Corrosion and Scaling in High Gas (25wt%) Geothermal Fluids

Abstract

The majority of New Zealand geothermal resources produce separated steam with moderate gas content, 3 to 5 wt%, with these gases being predominantly CO2 but with significant concentrations of H2S and NH3. The ratios of these gases are relatively constant in the New Zealand fields giving near neutral to alkali chloride waters. Corrosion in the moderate gas geothermal fields has been extensively studied. Corrosion behaviour in high gas geothermal fields such as at Ngawha having up to 10 wt% gas was initially predicted to be equally low and acceptable as for moderate gas fields. This paper describes work aimed at determining the corrosivity of a high gas geothermal fluid. A mobile corrosion test facility, developed in house, was used to measure the corrosion rates of three common construction materials; a carbon steel, a 12% Cr stainless steel and 316 stainless steel in a high gas geothermal fluid. The tests were completed by mixing gas separated from Broadlands/Ohaaki Well BR3 into Well BR22 two-phase produced fluid to give a total gas content in separated steam of 25 +/- 5 wt%. The measured corrosion rates in separated water and separated steam were low and acceptable and comparable to those in fluids having more moderate gas concentrations. In two phase fluid the test results showed sensitivity to erosion corrosion.