Hot dry rock geothermal energy

Hot dry rock (HDR) is an extremely abundant source of geothermal energy that is difficult to access. A vast store of thermal energy is contained within hot – but essentially dry and impervious crystalline basement rocks found almost everywhere deep beneath Earth's surface.Armstead, H. C. H., and Tester, J. W., 1987. Heat Mining, E. & F. N. Spon, London and New York, pp. 34–58 A method for the extraction of useful amounts of geothermal energy from HDR originated at the Los Alamos National Laboratory in 1970, and Laboratory researchers were awarded a US patent covering it.Potter, R. M., Smith, M. C., and Robinson, E. S., 1974. “Method of extracting heat from dry geothermal reservoirs,” U. S. patent No. 3,786,858

This technology has been tested extensively with multiple deep wells drilled in several field areas around world including the US, Japan, Australia, France, and the UK and investment of billions of research funds. It continues to be the focus, along with a related technique called Enhanced Geothermal System (EGS), for sizable government-led research studies involving costly deep drilling and rock studies. Thermal energy has been recovered in reasonably sustainable tests over periods of years and in some cases electrical power generation was also achieved. However no commercial projects are ongoing or likely due to the high cost and limited capacity of the engineered reservoirs, associated wells, and pumping systems. Commonly tests have opened just one or more fractures such that the reservoir surface heat exchange areas are limited. For this technology to successfully compete with other energy sources, drilling costs would have to drop drastically or new approaches that result in much more extensive, complex, and higher rate flow paths through actual fracture networks would have to be established. The enthusiasm in the research community is justified by the vast extent of the energy supply and the low environmental impact of the method, however significant breakthroughs will be required to make this a commercial energy resource.

Overview Although often confused with the relatively limited hydrothermal resource already commercialized to a large extent, HDR geothermal energy is very different.Brown, D. W., 2009. "Hot dry rock geothermal energy: Important lessons from Fenton Hill," in Proceedings, 34th workshop on geothermal reservoir engineering (February 9–11, 2009: Stanford, CA). SGP-TR-187, pp. 139–142 Whereas hydrothermal energy production can exploit hot fluids already in place in Earth's crust, an HDR system (consisting of the pressurized HDR reservoir, the boreholes drilled from the surface, and the surface injection pumps and associated plumbing) recovers Earth's heat from hot but dry regions via the closed-loop circulation of pressurized fluid. This fluid, injected from the surface under high pressure, opens pre-existing joints in the basement rock, creating a man-made reservoir which can be as much as a cubic kilometer in size. The fluid injected into the reservoir absorbs thermal energy from the high-temperature rock surfaces and then conveys the heat to the surface for practical use.

History The idea of deep hot dry rocks heat mining was described by Konstantin Tsiolkovsky (1898), Charles Parsons (1904), and Vladimir Obruchev (1920).

In 1963 in Paris, a geothermal heating system that used the heat of natural fractured rocks was built.

The Fenton Hill project was the first system for extracting HDR geothermal energy from an artificial formed reservoir; it was created in 1977.

Technology

Planning and control

As the reservoir is formed by the pressure-dilation of the joints, the elastic response of the surrounding rock mass results in a region of tightly compressed, sealed rock at the periphery—making the HDR reservoir totally confined and contained. Such a reservoir is therefore fully engineered, in that the physical characteristics (size, depth at which it is created) as well as the operating parameters (injection and production pressures, production temperature, etc.) can be pre-planned and closely controlled. On the other hand the tight compression and confined nature of the reservoir severely limits that amount and the rate at which energy can be extracted.

Drilling and pressurization As described by Brown,Brown, D. W., 1990. “Hot dry rock reservoir engineering,” Geotherrm. Resour. Counc. Bull. 19(3): 89–93 an HDR geothermal energy system is developed, first, by using conventional drilling to access a region of deep, hot basement rock. Once it has been determined the selected region contains no open faults or joints (by far the most common situation), an isolated section of the first borehole is pressurized at a level high enough to open several sets of previously sealed joints in the rock mass. By continuous pumping (hydraulic stimulation), a very large region of stimulated rock is created (the HDR reservoir) which consists of an interconnected array of joint flow paths within the rock mass. The opening of these flow paths causes movement along the pressure-activated joints, generating seismic signals (microearthquakes). Analysis of these signals yields information about the location and dimensions of the reservoir being developed.

Production wells

Typically, an HDR reservoir forms in the shape of an ellipsoid, with its longest axis orthogonal to the least principal Earth stress. This pressure-stimulated region is then accessed by two production wells, drilled to intersect the HDR reservoir near the elongated ends of the stimulated region. In most cases, the initial borehole becomes the injection well for the three-well, pressurized water-circulating system.

Operation

In operation, fluid is injected at pressures high enough to hold open the interconnected network of joints against the Earth stresses, and to effectively circulate fluid through the HDR reservoir at a high rate. During routine energy production, the injection pressure is maintained at just below the level that would cause further pressure-stimulation of the surrounding rock mass, in order to maximize energy production while limiting further reservoir growth. However, the limited reservoir size limits reservoir energy. Meanwhile high pressure operation adds significant cost to piping and pumping systems.

Productivity

The volume of the newly created array of opened joints within the HDR reservoir is much less than 1% of the volume of the pressure-stimulated rock mass. As these joints continue to pressure and cooling -dilate, the overall flow impedance across the reservoir is reduced, leading to a high thermal productivity. If the cooling leads to cooling fractures in a way that exposes more rock then it is possible that these reservoirs may improve over time. To date reservoir energy growth is only reported to come from new expensive high pressure well stimulation efforts.

Feasibility studies The feasibility of mining heat from the deep Earth was proven in two separate HDR reservoir flow demonstrations—each involving about one year of circulation—conducted by the Los Alamos National Laboratory between 1978 and 1995. These groundbreaking tests took place at the Laboratory's Fenton Hill HDR test site in the Jemez Mountains of north-central New Mexico, at depths of over and rock temperatures in excess of 180 °C.Brown, D. W., Duchane, D. V., Heiken, G., and Hriscu, V. T., 2012. Mining the Earth’s Heat: Hot Dry Rock Geothermal Energy, Springer-Verlag, Berlin and Heidelberg, 655 pp The results of these tests demonstrated conclusively the engineering viability of the revolutionary new HDR geothermal energy concept. The two separate reservoirs created at Fenton Hill are still the only truly confined HDR geothermal energy reservoirs flow-tested anywhere in the world. Although these tests demonstrated that HDR systems could be constructed, the flow rates and energy extractions rates did not justify the cost of the wells.

Fenton Hill tests

Phase I The first HDR reservoir tested at Fenton Hill, the Phase I reservoir, was created in June 1977 and then flow-tested for 75 days, from January to April 1978, at a thermal power level of 4 MW.Dash, Z. V., Murphy, H. D., and Cremer, G. M. (eds.), 1981. “Hot dry rock geothermal reservoir testing: 1978–1980,” Los Alamos National Laboratory Report LA-9080-SR, 62 pp The final water loss rate, at a surface injection pressure of , was (2% of the injection rate). This initial reservoir was shown to essentially consist of a single pressure-dilated, near-vertical joint, with a vanishingly small flow impedance of .

The initial Phase I reservoir was enlarged in 1979 and further flow-tested for almost a year in 1980.Brown, D. W., and Duchane, D. V., 1999. "Scientific progress on the Fenton Hill HDR project since 1983,” Geothermics 28(4/5) special issue: Hot Dry Rock/Hot Wet Rock Academic Review (Abe, H., Niitsuma, H., and Baria, R., eds.), pp. 591–601 Of greatest importance, this flow test confirmed that the enlarged reservoir was also confined, and exhibited a low water loss rate of 6 gpm. This reservoir consisted of the single near-vertical joint of the initial reservoir (which, as noted above, had been flow-tested for 75 days in early 1978) augmented by a set of newly pressure-stimulated near-vertical joints that were somewhat oblique to the strike of the original joint.

Phase II A deeper and hotter HDR reservoir (Phase II) was created during a massive hydraulic fracturing (MHF) operation in late 1983. It was first flow-tested in the spring of 1985, by an initial closed-loop flow test (ICFT) that lasted a little over a month.Dash, Z. V., et al., 1989. "ICFT: an initial closed-loop flow test of the Fenton Hill Phase II HDR reservoir,” Los Alamos National Laboratory report LA-11498-HDR, Los Alamos NM, 128 pp Information garnered from the ICFT provided the basis for a subsequent long-term flow test (LTFT), carried out from 1992 to 1995.

The LTFT comprised several individual steady-state flow runs, interspersed with numerous additional experiments.Brown, D. W., 1993. “Recent flow testing of the HDR reservoir at Fenton Hill, New Mexico,” Geothermal Program Review XI, April, 1993. U.S. Department of Energy, Conservation and Renewable Energy, Geothermal Division, pp. 149–154 In 1992–1993, two steady-state circulation periods were implemented, the first for 112 days and the second for 55 days. During both tests, water was routinely produced at a temperature of over 180 °C and a rate of , resulting in continuous thermal energy production of approximately 4 MW. Over this time span, the reservoir pressure was maintained (even during shut-in periods) at a level of about 15 MPa.

Beginning in mid-1993, the reservoir was shut in for a period of nearly two years and the applied pressure was allowed to drop to essentially zero. In the spring of 1995, the system was re-pressurized and a third continuous circulation run of 66 days was conducted.Brown, D. W., 1995. “1995 verification flow testing of the HDR reservoir at Fenton Hill, New Mexico,” Geothermal Resources Council annual meeting (October 8–11, 1995: Reno, NV) Trans. Geotherm. Resour. Counc. 19:253–256 Remarkably, the production parameters observed in the two earlier tests were rapidly re-established, and steady-state energy production resumed at the same level as before. Observations during both the shut-in and operational phases of all these flow-testing periods provided clear evidence that the rock at the boundary of this man-made reservoir had been compressed by the pressurization and resultant expansion of the reservoir region.