Mechanism study on casing failure of mudstone layers caused by waterflooding development

After waterflooding, mudstone layers are softened by water injection, as a result, asymmetry load distribution of external creep deformation is changed. So casing failure is caused by the diameter reduction and deformation at radial direction. Through the establishment of the “casing-cement mantle-surround rock” 3D finite-element mathematical model, the effect of mudstone creep deformation on casing resulted from waterflooding development is analyzed quantitatively. The mechanism of casing failure of mudstone layers caused by waterflooding development is presented. Study shows that the time of creep load increasing from 0 to the maximum is related to the creep characteristics of mudstone, about 400 d are needed generally. When the elastic coefficient decreases from 10 GPa to 6 GPa, the load on casing at radial direction will be increased to 68.36 MPa from 61.95 MPa. When the mudstone gets softened by water injection, the additional load on casing will witness an obvious increase with the decrease of elastic modulus. That is to say, as the mudstone gets softened under high water injection pressure, the heterogeneity of creep load distribution is intensified and the creep load exceeds the resistance to external pressure. As a result, the casing string becomes deformed at radial direction.