Issue 45

Q.-C. Li et alii, Frattura ed Integrità Strutturale, 45 (2018) 86-99; DOI: 10.3221/IGF-ESIS.45.07 97 (4) It is impractical to investigate borehole collapse using the simplified model that neglects both the seepage and the heat transfer (i.e., Model 2 in this paper). By comparison, the coupled finite element model and the investigation method established herein are feasible for simulating borehole instability during the drilling operation in hydrate-bearing sediments. (5) Poor mesh quality is not conducive to realistically simulating the hydrate dissociation during the drilling operation and the resulting wellbore collapse. Mesh size is also an important factor when the FEM was used to investigate wellbore stability in hydrate-bearing sediments. Although the research method proposed in this paper has some advantages compared with previous studies, there are still many shortcomings and need further improvement: (1) Effect of the anisotropy of reservoir properties on borehole collapse needs to be investigated. (2) Effect of hydrate dissociation on temperature and pore pressure must be considered to realistically simulate the actual situation. (3) A temperature change of drilling mud during mud circulation also a limitation requires to be considered. F UNDING his work is supported by Program for the Changjiang Scholars and Innovative Research Team in University (IRT_14R58) ， the National Natural Science Foundation Project of China (51704311), the Fundamental Research Funds for the Central Universities (Grant No. 16CX06033A), National Key Research and Development Program (Grant No. 2016YFC0304005), the National Basic Research Program of China (973 Program) (Grant No. 2015CB251201) and Qingdao Science and Technology Project (Grant No. 15-9-1-55-jch). A CKNOWLEDGEMENTS his work should be thankful for the help from Rock Mechanics Laboratory in China University of Petroleum (East China). R EFERENCES [1] Gai, X. and Sánchez, M. (2017). A geomechanical model for gas hydrate-bearing sediments, Journal of Environmental Geotechnics, 4(2), pp.143-156. DOI: 10.1680/jenge.15.00050. [2] Klar, A., Soga, K. and Ng, MYA. (2017). Coupled deformation–flow analysis for methane hydrate extraction, Géotechnique, 60(10), pp.765-776. DOI: 10.1680/geot.9.P.079-3799. [3] Lu, S. (2015). RETRACTED: A global survey of gas hydrate development and reserves: Specifically in the marine field, Renewable & Sustainable Energy Reviews, 41, pp.884-900. DOI: 10.1016/j.rser.2014.08.063. [4] Li, G., Li, X. Zhang, K. Li, B. and Zhang, Y. (2013). Effects of Impermeable Boundaries on Gas Production from Hy drate Accumulations in the Shenhu Area of the South China Sea, Energies, 6, pp. 4078-4096. DOI: 10.3390/en6084078. [5] Merey, S. (2016). Drilling of gas hydrate reservoirs, Journal of Natural Gas Sciences & engineering, 35, pp.1167-1179. DOI: 10.1016/j.jngse.2016.09.058. [6] Milkov, AV. (2004). Global estimates of hydrate-bound gas in marine sediments: how much is really out there, Earth Science Reviews, 66(3), pp.183-197. DOI: 10.1016/j.earscirev.2003.11.002. [7] Zhang, W., Bai, F. Shao, M. and Tian, Q. (2017). Progress of offshore natural gas hydrate production tests in Japan and implication, Marine and Geology Quaternary Geology, 37(5), pp.27-33. (In Chinese) [8] Li, G., Moridis, GJ. Zhang, K. and Li, XS. (2011). The use of huff and puff method in a single horizontal well in gas production from marine gas hydrate deposits in the Shenhu Area of South China Sea, Journal of Petroleum Science & Engineering, 77(1), pp.49-68. DOI: 10.1016/j.petrol.2011.02.009. [9] Cha, Y., Yun, TS. Kim, YJ. Lee, JY. and Kwon, T. (2016). Geomechanical, Hydraulic and Thermal Characteristics of Deep Oceanic Sandy Sediments Recovered during the Second Ulleung Basin Gas Hydrate Expedition, Energies, 9(10), pp. 775. DOI: 10.3390/en9100775. T T