Issue 45

Q.-C. Li et alii, Frattura ed Integrità Strutturale, 45 (2018) 86-99; DOI: 10.3221/IGF-ESIS.45.07 94 From Fig.7, the temperature front gradually moves forward from the borehole, but its speed gradually slows down. When the formation is drilled for 10 min, the temperature front reached a position of 4.30 cm from the borehole. However, the temperature front reaches the position of 35.72 cm when the hydrate-bearing sediment has been drilled for 3 hours. However, the temperature disturbance in the hydrate reservoir does not imply the hydrate dissociation. Fig.8 shows the dissociation range of hydrate-bearing sediments within the near-wellbore area at different times. It can be seen from Fig.8 that drilling operation only results in the hydrate dissociation within a range of 17.94 cm around the borehole in the near- wellbore area, which indicates the fact mentioned above that the temperature disturbance in the hydrate reservoir does not imply the hydrate dissociation. Therefore, only when both the pore pressure and temperature reach the dissociation condition of methane hydrates, hydrate dissociation can occur. By comparing the simulation results shown in Fig.7 and Fig.8, it can be seen that the dissociation speed of seawater hydrates in hydrate-bearing sediments during the drilling operation is similar to that of temperature front, and both speeds slow down as drilling operation continues. Figure 8 : Dissociation range of hydrate-bearing sediments within the near-wellbore area. Yield area caused by hydrate dissociation The area where the equivalent plastic strain (PEEQ) generated is the area that borehole collapse may occur. Therefore, the equivalent plastic strain can be used as an important parameter to describe the borehole collapse/instability during a drilling operation in hydrate reservoirs. The evolution of yield range (where the PEEQ value is positive) within the near- wellbore area during the drilling operation in hydrate-bearing sediments of the South China Sea is demonstrated in Fig.9. As can be seen from Fig.9, within half an hour of the whole drilling operations, not only the yield range of the near- wellbore region, but also the maximum equivalent plastic strain continuously increase as drilling operation continues. However, although the yield range within the near-wellbore area changes little after half an hour of the drilling operation, the equivalent plastic strain (PEEQ) continues to increase rapidly. In spite of this, larger yield area where borehole collapse may occur can present with the increase of the dissociation area. Superiority and applicability of the coupled FE method In order to verify the superiority of the simulation method developed herein, results obtained from two different FE models are compared. Among them, a model is the finite element model proposed in this paper that couples seepage, deformation and heat transfer together (which is named as “Model 1”). The other is a simplified model that ignores seepage and heat transfer (which is defined as “Model 2”). Fig.10 shows the comparison results of equivalent plastic strain at last between these two different models described above. From Fig.1, the maximum collapse area should present at the direction of the minimum horizontal principal stress. However, as shown in Fig.10b, when the FE model neglects seepage and the heater transfer (Model 2), the yield area is evenly distributed along the borehole. Therefore, as can be seen from Fig.10, simulation of borehole collapse during the drilling operation in hydrate reservoirs is more realistic using the coupled FE model (Model 1) that integrates seepage, deformation and heat transfer. In addition, borehole collapse simulation with the FE model neglects seepage and the heater transfer (Model 2) overestimates the borehole collapse. Therefore, all these comparison results indicate the applicability of both the coupled FE method and the investigation method established herein.