Borehole casing failure analysis in unconsolidated formations: A case study

A considerable number of oil and gas reservoirs in the world are located in unconsolidated sands, in which casing failure and sand production are the major problems impacting oil production. This paper presents field-observed results of casing damages in unconsolidated sandstones in the Gudao reservoir of the Shengli Oilfield, China. Three-dimensional failure characteristics of borehole well casings were obtained using the ultrasonic wellbore televiewer. The downhole observation results show that casing failures primarily occurred in unconsolidated sandstones, and were caused by sanding-induced cavities. The results also reveal that most failures were caused by the casing buckling and fracturing due to the cavities and non-uniform stress distribution in the unconsolidated formation. Numerical modeling is applied to analyze the mechanism of casing failures in such a formation. The interactions between the casing and surrounding formation are simulated, and effects of sanding-induced cavities on the casing are also considered. The numerical results show that the cavities in the formation due to sanding cause the formation more likely to fail and the casing to suffer much larger deformations. Furthermore, casing buckling, axial compressive yield and load-deformation relationship are analyzed. According to these analyses, the casing damage mitigation methods are presented.