Directional Resistivity Imaging with Tilted Coils
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Solution Overview
Problem
Existing electromagnetic logging tools face challenges in accurately imaging the resistivity of subsurface formations while drilling, particularly in anisotropic formations, due to limitations in directional measurement capabilities and data quality during the drilling process.
Innovation Solution
A downhole logging tool with tilted or transverse coils is used to make directional measurements by recording voltage ratios at different rotation angles, allowing for the production of improved resistivity images through symmetrization and normalization, which enhances the detection of bed boundaries and conductivity differences.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional electromagnetic logging tools are used to measure resistivity, then basic resistivity data can be obtained, but the ability to accurately image anisotropic formations and detect bed boundaries is insufficient
Solution Approach 1:
The tool divides the measurement system into multiple independent transmitter-receiver coil pairs, each capable of making directional measurements. This segmentation allows the complex task of anisotropic formation imaging to be broken down into multiple simpler directional resistivity measurements that can be processed independently and then combined to create comprehensive formation images.
Solution Approach 2:
The patent introduces directional measurements by tilting coils transverse to the tool axis, adding an azimuthal dimension to the traditional vertical resistivity measurements. This dimensional extension enables the system to resolve horizontal conductivity variations and bed boundary orientations that were previously undetectable with conventional tools.
2Measurement precision
If directional measurements are made with tilted or transverse coils, then bed boundary detection improves, but the complexity of data processing increases
Solution Approach 1:
The patent uses asymmetric coil orientations (tilted or transverse coils at specific angles) to create directional sensitivity. By orienting coils at non-standard angles relative to the tool axis, the system generates asymmetric measurement patterns that are highly sensitive to bed boundary orientations, enabling accurate detection through the analysis of these asymmetric response patterns.
Solution Approach 2:
The system incorporates reference receiver coils that provide feedback signals used to normalize and symmetrize the measurements from tilted or transverse coils. This feedback mechanism allows the complex directional measurements to be processed by comparing against known reference patterns, simplifying the extraction of bed boundary information while maintaining high detection accuracy.
3Measurement precision
If multiple transmitter-receiver pairs are used for comprehensive imaging, then formation characterization improves, but the measurement time and data volume increase
Solution Approach 1:
The tool is pre-configured with multiple transmitter-receiver coil pairs positioned at specific spacings and orientations during manufacturing. This preliminary arrangement allows the system to perform comprehensive directional measurements in a single tool deployment without requiring time-consuming reconfiguration, as all measurement geometries are already in place to characterize the formation.
Solution Approach 2:
The patent combines multiple measurement functions into a single integrated tool operation by simultaneously activating multiple transmitter-receiver pairs with different spacings and orientations. This merging allows the system to acquire comprehensive directional resistivity data across multiple depths and azimuths in a single measurement sequence, reducing total measurement time while maintaining comprehensive formation characterization.
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
The method provides sharper and more accurate resistivity images, enabling effective visualization of formation layers and their orientation, even in anisotropic formations, with improved detection capabilities at bed boundaries and conductivity differences.
Implementation Method 1
With propagation tools, typically a measurement of the attenuation and phase shift of an electromagnetic signal that has passed through the formation is used to determine the resistivity.
Implementation Method 2
The magnetic moment of these coils has a component perpendicular to the tool axis.
Data Source
AI summary
The present disclosure relates to a method to produce an image of a subsurface formation using directional measurements. A downhole logging tool having one or more transmitters and one or more receivers, and being capable of making directional measurement, is used to measure the voltage in a particular receiver due to a particular transmitter for one or more transmitter/receiver pairs, at least one of those voltage measurements being a directional measurement. The complex (phasor) voltage recorded on a receiver coil is divided by the complex voltage recorded at another reference receiver coil. Alternatively, we can use the ratio of a receiver voltage at a particular rotation angle of the tool divided by the voltage on the same receiver when the tool has rotated by an angle of 180 degrees. The information in those ratios is combined to produce images of the resistivity of the subsurface formation surrounding the tool.