Research On Wellbore Temperature Control Method And Thermal Stress Near Wellbore

When drilling deep oil and gas reservoirs,engineering and technical challenges such as abnormal high temperature（greater than 170℃）,high pressure,and narrow safety density windows are often encountered.The high-temperature environment underground seriously affects the performance of drilling fluid,limiting the use of drilling guidance and measurement equipment,and the thermal stress near the wellbore also has a significant impact on the stability of the wellbore.Therefore,accurately predicting and reducing wellbore temperature,clarifying the distribution pattern of thermal stress near the wellbore,is of great practical significance for safe and efficient drilling.Based on the background of ultra deep wells in a certain oilfield,the following studies were mainly conducted:1.Based on the law of conservation of energy and the theory of fluid mechanics,taking full account of the wellbore structure,drill string assembly and heat sources generated during drilling,combined with the mutual coupling effect of the physical parameters of drilling fluid and the temperature and pressure in the wellbore,a coupling model of temperature and pressure fields in ultra deep wells was established.The rationality and accuracy of the model were verified by using field drilling data validation,and the distribution law of temperature and pressure in high-temperature wells was studied.（See Chapter 2）2.Through sensitivity analysis,it is concluded that the main influencing factors of ultra deep well wellbore temperature field are drilling fluid circulation time,displacement,specific heat capacity of drilling fluid and thermal conductivity of drill pipe;On this basis,the overall idea of wellbore temperature control method is provided;A method of using nano Ti O₂ as a drilling fluid additive and double wall insulated drill rods to reduce bottom hole temperature was proposed,and the cooling effect and feasibility of the above methods were analyzed.（See Chapter 3）3.The coupling model of wellbore temperature and pressure field is used to study the temperature distribution law of the wellbore and the formation near the wellbore.Based on the thermoelastic theory,the thermal stress model near the wellbore is derived,the thermal stress distribution law is studied,and the influence law of thermal stress on the stress of the surrounding rock of the wellbore is analyzed;The calculation results show that the thermal stress near the wellbore is tensile stress,and the circumferential and vertical thermal stresses reach their maximum values respectively at the wellbore.The thermal stress increases with the increase of cycle time,reducing the effective stress of the formation,making the wellbore more prone to fracture,and increasing the risk of leakage.（See Chapter 4）4.On the basis of clarifying the distribution law of wellbore surrounding rock under additional thermal stress conditions,a wellbore stability model under thermal coupling was derived using the Mohr Coulomb criterion,and the influence of thermal stress on the safe density window of drilling fluid was studied;The calculation results indicate that the safety density window of drilling fluid is positively correlated with the wellbore temperature.Therefore,it can be considered that when drilling in formations with strong pressure bearing capacity,the above wellbore temperature control method can be used to reduce the bottom hole temperature and achieve the goal of improving wellbore stability.（See Chapter 4）...