Cooper Overpressure, Natural Fractures and Velocity

Abstract

Recent drilling in the Nappamerri Trough of the Cooper Basin has confirmed continuous gas, no water and variable overpressure. These observations are seen in all wells including those outside of structural closure. We speculate that the overpressure is dynamic; it is caused by the rate of current day gas generation exceeding the rate of gas escape from these tight gas sandstones and shales. In a system like this sweet spots coincide with the presence of overpressure. The objective of this research is to document overpressure, open versus closed natural fractures and their geophysicals signatures in the Cooper Basin. Fracture analysis on image logs was performed at several normally pressured and overpressured wells. Results were then compared with seismic velocity analysis and dipole sonic logs. Preliminary results show many more open fractures associated with overpressure. Closed resistive fractures were observed only in a normally pressured well. Large velocity anisotropy signature observed on dipole sonic log coincides with high fracture intensity documented on the image log. Furthermore borehole breakouts exist only on sand and not in lower compressive strength shales. Results will allow one to understand if overpressure is associated with open natural fractures and expected enhanced gas flow rate. Results can be applied for choosing optimum well location and assisting well design and hydraulic fracture stimulation design.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015