Feasibility of Seawater Reinjection In Geothermal Systems

One of the main challenges in geothermal exploitation is related to sustainability of reservoirs that usually undergo significant pressure drawdown after a variable span of time, mainly depending on rate of production and geological features. Reinjection is a key factor that usually determines the sustainability of a field through the time and represents the main tool to prevent drawdown and to recover production of geothermal systems. However, water supply represents a big deal for many of the geothermal sites worldwide, generally because fresh water is an essential resource for drinking purpose at first and for many industrial activities as well. Seawater could represent an important alternative that need to be studied in detail. The use of seawater as reinjection fluid presents some problems that need to be dealt with, in particular in a high temperature, steam dominated, geothermal field. The main problem we forecast are as follows: i) due to the seawater flashing close to reinjection well, the halite deposition could greatly reduce the injectivity of the system up to complete clogging the reservoir; ii) the high Ca-Mg-sulphate content of seawater may produce other secondary precipitation thus increasing the risk of permeability reduction; iii) the handling of geothermal fluids within the reservoir may become more difficult in case of a formation of a high salinity brine after the flashing, which fate in to the reservoir need to be assessed and lastly iv) the high temperature fluid rock interaction between the newly added seawater and the reservoir rock should be investigated, not only for the purpose of permeability change but also with the aim to identify potentially corrosive fluids (gas) originating after the seawater injection. The knowledge on seawater geothermal systems is also limited compared to regular water systems, and we need also to tackle the environmental impact assessment for a sustainable production and resource conservation, that are in some instances intermixed. To deal with the problematic described so far, that arise from the idea to use seawater reinjection to improve the geothermal field productivity, we plan a modeling study carried out with TOUGHREACT in which we simulate some scenario of reinjection wells at different depth and as flow in a fracture. To increase the accuracy of the model, the chemical and mineralogical analysis of some core drilled samples from possible reinjection points will be carried out, as well as the investigation of seawater-rock interaction carried out in an high P,T Parr reactor. The experimental results are modeled with TOUGHREACT as a means to calibrate the reaction model used to simulate the different injection scenario to evaluate the feasibility of seawater use as reinjection fluid to improve the geothermal field productivity.