| Title | Jolokia EGS – the limits of hydraulic stimulation |
|---|---|
| Authors | Hogarth, R; Baisch, S; Holl, H-G; Jeffrey, R; Jung, R |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | enhanced, geothermal, granite, in-situ stress, fracture, fault, shearing, Habanero, Australia |
| Abstract | Geodynamics Limited began developing Enhanced Geothermal Systems (EGS) in hot granite beneath the Cooper Basin in Central Australia in 2002. Successful drilling and stimulation at Habanero lead to drilling of an exploration well, Jolokia 1, located 10 km west of Habanero. Jolokia 1 penetrated hot granite similar to that encountered at Habanero and reached total depth of 4,911 mRT in 8½” hole without any major drilling issues. The well was completed with a liner set at 4,320 m, leaving 591 metres of open-hole, all in granite. Preparations for stimulation of the Jolokia open-hole section were based upon experience from Habanero, where injection rates of up to 53 L/s (20 bbl/min) had been reached. However, the stimulation at Jolokia only ever achieved very low injection rates. During most of the stimulation, injection rates averaged 1 L/s (0.4 bbl/min), despite using NaCl brine to increase bottomhole pressures and applying surface pressure of up to 69 MPa (10,000 psi). A small quantity (21 m3) of saturated NaBr brine was used to further increase bottomhole pressures, resulting in the injection rate briefly increasing to 7 L/s (2.6 bbl/min). However, injection rates subsequently declined once the dense brine had left the wellbore, returning to rates similar to those before the NaBr stage. At both Jolokia and Habanero, the granite is known to be in a thrust stress regime, where the minimum principal stress is vertical and where critically stressed fractures, which are prone to shearing, are oriented close to horizontal. At Habanero, stimulation had initiated tens of thousands of micro-seismic events from shearing in a fault zone dipping at 10° from horizontal. During most of the two-week stimulation of Jolokia, estimated bottomhole pressures were greater than the estimated minimum (vertical) principal stress. Even so, at Jolokia, only 234 microseismic events were recorded during and after the stimulation. These were clustered into three distinct groups and indicate that the shearing was occurring on three steeply dipping fracture surfaces. The stimulation rate and pressure history and the micro-seismic data are analysed. A summary of the main conclusions from this unsuccessful attempt to create an Enhanced Geothermal System are provided, along with a discussion of the possible implications for attempts to ‘engineer’ fractures in unfractured basement rocks.. |