Sanyal Temperature Classification
The information for this page was taken directly from Subir Sanyal's paper, Classification of Geothermal Systems: A Possible Scheme.
At the request of the United States Department of Energy, the author was asked by the Geothermal Energy Association (Washington, D.C.) to prepare a white paper on the subject (in connection with a new national assessment of geothermal resources). This paper offers a possible scheme in which geothermal resources are classified into seven categories based on temperature.
This scheme is based not only on temperature but also according to a set of additional attributes important for practical utilization of geothermal energy: (a) steam fraction in the mobile fluid phase in the reservoir (a controlling factor in reservoir performance), (b) type of power generation technology applicable, (c) production mechanism and the state of the fluid at the wellhead (which influence operational economics), (d) factors other than temperature that control well productivity (these factors affect the optimization of field development and operation), and (e) unusual operational problems that impact power cost (such as scaling, corrosion, high content of non-condensable gases, etc.).
Table 1. A Possible Classification Scheme for Geothermal Resources
|Class of Resource||Reservoir Temperature||Mobile Fluid Phase in Reservoir||Production Mechanism||Fluid State at Wellhead||Well productivity and Controlling Factors other than temperature||Applicable Power Conversion Technology||Unusual Development or Operational Problems|
|Extremely Low Temperature*||< 100°C||Liquid water||Artesian self-flowing wells; pumped wells||Liquid water||Well productivity dependent on reservoir flow capacity and static water level||Direct Use; Binary (in areas of very low air temperatures)*|
|Very Low Temperature||100°C to 150°C||Liquid water||Pumped wells||Liquid water (for pumped wells); steam-water mixture (for self-flowing wells)||Typical well capacity 2 to 4 MWe; dependent on reservoir flow capacity and gas content in water; well productivity often limited by pump capacity||Binary|
|Low Temperature||150°C to 190°C||Liquid water||Pumped wells; self-flowing wells (only at the higher-temperature end of the range)||Liquid water (for pumped wells); steam-water mixture (for self- flowing wells)||Typical well capacity 3 to 5 MWe; dependent on reservoir pressures, reservoir flow capacity and gas content in water; productivity of pumped wells typically limited by pump capacity and pump parasitic power need; productivity of self-flowing wells strongly dependent on reservoir flow capacity||Binary; Two-stage Flash; Hybrid||Calcite scaling in production wells and stibnite scaling in binary plant are occasional problems|
|Moderate Temperature||190° to 230°C||Liquid water||Self-flowing wells||Steam-water mixture (enthalpy equal to that of saturated liquid at reservoir temperature)||Well productivity highly variable (3 to 12 MWe); strongly dependent on reservoir flow capacity||Single-stage Flash; Two-stage Flash; Hybrid||Calcite scaling in production wells occasional problem; alumino-silicate scale in injection system a rare problem|
|High Temperature||230°C to 300°C||Liquid water; Liquid-dominated two-phase||Self-flowing wells||Steam-water mixture (enthalpy equal to or higher than that of saturated liquid at reservoir temperature); saturated steam||Well productivity highly variable (up to 25 MWe); dependent on reservoir flow capacity and steam saturation||Single-stage Flash; Hybrid||Silica scaling in injection system; occasionally corrosion; occasionally high NCG content|
|Ultra High Temperature||> 300°C||Liquid-dominated two-phase||Self-flowing wells||Steam- water mixture (enthalpy equal to or higher than that of saturated liquid at reservoir condition); saturated steam; superheated steam||Well productivity extremely variable (up to 50 MWe); dependent on reservoir flow capacity and steam saturation||Single-stage Flash||High NCG content; silica scaling in injection system; occasionally corrosion; silica scaling potential in production wells at lower wellhead pressures|
|Steam Field||240°C (33.5 bar; 2,800 kJ/kg)||Steam||Self-flowing wells||Saturated or superheated steam||Well productivity extremely variable (up to 50 MWe); dependent on reservoir flow capacity||Direct steam||Occasionally high NCG content or corrosion|
* Since Sanyal created this classification system, the Chena geothermal area in Alaska has successfully generated electricity. This proves that geothermal waters below the boiling temperature (~75°C in the case of Chena) can host electrical grade, binary power plants, in low air temperature conditions. For this reason, we have modified Sanyal's classification of "non-electrical grade" to "extremely low temperature".
- Subir K. Sanyal. 2005. Classification of Geothermal Systems: A Possible Scheme. In: Thirtieth Workshop on Geothermal Reservoir Engineering; 2005/02/02; Stanford, California. Stanford, California: Stanford University; p. SGP-TR-176