Casing point selection at shallow depth

Abstract

Different oil companies place shallow casing strings at different depths. Apparently there is no general method or procedure that gives the optimum location. This paper is an attempt to define this optimum casing seat location. The potential benefits will be improved safety and economy of the operation. Two main factors determine the depth of the casing shoe, namely, the fracture pressure and the pore pressure. A third factor is the lithology, because it is desirable to place the casing shoe in a competent shale section. Methods to predict fracture gradients for deeper wells already exist. In this paper a method to predict fracture gradients for shallow well has been derived. This method is combined with kick tolerance criteria to yield a casing depth selection method. Also, the variation in fracture pressures at any depth has been investigated. A case study of a field offshore Norway has been performed. The preliminary conclusions are that the 30-in casing setting depth is difficult to model, whereas the 20-in casing string could be set at shallower depths. The large spread in leak-off pressures of the field has been investigated further. Some of the spread is found to be caused by the failure mechanisms of the rock and is therefore unpredictable. Part of the spread in observed fracture gradients, however, seems to correlate with mud properties. Improved hole strength is, therefore, possible by formulating a proper mud.