Study On Wellbore Instability Mechanism And Extension Limit Of High Temperature And High Pressure Horizontal Wells In Yingqiong Basin

With the development of of oil and gas resources in the South China Sea,high temperature high pressure(HTHP)oil and gas field has become the focus of oil and gas exploration and development in the future.Yingqiong Basin is one of the world’s three offshore HTHP oil and gas basins.The drilled well bottom temperature in the basin is over 234℃ and the formation pressure factor is as high as 2.35.High temperature overpressure is widely distributed in the basin,and the geological structure and pressure causes are complex and diverse,offshore HPHT drilling and completion are faced with harsh geological environment and engineering problems,such as large environmental temperature difference,narrow drilling fluid safety density window,unstable borehole wall and limited horizontal extension.Therefore,it is the key to ensure drilling safety and improve drilling efficiency to explore the distribution characteristics of temperature and pressure around borehole in Yingqiong Basin,reveal the influence law of temperature and pressure factors on borehole stability,increase the extension limit of horizontal open hole in HPHT formation.This study firstly uses the API RP 13 D model in combination with the Modified circulating temperature-fields model to obtain a set of fully transient analytical solutions to circulating temperature fields,including the four types of the temperature inside the drilling pipe,borehole annulus,at the borehole wall,and formation for drilling an arbitrary inclined borehole in HPHT formation.The modified model introduces the effect of the riser,casing,and cement on the circulating temperature fields.Furthermore,under LTE(local thermal equilibrium)condition,this paper provides One derives a porothermoealstic solutions to the field variables around an arbitrarily inclined borehole based on the coupled thermal-hydro-mechanical model and the dynamic temperaturedifference boundary.Performing an inverse transform to the porothermoelastic solutions in a form of Laplace type can study the time-dependent variation of the filed variables.The field variables include the displacements,stresses,and pore pressure.The analytical solutions in the limiting case(long time)related to the static temperature-difference boundary condition verify the model.This model also enables the drilling engineer to more appropriately consider the different time-domain poroelastic(or porothermoelastic)solutions including the instantaneous,very short time and long-time ones in designing the mud weight.Furthermore,combining the porothermoelastic solutions with strength criteria including the shear failure and tensile fracture of porous rock studies the wellbore stability drilling through the high-pressure and high-temperature formation in Yingqiong Basin.Accordingly,according to the analytical solutions in different time domains(instantaneous,extremely short and long time),combined with the porothermoelastic solution(dynamic temperature difference boundary)of the rock field variables around the well and the strength failure criterion,the calculation models of formation fracture pressure and collapse pressure in the horizontal section of the circulating temperature field are obtained,and the equivalent static density model of drilling fluid under the circulating temperature field and the calculation model of circulating pressure loss in each well section are established,The calculation method of open hole extension limit for offshore HTHP wells has been formed.Based on the established extension limit theory,the optimization method for drilling parameters such as wellhead drilling fluid density and drilling fluid displacement is proposed.The following conclusions are drawn from the analysis of wellbore stability and open hole extension limit of HTHP formations in Yingqiong Basin:At the borehole bottom,the temperature difference that the drilling mud is lower than the formation increases with increasing time.Thus,the increased cooling effect increases the risk of the drilling-induced tensile wall-fractures.When the constant temperature-perturbation boundary condition chooses a larger temperature difference of the transient case at a later time,it easily overeastate the safe mud-weight window.When the hydro-mechanical coupling and coupled therma-mechanical as well as circulating temperature field are considered,the drilling mud-weight window narrows with increasing time.This study provides the theoretical and technical instruction for drilling engineers to appropriately design the pre-drilling time-dependent safe-mud-weightwindow.Under the condition of thermo-hydro-mechanical coupling,with the increase of drilling fluid circulation time,the equivalent static density of drilling fluid increases,the fracture pressure of horizontal section decreases,and the open hole extension limit decreases continuously.However,the range of change decreases with the increase of circulation time.After the circulation time exceeds 48 h,the equivalent static density of drilling fluid,the fracture pressure of horizontal section,and the open hole extension limit value tend to be stable.Drilling design based on the open hole extension limit value neglecting the effect of circulating temperature field will cause drilling risks.This study provides theoretical and technical guidance for reasonable design of "safe density window with time correlation" before drilling,and provides theoretical support for meeting the requirements of extension reach wells in Yingqiong Basin to increase their open hole horizontal length in HTHP formations.