Habanero Field - Structure and State of Stress

Abstract

Habanero is an enhanced geothermal system (EGS) in the Cooper Basin in northeast South Australia operated by Geodynamics Limited. The geothermal reservoir is water-saturated, naturally fractured basement granite (264°C at about 4600 meters). The reservoir is interpreted to be a fault zone and its permeability has been effectively enhanced by stimulation. There is approximately 3650 m of sedimentary cover above the granite which provides effective thermal insulation on top of the heat producing granite. A series of flow tests and stimulations were conducted from October to December 2012. A total volume of 36.5 ML of fresh water was injected into the Habanero reservoir during the stimulations. The seismicity commenced around Habanero 4 well and grew elliptically but was constrained by an apparent boundary along its eastern edge. The state of stress at reservoir depth has been numerically modelled using wellbore breakouts indicating that the least horizontal stress, Shmin is intermediate between SHmax and Sv. Fracture geometry was analysed using image logs and was used to predict slip likelihood depending on the orientation of the structures in the recent stress field at reservoir depth using the Coulomb Failure Criterion. Pressure response on failure proven by seismic events is well documented over the stimulation interval. Typically an immediate pressure decline is detectable after higher magnitude events. The opposite case was detected less commonly. Sudden increase of pressure after seismic events is interpreted as destruction/erosion of asperities which rapidly reduces pore space. Permeability thickness was increased fivefold up to 10Dm after stimulation and injectivity was up to 4 times higher than achieved in the wells drilled before Habanero 4. The reservoir is confined to a critically stressed low angle fault zone which is prone to slip triggered by injection. A fracture network within the granite succession is proven by image logs but it does not contribute to production.