Castelnuovo- Qualtra test site is located in Central Italy, southern Tuscany, a few Km northeast from the Larderello geothermal area. This region is part of the inner zone of the Northern Apennines, a fold-and-thrust belt originated from the collision (Cretaceous-Early Miocene) of the African and the European plate. The geological setting is defined by the a thinned tectonic pile composed of the Ligurian Units directly overlying on the Late Triassic evaporites, representing the basal stratigraphic succession of the Tuscan Nappe, and/or the metamorphic succession.
The metamorphic succession consists of Triassic and Palaeozoic metamorphic successions, affected by HP-LT and green-schist facies metamorphism. The tectonic units geometrical setting was strictly controlled by both thrusts and following extensional detachments. The main fault is represented by the so called Anqua fault which was intensely explored and exploited for geothermal purposes. The relationships between the different tectonic units and/or stratigraphic complex are resumed in the fig. 1.
Figure 1: Conceptual block-diagram showing the geometrical setting of the main geological bodies occurring
in the Castelnuovo site with the relationships between different units
The Castelnuovo project aims to exploit a deep reservoir for 5 MWe binary (zero emission) ORC power plant and closed loop system for avoiding the emissions of NCGs by drilling two production wells and one reinjection well to a depth of 3500 m with a temperature >250 °C and 70 bar. A total NCGs reinjection system has been designed through an annular injection of liquid from production well and injection of NCG through a tubing. Laboratory experiments has been performed to characterize the kinetics of fluid mixing to determine the right operating conditions to prevent gas formation in the well with the development of a production-reinjection well test device. The scheme of the wells/power cycle arrangement is shown in Figure 2. A subcritical recuperative ORC power cycle using R1233zd(E) as working fluid is fed by the geo-fluid through a condensing heat exchanger. The NCGs are released at the top while the condensed brine is released from the bottom and directed to the reinjection wells.
Figure 2 – Schematic of Castelnuovo power plants and wells/NCG reinjection arrangement
The well completion considered is made up of a central tubing and a casing (Fig. 3). Along the inner tubing several points of injection are installed to connect the annular space and the tubing. Water is injected in the tubing, while gas in the annular space flows through valves and is mixed at depth with water in the tubing. Depending on the reservoir injectivity and the initial reservoir pressure that may be largely lower than the hydrostatic pressure for geothermal steam reservoirs the water column may not reach the wellhead, when water is injected at constant flow rate. To allow gas injection in the water phase, at least one open injection valve has to be located below the water level. The use of multiple injection points provides flexibility to operate in different situations, to facilitate the start-up operations and to secure the gas entrainment with the gas-liquid downward flow.
Figure 3 – Injection well configuration