Estimation of Leakoff Pressure Gradient from Cementing and Casing Data

Abstract

Abstract The paper describes the development and use of a new procedure for finding safe value of casing shoe leak-off pressure in surface holes in the shallow-marine sediments (SMS). Presented is a theoretical proof, supported by field data from leak-off tests, that, in SMS, annulus channeling outside cement occurs more likely at pressure values smaller than the rock fracturing pressure. Hence, for upper well sections in SMS the prediction of casing shoe strength should be based on critical conditions for annulus channeling rather than rock fracturing. It is also shown that pressure required for casing shoe leak-off behind cement 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. Set up is a mathematical model of contact stress around casing shoe based on cement volume reduction and compensation from casing string, cement compressibility and wellbore. Using readily available data of well geometry, casing properties and cementing operation, simple procedure, explained in this paper, gives a good estimation of cement fracture compared with actual leak-off test values. The study identifies two factors, related directly to drilling technology, that control critical pressure of cement channeling in SMS: contact stress at casing shoe – resulting from cementing operations, and rock penetration by liquid – an invasion of drilling fluid into the rock around the casing shoe. It is shown in the paper that changes in cementing and drilling practices may increases casing shoe integrity and reduce the need for cement squeeze treatments.