Cement Seal Failure at Casing Shoe in Shallow Marine Sediments

This paper describes the hypothetical mechanisms of cement seal failure during casing shoe leak-off testing (LOT) of surface holes in the shallow-marine sediments (SMSs)—parting of cement sheath from formation by LOT wellbore pressure. Presented is a theoretical proof—supported by LOT field data—explaining why, in SMSs, casing shoe failure occurs at pressures smaller than the rock fracturing pressures. Hence, for upper well sections in SMSs the prediction of casing shoe strength should be based on critical conditions for cement seal failure rather than rock fracturing. It is also shown that pressure required for cement seal failure can be determined from contact stress between cement and wellbore. Contact stress develops during the process of cement setting as a result of volumetric changes in the annulus. Proposed is a mathematical model of contact stress based on cement volume reduction compensated by compressibility of the casing string, cement, and wellbore. An example demonstrates potential application of the model. The study identifies two factors resulting from drilling process, which may control the critical pressure of cement seal failure in SMSs: contact stress at casing shoe, from cementing operations, and liquid penetration, an invasion of drilling fluid into the cement-rock interface around the casing shoe. It is shown that changes in cementing and drilling practices may increase casing shoe integrity and reduce the need for cement squeeze treatments.