Comparison of Overpressure-Detection Techniques and a Determination of Overpressure Causes in Taranaki Basin, New Zealand

Overpressure, the condition in which pore pressure in a geologic formation is greater than the hydrostatic pressure, is an important drilling hazard, and can strongly influence hydrocarbon production techniques. This study looks at logs six wells in the deepwater Taranaki Basin in New Zealand. The wells were evaluated using three different approaches to determination of overpressure, with the goal of establishing the reliability of the overpressure determination from well logs using some conventional approaches. The first step was to determine using the well logs for density, velocity, and resistivity, established overpressure zone using four techniques. The second step is the calculation of overpressure values of overpressure zone. The final step, these results with indications of overpressure observed on the drilling reports, mud logs, and other post-drilling data. Measured pore pressure, log data, temperature data, drilling data, and mud log data were analyzed for selected wells, including the end-of-well reports. The four different pore pressure calculation methods used were “Eaton Ratio Resistivity” method, “Eaton Ratio Sonic” method, and “Bowers Sonic” method, and “the Equivalent Depth” method. The methods each provided reasonable estimates, once properly calibrated, although the Eaton Ratio method appears to have provided results that are most consistent with other indicators. The intervals identified by log interpretation as being overpressured are consistent with drilling data, mud log data and other information from the end-of-well reports