Unlike hydrothermal EGS, there are, as yet,
no commercial HDR fields, so experience with these systems has been confined primarily to pilot projects. Of particular importance to the concept is an extended study at Fenton Hill—the first HDR project—that began in the early 1970s. The Fenton Hill HDR site is about 64 km [40 mi] west of Los Alamos, New Mexico, USA. It includes
two confined reservoirs created in crystalline rock at 2,800 and 3,500 m [9,200 and 11,480 ft] with reservoir temperatures of 195°C and 235°C [383°F and 455°F], respectively. Flow tests were conducted in each of the reservoirs for almost a year. The project, conducted over a period of about 25 years, ended in 1995.
HDR systems are essentially reservoir-
creation projects. One of the most important les- sons learned at Fenton Hill is that it is nearly impossible to connect two existing boreholes by creating a hydraulic fracture between them. Reservoirs should therefore be created by stimu- lating or creating fractures from the initial bore- hole and then accessing them by two production boreholes (left).17 Work at Fenton Hill also advanced the case
Heat Central monitoring exchanger Cooling Power
generation Heat
distribution
Injection well
Production well
Makeup water reservoir
for HDR fields by defining which critical factors in their construction are controllable. For exam- ple, the reservoir’s size is a direct linear function of the amount of fluid injected into it (next page). Similarly, temperature, injection pressure and flow rate, production backpressure, and the num- ber and placement of wells are all manageable variables within HDR field development. While many of the technological questions
associated with HDR systems were answered through the work at Fenton Hill, uncertainties about reservoir creation remain. Although a rela- tionship can be established between fluid volume injected and resulting volume made available for heat exchange, the fractured surface area within that volume of rock is more difficult to quantify. One approach renders an order-of-magnitude
estimate of the rock volume required. This is obtained by equating the heat flow rate from the reservoir with the change in stored thermal energy, assuming uniform extraction of heat throughout the volume. The heat flow rate is a function of rock density, volume and heat capacity, and the change in rock temperature over time. A numerical simulation study by Sanyal and
Butler suggests the electrical power generation rate achievable on a unit rock volume basis is 26 MWe/km3 [106 MWe/mi3].18
This power-
> The EGS concept as applied to HDR. Fractures are generated from an injection well (blue) drilled into a low-permeability reservoir of deep crystalline rock. Production wells (red) are then drilled into the fractured zone. Injected water is heated as it flows from the injection well to the production wells.
production correlation requires a volume of roughly 0.19 km3 [0.05 mi3] to generate 5 MWe. Such a cube would measure 575 m [1,886 ft] on each side, and the simulation is based on an assumption of uniform properties, including permeability, within the stimulated region. The study concluded that if constant pro-
17. Brown DW: “Hot Dry Rock Geothermal Energy: Important Lessons from Fenton Hill,” Proceedings of the Stanford University 34th Workshop on Geothermal Reservoir Engineering, Stanford, California (February 9–11, 2009).
18. Sanyal SK and Butler SJ: “An Analysis of Power Generation Prospects from Enhanced Geothermal Systems,” Proceedings of the Stanford University 34th Workshop on Geothermal Reservoir Engineering, Stanford, California (February 9–11, 2009).
MWe stands for electrical megawatt.
19. Polsky Y, Capuano L Jr, Finger J, Huh M, Knudsen S, Mansure AJC, Raymond D and Swanson R: “Enhanced Geothermal Systems (EGS) Well Construction Technology Evaluation Report,” Sandia Report SAND2008-7866: Sandia National Laboratories, December 2008.
12 20. Polsky et al, reference 19.
21. Kumano Y, Moriya H, Asanuma H, Wyborn D and Niitsuma H: “Spatial Distribution of Coherent Microseismic Events at Cooper Basin, Australia,” Expanded Abstracts, 76th SEG Annual Meeting and Exhibition, New Orleans (October 1–6, 2006): 595–599.
Microseismic multiplet analysis, based on a high- resolution relative hypocenter location technique, uses waveform similarity to identify events located on geometrically or geophysically related structures.
22. Petty S, Bour DL, Livesay BJ, Baria R and Adair R: “Synergies and Opportunities Between EGS Development and Oilfield Drilling Operations and Producers,” paper SPE 121165, presented at the SPE Western Regional Meeting, San Jose, California, March 24–26, 2009.
AUT09–RVF–10 Oilfield Review
duction is maintained, generation capacity is pri- marily a function of the stimulated rock volume. Other considerations may include well configura- tion, number of wells within a reservoir volume, reservoir mechanical properties, reservoir stress state and natural fracture features. These char- acteristics collectively determine how the reser- voir is best stimulated to create the requisite volume and the flow paths necessary for effective heat extraction.19
500 to 1,000 m
500 to 1,000 m
4,000 to 6,000 m
Crystalline rocks
Sediments
Stimulated
fracture system
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