超深井筒温度分布及其对围岩力学性质的影响研究

Study on temperature distribution of ultra-deep wellbore and its effect on mechanical properties of surrounding rock

钻井液停止循环后井底温度回升,极易发生由于温度升高而造成的井壁岩石延迟破坏。为探究钻井液循环过程中,由于井筒温度变化而造成的井壁失稳问题,首先通过基于非稳态温度分布的数值计算方法,探究时间、工程参数对井筒温度分布的影响;此外,借助高温高压岩石力学试验机,进行不同温度下的细砂岩的强度实验,探究温度变化对岩石强度的影响规律。实验结果表明:钻井液开始循环后,井口附近温度随时间增加呈上升趋势,井底附近呈下降趋势,存在一个与时间有关的临界井深;温度变化主要集中在井筒附近很小的区域,并且在钻井液循环初期温度迅速降低;钻井液的排量、黏度、密度越大,井底附近的井壁温度降低幅度越大,其中,排量改变对井壁温度分布影响最为明显;温度增加使含裂缝岩样的破坏程度加剧,除滑移破坏,伴有断面破坏和端面破坏发生,在裂缝倾角–抗压强度关系中,温度升高对发生滑移的区域影响不明显;温度升高,岩样的抗压强度明显降低,应力–应变曲线变得曲折,存在明显的塑性变形段,破坏过程变得复杂。

After the drilling fluid stops circulating,the bottom hole temperature rises again,which is very easy to cause delayed damage of the wellbore rock caused by the temperature rise.The wellbore instability caused by the change of wellbore temperature during drilling fluid circulation was analyzed.Firstly,the influence of time and engineering parameters on the temperature distribution of wellbore was studied by numerical calculation method based on unsteady temperature distribution.In addition,the strength experiments of fine sandstone at different temperatures were carried out by means of high temperature and high pressure rock mechanics testing machine,and the influence law of temperature change on rock strength was researched.The experimental results show that after drilling fluid circulation,the temperature near the wellhead increases with time,and the temperature near the bottom of the well decreases with time,and there exists a critical well depth related to time.The temperature changes mainly concentrate in a small area near the wellbore,and the temperature decreases rapidly in the initial stage of drilling fluid circulation;the larger the displacement,viscosity and density of drilling fluid,the greater the decrease of wellbore temperature near the bottom hole,and the change of displacement has the most obvious effect on the temperature distribution of wellbore;the temperature increases,which aggravates the damage degree of fractured rock samples and eliminates slip damage.With the occurrence of cross-section failure and end-face failure,in the fracture dip-compressive strength diagram,the influence of temperature rise on the slip area is not obvious;with the increase of temperature,the compressive strength of rock sample decreases obviously,and the stress-strain curve becomes tortuous,there are obvious plastic deformation sections,and the failure process becomes complex.