Changes in Shale Rock Properties and Wave Velocity Anisotropy Induced by Increasing Temperature
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Original Paper
Changes in Shale Rock Properties and Wave Velocity Anisotropy Induced by Increasing Temperature Yu Suo ,1,2 Zhixi Chen,1 and Sheikh S. Rahman1 Received 22 October 2019; accepted 1 May 2020
Water saturation, organic matter content, and fractures are important factors influencing the physical properties of shale. In this study, shale samples were subjected to heating at 1 °C/ min and then gradual cooling. An initial suite of tests was conducted at temperatures of 100 °C, 200 °C, 300 °C, 400 °C and 500 °C. The P-wave and S-wave velocities were measured, rock mechanics parameters were calculated, and then the uniaxial compressive strength test was performed. The mineral composition changed at 500 °C and the montmorillonite transformation of the shale to illite and rock diagenesis were gradually enhanced. Under the condition of heating, shale water saturation and organic matter content were reduced. The difference in the thermal mineral grain (thermal expansion anisotropy and inhomogeneous thermal expansion.) caused the corresponding changes in the internal structure of the rocks. When the changes exceeded those of the rock itself to some extent, they tended to produce microcracks within the rock, resulting in a change in the rock mechanics parameters. The results provide the fundamental parameters for rock mechanics and an important guide for high-temperature wellbore stability and hydraulic fracturing. KEY WORDS: Shale, Thermal, Rock property, Wave velocity, Anisotropy.
INTRODUCTION Shale gas has been included in the strategy for the development of oil and gas resources in China. The very low matrix permeability of shale reservoirs requires hydraulic fracturing to form a large-scale industrial production. Shale reservoirs are extremely prone to water-phase trap damage because of localized ultralow water saturation phenomena. High temperatures can relieve water-phase trap damage and generate new cracks to improve production. Shale is a sedimentary rock with bedding or thin layers of mainly clay deposits that may fracture
1
School of Minerals and Energy Resources Engineering, University of New South Wales, Sydney, NSW 2052, Australia. 2 To whom correspondence should be addressed; e-mail: [email protected]
due to formation pressure. Rock properties are essential in reservoir evaluation because they have a great influence on fracture initiation and propagation (Zomeni et al. 1997; Sondergeld et al. 2010; Waters et al. 2011; Li et al. 2013). Therefore, understanding the rock properties of shale gas reservoirs under high temperatures has great significance for efficient drilling completion and hydraulic fracturing. Rock mechanics properties reflect mainly heterogeneity, anisotropy, and brittleness. Shale properties are generally evaluated by stress–strain curves, ultrasonic wave velocity, acoustic emission and other experimental methods (Fang and Harrison 2002; Wang et al. 2015, 2016a, b; Yu et al. 2018). Rock anisotropy is a reservoir property that is due to the existence of bedding and oriented fracture
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