Investigation of casing deformation characteristics under cycling loads and the effect on casing strength based on full-scale equipment

Abstract

Production casing deformation has hindered the safe and economic development of shale gas resources, especially in Weiyuan-Changning Shale Gas field, China. In terms of current literatures, this problem should be attributed to the casing failure induced by varying high loads during the multi-fracturing process. However, the characteristics of casing deformation under cycling loads have hardly been investigated in previous work. In order to realize the casing deformation mechanism during multi-fracturing, the casing deformation real-time monitoring system is improved, based on full-scale casing-cement sheath-formation system (CCFS) combination device. Further, the deformation testing experiments of P110 casing under cycling inner pressure loads, for four conditions (1#-standard condition, 2#-higher inner pressure, 3#-thinner casing, 4#-thicker cement sheath), are carried out … Besides, the reliability of experiment approach is verified, by the consistency of effective circumference strain values in the loading process of first cycle between calculated by elastic theory and determined in this paper. Further, the casing strength under cycling loads is analyzed, to evaluate the casing failure in shale gas well. Results illustrate that the casing under cycling loads are available to generate residual strain, which are reliable to reach 0.035%, 0.104%, 0.042% and 0.027%, respectively, based on above mentioned four testing parameters under 16 cycles. The higher cycling inner pressure significantly increases the residual strain of casing, which is harmful for the casing protection. Increasing the thickness of casing and cement sheath is beneficial to reduce the residual strain of casing. The casing collapsing strength is linearly lowered with cycling number, due to the growing unrecovered strain of casing during multi-fracturing. It precisely explains the phenomenon that casing deformation frequently occurs in the last few fracturing stages of shale gas wells. The casing strength lines from experiment results demonstrate higher accuracy than API (American Petroleum Institute) standard, in evaluating the casing failure of field cases. This work will provide a new criterion for the parameters designs of casing and fracturing in shale gas wells, to reduce the risk of casing deformation during multi-fracturing. • The measurement of casing strain under cycling loads is achieved. • Casing under cycling loads are available to generate residual strain rather than perfect elastic strain. • The equations predicting residual strain of casing are raised by fitting experiment data. • Casing strength lines determined in this paper demonstrate higher accuracy than API standard in shale gas wells.