Borehole-to-Surface Electromagnetic Survey Transfer Function
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Solution Overview
Problem
Existing surface-to-borehole electromagnetic methods face challenges in depth resolution due to attenuation losses at higher frequencies and logistical issues with high-power transmitters, particularly in difficult terrain, and lack effective methods for combining downhole and surface magnetotelluric measurements.
Innovation Solution
A method and system for analyzing electromagnetic fields by estimating a transfer function between downhole and surface locations using simultaneously recorded data, allowing for more sensitive detection of changes at depth through borehole-to-surface transfer function estimates, which can be applied in various well configurations and frequencies below 10 kHz.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a large loop source is used on the earth's surface with high frequency to achieve significant inductive scattering detection, then detection sensitivity is improved, but attenuation losses increase and limit the depth to which the method can be employed
Solution Approach 1:
The patent inverts the conventional surface-to-borehole measurement approach by implementing borehole-to-surface measurements. Instead of placing the source at the surface and measuring downhole, the source is placed in the borehole and measurements are taken at the surface, thereby avoiding the attenuation problems associated with high-frequency signals traveling from surface to depth.
Solution Approach 2:
The patent changes the frequency parameter from high frequency (used in conventional methods to achieve inductive scattering) to low frequency (below 10 kHz, preferably below 1 kHz), which reduces attenuation losses while still enabling effective detection when combined with the inverted measurement geometry.
2Adaptability or versatility
If a grounded source is employed at any frequency to avoid inductive scattering limitations, then frequency flexibility is improved, but good electrical contact with the ground is required which creates logistical problems
Solution Approach 1:
The patent inverts the measurement configuration so that the grounded source is placed in the borehole rather than at the surface. This eliminates the need for good electrical contact with the ground at the surface, removing the logistical problems associated with deploying grounded sources in difficult terrain while maintaining frequency flexibility.
3Power
If a high-power transmitter is used to generate detectable electromagnetic signals at the surface, then signal strength is improved, but transport in difficult terrain becomes problematic
Solution Approach 1:
The patent inverts the source and receiver locations, placing the high-power transmitter in the borehole where it can be deployed using existing well completion equipment rather than transporting it across difficult terrain. The receiver is placed at the surface where logistics are simpler, thereby maintaining signal strength while eliminating transport difficulties.
4Ease of operation
If surface-only electromagnetic methods are used to avoid logistical issues, then ease of deployment is improved, but depth resolution is reduced
Solution Approach 1:
The patent inverts the conventional approach by placing the source in the borehole and receiver at the surface, rather than using surface-only methods. This configuration maintains the ease of deployment associated with surface measurements while achieving improved depth resolution because the source is positioned much closer to the subsurface targets of interest.
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 sensitivity to subsurface changes, improving depth resolution and overcoming logistical challenges by synchronizing surface and downhole measurements, leading to more accurate subsurface resistivity structure interpretation.
Implementation Method 1
receiving downhole electromagnetic survey data of electromagnetic fields obtained using a downhole receiver deployed at a first location in a borehole
Implementation Method 2
A second set of electromagnetic survey data of the electromagnetic fields is also received that has been obtained using a receiver deployed at a second location
Implementation Method 3
A second set of electromagnetic survey data of the electromagnetic fields is also received that has been obtained using a receiver deployed at a second location. A transfer function is estimated between the first and second locations for at least portions of the electromagnetic fields based on at least portions of the two sets of electromagnetic survey data
Data Source
AI summary
Methods and related systems are described for measuring naturally occurring electromagnetic fields both at the earth's surface as well as downhole. These fields originate from currents in the ionosphere above the earth, and are the same fields as employed by known magnetotelluric geophysical methods based on surface measurements. Some embodiments are especially useful in horizontal wells that are uncased at depth, although some embodiments are also useful in normal vertical wells that are both uncased or cased with a conductive liner. The method includes receiving downhole electromagnetic survey data of the naturally occurring electromagnetic fields obtained using a downhole receiver deployed at a first location in a borehole. A second set of electromagnetic survey data of the naturally occurring electromagnetic fields is also received that has been obtained using a receiver deployed at a second location. A transfer function is estimated between the first and second locations for portions of the electromagnetic fields based on the two sets of electromagnetic survey data.