Part of Australia’s effort to develop alternative energy sources includes developing the unconventional geothermal systems in the Cooper Basin. In the first stage of the Hot Fractured Rocks Project, Geodynamics Ltd has drilled three vertical wells. During the first well, Habanero #1, the granite basement was unexpectedly discovered to be overpressured by as much as 5200 psi. This and the existing stress regime of the granite created unforeseen conditions that lead to costly kicks, losses, and high mud costs. They saw Managed Pressure Drilling (MPD) as a solution to control the delicate balance between the overpressure and fracture gradient.

In Habanero #2 and #3, mud weight, equivalent circulating density (ECD), and annular surface backpressure were used to maintain balance just above the reservoir pressure at the first productive fracture drilled in the 6” interval in the #2 well, and in the 8.5” interval in the #3 well. To minimize the overbalance in subsequently drilled permeable fractures the static mud weight was reduced by up to 1 ppg below pore pressure. The ECD was continuously managed by applying, while drilling, between 220 and 295 psi and, during connections, between 525 and 625 psi.

Geodynamics elected to use the automated Dynamic Annular Pressure Control (DAPC) system to balance the ECD with the formation pressure and respond to, and control any fluid influx. The DAPC system was used on Habanero #2 in 2004 and again most recently on Habanero #3. It gave the operator the ability to reduce drilling costs by lowering the mud weight and mitigating the risks experienced in Habanero #1 with conventional drilling.

At one point during the drilling of the Habanero #3 well, an influx occurred while the DAPC system was actively managing pressure. The system was used to gain control of and kill influx within 90 minutes. Once under control, the mud weight was increased by 0.7 ppg and drilling proceeded with no other events. By comparison, during the drilling of the Habanero #1 well, without MPD, an influx occurred which required 4.0 ppg of additional mud weight to kill and considerably more time to control.