Shallow casing shoe integrity interpretation technique

Abstract

Unlike the leak-off testing (LOT) in deep wells, the LOT plots from shallow well sections subsea are often inconclusive due to the lack of an obvious point of deflection from their initial linear behavior. In the paper, simultaneous effects of geo-mechanical factors and fall-off mechanism have been combined to explain and analyze the tests in shallow marine sediments (SMS). Based on theoretical analysis, three main stages of leak-off plot from SMS, pressure buildup, stabilization and fall-off are identified and explained. Non-linearity of the pressure buildup and mechanisms controlling pressure stabilization are explained and used to develop a method to analyze LOT. The presented interpretation procedure considers separately the pressure stabilization and the pressure buildup and fall-off sections of the LOT plot. The procedure combines well data with pressure testing record. Well data includes overburden pressure — calculated from correlations specific for SMS, and the maximum cementing pressure at the time of slurry placement. The stabilized LOT pressure analysis identifies the maximum strength of the casing shoe controlled by the rock stress (overburden pressure) and the failure by rock fracturing. Also presented is a graphical procedure for comparing the plots of actual and equivalent pressure fall-off. The graphical procedure provides a pattern-recognition method to distinguish the mechanism of cement–rock parting from the fluid loss mechanism.