BACKGROUND OF THE |
Hot Dry Rock (HDR) technology started from an idea to help fulfill mankind's future energy needs as the fossil and other known fuels begins to run out or deplete. The HDR concept itself is very simple but the development of the associated technology has taken significantly longer than anticipated. Anyone with experience of natural materials like rocks knows that there are always imponderables that have not been really understood and indeed cannot at present be dealt with in a fully satisfactory manner. Furthermore, geology always has a habit of presenting us with new problems. Under these auspices and considering the limited funds which have been made available, it is encouraging to note that at long last light is visible at the end of the long tunnel of uncertainty. Certainly the results of the 1997 circulation test at Soultz-sous-forets (France) seem to show that we may have come up with a type of concept and an appropriate set of background site conditions to advance the technology.
The Concept of an HDR reservoir has evolved from that of a single penny-shaped fracture borrowed from the oil industry to the present Graben or HWR (Hot Wet Rock) concept. International Co-operation has been a key issue so far and the expensive nature of this research demands that co-operation of this type continues to break new ground in the future.
The necessary supporting technology has also evolved and the time appears to be ripe for taking advantage of this new and exciting development. Not all the answers are known but at least, we know now which questions to ask. It is still worth remembering that so far, there is not a single commercial HDR plant in existence to provide real data on building, operating and maintenance costs for planning a new unit. This should not be regarded as an insuperable problem. If such were the case, than we should not have any aircraft, steel, shipping, telecommunication or nuclear industries.
Many people feel that HDR technology will be needed sooner or later and the important question now is how quickly can it be put in to practice when the need does arise! Recent moves to form a consortium for this next step at the Soultz site are very encouraging and show the promise and confidence that commercial and industrial interests it's future.
Following the oil crisis of 1974 it became clear that both alternative sources of energy and economies in the usage of energy were necessary, particularly on the part of the industrialised countries who were consumers of a significant proportion of the world's energy resources. One potentially attractive source was the vast quantity of thermal energy in deep-lying, hot rock masses if it could be accessed by artificially created flowpaths and heat exchange surfaces. For such a reservoir to be useable as an energy source requires the following conditions to be satisfied:
a) A substantial mass of hot rock should be available at a reasonable depth
b) A fluid flowpath and heat exchanger or a permeable zone must be created in the rock mass having
An underground fluid circulation system fulfilling these conditions by enhancement of the natural permeability is the most general definition of an HDR geothermal system. The creation or the enhancement of natural situation to create the artificial heat exchanger (reservoir) forms the heart of HDR technology. Subsequently, it was felt that deep hot rocks with some degree of initial permeability should fall within the terms of HDR. In Japan this variant was termed "Hot Wet Rock", and is rather associated with the margins of hydrothermal fields. In Europe it was proposed as the "Graben Concept" and is associated with areas of Graben where the regional stresses tend to be low, allowing stimulation and circulation to be carried out at relatively moderate pressures.
All concepts (HDR, HWR, HFR, EGS) are by no means mutually exclusive, since there exists a continuous spectrum of site characteristics ranging from absolutely dry rock masses to the highly permeable margins of hydrothermal systems.
Early research carried out in mid 1970's in the USA, the UK, France and the Federal Republic of Germany adapted techniques from oil well stimulation and idealised the reservoir concept as a "penny-shaped" fracture (fig. 1A) formed in a rock mass which behaves as an isotropic continuum within a uniform stress field. The implication of creating a reservoir in such a medium was that the most likely effect of water injection under high pressures would be to create a new fracture and "jack" the new fracture open, thus forming the heat exchanger surface.
By the early 1980's research at various sites confirmed that the creation of new hydraulic fractures was not the dominant process but that the shearing of natural joints, favourably aligned with the principal stresses of the local stress field was a more important mechanism. These joints fail in shear because the fluid injection reduces the normal stress across them, but at the same time only marginally affects the magnitude of the shear stress. The shearing mechanism allows frictional slippage to occur before jacking and therefore there will be a component of shearing ahead of any "jacked" zone.
One of the most significant outcomes of the various international research projects to date has been this realisation that shearing on existing joints constitutes the main mechanism of reservoir growth. This has led to a basic change in our vision of an HDR reservoir. It has led to a departure from the conventional oil field reservoir development concepts and techniques towards a new technology related to the uniqueness of any jointed rock mass subjected to a particular anisotropic stress regime (Fig. 1B).
In the last five years or more, the reservoir concept has shifted again to take advantage of Graben settings (the Soultz concept) where higher temperatures can be encountered at shallower depth, a relatively low minimum stress gradient is likely and the probability of finding joints / faults at depth, which are already partially open is high. In an open system like that at Soultz, an HDR reservoir module will probably consist of three wells, one injector bounded by two producers (Fig. 1C). The two production wells will contain downhole pumps to assist production. The three wells are likely to be aligned approximately parallel to the direction of the maximum horizontal stress.
HWR concepts are already in use in a number of geothermal fields in Italy, Japan, Iceland and USA.
Fig 1: The concepts that have evolved in the last 25 years for HDR