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Understanding Pressure Compartmentalization in Ultra Deep-Water Drilling Off Mexican Gulf Coast: A Case Study


High formation pressures and narrow safe mud weight window are prime concerns while designing a well in southern part of ultra deepwater off Mexican Gulf coast. In these wells the stable window is predicted to be quite narrow and is further reduced by higher Effective Circulating Density (ECD). Pre-drill wellbore stability (WBS) modeling helps to assess the feasibility of these complex wells, although such models serve to support overall well design. However, controlling WBS during well construction often requires a more interactive approach between geomechanics, Geology, petrophysics and drilling operations to meet the well objectives. Pressure data acquired in offset wells suggests that formations above Miocene are hydrostatic in nature and pressure gradually increases in Middle Miocene where pressure gradient has been observed up to 0.63psi/ft. Drilling challenges arise while entering lower Miocene with a steep pressure ramp of ∼3psi/ft has been encountered. Although, overpressure can be estimated using logging-while-drilling (LWD) data, it was quite complex a situation to ascertain the pressure transition to set the casing at optimum depth to reach the objectives. Pressure compartmentalization of this order is critical to understand, not only for optimal drilling but also for evaluation of the reservoirs. In this study, a multi-disciplinary approach is used to understand and identify the pressure seals and a solution has been proposed. Background shale pressure was compared with the measured pressure in sands to understand the sand-shale equilibrium; in conjunction with petrophysical analysis of Lower and Middle Miocene formations to delineate the pressure transition. While exiting Miocene and entering Oligocene formations, pressure gradient reduces to ∼0.8psi/ft. Geological past was also reconstructed to see the potential of fluid retention capacity as well as fluid restriction capability of the formations. Real-time quality check on LWD data was used to correlate with geomechanical analysis to support the drilling operations and optimize the section depth. The case study discusses the impact of pressure compartmentalization in well design and mitigating associated risks in drilling operations. This also offers better understanding of the expected sub-surface behavior to drilling, leading to successful exploration in over pressured Oligocene formations in southern part of Mexican Gulf Coast.