Increasing Permeability in Deep Geothermal Wells Through Water Injection for the Las Pailas and Borinquen Geothermal Fields, Costa Rica

Abstract

The permeable conditions found in some wells of the Las Pailas and Borinquen Geothermal Fields has been very limited, so ICE`s decided to use an easy and cheaper method of stimulation, to try to improve the permeability conditions of the wells. The PGB-01 was the first vertical deep well drilled in the Borinquen area to begin the pre-feasibility studies. The temperatures measured in the well were very high (275°C), nevertheless, the tests carried out during drilling defined an injectivity index of 1.4 l/s/bar. It was considered that these conditions could be partly responsible for the low productive characteristics obtained during the output tests carried out in the well. By the other hand, the PGP-08 was a vertical well drilled in the heater area of Las Pailas, with the objective of finding mass to use in the binary plant located in this field. However, despite showing good temperature conditions (240 ºC), the permeability was low with an injectivity index of 1.9 l/s/bar. Because of these characteristics, several output tests showed low conditions of mass produced and wellhead pressure. With the objective to improve the permeability in both wells, ICE conducted several water injection tests for extended periods in each well. The main factors to consider for the possible increase of the wells permeability, are forced cleaning of the permeable zones, thermal impact caused by the temperature difference between the injected water and the bottom of the well, and the pressure increase generated for the water in deeper permeable zones. Injection tests led to a significant increase in permeability and further improved production conditions, in mass produced and operating pressures. Due to the positive results obtained in the two wells tested, it was decided to carry out similar tests in others wells drilled in Miravalles, Las Pailas and Borinquen, in order to increase the permeability of the wells.