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Geothermal reservoir engineering
Geothermal reservoir engineering, having its roots in petroleum reservoir engineering, has historically relied on conventional petroleum methods with slight modifications to account for inherent differences in conditions. It was not until the late 1960s and early 1970s that engineers recognized they must include a rigorous energy balance to account for interphase mass and energy exchange and other heat transfer mechanisms that arise from vaporization of fluid during extraction operations.
Differences between petroleum and geothermal reservoir engineering
There are a variety of phenomena that make geothermal reservoir engineering unique compared to conventional reservoir engineering, including:
- The reservoir fluid has no inherent value in and of itself. The fluid (either liquid or vapor) can be viewed as a working fluid whose sole value is the energy (heat) it contains.
- Geothermal reservoirs in the native state are rarely static and are usually neither isothermal nor of uniform fluid composition. Large spatial variations in pH occur. Highly-corrosive reservoir fluids are not uncommon and lead to additional expense of drilling, completions, and production.
- Geothermal reservoirs are rarely completely closed. More often, a zone of recharge and multiple zones of discharge (including springs and fumaroles) are associated with the resource.
- Phase behavior is deceptively complex. In its simplest form, the reservoir fluid is a single component that may partition into up to three phases: liquid, vapor, and adsorbed phases. Usually, there are additional components such as noncondensible gases (CO2, H2S, etc.) and salts.
- Geothermal reservoirs are typically found in highly fractured igneous or metamorphic rocks; very few are found in sedimentary rocks worldwide. While rock matrix properties would make the resource commercially unattractive as a petroleum reservoir (e.g., permeability can range as low as a 10–20 m2, porosity is in the 0.02 to 0.10 range), the relatively large dimensions (thickness may range to thousands of meters) ensure a substantial resource (heat) is in place.
While there are important distinctions between classical petroleum engineering and geothermal reservoir engineering, much of the latter can be considered an extension of the former.
Additional aspects of reservoir engineering
Additional areas where there are important differences or considerations for geothermal operations include:
- Geothermal reservoir characterization
- Tracer testing in geothermal reservoirs
- Modeling geothermal reservoirs
- Whiting, R.L. and Ramey Jr., H.J. 1969. Application of Material and Energy Balances to Geothermal Steam Production. J Pet Technol 21 (7): 893-900. SPE-1949-PA. http://dx.doi.org/10.2118/1949-PA.
- Ramey, H.J. A Reservoir Engineering Study of The Geysers Geothermal Field. Testimony for the Trial of Reich and Reich vs. Commissioner of the Internal Revenue, Tax Court of the U.S., 52 T.C., No. 74.
Noteworthy papers in OnePetro
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