Prediction And Mitigation Mechanism Of Casing Annular Pressure In Deepwater Wells

The well integrity is an important and necessary prerequisite for the safe and efficient exploration of deepwater oil and gas resources.Due to complex well structure and limited cement technology,multiple annuli are formed in the deepwater wells and filled with liquid.As the temperature redistributes in the production process,annular liquid expands due to temperature increase and brings about trapped annular pressure.Trapped annular pressure seriously threatens the well integrity of deepwater wells and many related accidents have been reported.To effectively reduce the risk caused by trapped annular pressure,the trapped annular pressure is studied in the fields of prediction,analysis,risk evaluation and mitigation.Based on the deepwater well complex well structure and the analysis of tubing fluid heat transfer in the production process,a model is established to calculate the trapped annular pressure and annular temperature change.The impact of annular liquid properties,formation features and production parameters are analyzed by this model.A new index,ratio of dispersion coefficient(shorted for RDC),is introduced to evaluate the sensitivity of controllable influencing factors.And then the well integrity is evaluated under the influence of trapped annular pressure.A model is built based on Mohr-Coulomb criterion and displacement continuity law to calculate the displacement of the casing-cement-formation system and analyze the sealed integrity casing-cement interface when trapped annular pressure fluctuates.Considering the impact of trapped annular pressure on casing triaxial stress and strength,safety factor is selected to evaluate the reliability of casing strength.The status of casing strength reliability is divided into safety,risk and danger.And then the casing integrity is studied.The available mitigation measures are classified and compare based on the evaluation result.The mitigation effect and key parameters of three representative measures are analyzedand optimized.A method is proposed to calculate the trapped annular pressure of deepwater wells with thermal-insulated pipes.The impact of setting depth,thermal-insulated property,formation temperature and production rate are studied.A new casing with double walls is developed to control trapped trapped annular pressure.This casing has an independent space to contain expanded annular liquid.The mechanism,feasibility and mitigation process are analyzed.And then the mitigation effect and design method are optimized.A time-iteration method is built based on volume balance and conservation of mass to analyze the changing law of trapped annular pressure when nitrogen gas is injected into trapped annulus.The impact of nitrogen volume,formation properties,production rate and water depth on mitigation effect are analyzed.Sustained casing pressure caused by gas channeling will happen when the cement sealed integrity is damaged by trapped annular pressure.On the basis of cement sealed integrity failure mechanism and high-pressure gas seepage process,a model is built to predict the sustained casing pressure when gas invades into the trapped annulus filled with liquid.And then utilize this model to study the impacts of different factors on the pressure rising process,thus proposing some measures to control the sustained casing pressure.The result indicates that the fundamental reason of trapped annular pressure is the contradiction between expanded annular liquid and limited volume of trapped annulus.The temperature increase and pressure accumulation mainly happens in the early stage of production process.And then the temperature and pressure increase gently.As heat transfer medium and pressure carrier,the properties of wellbore fluid have remarkable impact on trapped annular pressure.Cement generates irreversible plastic deformation under the effect of trapped annular pressure.As a result,micro annulus appears between casing and cement and the cement sealed integrity fails.The weak point of casing collapse and yield deformation is located nearby the wellhead.The casing string will gradually turn into danger status from top to bottom as the trapped annular pressure increases.Most of the available measures mitigate trapped annular pressure by controlling the annular liquid temperature,releasing expanded annular liquid and increasing the annular liquid compressibility.And these measures are different in the cost,mitigation effect,operation difficulty and reliability.The research shows that thermal-insulated pipes,double-wall casing and nitrogen injection caneffectively control trapped annular pressure.However,optimal setting depth exists for the thermal-insulated pipes and there is a limit value for the mitigation effect of nitrogen injection.The double-wall casing can completely eliminate trapped annular pressure.The mitigation effects of the above three measures are all impacted by formation properties,production rate and related key parameters.Meanwhile the mitigation effect of nitrogen gas is influenced by water depth.The low cost and efficient mitigation can be realized through optimization design.The increase of sustained casing pressure can be divided into rapid rising stage and stable rising stage.The increasing speed and maximum value can be reduced by optimizing the annular liquid properties and cement parameters.