Multi-coupling stress field and evaluation of borehole stability in deep brittle shale
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ORIGINAL PAPER
Multi-coupling stress field and evaluation of borehole stability in deep brittle shale Teng Liu 1 & Houbin Liu 1 & Yingfeng Meng 1 & Xu Han 1,2 & Shuai Cui 1 & Anran Yu 1 Received: 31 January 2020 / Accepted: 19 October 2020 # Saudi Society for Geosciences 2020
Abstract In drilling operations of deep brittle shale, borehole stability problems occur frequently, resulting in development difficulties and increased costs. Therefore, studying borehole stability of deep brittle shale is particularly important. The composition of deep brittle shale was analyzed by component content and microscopic observations, and its mechanical strength was experimentally determined by direct shear and triaxial mechanical tests. The multi-coupling stress field was established, and the stability of shale was evaluated using the Jaeger weak-plane strength criterion. Deep brittle shale is mainly composed of quartz (~ 40%), and the hydration effect has a minor influence on its mechanical strength. The rock mass develops natural fractures interlaced with the bedding plane. The cohesive force in the fractures is only 2.02 MPa, and the internal friction angle is 33.39°. There is almost no connection force, which leads to breaking of the rock mass. This study shows that an increase in the number of weak planes, the influence scale of mechanical strength, and collapse pressure increases gradually, and when the number of weak faces increases indefinitely, mechanical strength of the rock mass is infinitely close to a greatly reduced homogeneous state. Further, when the well deviation is 90°, influence from the number of weak planes is most obvious. The weak-plane effect is the main factor influencing stability of the deep brittle shale borehole. Experimental conclusions provide a guiding role in the construction of and research on deep brittle shale boreholes. Keywords Stress field . Shale . Mechanical strength . Weak plane . Borehole stability
Responsible Editor: Santanu Banerjee * Houbin Liu [email protected] Teng Liu [email protected] Yingfeng Meng [email protected] Xu Han [email protected] Shuai Cui [email protected] Anran Yu [email protected] 1
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Xindu Avenue 8#, Xindu District, Chengdu City, Sichuan 610500, People’s Republic of China
2
South Sichuan Gasfield, PetroChin, Jiangyang District, Luzhou City, Sichuan 646000, People’s Republic of China
Introduction With the advancement of oil and gas exploration and development, shale gas has become a potential resource, receiving worldwide attention (B. Curtis, 2002). However, during the development of and drilling for shale gas, especially in deep brittle shale, borehole wall collapse occurs frequently, greatly increasing the cost of drilling and hindering later construction (Bowker, 2007; Li et al., 2015). Considerable research has been conducted on shale borehole wall stability. Through the study of shale wells, Gholami et al. (2017) established a new method for analy
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