Issue 45

Q.-C. Li et alii, Frattura ed Integrità Strutturale, 45 (2018) 86-99; DOI: 10.3221/IGF-ESIS.45.07 87 and the vast reserves. Although the estimation of global hydrate reserves is uncertain, according to incomplete statistics, the energy stored in natural gas hydrates all over the world is at least twice as much as that stored in fossil fuels currently known [4-6]. Although various countries have conducted a series of hydrate production test activities recently (especially China and Japan), the current research on methane hydrates is generally in the stage of theoretical research and laboratory exploration. The production test activities are listed in Tab. 1. Canada U.S.A Japan China 1 st test 2 nd test 1 st test 1 st test 2 nd test 1 st test Time 2002 2007 2012 2013 2017 2017 Test area Mackenzie Delta North Slope of Alaska Nankai Trough Shenhu area Lithology Sandy Sandy Sandy Clayey silt Method Hot water circulating Depressurization Replacement+ Depressurization Depressurization Fluid extraction Production time 5d 6d 30d 6d 36d 60d Cumulative gas 516m 3 13000m 3 24000m 3 120000m 3 235000m 3 309000m 3 Reasons for stop - - - Sand Production Table 1 : Production test activities of natural gas hydrates Worldwide [7]. Some development strategies have been studied by numerous experts for the development of natural gas hydrates, of which the depressurization method is the most effective and the most commonly used one [8]. However, hydrate reservoirs in deep water are generally low-strength formations, which make the wellbore collapse to be a commonly occurred issue during hydrate production process [9-12]. In addition, hydrate dissociation during hydrate production operation is another contributor to both the complex accidents within the borehole (i.e., borehole instability and casing damage, etc.)[13-17] and the geological disasters (such as slope slide and submarine earthquakes)[18, 19]. Hydrate dissociation at high temperature and/or low pressure determines that drilling operation in hydrate reservoirs is more difficult than conventional oil and gas reservoirs. The caliper logging curve at the SH4 site in Shenhu area of the South China Sea shows that serious borehole collapse and borehole enlargement has emerged during the drilling process within the hydrate interval [20]. Therefore, exploration of a method for borehole stability investigation in hydrate reservoirs and thorough analysis of borehole collapse mechanism are of great importance to ensure the construction safety in hydrate reservoirs. Up to now, the main focuses of numerous scholars have been paid on gas production from hydrate-bearing sediments, and different production strategies have been investigated in detail [7, 21-25]. However, the threat of hydrate dissociation to the stability of both the reservoir and wellbore is an important research topic in the development of natural gas hydrates. On the one hand, hydrate dissociation leads to changes in both the pore pressure and the effective stresses within hydrate reservoir. On the other hand, hydrate dissociation also weakens the reservoir strength, which severely affects the strata stability. In spite of this, the investigations of wellbore behavior caused by hydrate dissociation during hydrate development in deep water only attract a few scholars’ interests [13-17, 26, 27]. Although these studies are valuable for studying borehole behavior during a drilling operation in hydrate formation, nearly all these related investigations did not integrate three physical fields of seepage, deformation, and heat transfer to thoroughly analyze borehole stability in hydrate reservoirs. In this paper, the finite element (FE) model used for investigating borehole stability during the drilling operation in hydrate reservoirs is established by integrating seepage, deformation, and heat transfer. Based on this, the results of borehole collapse are compared when the coupled FE model (investigated herein) and the simplified model (both seepage and heat transfer are neglected) are used respectively, then the applicability of the investigation method can be verified. Moreover, the yield range evolution within the near-wellbore area is also investigated with the coupled FE model.