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Chapter 3 Recoverable EGS Resource Estimates than one producer for each injector reduces the amount of “dead” fractured volume, in which the rock 3.3.3 Fracture spacing Earlier researchers cited the importance of reservoir geometric structure on heatremoval effectiveness (see Kruger and Otte, 1972; and Armstead and Tester, 1987). Later, Sanyal and Butler (2005) found that, while the fractured volume had the largest effect on recovery factor of the parameters they studied, fracture spacing also had a measurable impact because it is part of determining the active reservoir volume. They investigated fracture spacings between 3 and 300 m. For reasonable fracture spacings of 3 to 30 m that might be realistically accomplished, there is little or no thermal interference, and the fracture spacing is largely irrelevant compared to the total 39 fractured volume in determining how much of the heatinplace will be recovered. However, for very is fractured but the fluid doesn’t circulate. See Chapter 5 for more details. 3.3.4 Fracture surface area large fracture spacings (~300 m) and a maximum possible flow rate determined by pump and pressure limitations, the recovery factor using a fivespot pattern with four producers per injector was 2.2%. A smaller fracture spacing of 30.5 m (again using a fivespot pattern and the same flow rate) yielded a 29.4% recovery factor. Lowering the flow rate from 500 kg/s to 126 kg/s per producer (with 30.5 m fracture spacing) increased the recovery factor from 24% to 42.5%, and maintained the reservoir life while still producing economic power output. Many researchers, typified by the work of Sanyal and Butler, identify fractured rock volume as the single most important parameter affecting thermal recovery. To reach this conclusion, they have implicitly assumed that the rock mass has been homogeneously fractured, which will certainly not be the case in practice. While large surface area and fractured volumes are needed to ensure longterm heat extraction at acceptable rates, their mere existence alone does not guarantee performance. Sufficient fracture density and size are needed and fluid must sweep across the fractured surface area reasonably efficiently for longterm performance to be realized. This has been one of the biggest engineering challenges for EGS, and will be discussed extensively in Chapters 4 and 5. The geothermal reservoir operates like an underground heat exchanger. Injected water is circulated through the reservoir and is exposed to the surfaces of hot rock allowing it to remove heat. The rate of heat transfer – and, consequently, the final temperature that the fluid achieves – is related to the mass flow rate of fluid and the surface area the fluid contacts. The heattransfer system can be thought of as similar to a series of flat plates with gaps (the fractures) between them and a semiinfinite conduction heat source surrounding each fracture. Heat is transferred by conduction through the rock, perpendicular to the surfaces of the fractures. Then heat is transferred by convection at the rock fluid interface to the fluid contained in the fracture. The larger that surface area is relative to the flow rate, the faster heat can be transferred to the fluid and still have its outlet temperature approach the original rock temperature with minimal thermal drawdown. (For more details concerning these coupled transport processes, see Armstead and Tester, 1987.) There are several parameters that affect this heattransfer area: • Well spacing – This is the distance between the wells in the active part of the reservoir. The well spacing controls the length of the fracture that is actively involved with fluid circulation. • Fracture spacing – The average distance between fractures that are open and accepting fluid. These are assumed to be connected to the production wells through the fractured rock volume. In reality, these may not act as separate discrete fractures, but as an overall fractured rock mass.PDF Image | Recoverable EGS
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