Research On Casing Leakage Prevention Technology Under Narrow Safety Density Window In Tarim Ultra-deep Well

Tarim oilfield plays an important role in the energy structure of China,which is called the energy economic artery of Western China by economists.Kuqa Piedmont deep high temperature and high pressure gas reservoir concentrates more than 60%of the natural gas reserves and production of Tarim Oilfield.However,due to the complex geological characteristics and narrow window conditions,it is difficult to guarantee the engineering quality of small clearance cementing operation in running liner for a long time due to irregular leakage during cementing,which seriously threatens the integrity of wellbore in safe well construction and development and production.To solve the problem of ultra deep well cementing quality,this paper treated the narrow window running casing cementing technology as the breakthrough of leakage prevention.Based on the research of domestic and foreign scholars,the quantitative effects of drilling fluid thixotropy and rheology,shear rate and annular clearance change on surge pressure are considered in this paper,and based on the principle of balancing surge pressure and wellbore pressure containment,the maximum casing running velocity model and the maximum mud circulation displacement model are established.Through the study of this paper,the following conclusions and understanding are obtained:(1)Complex geological characteristics such as high sensitivity of formation pressure and narrow density window are the objective factors causing running casing leakage.Improper design of running casing engineering parameters due to inaccurate wellbore pressure containment acquisition,narrow annulus clearance in casing casing and rheological and thixotropic effects of high-density oil-based drilling fluid are the main factors causing well leakage.(2)The change rule of the thixotropy of conventional and high density oil-based drilling fluids with the extend of time was studied.Experiments show that the static shear stress of drilling fluid increases with the increase of standing time,and finally tends to a constant value;it may cause well leakage if the surge pressure is calculated by the ultimate static shear stress.(3)Based on the rheological test of drilling fluid under variable temperature and pressure,the prediction model of relevant rheological parameters in the range of full shear rate and low shear rate of high-density oil-base drilling fluid was established.The results show that it is safer to calculate the surge pressure under the range of low shear rate.(4)One model of calculating the surge pressure of the Herschel-Bulkely fluid is established,which takes into account the variation of the annulus clearance during liner cementing.The results show that this model can better reflect the actual change rule of the surge pressure with the depth of casing running than the conventional Bingham model and the Herschel-Bulkely model which without considering the change of annular clearance.(5)This paper established the maximun casing running velocity model and the maximum mud circulating displacement model considering wellbore pressure containment,rheology,shear rate and the variation of annular size on the use of constraints of the value of annulus circulation pressure loss ofthe thief zone.Through this study,a set of running casing leakage prevention technology was developed for ultra-deep wells with narrow density window,and it was applied in the field of KS605 well.Through this technical method,the maximum casing running velocity and the maximum mud circulation displacement of liner cementing operation are calculated accurately,and no leakage occurs in the whole process,indicating that this technology is feasible in the field.