A Study on Stress Corrosion Crack of Thick-Walled Elbow in Manifold for Acid Fracturing

Acid fracturing, with a pumping pressure up to 140 MPa for fractural liquid injection, is commonly used in enhanced oil recovery. Subjected to severely high hoop stress, stress corrosion cracking (SCC) often occurs at cracks inside manifold elbows which are due to acid corrosion and fracturing propping agent erosion. To avoid the occurrence of SCC, it is necessary to establish a substantial understanding of the fracture strength of thick-walled elbow, including the stress state distribution in elbow and SCC properties of elbow material. Base on the hoop stress calculation for thin wall straight pipe, a formula for hoop stress of thick-walled elbow is derived and also verified with finite element method (FEM). This is accomplished by introduce curvature factor and wall thickness factor into the formula. Furthermore, the critical stress intensity factor, K_ISCC, is determined subsequently in simulated acid fracturing corrosion condition. However, part of the considerable contribution of this work is to scale the critical crack depth. A simplified hoop stress formula proposed in this paper can be used in design of elbow wall thickness and strength evaluation. The established critical crack depth calculation formula can be applied to safety assessment of elbows with initial erosion cracks. The proposed method has been proved to be a simple and useful method to calculate the stress and fractural strength of thick-walled elbow.