Neutralization process of acid wells : design of the sampling system, Miravalles geothermal field, Costa Rica

Abstract

In the year 2000, downhole pH neutralization was started in the Miravalles geothermal field with well PGM-19. Since then, three additional systems have been installed in the acid wells PGM-02, PGM-06 and PGM-07. Several changes have since been made in order to improve operative conditions. The online sampling process has not been implemented successfully but that is essential for the control and development of the neutralization and, subsequently, the exploitation of the acid sector of the field. In this report, three scenarios for on-line pH monitoring were studied, two of them based on a flashing process; the third on a cooling process using a heat exchanger, where flashing is minimal. The computed saturation index for amorphous silica (log(Q/K)) indicates problems with silica scaling in most of the exchanger. The last scenario also needs fresh water and possibly other equipment to keep the readings continuous (another exchanger in parallel). However, this option would give the best results of pH readings as it minimizes the losses of volatile components that affect the value of the acidity, and approaches the real values inside the pipe. The disadvantages presented in the heat exchanger are not significant when compared with possible costs of an uncontrolled system: loss in production, the cost of mechanical intervention, the cost of the wells being out of production, associated costs in damages to casing, pipes and superficial stations and possible loss of a well. Hence, the cooling process (heat exchanger) should be selected.