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418 Chapter 4 Review of EGS and Related Technology – Status and Achievements • There are always critically oriented natural fractures, so while it is easy to stimulate with lower pressures, it is also very easy to apply too high a pressure. • The downward growth at Rosemanowes (Figure 5.1 in Chapter 5), and that observed at Fenton Hill, are the result of the combination of in situ stress and stress gradient changes caused by imposed temperature and pressure changes in the rock. • In the general case, a prediction of the direction of fracture growth is difficult in the absence of precise downhole data. Even with nearwellbore data from image logs, the fractures may not grow exactly as predicted. As a result, it is better to create the reservoir first, and then drill into it (Batchelor, 1987). • Pressure drop through the system (“impedance”) was a major problem. This was caused both by the low permeability of the reservoir and by frictional losses in the wellbores and in the nearwellbore area. Pressure drop is a critical parameter for two reasons: (i) the higher the pressure drop, the greater the pumping power required – and, hence, the greater the parasitic losses (an economic issue); and (ii), more important, a high impedance requires high downhole pressures to achieve the required flow rate, and these could easily exceed the levels at which runaway fracture growth and consequent water losses are incurred. • One way to increase reservoir permeability is to choose areas where there are low permeability, preexisting fractures closely spaced in the wellbore and that are oriented so that they will be likely to fail during stimulation (Batchelor, 1989). • Nearwellbore permeability reduction (“skin effect”) can increase pressure drop and decrease flow rates. Placing proppants in this nearwellbore area in the injector may require high pressures and flow rates that increase the likelihood of short circuits. • Probably the most important single lesson from this experiment is that hydrofracturing and artificial fractures are almost irrelevant. The natural fracture system dominates everything (Batchelor, 1989). • Natural fractures are pervasive in crystalline rocks at all depths and all locations we have investigated. Even if one does generate an artificial fracture, deliberately by hydrofracturing – or, more often, accidentally while drilling – it will intersect the natural system within meters, and from there on the behavior is dominated by the natural system. • Overstimulating preexisting fractures can result in a more direct connection from injector to producer than is desired, so that cool fluid can “shortcircuit” through the reservoir resulting in a lower production temperature. • At Rosemanowes, it became clear that everything one does to pressurize a reservoir is irreversible and not necessarily useful for heat mining. For example, pumping too long at too high a pressure will cause irreversible rock movements that could drive short circuits as well as pathways for water losses to the far field.PDF Image | Review of EGS and Related Technology
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