In-situ Yield Stress Estimation via Acoustic Inversion
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Solution Overview
Problem
Current methods for determining in-situ rock yield stresses in viscoelastic formations, such as salt and shale, are unreliable and costly, leading to challenges in avoiding borehole shrinkage or expansion during drilling, which can result in stuck pipe or heavy fluid loss.
Innovation Solution
A novel technique that estimates in-situ rock yield stresses by analyzing radial variations in shear slowness using cross-dipole dispersions and multi-frequency inversion, allowing for the calculation of elastic and plastic limits of octahedral stress, thereby determining a safe mud weight window.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If core samples are tested in a triaxial cell to obtain limiting octahedral stresses, then some stress data can be obtained, but the method is costly and unreliable for in-situ conditions
Solution Approach 1:
The patent replaces the mechanical triaxial cell testing system with an acoustic wave propagation measurement system. By measuring compressional and shear wave velocities in the borehole and inverting the data, the method obtains formation stress parameters without physical core sampling and laboratory testing, thereby reducing cost while improving reliability for in-situ conditions
Solution Approach 2:
The patent introduces acoustic wave propagation as an intermediary measurement method. Instead of directly measuring stress through mechanical testing, the system uses wave velocity measurements as an intermediate parameter that can be inverted to obtain stress information, enabling non-intrusive in-situ stress characterization
2Measurement precision
If acoustic wave propagation is used to estimate formation stress parameters, then in-situ stress data can be obtained, but the measurement and inversion process is complex
Solution Approach 1:
The patent employs a dipole sonic logging tool that can measure multiple wave modes (compressional and shear waves) with a single device. This multi-functional approach allows simultaneous acquisition of different velocity data needed for stress parameter inversion, reducing the need for multiple separate measurement systems while maintaining measurement precision
Solution Approach 2:
The patent utilizes changes in wave propagation parameters (compressional and shear wave velocities) as indicators of formation stress state. By measuring how these acoustic parameters vary with stress conditions and inverting the relationship, the system achieves precise in-situ stress measurement through parameter transformation rather than direct mechanical measurement
3Loss of information
If conventional sonic logging is used to measure wave velocities, then basic formation characteristics can be obtained, but yield stress state information cannot be determined
Solution Approach 1:
The patent implements an inversion process that uses measured wave velocity data as feedback to iteratively determine formation stress parameters. The measured compressional and shear slowness data are fed into an inversion algorithm that estimates stress parameters, creating a closed-loop system that extracts complete formation information including yield stress state from conventional sonic logging measurements
Solution Approach 2:
The patent performs preliminary inversion of wave velocity data to estimate formation stress parameters before drilling operations. By pre-determining the yield stress state and elastic/plastic limits from acoustic measurements, the system enables proactive wellbore stability assessment and mud weight window determination, preventing information loss rather than recovering it after problems occur
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 provides a reliable and cost-effective method for estimating in-situ rock yield stresses, enhancing wellbore stability by predicting viscoelastic deformation and preventing borehole instability during drilling.
Implementation Method 1
mechanical disturbances can be used to establish elastic waves in earth formations surrounding a borehole
Implementation Method 2
A standard sonic measurement system consists of placing a piezoelectric source and an hydrophone receivers inside a fluid-filled borehole
Implementation Method 3
A standard sonic measurement system consists of placing a piezoelectric source and an hydrophone receivers inside a fluid-filled borehole
Implementation Method 4
mechanical disturbances can be used to establish elastic waves in earth formations surrounding a borehole, and the properties of these waves can be measured to obtain important information about the formations
Data Source
AI summary
Determination of in-situ rock yield stress state of a geological formation surrounding a borehole includes determining a profile for each of an axial effective, a radial effective, and a hoop effective stress within at least one axial plane containing a borehole axis. A predicted radial shear response radial profile is calculated from the effective stresses within the at least one axial plane. A measurement-based estimate of a shear response radial profile within the at least one axial plane is determined from measured data. A maximum radial distance at which a difference between the predicted and measurement-based shear response radial profiles is identified within the at least one axial plane as being greater than a difference threshold. The respective axial, radial, and hoop stresses, are determined at the identified maximum radial distance. The identified stresses are indicative of an in-situ yield stress state of the rock.