Drillpipe failure is an outstanding issue in drilling engineering, often involving great financial losses. In view of the special features of the flow channel in the high failure zone, this article analyzes the drillpipe failure mechanism from the point of view of flow induced corrosion. Based on the Eulerian-Langrangian method and the discrete phase model, a numerical simulation method is used to investigate the flows of the drilling fluid in the drillpipe connector during the operation of three typical drilling methods （mud drilling, air drilling and foam drilling）. From the flow field in the drillpipe connector, especially, the velocity and pressure distributions in the threaded nipple and the thickened intermediate belt, one may detect the existence of the flow induced corrosion. Then, some structural optimization measures for the drillpipe connector are proposed, and the optimization effects are compared.
ZHU Hong-jun LIN Yuan-hua ZENG De-zhi YAN Ren-tian
Sustained casing pressure （SCP） in gas wells brings a serious threat to worker safety and environmental protection. According to geological conditions, wellbore structure and cement data of gas wells in the Sichuan-Chongqing region, China, the position, time, environmental condition and the value of SCP have been analyzed. On this basis, the shape of the pressure bleed-down plot and pressure buildup plot were diagnosed and the mechanism of SCP has been clarified. Based on generalized annular Darcy percolation theory and gas-liquid two-phase fluid dynamics theory, a coupled mathematical model of gas migration in a cemented annulus with a mud column above the cement has been developed. The volume of gas migrated in the annulus and the value of SCP changing with time in a gas well in Sichuan have been calculated by this model. Calculation results coincided well with the actual field data, which provide some reference for the following security evaluation and solution measures of SCP.
Failure data from oilfield showed that casings which were designed according to API standards were deformed and collapsed in salt formations. The main reason for decrease in strength may be caused by non-uniform loading(NUL) that was not considered in traditional casing collapsing strength design or that the designing method should be improved and developed. Obviously, the calculation of casing collapse strength is one of the key factors in casing design. However, the effect of NUL on casing collapse strength was generally neglected in the present computational methods. Therefore, a mechanical model which can calculate casing collapse strength under NUL was established based on the curved beam theory of the elasticity and was solved using displacement method. Simultaneously, three anti-collapse experiments were performed on C110 casing under NUL, and the strain and deformation laws of three casings in the process of collapse were obtained by the electrical method. Yield limit of every casing was obtained by analyzing those data. Experimental results are consistent with the results of calculation of new model. It indicates that the model can be used to calculate yield limit loading of casings under NUL.
