Study On Risk Assessment And Countermeasures For The Wellbore Integrity Of Natural Gas Hydrate

Natural Gas Hydrate（NGH）,a kind of crystalline material being made up of gas and water,with its structure similar to ice,forming under the environment of high pressure and low temperature below 200m under the seafloor in deep-water seas,is very easy to burn because of its high energy density.According to concerning surveys,as a new kind of substituted energy,Natural Gas Hydrate may exist in about 90%of all marines in the whole word,with very considerable reserves of 21×10¹⁵m³,which doubles of the total reserves of coal,oil and gas.NGH has a very high commercial value due to many advantages,such as the wide range of distribution,shallow burying and clean and green energy.Technical problems during the process of NGH drilling operations may be caused of special physical properties,which can be solved by good management of wellbore integrity.But if we fail to manage the integrity of the wellbore,including uncontrolled transformation of pressure and temperature in the wellbore,the micro annulus of cement rings,fault shut-in of the BOP,serious accidents may occur,with serious economic losses,environment pollution and casualties.In order to ensure the safety of NGH drilling operation,we must carry out the risk assessment of the wellbore integrity and some chain accidents it may cause,and then establish monitoring system and safety barrier system of wellbore integrity.The dissertation combined with the sub-topics―Study on risk management and control measures for the deep-water oil and gas drilling in South Sea Area in China‖of the project―A group of Four‖of state administration of supervision and management for safety production,is aimed at carrying out the risk assessment of the wellbore integrity and some chain accidents it may cause,providing theoretical and technical support for NGH drilling operation.On the basis of extensive study on methods of wellbore integrity analysis of conventional deep-water drilling and the last technology of NGH drilling operation,the right method was chosen about NGH drilling.The corresponding structure of wellbore was chosen to conduct its risk assessment.In this dissertation,a kind of NGH drilling method called buck and heat injection is considered to be the most appropriate one,combing a kind of drilling technology called Control pressure drilling.In order to establish the model of the failure of wellbore integrity in the process of NGH drilling operation,a kind method of risk assessment called FTA（Fault Tree Analysis）was used.And then,using a kind of method combined with AHP（Analytic Hierarchy Process）and fuzzy comprehensive evaluation to value the risk level of the wellbore integrity system.Furthermore,analyze what chain accidents would be caused by the failure of wellbore integrity,how these chain accidents occur,and the main reasons and risk factors which can affect these chain accidents.Next,a drilling process model was established by a kind of software of multiphase flow transient simulation named of OLGA and PVTsim.Then the factors leading to the blowout accident and wellbore plugging accident,which is to say the breakdown and regeneration of Natural Gas Hydrate,were figured out by the calculation of the model established through OLGA.The main factors include temperature,pressure,and mud displacement.Then,establish monitoring system scheme of wellbore,monitoring the temperature and pressure in the wellbore,the decomposition and regeneration of GNH,and the deformation cement ring and casing.Finally,establish safety barrier system based risk assessment,followed by risk and accidents control measures of wellbore integrity,providing technical support for both wellbore integrity management and the safety of the drilling operation of Natural Gas Hydrate.