Risk Assessment Methods Of Deepwater Drilling Related To The Wellhead And Wellbore Safety

The complex environment load and properties of subsea shallow formation will lead to the risk of subsea wellhead instability in deepwater drilling;At the same time,factors such as the deep water and high static pressure environment,subsea cryogenic,shallow geological disasters,narrow safe density windows,gas hydrates,and uncertain drilling geological parameters will lead to the risk related to wellbore safety in deepwater drilling.However,factors such as empirical coefficients and geological parameters are still constant values in current related design or calculation models,and the risks caused by the uncertainty of parameters are not taken into consideration.So it’s necessary to study the subsea wellhead mechanical stability,wellbore temperature and pressure field calculation under gas cutting condition,prediction of hydrate formation region,and casing program design method and so on.On this basis,those with higher risks such as wellhead instability,gas cutting and blowout,gas hydrate formation and casing program design should be analyzed.And the characteristics and influence rules of uncertain parameters and empirical coefficients should be analyzed emphatically by using methods such as probability statistics,and the risk assessment methods of deepwater drilling related to the wellhead and wellbore safety which consider the uncertain parameters can be established.Thus the categories and probability of various risks can be quantitatively evaluated and analyzed.Meanwhile,the corresponding risk avoidance recommendations or solutions can be proposed.And then a systematic risk assessment platform of deepwater drilling related to the wellhead and wellbore safety can be established which will provide technical supports for risk prevention and control,optimizing design and construction programs in deepwater drilling.For the subsea wellhead mechanical stability in deepwater drilling,the influences of various factors on the characteristic parameters of wellhead tilt and sink were studied.And through sensitivity analysis,factors which have greater impact on and are more sensitive to the characteristic parameters of wellhead tilt（transverse mechanical stability）and sink（vertical mechanical stability）were selected.On this basis,combined with the uncertainty analysis of sensitive factors,the wellhead transverse and vertical instability risk identification methods which take the uncertain factors into account were established.The subsea wellhead mechanical stability of example was analyzed and the recommendations how to improve its stability was proposed.This method can not only assess the wellhead instability risk before drilling processes,but also achieve real-time monitoring and assessment of wellhead transverse instability using the surface current velocity u_wA and the average amount of platform drift S_wA which are calculated under condition of risk threshold J_A.In order to solve the problem that the risk of gas cut early detection and well control is greater in deepwater drilling,the risk factors and influence law of well killing were analyzed,and it can be found that the overflow rate when killing and formation pressure prediction errors have a great impact on the safety of killing,and there are problems of uncertainty.Through the introduction of factors such as the maximum allowable killing casing pressure,the annulus gas position,and the formation pressure prediction methods with credibility,the precise design method of overflow warning value was established.Through using this method,the overflow warning value for the specific well location and well depth can be accurately calculated.On this basis,through the analysis of the relationship between gas cutting law and overflow rate in deepwater drilling,the gas cut early detection and risk assessment method based on gas-liquid tow phase flow identification in riser was proposed,and its feasibility and advantages were demonstrated.The feasibility and effect of gas cut detection at riser using ultrasonic transmission method were discussed though experimental studies and numerical simulation.The quantitative description method of wellbore gas cut extent based on gas cutting monitoring data in riser was proposed.By using this method,it is expected to accurately detect the gas cut as well as quantitatively evaluate the wellbore gas cut extent in real time,which will provide data support for the reasonable well control.For the formation risk of gas hydrate in wellbore in deepwater drilling,the quantitative relationship of construction parameters and hydrate formation risk was established by introducing the supercooling degree density,which solved the difficulty of quantitatively evaluating the hydrate formation risk.Through the calculation of supercooling degree density under different construction parameters,and by using the dimensionless analysis,the quantitative risk assessment method based on construction parameters was established,which could quantitatively assess the impact of construction parameters on the hydrate formation risk.For the example well,some measures should be taken to reduce the risk of gas hydrate formation in the wellbore.For instance,the drilling fluid thermal conductivity should be reduced,and the drilling fluid inlet temperature,the displacement and NaCl concentration should be increased according to the sensitive factors and actual situation.Meanwhile,the semi-quantitative assessment method was established based on AHP and the risk degree classification in predicted hydrate formation region.This method took the on-site experience of experts and the impact of other factors into account,and the calculated order and its weight of each factor were conducive to optimize the drilling design and take construction remedies.In order to solve the problems such as narrow safe density window,formation pressure uncertainty and risk of casing program design in the deepwater drilling,the method of formation pressure prediction while drilling based on probability and statistics was established.This method could correct the lower undrilled formation pressure distribution belt with credibility in real time according to the LWD data,and improved the prediction accuracy of undrilled formation pressure.On this basis,the dynamic risk assessment method of casing program design was established based on the constraint criteria of pressure balance,which could make real-time quantitative evaluation of project risks of the casing program of lower undrilled formation such as leak,kick,collapse,and sticking.Meanwhile,through analyzing the safe density window constraints of current casing program,it can be found that its empirical safety factors are too large and have not considered the leak proof while circulation and drilling,which is not conducive to the narrow safe density window formation in deepwater drilling.Thus,the predrill precise casing program design method,which considered ECD and accurately calculated the design factors,was established;at the same time,the dynamic casing program design method while drilling was proposed,this method could design the casing program for the lower undrilled formation in real time,which could avoid the project risks in the greatest extent at the design stage.Based on the above theoretical research,the software of risk assessment methods of deepwater drilling related to the wellhead and wellbore safety was developed.And then a risk assessment platform for pre-drill design and drilling process which was systematic,suitable for deepwater drilling and related to the wellhead and wellbore safety was established.It could systematically assess the related project risks in deepwater drilling.