Multifrequency Conductivity Inversion for Shale Organic Maturity
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Solution Overview
Problem
Current methods for estimating the level of organic maturity (LOM) in shale reservoirs are limited by the interference of pyrite, graphite, and turbostatic carbon nanostructures, which affect resistivity interpretations and make it difficult to accurately determine the volume of kerogen and hydrocarbon content, especially in overmature sediments.
Innovation Solution
A method using multifrequency complex conductivity measurements to estimate the volume fractions of pyrite, graphite, and turbostatic carbon nanostructures, which involves inverting conductivity data to improve resistivity interpretations and provide a continuous real-time log for directional drilling and well placement, enabling better identification of hydrocarbon-rich zones and avoidance of overmature rocks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional resistivity interpretation techniques are used to evaluate water saturation, then water saturation can be estimated, but the effects of pyrite, graphite, and other constituent minerals are not included, leading to inaccurate TOC and LOM determination
Solution Approach 1:
The patent segments the complex resistivity measurement into multiple frequency components (e.g., low frequency, mid frequency, high frequency) to separately characterize different mineral phases. By inverting multifrequency complex conductivity data, the method isolates the contributions of pyrite, graphite, and other minerals, enabling accurate TOC estimation while accounting for mineral interference.
Solution Approach 2:
The patent changes the measurement parameter from single-frequency resistivity to multifrequency complex conductivity. This parameter change allows the system to capture frequency-dependent dispersion effects caused by different minerals, improving the accuracy of TOC and LOM determination while providing additional information for differentiation.
2Measurement precision
If spectral elemental analysis using geochemical logs is used, then elements present in the formation can be identified, but the volume of kerogen cannot be determined due to the presence of carbon in various other minerals
Solution Approach 1:
The patent segments the total carbon signal into contributions from different sources (kerogen vs. carbon-containing minerals) by utilizing frequency-dependent electromagnetic responses. Different carbon sources exhibit distinct dispersion characteristics across the frequency spectrum, allowing the inversion process to separate and quantify kerogen volume independently from other carbon-bearing minerals.
Solution Approach 2:
The patent introduces multifrequency complex conductivity measurements as an intermediary parameter to indirectly determine kerogen volume. Instead of directly measuring carbon content, the method uses electromagnetic wave interaction with different carbon sources at multiple frequencies to infer kerogen volume, bypassing the limitation of spectral elemental analysis.
3Measurement precision
If the Passey method is used for organic content evaluation, then organic content can be assessed, but the method is less accurate for sediments that are overmature, such as shale gas formations
Solution Approach 1:
The patent changes from using single-parameter resistivity measurements to multifrequency complex conductivity measurements. This parameter change enables the method to detect subtle variations in electromagnetic response caused by different organic matter maturity stages, including overmature shales, thereby extending accurate organic content evaluation across the full range of thermal maturity.
Solution Approach 2:
The patent introduces dynamic frequency sweeping to capture the time-dependent polarization responses of different minerals and organic matter. By measuring conductivity across a range of frequencies rather than at a single frequency, the method adapts to different maturity stages where mineral and organic matter properties vary, maintaining accuracy from immature to overmature sediments.
4Measurement precision
If multifrequency complex conductivity measurements are used to estimate volume fractions of minerals, then TOC estimation accuracy is improved, but the inversion process and validation requirements increase computational complexity
Solution Approach 1:
The patent performs preliminary constraints and validations during the inversion process by incorporating geologic knowledge and expected mineral phase relationships. The inversion algorithm includes built-in checks for physical合理性 and uses preliminary estimates of mineral volumes to guide the optimization, reducing computational complexity while maintaining accuracy in TOC and LOM determination.
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 TOC estimation and hydrocarbon saturation assessment by accounting for interfacial polarization effects, allowing for more precise well placement and improved hydrocarbon reserve exposure.
Implementation Method 1
The presence of pyrite, graphite, and turbostatic carbon nanostructures ('TCN') produces interfacial polarization that results in frequency-based dispersion of effective electrical properties of shale formations.
Data Source
AI summary
A method for determining a level of organic maturity of a shale gas formation includes inverting multifrequency complex conductivity data to estimate a volume fraction of graphite, turbostatic carbon nanostructures, and pyrite. The inversion is validated using estimates of the volume fraction of graphite, turbostatic carbon nanostructures, and pyrite. The volume fraction of graphite and turbostatic carbon nanostructures is correlated to a level of organic maturity log of the shale gas formation. The level of organic maturity log is validated using sulfur content obtained from pyrolysis or vitrinite reflectance. A variation of an electromagnetic response due to the volume fraction of graphite, turbostatic carbon nanostructures, and pyrite is quantified. The electromagnetic response is modified by removing the quantified variation to obtain resistivity and permittivity values.