reservoirs. There is, however, general agreement on several major issues. First, the resource base (Table 8–1) is large enough to deserve a thorough investigation. On the other hand, the relatively low temperatures and the high cost of drilling the necessary deep holes make it unlikely in the near future to become an economical source of heat for generating electricity. If, however, the geopressured brine is nearly or completely saturated with natural gas and if this can be economically separated, dried, and pipelined, then the credit for natural gas recovery may make this a very economical source of relatively low-grade heat.
It is anticipated that by about 1981 enough information will have been collected to permit an intelligent decision on the feasibility of constructing electrical generating plants using geopressured reservoirs. As much as 25,000 MWe of generating capacity might eventually be installed along the Gulf Coast. It is, of course, also possible that the results of research now beginning will discourage this.
There is little information on the chemical compositions of these brines beyond their salinities (1.5–9.0 percent), but their geological origin as coastal sediments and the presence of large amounts of methane strongly suggest large concentrations of hydrogen sulfide, a harmful air pollutant. A more certain and ultimately even more serious environmental problem is the hazard of subsidence, since the pressure in these reservoirs is due simply to the weight of the overlying land. It seems clear that subsidence problems will limit exploitation. To be sure, full reinjection of the brine might prevent subsidence, but the pumping required would use up all the captured mechanical energy and more.
Particularly where the crust is thin or has been disturbed by volcanism or faulting, higher than normal geothermal gradients are often encountered. This offers the possibility of reaching usefully high temperatures with shallower, less expensive holes than would be needed where the geothermal gradient is normal (about 30°C/km of depth). For purposes of discussion, a gradient of 40°C/km is the dividing line between the normal gradient and hot-dry-rock resource types.
The main technical barrier to exploiting this part of the geothermal resource is the lack of a method for extracting heat from deeply buried dry rock. The approach most widely investigated is to use water as a working fluid. However, much of the hot-dry-rock resource is embodied in impermeable rocks, which lack channels through which the water can flow