Steam and brine gathering system design for Cachaços-Lombadas new production wells in Ribeira Grande geothermal field

Abstract

The aim of this study is the development of a conceptual design of a steam and brine gathering system for the Cachaços-Lombadas new production wells and the separation station equipment. The typical configuration in Ribeira Grande geothermal field, consisting of a vertical steam separator and a brine accumulator tank for each well, is evaluated and compared to a single horizontal gravity separator serving all wells. This study establishes design criteria for the pipe and separation station equipment sizing to try to solve some operational difficulties of existing wells in the field. The steam and brine design flow rates are assumed to be similar to the last make up well drilled in Cachaços-Lombadas, CL7, which is the most productive well of the field, having the expected total production of around 120 ton/h of saturated steam and 450 ton/h of brine. Aiming to minimize steam and brine gathering system piping pressure losses, a maximum velocity criterion of 30 m/s for steam and 1 m/s for brine was established. It was found that the gathering system nominal pipe sizes are DN 700 for steam and DN 400 for brine. The calculated pressure loss in the estimated 1400 m pipe route from the well pad to the plant is 0.7 bar in the steam line and 0.6 bar in the brine line.

Some of the wells in Cachaços-Lombadas sector are subject to wellhead pressure fluctuations which results in flow rate changes that cannot be managed by the separation station equipment, namely the brine accumulator tank. Therefore, each accumulator tank is sized to act as a buffer to accommodate possible fluctuations coming up in the well. It has also been noticed that the steam quality decreases at the outlet of the steam separators in the well field, maybe due to undersize. To minimize this occurrence, the separator annular cross section area is sized to keep the upward steam velocity below 2 m/s and so enhance the gravity role in the separation efficiency. The common horizontal gravity separator for the three wells is sized accordingly to the criteria’ established by Verkís for Icelandic geothermal power plants. Its higher volume is better suited to accommodate possible fluctuations while its efficiency is less dependent on the fluid inlet velocity which allows to operate the three wells in any combination possible. The overall project cost is estimated to be € 6,100,000 Vieira 756 Report 33 while differences between installing three individual vertical separator stations and one single horizontal gravity separator is about € 92,500, which is not relevant or decisive in the overall estimated cost but the evaluation of technical matters between both types of separation shall be developed in a later stage of the project.