BOREHOLE FAILURE RESULTING FROM FORMATION INTEGRITY (LEAK-OFF) TESTING IN UPPER MARINE-SEDIMENTS OFFSHORE

This paper presents the results of a theoretical study, supported with the finite element analysis, into potential loss of external integrit y around a casing shoe resulting from leak-off testing (LOT) in upper marine sediments (UMS). Three types of possible failures from LOTs wer e considered: vertical fracture, horizontal fracture, and a channel ou tside cemented annulus. It is proved in the paper that vertical fractu re is the most unlikely failure of the three. The other two types of f ailure can be distinguished by different values of propagation pressur es. Although horizontal fractures are initiated at low pressure in the plastic zone around the wellbore, they cannot propagate beyond the pl astic zone until wellbore pressures exceed overburden pressures. Annul ar channels, on the other hand may propagate upwards at pressures lowe r than overburden pressure. The paper shows that these channels are in itiated at pressures equal to the contact stress between cement and ro ck and their propagation pressures are on average 3.5-fold greater tha n contact stress. It is also explained how to identify the UMS with hi gh risk of annular channeling during LOTs.