Title:

GEOCHEMICAL MODELLING OF THE SOULTZ-SOUS-FORETS HOT DRY ROCK SYSTEM. BRINE ROCK INTERACTIONS IN A DEEP HOT FRACTURED GRANITE RESERVOIR

Authors:

Pierre Durst and FranÁois-D. Vuataz

Key Words:

water-rock interaction, geochemical modelling, reservoir modelling, fractured granite, Hot Dry Rock, Soultz-sous-ForÍts

Geo Location:

Soultz-sous-Forets, France

Conference:

Stanford Geothermal Workshop

Year:

2001

Session:

hot dry rock

Language:

English

File Size:

75KB

View File:

Abstract:

The European Hot Dry Rock program is located in the Rhine Graben at Soultz-sous-ForÍts, in the north eastern part of France. A future pilot plant for generating electricity will be built on the basis of three 5,000 m deep wells, one for injection and two for production, all drilled in fractured granite and reaching a bottom hole temperature of 200?C. During a previous phase of the project, in 1997, a four-month circulation test was conducted between two boreholes (GPK1, 3590 m and GPK2, 3876 m), separated from each other by a horizontal distance of 450 m in hydraulically fractured granite. A hydraulic connection between the two wells was proven and a production flowrate was kept between 20 and 25 kg/s. The wellhead temperature reached 142?C, whereas the produced fluid and gases were totally reinjected after cooling at 65 ?C.

The chemical data obtained from this test as well as the mineralogy given by core samples have been used to build a numerical model of the water rock interactions in the reservoir. The high salinity of the fluid (TDS = 100 g/l) and the reservoir temperature (165?C) preclude the use of a classical geochemical model. Therefore a new model based on a Pitzer approach has been developed and introduced in the code CHEMTOUGH.

Thermodynamic equilibrium simulations show that, at the vicinity of the bottom of the injection well and during one year of fluid circulation at a rate of 25 kg/s, a maximum of 250 tons of calcite and 290 tons of dolomite are potentially dissolved within the reservoir, but 100 tons of quartz and 40 tons of pyrite can be potentially precipitated.

Kinetic simulation however indicates that the carbonates behaviour is the most relevant process for permeability variations during the time scale considered here (30 years). Simulations carried out for a 1-D fracture during a period of flow between 1 and 12 months do no display significantly different results. Calcite, dolomite and quartz reaction rates allowed calculating respective porosity increase or decrease in the system. This approach is still under work and the geochemical section of the program will then be coupled with the thermo hydraulic code FRACTure. This procedure will allow to simulate the flow evolution within the reservoir and to estimate the variation of the system impedance, as well as the production fluid temperature.


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