Downhole Spontaneous Potential Measurement Using Differential Magnetometers
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Solution Overview
Problem
Current methods for determining downhole formation parameters using spontaneous potential are limited in accuracy and effectiveness, particularly in measuring the magnetic fields generated by currents flowing through drill strings, which affects the reliability of parameter determination.
Innovation Solution
The method involves disposing first and second magnetometers at axial locations around a member in a borehole, oriented tangentially to the member's circumference, to measure magnetic fields resulting from both the spontaneous potential and the earth's magnetic field, allowing for the calculation of the spontaneous potential and subsequent determination of formation parameters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single magnetometer is used to measure the magnetic field, then the measurement process is simple, but the accuracy of determining spontaneous potential is insufficient due to inability to separate earth's magnetic field component
Solution Approach 1:
The measurement task is segmented into two separate measurements taken by two magnetometers positioned at different azimuthal locations. The first magnetometer measures the combined field (spontaneous potential + earth's field) while the second measures a different combination. This segmentation allows the earth's magnetic field component to be mathematically separated from the spontaneous potential component, resolving the contradiction between measurement simplicity and accuracy.
Solution Approach 2:
The second magnetometer acts as an intermediary measurement device that provides the additional data needed to solve for the earth's magnetic field component. By introducing this intermediate measurement at a different azimuthal position, the system can isolate and remove the earth's field contribution, thereby improving the accuracy of spontaneous potential determination without requiring direct measurement of the spontaneous potential alone.
2Reliability
If magnetometers are positioned to measure only the spontaneous potential magnetic field, then the measurement is direct, but the earth's magnetic field interference cannot be eliminated
Solution Approach 1:
The earth's magnetic field component is extracted from the total magnetic field measurement through a mathematical process. By taking measurements at two different azimuthal locations and using the known geometric relationship between these positions, the earth's field contribution is separated and removed, leaving only the spontaneous potential component. This extraction process eliminates the harmful interference while maintaining measurement reliability.
Solution Approach 2:
The two magnetometers are positioned asymmetrically at different azimuthal locations around the drill string rather than symmetrically. This asymmetric positioning creates different measurement geometries that allow the system to distinguish between the earth's magnetic field (which has a consistent directional component) and the spontaneous potential field (which is localized to the drill string). The asymmetric configuration enables mathematical separation of these two field sources.
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 enhances the accuracy of determining downhole formation parameters by isolating the magnetic fields from the spontaneous potential and the earth's field, providing a more reliable measurement of formation characteristics such as resistivity and permeability.
Implementation Method 1
measuring a first magnetic field at the first magnetometer that is a combination of a magnetic field resulting from a current flowing through the member due to a spontaneous potential
Implementation Method 2
current may thus flow through the drill string. The magnitude of the current is generally related to a spontaneous potential that is in the formation
Data Source
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AI summary
A drilling system, method and drilling apparatus for determining a parameter of a formation at a downhole location is disclosed. A member having a longitudinal axis is disposed in a borehole at the downhole location. A first magnetometer and a second magnetometer are disposed at an axial location of the member as separate azimuthal locations around the member and oriented tangential to a circumference of circle in a plane transverse to a longitudinal axis of the member. Magnetic fields resulting from a current flowing longitudinally through the member due to a spontaneous potential in the formation are measured to determine the spontaneous potential. The parameter of the formation is determined from the determined spontaneous potential.