Study On Risk Assessment Of Sustained Annular Pressure In Deepwater Wells

The ocean contains more than 70%of the world's oil and gas resources.In recent years,deepwater oil and gas reserves account for more than half of the new global oil and gas discoveries.China's offshore oil and gas resources are very rich.Only the South China Sea accounts for 1/3 of the China's oil and gas resources,but 70%of them are stored in deep water area.Compared with onshore and shallow oil and gas wells,deepwater wells face special marine environment and formation conditions.Once the annulus pressure of deepwater wells exceed the maximum allowable annular pressure(MAWOP),wellbore integrity will be faced with failure risk and may lead to major safety accidents.Therefore,it is of great significance to carry out the study on the risk assessment of the sustained annular pressure(SAP)in deepwater wells to ensure the safe and efficient production of deepwater wells.Through the thorough investigation of the relevant literature at home and abroad,the pressure field model of unidirectional flow and gas-liquid two-phases flow is established,which realizes the accurate solution of fluid pressure in the tubing.Based on the law of conservation of energy and the heat transfer principle of multi-layer cylinder walls,a prediction model for the temperature field of the wellbore in deepwater wells is established.Based on the temperature field of the wellbore and according to the principle of volume compatibility,the prediction model of the annular thermal expansion pressure is established.For the actual drilling and production conditions in the field,considering the wear of the drill pipe to the casing and the corrosion of CO2 gas to the production pipes,a calculation model for residual wall thickness and strength of tubing and casing is established,which provides basic parameter supports for calculating the reasonable annular pressure range of deepwater wells.Combined with related standards and recommendation practices,the reasonable annular pressure range calculation model for the deepwater wells is established which takes the bearing capacity of surrounding pipe columns of annulus and the checking of underground key nodes into consideration.The former calculation is mainly for the corresponding tubing and casing,while the latter is mainly for wellhead device,packer,downhole safety valve and liner hanger.For deepwater high pressure gas wells,some pressure should be reserved to ensure the safe operation of the downhole pipe columns and tools under some working conditions.Therefore,the calculation model of the minimum reserved pressure for the annular space is established and a set of SAP management chart is formed.Besides,the example calculation and analysis based on the above researches are carried out.The calculation of reasonable annular pressure range of each annulus without the consideration of wall thickness reduction,with the consideration of wall thickness reduction and the corresponding SAP management chart are all obtained.The risk reduction measures for reducing SAP in deepwater wells are carried out,mainly from the following two steps:reducing annulus temperature and reducing annulus stiffness.As for the reduction of the risk of SAP through reducing the temperature of the annulus,the vacuum insulation pipe column and heat insulation fluid are mainly studied.As for the reduction of the risk of SAP through reducing annulus stiffness,the additional chamber on casing and rupture disk are mainly studied.The applicable conditions and the working pressure range of the measures to reduce the risk of annular pressure are given.Moreover,the effects on the reduction of the risks of annulus pressure are evaluated separately and superimposly.The results can provide a reference for reducing the risk of SAP in deepwater wells.Based on the above theoretical research,a software for calculation of reasonable annular pressure range in deepwater well is developed,which includes wellbore temperature field and tubing pressure calculation module,annulus thermal expansion pressure prediction module,the calculation module of reasonable annular pressure range of deepwater wells and output interface of annular pressure management chart.Through the application of the software,it can control the risk of the SAP in deepwater wells better and ensure the wellbore integrity of deepwater wells.