| Title | Turbulence Mediated Rapid Alumuinosilicate Formation in Hydrothermal Vein Breccia: Ngatamariki Geothermal Powerstation, New Zealand |
|---|---|
| Authors | C.J. Newton |
| Year | 2020 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Aluminosilicate, aluminous, silica boehmite, aluminium, scale, precipitation, binary, hydrothermal |
| Abstract | A recent cleanout of a geothermal brine-pentane heat exchanger at the Ngatamariki geothermal power station provides valuable insight into the in-situ formation processes of both hydrothermal vein breccias and geothermal aluminosilicate scales. Samples of the scale breccia taken during the cleanout primarily consist of broken fragments of a dark-coloured amorphous aluminosilicate pipe scale, which was remobilised and redeposited within the heat exchanger over a period of ~12-hours during the early stages of the breccia forming event. This dark coloured clastic component was subsequently cemented with a white-coloured, massive to very fine-grained amorphous aluminosilicate matrix over the course of weeks. Scanning Electron Microscope (SEM) analyses indicate that the dark component of the breccia formed relatively slowly via colloid impact as a pipe-wall scale before separating from the pipe wall prior to its remobilisation. In contrast, the white fraction of the breccia appears to have precipitated quasi-instantaneously in place, likely as a result of the turbulence induced flashing caused by the earlier deposited clastic scale obstructing flow into the heat exchanger tubes. The rapidly formed white aluminosilicate fraction served to seal voids within the breccia, forming a strong cementing matrix. The volume and rate of white amorphous aluminosilicate precipitation was significant, with the breccia consisting of up to ~20vol.% white aluminosilicate matrix. A fine white aluminosilicate powder discovered immediately downstream of the breccia appears identical to the white aluminosilicate breccia cement, although unlike the aluminosilicate cement it contains ≥4 w.t.% of the aluminium oxyhydroxide mineral, boehmite. Knowledge gained from the investigation of this event yields invaluable information, both for the prevention of similar scaling events in the future, and the understanding the formation mechanisms of geothermal aluminosilicate scales generally. Furthermore, this event provides unique insight into the interaction between some physical and chemical processes which also occur within active natural hydrothermal systems. |