Distributed In-Situ Stress Measurement Using Fiber Optic Sensing
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Solution Overview
Problem
Current methods for measuring in-situ stress in substrates, such as rock formations, lack high spatial resolution and accuracy, particularly over wide ranges and at significant depths, which is crucial for applications like well design, rock stability, and hydraulic fracturing, as they can only measure minimum stress and require fluid injection and borehole sealing.
Innovation Solution
A high-definition fiber optic sensing technology integrated with an expandable element that applies pressure to the substrate's wall, using optical frequency domain reflectometry to detect strain and determine both minimum and maximum principal stresses without fluid injection or borehole sealing, enabling precise distributed stress measurements along a hole.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional stress measurement methods are used, then measurement simplicity is maintained, but measurement precision and spatial resolution are insufficient
Solution Approach 1:
The patent replaces conventional mechanical stress measurement systems with an optical-based fiber optic sensing system. The fiber optic sensor uses optical frequency domain reflectometry (OFDR) to detect strain-induced changes in reflected light, thereby measuring stress without mechanical contact or fluid injection. This substitution achieves high measurement precision while avoiding the complexity of mechanical injection systems, pressure seals, and fluid handling apparatus.
Solution Approach 2:
The patent introduces an expandable element as an intermediary between the fiber optic sensor and the substrate wall. This expandable element transfers the mechanical stress from the substrate to the fiber optic sensor through controlled expansion, enabling indirect but precise stress measurement. The intermediary approach allows the sensor to measure stress without direct contact or fluid injection into the substrate, maintaining measurement precision while simplifying the overall system.
2Adaptability or versatility
If conventional methods are used, then equipment simplicity is maintained, but the ability to measure both minimum and maximum stress is limited
Solution Approach 1:
The patent creates a universal measurement system that can determine both minimum and maximum principal stresses using a single fiber optic sensor and expandable element configuration. The system measures stress in multiple directions by detecting strain patterns during expandable element expansion, eliminating the need for separate measurement devices for different stress components. This multi-functional capability enhances adaptability while keeping the equipment relatively simple compared to conventional multi-device approaches.
3Measurement precision
If fluid injection methods are used, then stress measurement is achieved, but fluid penetration and environmental contamination occur
Solution Approach 1:
The patent replaces fluid injection mechanisms with a solid-based expandable element that uses controlled mechanical expansion to transfer stress to the fiber optic sensor. This substitution eliminates fluid penetration into the substrate, preventing contamination of the measured environment while maintaining accurate in-situ stress measurement. The expandable element achieves the necessary stress transfer through solid mechanics rather than fluid pressure.
4Measurement precision
If conventional sensing technology is used, then system simplicity is maintained, but spatial resolution and measurement range are insufficient
Solution Approach 1:
The patent employs optical frequency domain reflectometry (OFDR) technology to replace conventional mechanical sensing systems. OFDR provides high spatial resolution by analyzing the phase and amplitude of reflected light along the fiber optic sensor length, enabling precise localization of stress measurements. This optical approach achieves superior spatial resolution without the mechanical complexity of segmented sensors or multiple measurement points, as the entire fiber acts as a distributed sensor array.
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 technology provides high-resolution strain measurements, allowing for accurate determination of stress directions and magnitudes, including minimum and maximum in-situ stresses, over extended distances without fluid penetration, enhancing the understanding and management of subsurface stress and seismicity.
Implementation Method 1
using optical frequency domain reflectometry to detect strain
Implementation Method 2
a pressurizing device coupled to the expandable element for expanding the expandable device when the expandable element is inserted in the hole in the substrate to exert pressure on a wall of the hole
Implementation Method 3
a fiber optic interrogator to provide light to the optical fiber sensor and to detect reflected light from the optical fiber sensor
Data Source
AI summary
A system for performing distributed measurements of in-situ stress includes an expandable element with at least one fiber optic sensor. The expandable element can be positioned at various depths in a hole in a substrate. A pressurizing device expands (and contracts) the expandable element when the expandable element is inserted in the hole in the substrate to exert pressure on the hole wall. A pressure sensor provides a sensor output indicative of a pressure applied to the hole wall by the expandable element. The fiber optic sensor and an optical interrogator measure strain along a length of the sensor in a continuous, high spatial resolution manner Based on the measured strain and pressure sensor output, the system determines various properties of the substrate such as, minimum principal stress, maximum principal stress, and/or principal stress direction associated with one or more fractures in the substrate, as well as substrate modulus.