Computer modelling of reinjection in geothermal fields

Abstract

This thesis describes a computer modelling study of reinjection into geothermal systems. The aim of this work was to decide on optimum reinjection strategies for various types of geothermal systems. First an idealized 3D closed model used by Sigurdsson et al. (1995) is extended to examine the effect of the natural recharge from groundwater, from the basement and laterally from the boundaries of the system. The results show that injection increases steam flow if recharge is small because the reservoir is acting as a closed system, or if the caprock is permeable and allows groundwater recharge. Otherwise injection may cause a decrease in steam production by suppressing hot recharge from depth or replacing lateral recharge by colder injected water. For hot-water reservoirs the effect of different well configurations on the production performance is examined with a model of the East Mesa field and the results show that deep far-infield reinjection provides an optimum strategy that supports reservoir pressures without causing an early thermal breakthrough. The impacts of different rates of infield and outfield reinjection on two-phase liquid-dominated reservoirs are investigated by using a model of Wairakei- Tauhara. The results show that outfield reinjection is a safe method for disposing of water. A high rate of infield reinjection prevents boiling in the reservoir and causes a drop in the production enthalpies. A significant decline occurs in the surface features which are close to the injection zones. Reinjection infield of 25% of the separated geothermal water appears to be a good strategy since it does not cause a significant pressure or temperature decrease. For two-phase vapour-dominated reservoirs reinjection impacts on steam production are investigated by using a model of Darajat. Investigation of various production/reinjection schemes show that; reinjecting 50% of the condensate above the production zones increases steam production significantly. However for higher reinjection rates, the steam production rate may decline owing to the breakthrough of cold reinjected water. If the production zones are deeper, reinjection is much more beneficial. Introducing a larger number of production and reinjection wells scattered throughout the field increases the reservoir life.