Publications

Exploration wells drilled in the Madura-Flores Sub-Basin frequently experience borehole instability and loss of circulation. These drilling hazards are primarily restricted to the shaly overburden section overlying the Oligo-Miocene carbonates play. They are interpreted to be the result of abnormal pressure regimes of the formation water along with mechanical instability of the shales, which result in narrow effective stress windows. In this context, well design of deepwater exploration wells proves itself a perilous exercise, and the risks of escalating well construction costs and loss of drilling time during the operations are relatively high. To minimize these exposures, the development of wellbore stability models is essential to determine the critical mud weights that can provide the support to the wellbore wall without invading the formations. Pore pressure constitutes one of the key parameters of these models and its accurate prediction in abnormal pressure regimes remains decisive in the forecast capabilities of the model. A pore pressure cube of part of the Madura-Flores Sub-Basin was established using basin modelling techniques. Basin modelling provides more flexibility than traditional quantitative seismic tomography methods in assessing the impacts on the pressure regime of events that could be below seismic resolution, especially the up-dip transfer of pressure within permeable beds. The 3D model was simulated through stratigraphic times in terms of sediment compaction and formation water flow in order to calibrate a series of critical pore pressure indicators extracted from key wells previously drilled in the area. The fluid flow simulations used a modified Darcy Flow equation integrating buoyancy and capillary pressure into a pressure-temperature 3D finite element solver. Different scenarios of permeable facies occurrence and distribution, shale compaction and permeability reduction, and water salinity were envisaged in order to capture the existing uncertainties in the characterization of the overburden section. Pore pressure profiles were then extracted from the 3D pore pressure cubes at the location of selected deep water prospects. The Min/Base/Max pore pressure predictions for each prospect location were subsequently used as an input into a wellbore stability model.