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310 Chapter 3 Recoverable EGS Resource Estimates 3.4 Determining the Recoverable Fraction As discussed above, Sanyal and Butler (2005) have modeled flow in fractured systems to determine the sensitivity of the recoverable heat fraction to several important parameters: rock temperature, fractured volume, fracture spacing, fluidcirculation rate, well configuration, and poststimulation porosity and permeability. They used a 3dimensional finite difference model and calculated the fraction of the heatinplace that could be mined as these important parameters were changed. They found that for a variety of fracture spacings, well geometries, and fracture permeabilities, the percentage of heat recoverable from a stimulated volume of at least 1 x 108 m3 under economic production conditions is nearly constant at about 40%, with a range between 34% and 47% (see Figure 3.1). This recovery factor is independent of well arrangements, fracture spacing, and permeability, as long as the stimulated volume exceeds 1 x 108 m3. This roughly corresponds to a block of rock approximately 500 m x 500 m x 500 m. Because Phase II of the Fenton Hill project, the Rosemanowes project, the Soultz project (both the shallow and deep stimulated volumes), and the Cooper Basin project have achieved fractured volumes based on acoustic emissions mapping of equal to or greater than or greater than 10 x 108 m3 (or 1 km3), this threshold has already been exceeded in practice. Because in the early stages of EGS technology development, short circuiting and other reservoir management problems will require extra fractured volume to counter toorapid temperature drop, it was assumed that two to three times the volume would be needed to guarantee a useful reservoir life. This provides sufficient volume of hot rock for extended development in the event of an irreparable short circuit. However, the excess rock volume effectively halves the recovery factor. The Sanyal and Butler (2005) study found recovery factors that ranged from 2.5% to 90%, with a typical recovery • Fracture length and width – The fracture length is related to, but not necessarily the same as, the well spacing between producer and injector. The fracture is not likely to be a flat plane, but will take a tortuous path through the rock. The path length will, thus, be longer than the well spacing in most cases. The fracture width is the lateral distance that the fracture extends and has active circulation. • Well configuration – The arrangement of the production wells in relation to the injector. The actively circulated fracture width is controlled, to some extent, by the geometry of the well configuration. To produce 50 kg/s from a 200°C body of rock, with no more than 10°C temperature drop in the produced fluid over a project life of 30 years, a large rock surface area relative to the mass flow rate of fluid is needed (see Armstead and Tester, 1987). For instance, with eight fractures being used for heat extraction, each must have a length and width sufficient to produce 125,000 m2 of surface area. If these fractures are 100 m apart, then 700 m or more of wellbore at the 200°C average reservoir temperature is required. To maintain the temperature for a longer life, we would need a longer fracture path length, larger fractures, or more fractures in the wellbore. Real fractures are certainly not the discrete, rectangular channels or circular discs assumed in this simple model. In real situations, fractures often have a greater surface area and path length than the distance between the wells would suggest. At Soultz, for example, in GPK3, about nine open fractures occur in the 540 m openhole section. However, one fracture at 4,760 m takes 70% of the total fluid flow. This channeling, if left uncontrolled, will effectively reduce the useful recovered thermal energy of the entire reservoir, because heat removal in the fracture that is accepting the higher flow rate is much higher than can be sustained by transient thermal conduction through the surrounding rock.PDF Image | Recoverable EGS
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