Accurate Prediction Of Safe Mud Weight Window In Deepwater Drilling

Deepwater is an important area for the increase of oil and gas resources in our country.Developing deepwater resources is an important way for the development of China’s petroleum industry.At present,there are many difficulties and challenges in the deepwater drilling.Due to the low overburden pressure,the safe mud density window is narrow.In addition,due to the difference between the low temperature of the seabed and the high temperature of the formation,and the development of abnormal high pressure in some target intervals,the safe mud density window is further narrowed.The risk of wellbore instability during drilling process is very high,and complex situations such as kick and leakage often occur due to inaccurate prediction of mud density window.Therefore,it is of great significance to carry out the accurate prediction of safe mud density window in drilling process.Based on logging while drilling data,the acoustic wave velocity-density crossplot of shale in the abnormally high pressure interval was constructed,and the genetic mechanism of the abnormally high pressure in the deepwater well was revealed.Combined with rock acoustic wave testing experiments,the functional equations of rock acoustic wave velocity and rock parameters(effective stress,shale content and porosity)under loading mechanism and unloading mechanism were established respectively.Based on the effective stress principle,a multi-parameter formation pressure prediction model based on loading and unloading mechanism was established.Based on the principle of energy conservation and the law of thermodynamics,the temperature control equations for the borehole,wellbore,seawater and formation were established,and the wellbore temperature field under the drilling cycle conditions of deepwater wells is constructed.And the wellbore temperature under circulating condition was calculated by fully implicit finite difference method.In addition,based on the linear elastic constitutive equation,considering the effects of thermal stress and seepage stress on the stress field around the wellbore,and combined with Mogi-Coulomb shear failure criterion and maximum tensile stress failure criterion,the prediction model of wellbore collapse pressure and fracture pressure based on transient heat-fluid-structure coupling were established.Based on the deep neural network model,nine characteristic parameters of leakage in the comprehensive logging data were selected for preprocessing and training,and a leakage identification model based on deep neural network was established.The model is used to accurately identify the leakage in the drilling process.Combined with logging while drilling data and cuttings logging data,the leakage position and leakage mechanism were clarified,and the formation leakage pressure was analyzed to further determine the upper limit of the safe mud density window in the drilling process.The accurate prediction method of the safe mud density window proposed in this paper has been applied in the X area,which significantly reduced the number of complicated situations in the drilling process.This method provided technical support for precise control of mud density during deepwater drilling process.