Downhole EM Interferometry Using Ambient Field Cross-Correlation
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Solution Overview
Problem
Existing downhole electromagnetic (EM) systems for oil and gas exploration face challenges such as the need for a sizable downhole energy source with limited lifetime and cumbersome power supply, as well as low signal strength and poor source localization when using ambient EM signals like Earth's magnetotelluric currents.
Innovation Solution
The use of downhole EM field sensors to measure and cross-correlate ambient EM fields, deriving responses to a virtual EM field source to determine properties and image the downhole environment, eliminating the need for a traditional EM source and improving signal processing through inversion techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a downhole EM source is used, then EM field measurements can be obtained, but the system requires a sizable energy source with limited lifetime or cumbersome power supply
Solution Approach 1:
The system uses ambient EM fields already present in the downhole environment rather than requiring an active EM source. The sensors detect and process these naturally occurring fields, allowing the system to serve itself by utilizing the environment's existing electromagnetic conditions without needing external energy input for signal generation.
Solution Approach 2:
The patent introduces cross-correlation processing as an intermediary technique that extracts useful signal information from ambient EM fields. By correlating signals from multiple sensors, the system effectively creates a virtual EM source through signal processing, eliminating the need for a physical downhole EM source and its associated power requirements.
2Use of energy by moving object
If ambient EM signals like magnetotelluric currents are used, then no downhole energy source is needed, but signal strength is low and source localization is poor
Solution Approach 1:
The system combines signals from multiple EM sensors through cross-correlation processing. By merging the information from multiple sensor measurements and correlating them in time and space, the system enhances the effective signal strength and improves source localization capability beyond what any single sensor could achieve alone.
Solution Approach 2:
The patent transitions from analyzing ambient EM signals in the time domain to processing them in the frequency domain through spectral analysis and cross-spectral density calculations. This dimensional transformation in signal processing space enables better signal characterization and source localization despite the inherent weaknesses of ambient signals.
3Measurement precision
If traditional EM source methods are used, then sufficient signal strength is achieved, but device complexity and power supply requirements increase
Solution Approach 1:
The system extracts useful EM field information from the ambient environment rather than generating it through a downhole source. By taking out only the sensing and signal processing components while removing the complex power supply and EM source generation equipment, the system achieves simplified device architecture with reduced power 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
This approach enables effective EM interferometry in drilling, wireline, and monitoring environments, providing accurate formation property analysis and optimizing production parameters without the need for a downhole EM source, enhancing signal quality and operational efficiency.
Implementation Method 1
a plurality of downhole EM field sensors to measure ambient EM fields
Implementation Method 2
The measured ambient EM fields are cross-correlated to derive responses of an EM field sensor array to a virtual EM field source
Implementation Method 3
The responses to one or more virtual EM field sources can be inverted to determine properties and/or to provide an image of the downhole environment
Data Source
AI summary
A disclosed electromagnetic (EM) interferometry system includes a first EM field sensor at a first position in a downhole environment. The system also includes a second EM field sensor at a second position in the downhole environment. The system also includes a processing unit that receives a first EM field measurement from the first EM field sensor and a second EM field measurement from the second EM field sensor. The processing unit derives a response of the first EM field sensor to a virtual EM field source at the second position by cross-correlating the first EM field measurement with the second EM field measurement. The processing unit also performs an inversion process based at least in part on the derived response to obtain a model of subsurface formation properties.