The Conceptual Model and Its Evolution Over 15 Years of Development and Operation: San Jacinto-Tizate Geothermal Field, Nicaragua
Julio Alberto Guidos PINEDA, Guillermo CHAVEZ, Alexis OSORNO, James V. LAWLESS, Gregory N. USSHER, Kenneth M. MACKENZIE, Ramonchito Cedric M. MALATE, Brian G LOVELOCK, Anna E. PRESTAGE
[Jacobs NZ Ltd, New Zealand]
Exploration began at the San Jacinto – Tizate geothermal system in the early 1950s and the field has been under production since 2005. Early exploration was undertaken by several different organisations until the geothermal concession was acquired by Polaris Energy Nicaragua S.A. (PENSA) in 2003. Jacobs has continued working as technical advisor since becoming involved in the project in 1999. Since the early days of exploration, the conceptual model has evolved considerably, from a relatively simple one, based on limited initial data, to a more thorough and nuanced model that has recently been updated annually. The model continues to be refined as new data is acquired through ongoing production and field management activities, and with the incorporation of results of makeup drilling and geoscientific work. The geothermal system is centred in a step-over along a NNE striking regional structure that results in N-S structural grain within the field with a component of E-W extension. The resource is recharged by a high temperature ( more than 300°C) neutral chloride water with salinity of about 5500 ppm and gas content of 0.25-0.30 wt%. This rises from depth along the eastern central margin of the production field and spreads to the west, north, and south. The field is bounded by low permeability zones to the north, east, and west, and historically outflowed to the South toward the village of San Jacinto. This has resulted in a NNE-SSW elongation of the system that is evident in the most recent MT geophysics survey. Additionally, there is a second, seemingly unconnected but potentially larger, geothermal system ~5 km to the west of the currently operating project. Since the field has come under increased production in 2012, when the installed capacity was expanded in two stages to 72 MWe, the historic outflow to the south has reversed due to pressure drawdown in the reservoir, causing the still hot outflow fluids to be pulled back towards the reservoir. The effects of pressure drawdown have resulted in the development of a steam cap in the central production area, which has created both challenges and opportunities in the ongoing management of the reservoir. Despite these changes in resource characteristics the field is performing well and the project operating close to its total installed generation capacity. While the conceptual model has been extensively tested and refined over the years, some uncertainties remain. The nature of structural controls within the production field have evolved from numerous discrete faults to considering more distributed fracturing as in a horsetail fault swarm. Our understanding of the structural controls in the very deep levels of the field and how they cause the westward trending rise of the geothermal fluids from depth are still being refined. The degree of connection between the northern lobe of the field and the central production field is currently being investigated.
|        Topic: Resource Assessment||Paper Number: 16088|