Study On The Mechanism Of Wellbore Instability In Ultra-deep Fractured Formation

The geological conditions of ultra-deep fracture reservoir rock are more complex than that of shallow layer.It has the characteristics of wide development of natural fractures,high bottom hole temperature,low permeability of reservoir matrix,large difference of structural stress,high pore pressure,etc.when drilling in this kind of layer,wellbore instability accidents occur frequently,which greatly restricts the efficient development of ultra-deep fracture tight reservoir.The traditional wellbore stability model considers the influence of temperature and weak surface on wellbore temperature,but it fails to consider fracture types and the dynamic change of wellbore temperature,the influence of temperature and pressure on the performance of drilling fluid,and cannot effectively describe the wellbore instability in ultra-deep fracture tight reservoir.In view of this,this paper uses the numerical method to solve the dynamic distribution of the temperature field around the wellbore in the process of drilling,calculates the change of the performance parameters of the drilling fluid with the temperature and pressure,determines the strength failure criterion of the rock in the fractured formation,and finally analyzes the influence of different fracture types on the wellbore stability by establishing the thermal-fluid-solid coupling model with the time-dependent temperature boundary.The specific research contents are as follows:1)Calculation method of wellbore temperature field in ultra-deep wellBased on the theory of unsteady temperature field,a wellbore temperature model considering the heat source term and wellbore structure caused by friction loss is established.The dynamic changes of temperature field around the wellbore while drilling fluid circulation and after stopping drilling are solved respectively.The influence of wellbore structure and construction parameters on wellbore temperature is analyzed,and the changes of drilling fluid performance with temperature and pressure are explored.It is found that the influence of construction parameters such as the displacement and density of drilling fluid on the temperature of drilling fluid is greater than that of the wellbore structure,and different wellbore structures have a greater influence on the temperature rise after the stopping drilling;the trend of the density and viscosity of drilling fluid is opposite to that of the temperature,and consistent with that of the pressure.2)Analysis of mechanical properties of fractured rock samplesThrough the RTR1500 high temperature and high pressure rock mechanics experimental machine to carry out the experimental research of fractured sandstone samples,the influence laws of different confining pressure,temperature,fracture dip angle and fracture filling mode on rock strength are analyzed,the failure criteria of fractured rock samples are explored based on the experimental results,the judgment basis of different failure modes of rock samples is given,and the relevant parameters are obtained for the wellbore stability model in Chapter 4.It is found that the fracture obliquity is the most important factor affecting the failure mode and compressive strength.The fracture obliquity is the most important factor affecting the failure mode and compressive strength.The failure mode of oblique fracture rock sample is sliding along the fracture surface,with the increase of fracture obliquity,the compressive strength first decreases and then increases,there is a minimum value;the coupling of high temperature and high pressure will make the rock more fragile,the rock sample changes from brittleness to plasticity,and the increase of confining pressure will inhibit the effect of fracture obliquity.3)Thermal-fluid-solid multi field coupled wellbore stability model considering fractureBased on Duhamel’s principle,the boundary conditions of wellbore temperature varying with time are coupled into the temperature-stress field,and the thermal fluid structure coupled wellbore stability model with time-dependent temperature boundary is established,the wellbore stability model with different fracture filling conditions is proposed,and the response mechanism of formation pore elasticity under different instantaneous permeability boundary is analyzed,the additional stress induced by temperature change at different temperature boundary,and the collapse condition of wellbore under different fracture filling conditions.Finally,taking Keshen block as an example,the model is used to analyze the wellbore collapse in ultra-deep fractured tight sandstone reservoir.Finally,the corresponding countermeasures will be put forward.