Casing Point Selection for Drilling Operations at Shallow Depths

In shallow sediments, unlike deep sediments with elastic behavior, the failure mechanism of the casing shoe is strongly affected by the plasticity of the rock. Hence, the common practice in casing design which is based on using the pore pressure and fracture pressure gradients plots is not applicable in shallow sediments. Moreover, because of the plastic behavior of the sediments, the interpretation of Leak-Off Tests (LOT) in Shallow Marine Sediments (SMS) could be inconclusive. Therefore, because of uncertainties in prediction of formation fracture and pore gradients, the conductor and surface casing setting depths have always been subject to debate. Also, incorrect interpretation of LOT would lead to costly problems that might jeopardize well progress such as; well control issues, unnecessary squeeze jobs, premature setting of casing, and lost circulation problems. Two of the most important factors in any design are safety and cost. Since safety is one of the most important concerns during drilling an offshore well, planning a design based on the well control aspects would be an appropriate approach to come up with a safe and better design. A well control simulator was used to plan for well control situations. In this paper, the results were generalized for different design scenarios and a simple design method is presented. Also, a new method, supported by field data from LOT in SMS, is presented to accurately analyze casing shoe leak-off pressure in the SMS. A safe design based on the optimum lengths of conductor and surface casing would enable the operator to handle possible formation kicks. Extension of this method to well design in general suggests the potential for safer drilling operations and cost optimization.