Hydraulics and well testing of engineered geothermal reservoirs

Abstract In most engineered reservoirs the water to extract the geothermal energy must be supplied, i.e., forced into the reservoir, rather than merely extracted as is common for conventional geothermal reservoirs. One operational hydraulic parameter of great interest is the impedance, which measures how much pressure is required to force water at unit flow rate through the reservoir. For practical purposes, the impedance should be less than 1 MPa s/l, and economic competitiveness with conventional reservoirs requires values one tenth as large. Most engineered reservoirs operated to date have met the upper requirement, and some are approaching the lower. Another parameter of great practical interest is the water loss rate, i.e. the difference between the rates of water injected and produced. Water losses and impedance are very pressure-dependent, but impedance is inversely so—high pressure increases water loss, but decreases the impedance. Hence operators may select strategies to meet their requirements. In some tight rock formations such as those at the Fenton Hill, USA, reservoir, the water loss rate may be so small that the pressure may be maximized to reduce impedance and increase energy production. In more permeable rock formations, the water losses may be quite high, so pressure and energy production may have to be limited. But in some cases, e.g. the reservoir at Soultz, France, there may be enough native permeability and water in place that more water actually may be extracted with downhole pumps from the reservoir than is injected.