PRESSURE PREDICTION AND UNDERBALANCED DRILLING IN THE DEEPWATER NIGER DELTA

Abstract

The mechanisms that cause overpressure can be broadly classified into two categories: loading and unloading. This study looks at eight wells from the deepwater Niger Delta; the wells were evaluated with three aims in mind. The first aim was to determine shale pore pressure with the density log using the equivalent depth approach. The second aim, using depth profiles of density, velocity, resistivity, and vertical effective stress, and cross-plots, was to infer the overpressure mechanism. The third aim was to validate underbalanced drilling zones identified on the pressure-depth plot with indications observed on the drilling report, mudlog, and other post-drilling data. To discriminate clean shales intervals, a density-velocity transform based on Gardner’s (1974) relationship and GR filters were applied to the well logs. Measured pore pressures, log data, temperature data, drilling data, and mudlog data, including the end-of-well-report, were analysed for selected wells. Based on pressure-depth plots, the top of overpressure in the wells lies in the range 500 – 1200 TVDml. The equivalent depth method using the density log, when properly calibrated with measured pore pressures, can give reliable pressure prediction results, especially in the shallow section at temperatures < 70°. However, in the deeper sections at temperatures >75°C, the equivalent depth method is unreliable. It was also noted that shale intervals may provide vertical permeability barriers and create pressure compartments in some of the wells. The predominant overpressuring mechanism in the shallow section, as evident from density and velocity reversals and, was found to be disequilibrium compaction. At greater depths (temperatures >75°C), a combination of disequilibrium compaction and unloading mechanisms appears to be responsible for overpressure in the wells. Evidence for unloading mechanisms includes formation temperatures of 80°C or greater, indications of smectite-to-illite conversion on density-sonic cross-plots, rapid increases in mud weight that sometimes approached fracture gradient, and depressed density reversals relative to velocity reversals. Finally, intervals identified as being drilled underbalanced on pressure-depth plots are consistent with drilling data, mudlog data, and information in the end-of-well reports. Specifically, increased gas (connection gas and background gas), hole washout, high torque, drilling break, and decreased D-exponent trend show strong association with zones of underbalanced drilling on pressure-depth plots.