Study On Wellhead Stability During Natural Gas Hydrate Production Tests By Depressurization

Natural gas hydrate has the obvious advantages of wide distribution,large amount of resources,clean and no pollution,and high energy density,which is widely recognized as the most valuable alternative to oil.However,compared with traditional energy sources,natural gas hydrate mostly exists in the shallow soil under the deep sea mud line,and its stability is greatly affected by changes in temperature and pressure.The drilling and exploitation of which may lead to submarine landslide,well wall and wellhead instability,well shaft failure and other accidents,which seriously restricts the commercial exploitation of natural gas hydrate.This dissertation is supported by the key project of National Natural Science Foundation "Interaction and Safety Design between Gas Hydrate Production Wells and Deep Sea Soil",and this thesis studied the key natural gas hydrate sites in the South China Sea by combining theoretical analysis and numerical simulation.Great progress has been made in mechanical properties of weakly cemented,low strength and high porosity reservoirs,the change of vertical and transverse bearing capacity of conductor,stratum subsidence and wellhead stability caused by the dissociation of natural gas hydrate.The relevant conclusions can provide reference for the safe and efficient exploitation of gas hydrate reservoirs in the South China Sea.The main research progress is summarized as follows:(1)The theoretical study on the variation law of physical parameters of hydrate reservoir during natural gas hydrate production test by depressurization was carried out,and the comprehensive dynamic equation of elastic modulus,permeability,porosity and cohesion of reservoir was established.(2)An analysis method suitable for changes in the mechanical properties of the reservoir during natural gas hydrate dissociation process was proposed,and the distribution of displacement,equivalent stress and plastic strain of the hydrate reservoir was analyzed during natural gas hydrate production test by depressurization,so as to define the stress concentration area and the location of the first sand production.The displacement and equivalent stress distribution law of casing and cement ring inner and outer walls were explored,and the position where casing is most likely to yield and cement ring is most likely to fail was identified.(3)Based on ABAQUS,a two-dimensional axisymmetric model was established to analyze the effects of the dissociation range of natural gas hydrate,the depth of the conductor into the mud and the size of the conductor on vertical bearing capacity.A three-dimensional axisymmetric model was established based on ABAQUS,and by using DISP subroutine programming to apply displacement boundaries equivalently to replace the dissociation effect of natural gas hydrate,the influence of seabed subsidence,vertical and lateral wellhead loads,conductor above-mudline height on lateral bearing capacity were analyzed.(4)A solid model of wellhead stability was established to explore the time-varying characteristics of stratum and wellhead displacement during natural gas hydrate production test by depressurization,and analyzed how changes in the reservoir affect the overlying formation and then affect the wellhead,thus revealing the interaction law between hydrate dissociation and wellhead stability.The wellhead subsidence threshold was set to obtain the critical time for natural gas hydrate test production under different reservoir thickness,reservoir depth,natural gas hydrate saturation and production pressure to avoid wellhead instability accident.