In shale gas development, the mechanical properties of shale are crucial in hydraulic fracture propagation, wellbore stability, and the productivity of a shale gas wells. In this dissertation, acoustic velocity tests, uniaxial compressive tests, and Brazilian tensile tests were conducted on Eagle Ford and Mancos shale to investigate gas shale mechanical properties, including dynamic mechanial properties and static mechanical properties (compressive and tensile mechanical properties). Water content, mineralogy, and anisotropic effects on shale mechanical properties were analyzed.
Ultrasonic velocity measurements were performed on Eagle Ford shale samples. Dynaimic elastic properties were determined according to the compressive- and shear-wave vleocities. The results showed that both P- and S-wave velocities increase as confining pressure increases. Horizontal elastic modulus, vertical elastic modulus, and shear modulus increase with increasing confining pressure. While horizontal and vertical Poisson’s ratio exhibited more or less invariant with confining pressure. Transverse isotropy is an appropriate model to characterize Eagle Ford gas shale. Elastic properties of Eagle Ford shale are direction-dependent. Horizontal Young’s modulus is higher than vertical Young’s modulus and horizontal Poisson’s ratio is higher than vertical Poisson’s ratio. Increasing water content reduce Young’s modulus and shear modulus significantly. Induced water can make the shale softer. Water increase Eagle Ford shale’s anisotropies. Both P- and S- wave velocities decrease with increasing of TOC and clay content. Dynamic Young’s modulus, shear modulus, and bulk modulus vary inversely with TOC and clay. Poisson’s ratio does not correlate with TOC or clay content for these test samples.
Static mechanical properties were investigated by conducting uniaxial compressive tests and Brazilian tensile tests on Eagle Ford and Mancos shale samples. A new method was developed to analyze tensile elastic behavior of materials. The imbibed water significantly reduces the uniaxial compressive strength. Young’s modulus of wet samples is lower for corresponding dry samples. The maximum Young’s modulus decrease is up to about 70%. The imbibed water makes the shale softer. Poisson’s ratio increase with water content. Bedding plane/laminations have a significant impact on Eagle Ford indirect tensile strength, but not on Mancos shale. The imbibed water significantly reduces tensile strength and tensile Young’s modulus, but increase tensile Poisson’s ratio. Low clay content in the Eagle Ford shale (around 6%) and high clay content in the Mancos (around 22%) might be the explanation for the overall lower tensile strength of the Mancos than Eagle Ford shale.
Static and dynamic elastic properties of Eagle Ford shale samples are compared. Static Young’s modululs is lower than dynamic Young’s modulus. There is no strong correlations between static and dynamic Poisson’s ratio observed for the tested samples. The relationship of compressive and tensile mechanical properties of Eagle Ford shale are investigated. Tensile Young’s modululs is 0.76 to 0.98 times lower than corresponding compressive Young’s modulus. There is either no strong correlations between tensile and compressive Poisson’s ratio observed for the tested samples.
Water weaken mechanism was analyzed. Three potentially major weakening mechanisms—chemical effects, water clay interaction, and capillary pressure increase—were discussed in detail.
|Commitee:||Feng, Yin, Gang, Daniel, Guo, Boyun, Hayatdavoudi, Asadollah, Seibi, Abdennour|
|School:||University of Louisiana at Lafayette|
|School Location:||United States -- Louisiana|
|Source:||DAI-B 78/12(E), Dissertation Abstracts International|
|Keywords:||Anisotropy, Mechanical properties, Mineralogy, Shale rocks, Water content|
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