Research On The Risk Assessment And Prevention Methods Of Annular Pressure Buildup In Deepwater Wells

Limited by the uncertainty of formation information and cementing technology conditions,deepwater oil and gas wells usually carry out complex well structure design,forming a multiple sealed annuli structure composed of free and cemented casing section.Along with the radial heat transfer of the high temperature fluid in the wellbore during the production process,the fluid medium in sealed annuli of each casing absorbs heat to generate the expansion pressure.Due to the particularity of subsea wellhead device,the fluid thermal expansion between sealed casings cannot be released,thus leading to annular pressure buildup phenomenon,which will cause the casing to collapse and deform,severely threatening the safety and integrity of the wellbore.In order to effectively assess and prevent the risk caused by annular pressure buildup,the prediction model of annular pressure buildup,simulation experiment,and foam casing pressure control technology are studied in this paper.Based on the analysis of typical wellbore schematics of deepwater wells and subsea wellhead structure features,this papar studied the source and formation conditions of annular pressure in deepwater wells,analyzed the variation of thermodynamic performance of annulus fluid with temperature,established a temperature-pressurevolume balance matrix equation under the influence of multiple sealed annuli,developed a matrix equation of thermal displacement function for free and cemented casing section,presented an annular temperature and pressure prediction model of the single-layer annulus,double-layer annulus,and triple-layer annulus wellbore structure that based on the wellbore-formation system with thermo-structural coupling effects,evaluated the impacts of controllable influencing factors such as well production measures,thermal properties of the annulus fluid,casing properties parameters and cement sheath properties parameters on annular pressure buildup.An experimental device for simulating the double-layer annulus wellbore structure was developed,and an annulus pressure simulation experiment was developed.By testing different cycle temperatures,circulation flow rates and cycle times,exploring the effect of high temperature fluid during production phrase on sealed annular temperature and pressure.To eliminate the error caused by the dissolved gas of annulus fluid in the vessel,established a calculation formula for the ratio of incremental pressure and temperature,analyzed the relationship between annular pressure and annular temperature increment in the experimental high temperature heating phases and natural heat dissipation phases,verified the applicability of annular pressure prediction model.The crushable syntactic foams with different hollow glass microsphere mass fractions are obtained through experiments.The hydrostatic compression tests of crushable syntactic foams were developed to analyze the relationship between foam volume compressibility and hydrostatic pressure under different hallow glass microsphere mass fractions and test temperature conditions.A calculation model of foam volume compression covering both thermal expansion effect and densification effect was established,and the foam casing pressure control calculation process was designed,which provided a basis for the prevention measures optimal design of annular pressure buildup in deepwater wells.The rationality and accuracy of the model were verified by the application of the annular pressure buildup risk assessment and foam casing pressure prevention method in the deepwater area of West Africa.