Geomechanical Analysis for Deep-Water Drilling

Abstract

Abstract In deep water the fracture resistance of the borehole is low. This result in low mud weight, which in turn bring up problems like hole collapse, hole cleaning and stuck pipe. Circulation losses and underground waterflows are other associated phenomena. In this paper a model for the fracture resistance of the borehole is derived. Based on a review of fracturing mechanics and on results from laboratory investigations, one concludes that soft soil near seabed behaves in a similar manner as deeper harder rocks. The fracturing process is basically stress-dominated. Leak-off and lost circulation data are collected world wide and used in developing a field model. The water depths varies from 135 m to 2071 m. By using the basic fracture model, and also by normalizing the data for various water depths, it is shown that all field data collected behaves similarly regardless of location. One conclusion is that the frac pressure is governed by the total overburden weight, which consist of water to seabed and rocks down to the bottom of the well. A new method to normalize leak-off data is also presented. Wells from varying water depths can be compared, and used to develop prognoses at other water depths. A review of the mechanics behind penetrating/non-penetrating borehole conditions shows that this causes large variation in the fracture initiation pressure. The difference in behavior between water- and oil-based drilling fluids is explained in terms of wettability and capillary effects. A field case presents two deep-water wells. P. 441