Issue 45

Q.-C. Li et alii, Frattura ed Integrità Strutturale, 45 (2018) 86-99; DOI: 10.3221/IGF-ESIS.45.07 96 dissociation. Conversely, the better mesh quality within the near-wellbore area means that the mesh size increases from the borehole radially outward, which makes the hydrate dissociation simulation closer to the actual situation. Figure 11 : Dissociation range of hydrate-bearing sediments within the near-wellbore area when the Bisa ratio is different. Fig.12 displays the equivalent plastic strain of the hydrate formation within the near-wellbore area at the last of the drilling operation when the mesh size is different. As can be seen from Fig.12, the equivalent plastic strain is larger when the mesh quality is good (when the Basi ratio is 5), this is related to the hydrate dissociation in Fig.11. When the mesh quality is poor, more hydrate dissociation leads to more severe wellbore collapse and instability. For example, when the Basi ratio is 10, the maximum equivalent plastic strain is 2.977×10 -2 , but the maximum equivalent plastic strain increases to 3.051×10 -2 when the Basi ratio changes to 5. In addition, the yield range caused by hydrate dissociation changes little for models with two different mesh sizes. All these results indicate that mesh size is an important factor when the FEM was used to investigate wellbore stability in hydrate-bearing sediments. Figure 12 : Comparison results of equivalent plastic strain at the last (drilling in hydrate formation for 3h) of the drilling operation between these two different mesh size within the near-wellbore area. (a) : PEEQ when Basi ratio is 10, (b) : PEEQ when Basi ratio is 5. C ONCLUSIONS AND SUGGESTIONS he major results are as follows: (1) The investigation method of borehole collapse in hydrate-bearing sediments is developed, and a two- dimensional seepage-deformation-heat transfer coupled finite element model is also established. (2) Hydrate stability depends on so many factors, such as the temperature and the pore pressure. Only when both the pore pressure and temperature reach the dissociation condition of methane hydrate, hydrate dissociation can occur. (3) Equivalent plastic strain caused by hydrate dissociation increases with the continuous drilling operation in hydrate- bearing sediments. And, borehole collapse first occurs at the intersection of the minimum horizontal principal stress direction with the borehole. T