Casing-Collapse Strength Reduction Under Lateral Loads From Yielding Shales in the Daqing Oilfield

Casing failure has been found in nearly 20% of production wells in the Daqing oil field after 45 years of oil-production history. Plastic failure of casing strings has been recognized in the zones of yielding shales overlying reservoir pay zones in the field. This type of casing failure is one of the most costly problems in the field. No research has been conducted to study the resistance of the casing string to the lateral loads from the yielding shales. This paper offers results from Such research. Assuming a lateral load uniformly distributed along a casing section from a clay zone clamped between an upper and a lower sandstone zone, a mechanistic model was developed in this study. Results of the model indicate that the resistance of casing string to the lateral loads drops sharply in the early stage of development of radial deformation. The resistance of the casing string to the lateral loads is not very sensitive to the longitudinal deformation. The remaining strength of the casing string also depends on properties of the casing string. A calculation example is illustrated in this paper. This work provides an analytical tool that can be used for optimizing casing programs against lateral loads from yielding-clay zones to minimize casing failure and field-operation cost.