Borehole-to-Surface EM Logging Through Steel Casing
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Solution Overview
Problem
Existing electromagnetic logging techniques face challenges in measuring formation resistivity in cased wells due to severe attenuation caused by standard magnetic steel casing, making it impractical for both wells to be cased or for successful logs to be obtained with steel casing, especially when using magnetic field transmitters and receivers.
Innovation Solution
The implementation of a borehole-to-surface electromagnetic induction logging method using a downhole magnetic dipole transmitter and an array of surface receivers, which includes optimizing the transmitter design for cased well applications, using high-power logging cable technology, and employing a local reference receiver station outside the casing to measure and normalize the effective magnetic moment, allowing for effective measurements even in steel-cased wells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If standard magnetic steel casing is used in both wells, then the wellbore integrity and structural strength are improved, but the electromagnetic signal attenuation increases severely making resistivity measurements impractical
Solution Approach 1:
The patent changes the operating frequency parameter to very low frequencies (0.1-10 Hz), which reduces the skin effect and electromagnetic signal attenuation in conductive steel casing, enabling measurements through cased holes that would otherwise be impossible at higher frequencies
Solution Approach 2:
The system uses a combination of magnetic field transmitter and electric field receiver (or vice versa) to create a hybrid measurement approach that can penetrate steel casing more effectively than single-field methods, utilizing the complementary properties of different electromagnetic field types
2Power
If high-power logging cable technology is used to increase transmitter power, then the signal strength and measurement range are improved, but the device complexity and cost increase
Solution Approach 1:
The patent applies excessive action by using very low frequencies (0.1-10 Hz) that are much lower than conventional electromagnetic logging frequencies, which excessively reduces the skin depth and allows signals to penetrate through steel casing with acceptable attenuation without requiring extreme power levels
Solution Approach 2:
The patent introduces an intermediary reference receiver that measures the transmitter signal through the casing to determine the effective magnetic moment, which then serves as a reference for normalizing formation measurements, eliminating the need for complex direct calibration methods
3Measurement precision
If a local reference receiver station is deployed to measure and normalize the effective magnetic moment, then the measurement accuracy is improved, but the field operation time and setup complexity increase
Solution Approach 1:
The reference receiver measures the effective magnetic moment of the transmitter through the casing before formation measurements are taken, establishing a baseline calibration that simplifies subsequent data processing and reduces the need for complex real-time corrections during formation logging
Solution Approach 2:
The system uses the actual measured effective magnetic moment from the reference receiver to self-calibrate the formation resistivity measurements, eliminating the need for separate calibration tools or complex theoretical correction models and allowing direct calculation of formation properties
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 efficient and cost-effective resistivity measurements by reducing the impact of casing on the magnetic moment, allowing for more accurate and comprehensive subsurface resistivity mapping with reduced rig time and increased lateral sensitivity, while maintaining the integrity of the wellbore.
Implementation Method 1
One technique to measure formation resistivity involves the use of electromagnetic induction via transmitters of low frequency magnetic fields that induce electrical currents in the formation. These induced electrical currents in turn produce secondary magnetic fields that can be measured by a magnetic field receiver.
Data Source
AI summary
The present disclosure relates to systems and methods for conducting an electromagnetic borehole-to-surface survey of a formation surrounding a borehole. Such methods include deploying a dipole transmitter into the borehole to a depth of investigation, deploying an array of electromagnetic receivers outside of the wellbore, and measuring a response of the formation at the array of electromagnetic receivers deployed outside of the wellbore, for example at the surface. From the response of the formation a property of the formation can be determined based on the response of the formation measured at the array of electromagnetic receivers. For the scenario of a cased well, a local reference receiver may be added at a location proximate the borehole to measure the effective magnetic moment of the transmitter inside the casing, and normalize the formation response in order for a more accurate determination of a formation characteristic, such as resistivity. These receivers can also be used for other types of surveys.