Abstract
(Englisch)
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Within the Soultz HDR programme, the main goal of this project was to create a simulation module capable to define and forecast the geochemical evolution of hypersaline fluids flowing in hot fissured granite. When the future pilot plant will be exploited, effects on the fractured reservoir under production/re-injection should be forecasted. Creation of this geochemical module was based on an existing code (Chem-TOUGH2), to be later coupled to the finite element code FRACTure, developed at the ETH-Zurich.
The first task of the geochemical modelling was to characterise the fluid-rock system and to determine the possible chemical evolutions. The system considered is formed by all mineral phases in contact with the fluid and most relevant aqueous species. Due to the very high salinity of the Soultz fluid, activity coefficients have to be calculated by the Pitzer equations using the code TEQUIL. The thermodynamic model could be used to estimate the potential for mineral precipitation and/or dissolution, when the fluid is cooled in the heat exchanger from 165 to 65°C, and then injected back into the reservoir: carbonate dissolution is more important than quartz and pyrite precipitation.
The next step is kinetic modelling, to determine what really happen. The complete modelling of the reaction rates requires the knowledge of many parameters which do not always exist for hot brines. The method chosen had to be simplified and was built on one kinetic law for each dissolution or precipitation reaction of the four main minerals considered (calcite, dolomite, quartz and pyrite). The results do not refute those of the thermodynamic modelling: that calcite and dolomite dissolution will be the major process and that quartz and pyrite precipitation will be a secondary phenomenon.
To develop a geochemical module to simulate fluid-rock reactions in the fractured reservoir, the reactive transport code Chem-TOUGH2 was adapted to the very high salinity fluid. Geochemical subroutines were set up and implemented into Chem-TOUGH2. Then, the geochemical module was extracted and coupled with FRACTure. The new code called FRACHEM calculates chemical reactions, advective transport of the chemical species and variation of porosity and permeability.
With the revised version of Chem-TOUGH2 a simple geometry was used to simulate the chemical reaction along a 1-D fracture with water injected at 65°C and heated to 165°C. Most of the circulation takes place in the fracture itself and the simulation considered a fracture length of 1200 m over a period of 10 years. The results show that the main process is the fluid-calcite reaction, including dissolution near the injection zone during the first month, and precipitation further in the fracture.
The first application of the new code FRACHEM consisted of a 2-D simplified reservoir model, with water injected at 65°C into an initial rock temperature of 165°C during a period of 6.4 years. Results show that calcite and dolomite are rapidly dissolved near the injection point. Then, re-heating induces carbonates precipitation on a wider zone. Later, carbonates being less available in the injection area, reactions rates decrease significantly. Quartz and pyrite precipitate in the fracture during the first year. Permeability evolution shows a high increase near the injection point, due to carbonates dissolution.
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