Hydraulics and well testing of engineered geothermal reservoirs

In most engineered reservoirs the water to extract the geothermal energy mu st be supplied, i.e., forced into the reservoir, rather than merely extract ed as is common for conventional geothermal reservoirs. One operational hyd raulic parameter of great interest is the impedance, which measures how muc h pressure is required to force water at unit flow rate through the reservo ir. For practical purposes, the impedance should be less than 1 MPa s/l, an d economic competitiveness with conventional reservoirs requires values one tenth as large. Most engineered reservoirs operated to date have met the u pper requirement, and some are approaching the lower. Another parameter of great practical interest is the water loss rate, i.e. the difference betwee n the rates of water injected and produced. Water losses and impedance are very pressure-dependent, but impedance is inversely so-high pressure increa ses water loss, but decreases the impedance. Hence operators may select str ategies to meet their requirements. In some tight rock formations such as t hose at the Fenton Hill, USA, reservoir, the water loss rate may be so smal l that the pressure may be maximized to reduce impedance and increase energ y production. In more permeable rock formations, the water losses may be qu ite 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 nativ e permeability and water in place that more water actually maybe extracted with downhole pumps from the reservoir than is injected. (C) 1999 CNR. Publ ished by Elsevier Science Ltd. AU rights reserved.