Study On Risk Of Deepwater Gas Hydrate Formation Instability And Well Integrity During Drilling And Production

The deepwater gas hydrate faces a quite promising prospect for its wide distribution,abundant deposits and clean & high effective properties.For the present,over 30 countries and regions have been involved in the research and exploration of the gas hydrate,and as a competitive member,China has launched relative study since the beginning of the 80 s,but limited by critical technology,development project and operation safety,the large-scale commercial exploitation of gas hydrate would still remain beyond reach.Considering the thermal decomposition characteristics of hydrate and complicated shallow unconsolidated formation,the deepwater gas hydrate drilling and production operations are relatively difficult and take more risk.Therefore,depending on the National Key Research and Development Program “Marine hydrate drilling & completion and safety monitoring technology”,the paper conducts the study on key risk factors and effective control measures for gas hydrate exploration and development,analyzes the risk of hydrate formation instability during drilling and production and carries out wellbore integrity research,providing support for reasonable risk prediction,safety assessment and control during the whole operation.1.Study on the key problems of deepwater gas hydrate before drillingOn the basis of defining the classification and exploitation practicability of gas hydrate reservoir,comprehensive assessments are carried out on the potential risk of the deepwater gas hydrate during drilling and production,including the failure of well control,instability of formation,subsidence around the wellhead,poor quality of cementing,submarine landslide and change of drilling fluid properties,followed by the identification of key risk factors applying risk matrix theory.And to make effective response to the above risks,the paper first proposes the relative information about deepwater gas hydrate should be collected from the perspectives of operation environment(data on wind,wave & current and formation soil),hydrate layer(seismic data and wireline logging),and simulation tools(conducted in the laboratory and by software).And thus,the field application scheme is explained in details to the necessity and feasibility of collecting information before drilling.2 Stability analysis of deepwater gas hydrate during drilling operationCombined fault tree and event tree,the Bow-tie model is established on hydrate formation instability to develop following risk analysis.Based on TOUGH+HYDRATE,commercial hydrate simulation software,the paper makes study on distribution law of hydrate saturation,formation temperature and pressure in the process of drilling fluid invasion,and determines the invasion range of hydrate formation.And then further simulation has been taken to analyze influence factors during drilling fluid invading process.Using the changing parameters derived from the above simulation and combined with FLAC 3D,the formation stress near wellbore is calculated,and the stress concentration area and most vulnerable position has been figured out.According to the above results,the proper drilling fluid density is recommended in the paper.3 Stability analysis of deepwater gas hydrate during productionThe advantages and disadvantages of hydrate exploitation methods are compared from five aspects,such as exploitation efficiency,technology reliability and cost.In the view of gas hydrate reservoir in the South China Sea,the appropriate method is selected to apply in the production.Same as the analysis process in Chapter 3,the distribution law of hydrate saturation,formation temperature and pressure is studied for the process of production.And on the basis of the above research,the paper analyzes the formation stress distribution and the time of shear failure to provide reference for stability maintenance of hydrate formation.As the hydrate formation instability would result in the gap between the formation and cement sheath,the study on the influence of gap(under different gap pressures and angles)on casing strength has been conducted subsequently based on ANSYS.4 Wellbore integrity assessment of deepwater gas hydrateIntegrity research is carried out from two aspects of the wellbore integrity,including the open-hole section and casing section.And for the former section,evaluation of drilling fluid allocation scheme is conducted based on factors affecting wellbore stability;meanwhile,for the casing section,the barrier function is defined and classification of wellbore integrity is analyzed.In view of the specific accident scene,the topological structure of the key wellbore integrity barriers is established under the full consideration of the non-controlled decomposition of the hydrate and failure of the accessory production equipment.The path of the barrier is identified in order to locate the weak link,and the control measures are proposed to reduce the risk.