Study Of Risk Assessment Methods And Control Measures Of Deepwater High Temperature High Pressure Well Completion Test

The natural gas has a great deal of resources in the deepwater high temperature and high pressure(HTHP)area of South China Sea,which is the main target of the energy exploration and development in our country in the future.Safe and efficient construction of production channel is one of the bottlenecks that restrict the effective utilization of deepwater HTHP natural gas resources.The environment of deepwater HTHP area is complex,thus the risk of drilling & completion technology in this area is far higher than that in conventional well.Particularly,the deepwater HTHP well completion test has prominent problem in the operation risk because of its special temperature and pressure environment.In order to guide the design,risk monitoring and avoidance of the deepwater HTHP well completion test,a set of comprehensive risk assessment method for deepwater HTHP well completion test is formed based on the analysis of the hydrate formation risk,test string failure risk,packer failure risk,failure risk of casing reliability,failure risk of cement sheath sealing integrity,and the relevant risk control measures are developed.Aiming at the deepwater HTHP well completion test,the hydrate thermodynamic model is established based on the phase equilibrium theory to obtain the phase equilibrium temperature curve.On the basis of calculating the temperature and pressure distribution inside test string,the hydrate formation range is predicted.Compared with the results of C-S model,the accuracy of this model is proved.Based on the concept of subcooling degree,the dimensionless subcooling coefficient is introduced as the risk assessment index to assess the hydrate formation risk in formation range.The research shows that,this method is a risk comparative assessment method,which can obtain the highest risk point in the hydrate formation range.Combine with this risk assessment method,the on-site prevention and control methods can accurately deal with the hydrate formation and blockage,which can prevent the excessive control.The hydrate formation range is determined by the temperature and pressure,which are by changing their own distribution and phase equilibrium temperature curve respectively.In order to reduce the hydrate formation risk,the gas production can be increased appropriately while ensuring the smooth operation,and the heat insulation materials such as heat insulation oil pipe can also be used.Aiming at the failure risk of deepwater HTHP well completion test string system,firstly,the test string is simplified as a thick wall cylinder with uniform internal and external load,and the mechanical model of the test string under the influence of high temperature is established.On the basis of establishing the calculation model of annulus temperature and pressure,it is clear that the annulus pressure accumulation in well completion test is mainly caused by the trapped annular pressure.Combine with the temperature and pressure distribution inside string,it is revealed that high temperature can significantly increase the stress of the test string,thus increasing the failure risk of the string and packer.Based on the generalized stress-strength interference theory,the failure risk assessment method of deepwater HTHP well completion test string is established to assess the failure risk of test string.It is found that the failure mode of the string is internal pressure resistance failure,and the risk points are mainly concentrated near the wellhead.Moreover,the influence of gas production,bottom pressure and test fluid density on the failure risk are analyzed.Based on envelope curve,a risk assessment method for packer failure is established,which reveals that packer failure is mainly affected by axial force in deepwater HTHP well completion test,and the gas production,annulus liquid expansion and compression properties,and string thermal expansion properties have great influence on axial force.Aiming at the failure risk of wellbore integrity in deepwater HTHP well completion test,the wellbore is divided into unsealed section and sealed section,and the mechanical model is established to analyze the stress of casing and cement sheath respectively.Based on the generalized stress-strength interference theory,the risk assessment method of wellbore integrity failure is established to assess the reliability of casing strength and the failure risk of cement sheath seal capacity.The research shows that high temperature will increase the stress of casing and cement sheath,and can also make the stress of sealing wellbore section more uniform.Compare with the safety factor method,the assessment standard of this method is more strict and has higher sensitivity to risk.During deepwater HTHP well completion test,the intense radial heat transfer results in the synchronous increasing of the inner pressure inside string and the annular pressure in the unsealed wellbore section,which has only small pressure difference on the casing in the unsealed wellbore section.The compressive and tensile failure of cement sheath results in the micro-annulus on the cementing interface and radial fracture on the cement sheath body respectively,and its ability of bearing tensile stress is lower than that of bearing compressive stress.Changing the elastic modulus and Poisson’s ratio of the cement sheath can effectively control its failure risk,and the sealing integrity can be improved by increasing the toughness of the cement sheath.Based on the established assessment method of hydrate formation risk,test string system failure risk,wellbore integrity failure risk,a set of comprehensive risk assessment method for deepwater HTHP well completion test is formed,in order to provide technical support for on-site risk aversion.Based on this comprehensive method,the risk of two deepwater HTHP gas wells during well completion test in Lingshui area of South China Sea are assessed.Then,the corresponding risk control plans are put forward according to the analysis of risk influencing factors.The corresponding control measures are proposed for deepwater HTHP well completion test.Based on the principle of volume compatibility,the pressure-volume balance equation is established to study the effect of nitrogen injection on the relief of trapped annular pressure.The analysis shows that the increase of nitrogen will greatly reduce the trapped annular pressure.If the annular pressure is still high after nitrogen injection,it indicates that the gas channeling exists and leading to the sustained annular pressure.Hydrate prevention and control methods are divided into injection chemical inhibitor method and physical method.The mitigation effect of expansion joint on the axial force of test string is analyzed,and the mitigation coefficient is introduced to provide reference for optimization of expansion joint.Moreover,the sensitivity of influencing factors of packer failure are analyzed based on the orthogonal test,and a set of workflow applied in the design stage is established as the packer failure control method.A new type of casing is designed to protect the annulus sealing integrity during deepwater HTHP well completion test,which is named CRP casing.The experimental results show that,compared with the conventional casing,the cement sheath in the wellbore with CRP casing can bear 19.4 MPa more casing pressure,which proves that CRP casing can effectively protect the annular sealing integrity.According to the experimental mechanism,the wellbore numerical model is established.The analysis shows that CRP casing can effectively reduce the force transmitted to the cement sheath and delay the plastic deformation of the cement sheath.The risk assessment method and control measures established in this study can effectively prevent accidents and reduce losses,and provide theoretical support for operation design and engineering risk monitoring,and avoidance measures formulation.