Research On The Prediction Model And Control Method Of Cuttings Transport And Wellbore Pressure In Long Horizontal Wells With Slim Hole

To meet the high efficiency development of deep oil and gas,many oil fields at home and abroad use the technique of slim-hole horizontal well drilling.This technology can not only increase the production of oil and gas wells,but also reduce the drilling cost,and has good economic benefits.However,during drilling in deep formation,we often face many difficulties and challenges,which are mainly reflected in the narrow window of formation safety density and the difficulty of wellbore pressure control,resulting in frequent downhole complications such as overflow,leakage and even"co-existence kick and leakage".At the same time,in the drilling process of long horizontal wells with a slim hole,the problem of blockage and sticking caused by unclean holes seriously restricts the drilling effect,and even causes malignant stuck accidents in drilling sites,which is difficult to ensure safe and efficient drilling.To solve the above problems,the following studies are carried out in this paper based on the main line of borehole cleaning and wellbore pressure control of slim hole horizontal well:1.Based on the theory of solid-liquid two-phase flow and finite volume method,a three-dimensional numerical model of annular solid-liquid two-phase flow was established for cuttings migration in slim-hole horizontal wells,and the effects of displacement,drilling pipe rotation speed,well inclination and drilling fluid rheology on borehole cleaning and annulus pressure loss in slim-hole horizontal wells were analyzed.The results show that in the process of drilling slim-hole horizontal wells,increasing the displacement and drilling pipe rotation speed are both beneficial to hole cleaning.And there are critical speed and critical displacement.When the rotation speed and displacement are both lower than the critical value,it is more difficult for cuttings to migrate in the highly-deviated section.Cuttings migration in horizontal section is more difficult when the rotation speed and displacement are above the critical value,which is different from the cuttings migration pattern in conventional horizontal wells.The influence of rheological parameters of drilling fluid on the borehole cleaning is not a single trend.The rheological parameters of drilling fluid have an optimal range under different displacement and drilling pipe rotation speed.2.Based on the conservation equations of mass,momentum and energy,and considering the effects of cuttings and temperature on annulus pressure(ECD),a transient model for fine prediction of ECD in long horizontal wells with slim-hole is established,which is suitable for multi-field coupling in deep high-temperature formation.By comparing with the field measured data,the error of the calculated results is within 0.003g/cm~3,and the accuracy and reliability of the model are verified.The results show that,compared with the conventional ECD prediction model,the developed model achieves the three-field coupling of wellbore annulus temperature-pressure-multiphase flow,and can dynamically and accurately describe the changes of ECD under different borehole cleaning conditions in slim-hole horizontal wells,which is more conducive to the fine control of wellbore pressure.When the effects of drilling pipe rotation speed,temperature and cuttings are not taken into account,the prediction error of ECD in long horizontal wells with slim hole is relatively large.In a certain range,increasing displacement,drilling pipe rotation speed and drilling fluid viscosity are all conducive to improving the borehole cleaning degree of slim-hole horizontal wells.However,compared with increasing displacement and drilling fluid viscosity,increasing drilling pipe drilling speed has significantly less impact on annular pressure loss and is more conducive to the control of wellbore pressure.3.Based on the fine prediction transient model of ECD for long horizontal wells with slim hole and with the constraint conditions of"drilling safety density window"and"cuttings bed height",a hydraulic parameter optimization method suitable for long horizontal wells with slim hole under the condition of narrow safety window is proposed,which provides a theoretical basis for drilling fluid density,rheology,displacement and drilling pipe rotation speed in long horizontal wells with slim hole.Proposed method is used to optimize the hydraulic parameters of several wells in Mahu oilfield before drilling.The results show that the hydraulic parameters are closely related and inseparable in the optimization process,which should be considered comprehensively.Improving the drilling pipe rotation speed is the key to ensure that the wellbore meets the requirements of pressure control and borehole cleaning.When using managed pressure drilling techniques,reducing the displacement and drilling fluid viscosity can expand the selection of drilling fluid density.To solve the problems of borehole cleaning and pressure control in long horizontal wells with slim hole,the cuttings migration law of long horizontal wells with slim hole is clarified,the transient model of fine annulus ECD prediction is established,and the hydraulic parameter optimization method is proposed,which has important theoretical and engineering significance for solving the problems of borehole cleaning and wellbore pressure control of horizontal wells with slim hole during drilling in deep formation.