Large Displacement Design Study Of The Well Casing Strength

With additional exploratory development, direction well, horizontal well and extended reach well became more and more, especially extended reach well, because of larger deviation angle and long open hole section, casing was also run deeper, load and deformation of casing was more complicated. Not only the friction drag was considered in process of casing design, but also additional axial force caused by casing bending and casing wore, casing dynamic load were considered. The casing axial force was only involved in the effect of buoyant weight in the oil and gas profession standard (SY/T5322-2000), while ignoring the effect of friction drag and dynamic load. So it was necessary to research into theory and method of casing strings strength design for extended reach well.The paper deduced stiff-string model of considering casing stiffness in the three dimensional wellbore by the theory of elastic mechanics. The side load between casing and wall of hole could be calculated, then axial load distribution along well depth would be simulated in the process of raising or running. The analytical calculation of casing strings friction drag provided accurate axial force distribution of casing strings for casing strings strength design.According to establishing calculative model of contact force between casing and drill strings in space sideways plane, contact force between casing and drilling was simulated in the process of running or raising casing. Casing wear thickness was predicted by crushing efficiency model in the process of drilling. The paper deduced calculative model of collapsing strength after casing had been worn, the calculative result had the important theoretic and practical signification to select casing steel grade and wall thickness for extended well and horizontal well.The paper deduced trixial stress model of casing strength, and calculated axial force,external pressure and intrinsic pressure distribution along well depth. The theory and method of casing strings strength design was established according to modified equal safety coefficient method. The strength design method was suitable for extended reach well.On the basis of theoretic research, the paper combined measured actual drilling trajectory of QHD-A25 well, and simulated the axial force distribution of casing strings in the behavior of running casing. Compared with measured data, the error of simulating the hook load was about 10%, which indicted that the researched friction drag was credible in the paper. With the consideration of casing crescent-sharped wear, collapsing strength of casing was calculated after casing was worn , the error compared with test value was in 5%. Simultaneously, wear thickness of casing for P32H well was predicted according to side load distribution and continuation of drilling time. Based on the above, casing strength of P32H extended reach well was designed and checked.