| Title | Finding the Productive Sweet Spots in the Vapour and Transitional Vapour-Liquid Dominated Geothermal Fields of Java, Indonesia |
|---|---|
| Authors | Ian BOGIE, Greg USSHER, Brian LOVELOCK and Ken MACKENZIE |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Indonesia, Java, sweet spot, vapour, transitional vapour-liquid |
| Abstract | Some of the most highly productive (up to 40 MWe) vapour-dominated geothermal wells in the world have been drilled into the vapour and transitional vapour-liquid dominated fields in Java, Indonesia. Such wells are highly permeable, but are localised to only parts of the vapour reservoir that form exceptionally productive zones often referred to as ‘sweet spots’. Counter-intuitively the sweet spots are not found in the deep steam upflows of the systems. While structural control is important in any one productive well the localisation of high permeability to a number of wells in a particular area cannot be satisfactorily explained on a purely structural basis. The sweet spots are located on the opposite side of deep steam upflows to where there are steam outflows; these are generally towards thermal features. The sweet spots are interpreted to be zones of condensation by cooling at the top of the reservoir with the greatest cooling occurring where the steam flow is lowest. The condensate formed is acid and under-saturated with respect to silica and anhydrite and is thus capable of not only dissolving calcite, but silicates and anhydrite, as it trickles down into the reservoir until eventually it becomes saturated with these minerals and starts depositing them at greater depths within the reservoir. The sweet zones are thus likely to be shallow zones outside of deep steam upflows with poorly permeable zones below them. The locations of surficial thermal features are known and can be subject to heat flow surveys to better characterize them. The location of deep steam upflows and outflows can be interpreted from MT geophysical surveys prior to drilling in combination with the heat flow data. Outflows without obvious surficial thermal features can be recognized on MT cross sections by the presence of slightly higher resistivity zones in the conductive cap. Thus, potential sweet zones can be drilled by excluding these areas and targeting the shallow part of the reservoir. It is likely that the more mature parts of geothermal systems on Java that have had major drilling campaigns have these zones, but in some of the systems high gas and magmatic acidity preclude them from being commercially exploitable. These targeting suggestions can thus be applied to make up wells in the low gas and non-acidic developed systems, as well as undrilled prospects that have thermal features with chemistries indicative of low gas and acidity. |