DRILLING IN OVERPRESSURED FORMATIONS IN AUSTRALIA AND PAPUA NEW GUINEA

Abstract

The accepted definition of an overpressured formation is one in which the pressure of the formation fluid is in excess of the theoretical hydrostatic pressure of the natural formation water if the formation water extended to surface. For the purposes of this paper, formations which have a pressure in excess of that corresponding to a gradient of 0.47 psi/ft from the surface to the depth of the formation are considered overpressured; this corresponds to the gradient of saturated saltwater. It is not necessary for a large reservoir to exist before drilling problems can be occasioned by abnormally pressured formations.

Various means of determining the presence of overpressured formations exist and these formations have been encountered in many Australian basins including the Papuan Basin, the Bowen Basin, the offshore portion of the Carnarvon Basin, the Bonaparte Gulf Basin and in the Perth Basin. The usual means of combating abnormal pressures are by addition of barytes to the mud, by back pressure drilling or by allowing the formation pressure to partially deplete. This last alternative is only practical in low volume "shale-gas" occurrences. In the offshore Papuan Basin, a modified form of back pressure drilling known as the floating mud-cap technique was used for a productive reservoir containing a lengthy gas column.