Research On Safety Evaluation Method Of Casing Fracturing Wellbore Strength

Casing fracturing is one of the main fracturing methods,and the load,cement ring defects and casing bending under fracturing conditions are the main factors affecting the strength safety of the wellbore.The fluid enters the wellbore from the wellhead to cause changes in temperature,pressure,friction,etc.,which affects the stress and deformation of the casing;Oil and gas entering the ring air and uneven cement flow rate cause incomplete penetration defects of cement ring,which increases the risk of arbitrage;The casing in the bending borehole will be affected by the action of bending moments,causing the casing to bend and flatten,affecting the integrity and safety and stability of the casing.Therefore,in order to establish the safety evaluation of casing fracturing wellbore strength,the following analysis is carried out.According to the missing morphology in the actual project of cement ring,the finite element analysis model of casing-cement ring-formation was established,and the effects of different missing area,relative position angle and number of missing strips on the strength and safety of wellbore were analyzed according to the first interface formed by casing and cement stone,the two interfaces formed by cement stone and formation,and the intermediate position of the two interfaces.For the oval deletion inside the cementing cement ring,when the number of missing strips is 6,the stress increases by 43.23%.For the loss of crescent at the first interface of cementing cement,when the missing area was 6%,the stress increased by 51.71%.For the convex moon loss at the second interface of cementing cement,the stress increased by 9.81%when the relative position angle was 180°.According to the theory of material mechanics,the four-point bending mechanical model of casing is established,the complex load under the action of internal and external pressure,temperature,friction resistance and bending stress of the casing is considered,the three-way stress under the complex load is analyzed,and the fourth strength theory is applied to obtain the analytical formula of the anti-extrusion strength of the casing.The finite element analysis model of bending casing was established,the extrusion strength of bending casing was analyzed,the analytical solution and numerical solution results were compared,and the analysis method of extrusion strength of bent casing was improved.Taking the 5 1/2″ × 10.54 mm P110 casing as an example,when the borehole curvature increased from 2°/30 m to 12°/30 m,the extrusion strength decreased from 100.66 MPa to 97.11 MPa,a decrease of 3.53%,and the 12.7mm wall thickness casing of the same steel grade decreased by 6.67%.It can be seen that with the increase of borehole curvature,the extrusion strength of casing gradually decreases.For the same steel grade casing,the larger the wall thickness,the stronger the sleeve extrusion resistance.When the borehole curvature increased from 2°/30 m to 12°/30 m,the collapse strength of the casing before flattening was reduced to 4.99 MPa,and the decrease value after flattening was 22.74 MPa.With the increase of borehole curvature,the flattening rate of casing section increases,and the collapse strength of casing after flattening is more affected by the curvature of the borehole,and the decrease value is more obvious.In this paper,considering the complex load under fracturing conditions,an analysis method for the influence of cement ring defects and casing bending on casing safety is established,and a refined analysis of fracturing casing and a wellbore strength safety evaluation method are formed by applying the derived concentrated load and stress formula of casing to a sample well in an oilfield.