Subsurface System Design Issues EGS vs. Hydrothermal Pool

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Subsurface System Design Issues EGS vs. Hydrothermal Pool ( subsurface-system-design-issues-egs-vs-hydrothermal-pool )

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5­26 Chapter 5 Subsurface System Design Issues and Approaches iii. Low­rate injection tests The main objective of the low­rate injection test is to determine the hydraulic properties of the unstimulated open­hole section of the well. The derived values will be used as inputs for numeric models, planning of the stimulation (pressure required for a stimulation), and subsequently for the assessment of the stimulation and identification of predominant flowing zones using a temperature or flow log. iv. Pre­stimulation test This test consists of injecting nearly 400­600 m3 of fluid at a constant flow rate of about 5­7 kg/s. Fresh water, or saturated brine, can be used. Saturated brine can be very useful in helping stimulation near the bottom of the well, but this depends on the state of the in situ stress. After the pre­stimulation test, the wellhead is shut in to see how the pressure declines. This will give some indication of the natural transmissivity, leak­off, or far­field connectivity. v. Main stimulation of the first well During the main stimulation, fresh water is injected in steps with increasing flow rates. Three to four flow­rate steps are normally used. The flow­rate steps may vary depending on the leak­off, and on whether it is a closed system or open system. Flow­rate steps of about 30, 40, 50, and maybe 70 kg/s are not unreasonable. Normally, the selected step of injected flow rate is continued until the wellhead or downhole pressure reaches an asymptote showing that the far­field leak­off is balanced by the injected flow. This is feasible in a relatively open system, but most observed EGS systems have poor far­field connectivity and, therefore, the wellhead pressure is likely to continue increasing. In this case, injection may be carried out at 30 kg/s for 24­30 hrs, 40 kg/s for 24­30 hrs, 50 kg/s for 24­ 30 hrs, and 70 kg/s for 3 days. The injected volume may vary between 28,000 m3 to 31,000 m3, depending on the flow and the injection period. vi. Post­stimulationtest A post­stimulation test is conducted to evaluate the enhancement in the permeability obtained during the main stimulation of the reservoir. Possible injection flow rates would be around 7, 30, 40 and 50 kg/s for about 12, 12, 24 and 12 hrs. The apparent reduction in the injection pressure, compared to the injection pressure required pre­stimulation for the same flow rate, will give quantitative indication of the improvement in the permeability of the stimulated rock mass. The total volume of water used for this test would be about 7,200 m3. vii. Drilling of the second well and follow­on testing Results of the testing of the first well and the stimulated reservoir are used to target the drilling of the second well. Following drilling of the second well, the same wellbore characterization diagnostics recommended for the first well should be run on the second well. A suite of flow tests (similar to those for the first well) also should be carried out, but this time the goal of the testing is to establish the connection between the first and second well. If the connection is not good or needs improvement, the second well may need to be stimulated one or more times. viii.Short­term circulation test between the first and second wells Once a hydraulic link between the two wells has been established, a small­scale circulation loop

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