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These features can be understood from the dependence of specific enthalpy of CO2 on temperature and pressure, see Fig. 3. For adiabatic (de-)compression processes, thermodynamic conditions will move along lines of constant enthalpy, indicating that compression starting from modest pressures and/or elevated temperatures will be accompanied by strong temperature increases. At temperatures below 50 ̊C, however, the isenthalps shown in Fig. 3 curve slightly backwards towards lower temperatures at high pressures, so that in this region isenthalpic compression will be accompanied by a temperature decline. Increased downhole temperatures in the injection well are favorable from the viewpoint of reservoir heat extraction, but they also reduce the pressure increase with depth in the injection well. This will reduce the buoyant pressure drive available for pushing CO2 through the HDR reservoir, and will increase the power requirements for maintaining fluid circulation. Analogous considerations apply to temperature and pressure behavior in production wells (Fig. 6). Here the isenthalpic decompression will cause CO2 temperatures to decline as it flows up the well. The temperature drop along the well becomes stronger for smaller downhole pressures; at TWB = 200 ̊C the temperature drops are ∆T = -22.6 ̊C for PWB = 550 bar, -25.7 ̊C at PWB = 500 bar, and -28.7 ̊C at PWB = 450 bar. Temperature declines become smaller for increased downhole temperature. In the production well it is of course desirable to reduce temperature decline during fluid upflow as much as possible. This can be achieved by increasing downhole pressures, which however will require increased power consumption in the fluid circulation systems. From this discussion it is apparent that optimal operation of a CO2-HDR system will involve complex tradeoffs between reservoir heat extraction and power consumption in the fluid circulation system. Fig. 7 presents static pressure profiles in CO2 injection wells for 80 bar wellhead pressure and different wellhead temperatures. Downhole pressures decrease with increasing wellhead temperatures, and more so for adiabatic than for constant temperature conditions. This is because for adiabatic conditions wellbore temperatures are larger, and accordingly fluid densities are smaller. The differences in downhole pressures range from 5.3 to 33.3 bar (Table 1). 0 0 -1000 -2000 -3000 -4000 -5000 100 200 300 400 500 600 isothermal adiabatic 10 ÞC 20 ÞC 30 ÞC 0 100 200 300 Pressure (bar) Figure 7. Static pressure profiles in CO2 injection wells for 80 bar wellhead pressure and different wellhead temperatures. Profiles are shown for isothermal as well as adiabatic conditions. Table 1. Downhole pressures in CO2 injection wells for 80 bar wellhead pressure and different wellhead temperatures, for adiabatic and isothermal conditions. 400 500 600 T ( ̊C) 10 20 30 Pad (bar) 574.09 545.57 500.79 Pisoth (bar) 579.42 558.58 534.13 ∆P (bar) 5.33 13.01 33.34 RESERVOIR HEAT EXTRACTION Production-Injection in Single Fracture In order to compare CO2 and water as heat transmission fluids, we consider an idealized fractured reservoir problem whose parameters were loosely patterned after conditions at the European HDR site at Soultz (see Table 2; Baria et al., 2005; Dezayes et al., 2005). We consider a horizontal fracture zone of 700x50 m2 areal extent, with injection and production occurring at a fixed pressure drop of 20 bar over 650 m distance. The wall rocks are modeled as semi-infinite half-spaces with uniform initial temperature of 200 ̊C, and the semi- analytical technique of Vinsome and Westerveld (1980) is used to calculate the heat transfer from the wall rocks to the fluid flowing in the fracture. Table 2. Specifications of fracture injection- production problem.PDF Image | Workshop on Geothermal Reservoir Engineering Stanford Univ
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