Date of Award
Doctor of Philosophy (PhD)
The leak-off test (LOT) is an in-situ method for testing casing shoe integrity in a well. It is used to evaluate the cement and formation integrity relative to the well plan specifications. The results determine whether any remedial cementing or corrective actions are required and are a basis for planning future wells in the area. Typical analysis of LOTs assumes an elastic wellbore and involves identification of linear trends on the recorded plots. However, LOTs recorded in shallow marine sediments (SMS) are inherently nonlinear and their analysis is a problem. Starting from the analyses of shallow soil properties, LOT plots, and leak-off pressure data, the dissertation presents the results of a theoretical study (analytical and numerical-finite element analysis) into potential for damage to cement integrity at the casing shoe resulting from leak-off test in SMS. Stress, strain and displacement around the open hole are analyzed before and during LOT. Three types of possible failures from LOTs were considered: vertical fracture, horizontal fracture, and cement parting. It is proved that vertical fracture is the most unlikely failure of the three. Although horizontal fractures are initiated at low pressure in the plastic zone around the wellbore, they cannot propagate beyond the plastic zone until wellbore pressures exceed overburden pressures. Cement parting, on the other hand, may propagate upwards at pressures lower than overburden pressure. The study identifies two factors, related directly to drilling technology, that control critical pressure of cement parting: contact stress and drilling fluid penetration. It is shown in the study that changes in cementing and drilling practices can improve casing shoe integrity and reduce the need for remedial cement squeezes. A general pressure-volume model of a LOT is presented including volumetric effects of wellbore expansion, mud loss into the rock, and propagation of both cement parting and plastic fracture. Software entitled LOTUMS was developed to simulate LOTs in SMS. A method is also proposed to identify the mechanisms controlling LOT results using known overburden pressure and the shape of LOT plot.
Zhou, Desheng, "Well Integrity Mechanism, Failure, and Testing in Shallow Marine Sediments." (2000). LSU Historical Dissertations and Theses. 7405.