Wellbore temperature distribution during drilling of natural gas hydrate formation in South China sea

The natural gas hydrate resources in the South China Sea alone are about 85 trillion cubic meters.In the drilling process of marine gas hydrate,the natural gas hydrate will decompose and produce gas,as the rising of temperature and dropping of the pressure in the annulus.This process will have a significant impact on drilling safety.Therefore,it is necessary to study the wellbore temperature distribution during the drilling of marine hydrate layer.In this paper,the wellbore temperature distribution of safe drilling in hydrated formation is taken as the research goal,and the research status of relevant domestic and international wellbore temperatures was investigated.According to the characteristics of the marine environment and reservoir-forming characteristics of hydrate reservoirs in the South China Sea,the wellbore temperature distribution model of offshore drilling wellbore under the condition of hydrate decomposition was established.The temperature distribution curve of drilling straight wellbore in hydrate layer of South China Sea was obtained.When drilling the hydrate reservoir,the distribution regularity of the wellbore temperature is similar to that of the conventional offshore drilling wellbore.However,the temperature of the wellbore annulus near the hydrate decomposition site is lower than the ambient temperature,mainly due to the hydrate decomposition endothermic.In this paper,the sensitivity analysis of several main parameters of the wellbore temperature distribution of drilling straight wellbore in hydrate layer of South China Sea was carried out.Through the conduction of experiment and numerical simulation,we have get some new findings:(1)The hydrate saturation has little effect on the wellbore temperature;(2)As the drilling fluid displacement increases,the annulus temperature of the wellbore above the mudline increases,and the temperature of the wellbore below the mudline decreases continuously;(3)As the density of the drilling fluid increases,the temperature at the wellhead decreases,and the temperature at the bottom of the well increases slightly;(4)The greater the rate of penetration of the well,the temperature at the upper part of the wellbore decreases,and the temperature at the bottom of the wellbore increases;Among them,the penetration rate has the most obvious effect on the annular temperature.The results are expected to be helpful to guide the drilling process of marine gas hydrate and offer some references.