Integrity of petroleum wells requires sealing off the casing/welbore annulus with cement to prevent migration of fluids. Leak-off test (LOT) is an in- situ method for testing casing shoe integrity. A conventional analysis of LOT data assumes elastic wellbore and involves finding the upper limit of linear relationship between the recorded pressure vs. pumped volume. However, LOT data from offshore well sections in shallow marine sediments (SMS) are inherently nonlinear and their analysis is a problem. Presented here is a study into the mechanism and effect of LOT on integrity of the casing shoe in SMS. The study begins with review of available leak- off pressure field data from SMS comparing to data from deep well sections. Then, using mechanical properties of soft soils, analytical models and numerical finite-element simulations the authors analyze stresses, strains and possible mechanisms of integrity failure resulting from shallow LOTs. This first monographic book on well integrity in shallow sediments subsea should be useful to petroleum students, researchers and oilfield professionals involved in offshore well drilling operations.