Kenya is increasingly going geothermal with prospects of having 1,900MW of geothermal power in the grid by 2016 and 5,500MW by 2030.The country has a geothermal potential of up to 10,00MW with less than a quarter of that currently exploited.
Geothermal is considered Kenya’s top renewable energy alternative. Exploitation of geothermal energy is largely within the East African Rift Valley, in it there is a complex volcanic system among other features dotting the Kenyan section.
Wells are dug in identified areas in a geo-field and well heads fixed for production of steam. The steam is then piped to respective stations to be used in the actual power production where the steam heats up water or isobutene which in turn run turbines. After sometime in production, a well if not viable in production is ‘killed’. Kenya Engineer took to study ‘the killing of geothermal wells’.
To kill a well is to simply cease its production. Whereas some can later on be revived and used again, some are fully killed. Instead of having the killed wells idle, a new technology was discovered of making them useful, reinjection. Once a well has ceased production, it can be used as a reinjection well.
Reinjection occurs when water (hot or cold) is pumped deep underground within the geothermal system itself (infield reinjection) or outside the system (outfield reinjection).
Historically, the waste water (brine) from the geothermal power plants was initially being disposed off to the rivers or open collection ponds near the power plant. This raised environmental concerns and it led to the birth of the reinjection system.
Geothermal reinjection systems are being employed in the geothermal fields as a method for waste-water disposal for environmental reasons. The waste water obtained at the steam separation system or the condenser blow-down obtained from generation plants is believed to contain dangerous inclusions like arsenic, H2S, lead metal, etc that are hazardous both to humans and the surrounding environment.
Recent developments have shown that reinjection is also being used to counteract pressure draw-down (water-level decline) due to long term production. The natural rate of recharge (replenishment by rainfall) would not be commensurate to the rate of extraction resulting in pressure draw-down.
Where to reinject
Because of different characteristics of the field, the most important task for the reinjection project is to seek the best injection strategy, which involves selecting the best location for the reinjection well, determining the depth of the well and estimating the maximum reinjection flow rate for a given system.
For hot-water systems, the danger of cooling due to reinjection can be minimized by locating injection wells far enough from production wells, while the main benefit from reinjection (pressure support) is maximized by locating injection wells close enough to production wells.
A proper balance between these two contradicting requirements must be found. The design of such a system depends on details of the structure of the field and the practicalities of well location. Therefore, careful testing and research are essential when planning reinjection.
Effects of reinjection
Reinjection however comes with both positive and negative effects. In some recorded instances, reinjection has seen to the increase in brine that has in turn resulted to increase in enthalpy. In other instances however, cold reinjection has seen to the increase in steam and water resulting to slight decline in enthalpy. The effects can differ depending on some factors and thus reinjection should be independently considered for every well.