Estimating Formation Stresses Using Radial Shear Moduli Profiles
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Solution Overview
Problem
Estimating formation stresses in subterranean formations is challenging due to the influence of horizontal fluid mobility and high clay content on the horizontal shear modulus C66, making it difficult to accurately determine the ratio of vertical to horizontal stress ratios.
Innovation Solution
A method and apparatus that determine radial profiles of Stoneley, fast dipole shear, and slow dipole shear slownesses to estimate maximum and minimum horizontal stresses by inverting differences in far-field shear moduli using difference equations obtained from radial profiles of dipole shear moduli C44 and C55, and borehole stresses proximate to the borehole.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sonic velocities are used to estimate formation stresses, then stress estimation can be performed, but the accuracy deteriorates due to influences on the horizontal shear modulus C66 from fluid mobility and clay content
Solution Approach 1:
The patent segments the shear modulus measurement into three distinct components: fast shear modulus (C55), slow shear modulus (C44), and horizontal shear modulus (C66). Each is measured using different wave types (dipole shear waves for C55 and C44, Stoneley waves for C66). This segmentation allows independent characterization of each modulus, enabling the system to identify and compensate for the specific effects of fluid mobility and clay content on C66 without compromising the accuracy of stress estimation using the other moduli.
Solution Approach 2:
The patent changes the measurement parameters by using multiple types of acoustic waves (compressional headwaves, shear headwaves, Stoneley waves, dipole flexural waves) with different sensitivities to formation properties. By measuring sonic velocities across different wave modes and frequencies, the system can detect parameter changes in C66 caused by fluid mobility and clay content, then compensate for these changes to improve stress estimation accuracy.
2Measurement precision
If standard sonic measurement systems are used, then compressional and shear wave velocities can be obtained, but accurate stress ratio determination becomes difficult without compensating for C66 changes
Solution Approach 1:
The patent makes the sonic logging system multi-functional by incorporating multiple source types (monopole and dipole sources) and multiple receiver configurations within a single tool assembly. The monopole source generates compressional and Stoneley waves for measuring C66, while the dipole source generates shear waves for measuring C55 and C44. This universal design allows the same device to perform multiple measurements that collectively enable accurate stress ratio determination without requiring separate compensation devices or procedures.
Solution Approach 2:
The patent implements self-service by having the sonic logging system automatically perform all necessary measurements (compressional slowness, shear slowness, Stoneley slowness) and calculations needed for stress estimation. The system internally processes the raw velocity data to derive the three shear moduli, applies the stress estimation algorithm, and compensates for C66 changes caused by fluid mobility and clay content, eliminating the need for external compensation procedures or additional equipment.
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 allows for the accurate estimation and indication of maximum and minimum horizontal stresses, improving the accuracy of stress estimation in subterranean formations by compensating for changes in shear moduli caused by fluid mobility and clay content.
Implementation Method 1
A standard sonic measurement system uses a piezoelectric source and hydrophone receivers situated inside the fluid-filled borehole
Implementation Method 2
It is known that elastic wave velocities change as a function of prestress in a propagating medium
Implementation Method 3
A standard sonic measurement system uses a piezoelectric source and hydrophone receivers situated inside the fluid-filled borehole
Implementation Method 4
The critical incidence angle θi=sin−1(Vf/Vc), where Vf is the compressional wave speed in the borehole fluid; and Vc is the compressional wave speed in the formation
Implementation Method 5
the shear headwave can be similarly excited by a compressional wave at the critical incidence angle θi=sin−1(Vf/Vs), where Vs is the shear wave speed in the formation
Data Source
AI summary
Maximum and minimum horizontal stresses, and horizontal to overburden stress ratio, are estimated using radial profiles of shear moduli. Inversion enables estimation of maximum and minimum horizontal stresses using radial profiles of three shear moduli associated with an orthogonal set of axes defined by the three principal stress directions. Differences in the far-field shear moduli are inverted together with two difference equations obtained from the radial profiles of the dipole shear moduli C44 and C55, and borehole stresses in the near-wellbore region. The horizontal to overburden stress ratio is estimated using differences in the compressional, dipole shear, and Stoneley shear slownesses at two depths in the same lithology interval where the formation exhibits azimuthal isotropy in cross-dipole dispersions, implying that horizontal stresses are nearly the same at all azimuths. The overburden to horizontal stress ratio in a formation with axial heterogeneity may also be estimated using the far-field Stoneley shear modulus C66 and dipole shear modulus C55 together with the radial variation of the dipole shear modulus C55 caused by near-wellbore stress concentrations.