EM Field Ratio Analysis for Casing Distortion Reduction
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Solution Overview
Problem
Surface-to-borehole and borehole-to-surface electromagnetic measurements are affected by metal casings, karsts, and unknown overburden resistivity, leading to inaccurate geological information and noise in hydrocarbon target imaging.
Innovation Solution
A method and apparatus that model and compare ratios of electromagnetic field components to minimize the effects of conductive casings, karsts, and overburden variability, using EM sources and receivers to emit and measure signals, and update models based on actual field component ratios.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If metal casing is present in the borehole, then structural support and protection are provided, but current channeling occurs along the casing leading to additive EM field contributions and measurement distortion
Solution Approach 1:
The patent extracts and removes the harmful effect of the metal casing by mathematically eliminating its contribution from the measured EM fields. Through ratio analysis of different EM field components (Ex/Ey, Hz/Hy) and iterative modeling, the casing-induced current channeling effects are separated and subtracted, leaving only the formation response signals for accurate hydrocarbon detection.
Solution Approach 2:
The patent introduces an iterative modeling and ratio analysis process as an intermediary between the raw EM measurements and the final formation interpretation. This intermediary process uses measured field ratios to update modeled responses and progressively eliminate casing effects, serving as a mediator that transforms distorted measurements into accurate formation characteristics.
2Loss of information
If karsts are present in the shallow subsurface, then geological features are captured, but current density channeling occurs yielding secondary sources that bias EM fields
Solution Approach 1:
The patent extracts and removes the harmful effect of karst-induced current channeling by mathematically eliminating their contribution from the measured EM fields. Through ratio analysis and iterative modeling, the secondary sources generated at karst boundaries are separated and subtracted, allowing accurate detection of deeper hydrocarbon targets despite the presence of shallow karst features.
Solution Approach 2:
The patent implements feedback through iterative modeling where measured EM field ratios are continuously compared with modeled ratios, and the model is updated based on the comparison. This feedback loop progressively refines the elimination of karst effects while preserving genuine formation signals, achieving accurate hydrocarbon detection despite shallow subsurface heterogeneities.
3Device complexity
If overburden resistivity is unknown, then measurement setup is simplified, but geological noise is introduced affecting hydrocarbon target imaging
Solution Approach 1:
The patent enables the system to self-determine overburden resistivity characteristics through ratio analysis of measured EM field components without requiring external information or complex preliminary surveys. The measured ratios of EM fields (Ex/Ey, Hz/Hy) inherently contain information about overburden properties, allowing the system to automatically compensate for overburden effects and achieve accurate deep formation imaging.
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
Reduces distortion from conductive casings and karsts by up to 80%, and minimizes the impact of overburden uncertainty, providing more accurate hydrocarbon target imaging by isolating the formation signal.
Implementation Method 1
the EM source is operated to emit an EM signal into the subterranean formation, and the EM receiver is operated to measure actual EM field components of the EM signal
Implementation Method 2
a metal casing in the STB or BTS borehole may provide a highly conductive medium that yields current channeling along the length of the casing
Implementation Method 3
This may result in an additive contribution to measured EM fields due to currents induced on, and leaking out of, the surface of the casing
Implementation Method 4
Finite size, three-dimensional, conductivity inhomogeneities in the shallow subsurface, known as karsts, may also yield channeling of the current density
Implementation Method 5
The channeling phenomena may be described to first order as an accumulation of charges at the boundaries of the 3D karsts, which may effectively yield secondary sources at the position of the karsts
Data Source
AI summary
Electromagnetic (EM) survey processing comprising operating an electronic device to model expected EM field components to be measured by an EM apparatus associated with a subterranean formation, wherein the EM apparatus comprises at least one EM source and at least one EM receiver. The electronic device may be operated to determine one or more ratios of the modeled EM field components. The at least one EM source may be operated to emit an EM signal into the subterranean formation, and the at least one EM receiver may be operated to measure actual EM field components of the EM signal. The electronic device may then be operated to compare the one or more ratios of the modeled EM field components with one or more ratios of the actual EM field components. The one or more ratios of the modeled EM field components may then be updated based on the comparison.