including an electrically conductive component, at least one electromagnetic transmitter located at a location along the tool, a first receiver located a first distance from the location, and a second receiver located at a second distance from the location. The method also includes applying a single rectangular current pulse having a substantially constant amplitude to the transmitter to generate an electric current in the formation, the current pulse having a specified time duration ΔT. The method further includes measuring transient signals during a time period subsequent to the time in response to the generated current, combining the transient signals to generate a combined transient signal, and estimating a resistivity of the formation based on the combined transient signal.