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The deepwater drilling and cementing techniques achieved significant development in recent years. However, the engineering response mechanism of natural gas hydrate sediment (NGHS) during drilling and cementing process is far from clear, which challenges the safe development of offshore petroleum resources. To reveal the influence of heat perturbation on NGHS of hydration exotherm of cement slurry and drilling fluid circular temperature increment in the process of subsequent drilling operation in shallow seabed, a mathematical model of hydrate dissociation is developed to simulate the influence by using the numerical simulation approach in this paper, and taking a natural gas hydrate reservoir at Liwan slope in the northern South China Sea as an example. The mathematical model comprehensively takes into account heat conduction, heat consumption of hydrate dissociation, hydrate dissociation dynamics, fluid seepage around the wellbore, and their coupling reaction. The analysis shows that the increase of heat transmitted from drilling and cementing process to NGHS facilitates the hydrate dissociation, leading to the growth of the sediment pore pressure in the near wellbore, the fluctuation range of pore pressure is rapidly diffused and significantly larger than the wellbore radius. The gas from the hydrate dissociation will permeate to the wellbore-distant area and reform hydrate with water. The gas will not to totally reformed hydrate in the short term, partially gathering around the wellbore, and the maximum gas saturation in the sediment is between 10% and 12%. The hydrate reformation area will be several times than the wellbore radius. But the maximum hydrate saturation will not exceed a certain value. The transition area of hydrate dissociation area to hydrate reformation area is quite narrow, and the range of high pore pressure is identical to that of the hydrate reformation. The hydrate dissociation will lead to NGHS instability, so the influence of heat perturbation on NGHS should be paid attention to in deepwater drilling and cementing process.

The influence of heat perturbation on natural gas hydrate sediment in deepwater drilling and cementing process