Study On The Stability Of Wellhead During Drilling In Permafrost Zone

Arctic,subarctic and qinghai-tibet plateau,xinjiang regions in China are widely distributed with large areas of permafrost.Other than oil and gas resources,these regions contains plenty of natural gas hydrate,where exists a great development value.However,the tundra drilling is faced with great technical challenges,and the wellhead stability is one of the key technical difficulties in the tundra drilling.Due to the heat exchange effect between drilling fluid and formation and melting permafrost temperature,the bearing capacity of permafrost will fallen sharply,wellhead and surface tube are prone to instability and sinking under the action of wellhead load,led to the suspension of operation,seriously affect the operation safety.Therefore,it is of vital importance to study the stability of the drilling process in the permafrost and provide reference for the drilling operation of permafrost.Based on the analysis of the basic characteristics of permafrost and the characteristics of tundra drilling and theoretical calculation,we carried out multiple sets of simulation experiment to study the temperature changes in the drilling process for the influence law of frozen soil bearing capacity in xinjiang karamay,under the temperature about 20 ?.Through the finite element analysis software ABAQUS,the permafrost temperature field distribution of the drilling process and the thawing settlement of surrounding soils wellhead laws are simulated.The results show that the variation trend of soil temperature can be divided into three parts: stable rising stage,rapid ascent stage and stable stage.The wellhead subsidence is divided into stable descent stage,rapid descent stage and stable consolidation section.As the formation temperature rises,the wellhead starts to sink.When the local layer rises to a certain temperature range,the wellhead starts to enter the subsidence stage,and the rapidly descending temperature interval is inversely related to the initial moisture content of the frozen soil layer and the wellhead load.Test and numerical simulation results show that: the bearing capacity of viscous permafrost decreased fastest between-4 and 0,and the wellhead was prone to severe melt and sink damage in this interval.So according to the different wellhead loads and the physical and mechanical parameters of frozen soil,we can improve the wellhead stability of permafrost drilling by controlling the temperature of drilling fluid or by increasing the depth of the surface ducts.