Determination of Calcite Scaling Potential in OW-903 and OW-914 of the Olkaria Domes field, Kenya

摘要

Non condensible gas is a major potential factor likely to affect the reservoir pressure in the Olkaria geothermal reservoir which has been commissioned by Kenya Electricity Generating Company LTD (KenGen) as the Olkaria IV Geothermal Power Project. Under static conditions, the pressure within the production zone ranges from 120 to 160 bars while reservoir temperatures range from 200 to 320°C at 2000 to 2400m. Based on gas pressures estimated from applying Henry's law, approximately 60 bar (at 0.02 kg NCG/kg reservoir brine) to 100 bar (at 0.05 kg NCG/kg reservoir brine) is dissolved non condensible gas pressure. The non condensible gas is over 90% carbon dioxide. The reservoir fluids at these wells are bicarbonate waters and thus likelihood to produce calcite scale in the wellbores at the depth of gas breakout during production. The gas breakout pressures (or bubble point) i.e. the pressure below which the fluid will begin to transform from 100% liquid to two-phase. Gas breakout pressure is the sum of the gas pressure and water pressure at the reservoir temperature. These values can be estimated using Henry's Law and the steam tables. In the deep reservoir of Olkaria, bubble points are estimated to be between 80 bar (at 0.02 kg NCG/kg reservoir brine) and 100 bar (at 0.05 kg NCG/kg reservoir brine). At the flow rates (<220 tph) that the dynamic surveys are run, measured pressure in the well falls below this gas breakout pressure between 900 and 1200 m. Wellbore simulation is used to estimate the depth of gas beak out at higher flow rates. However, since the gas breakout occurs at greater depths at higher flow rates, it is important to estimate the depth of gas breakout at multiple mass flow rates to manage the potential effect of scaling in the feed zone as well as the depth of scale inhibitor injection.