Die Geothermie-Bohrung Hamburg-Allermöhe : eine Analyse mit Hilfe reaktiver Transportsimulation

Precipitation of minerals in hydrothermal reservoirs can decrease porosity and permeability significantly. In these cases, the amount of hot water produced by hydrothermal heat mining installations is far too low for an economical use of the resource. Massive anhydrite cementation of formerly highly porous sandstones was found at two locations within the North German Basin. These examples illustrate that the processes causing pore space clogging need to be understood in more detail in order to reduce the exploration risk with respect to heat mining. Due to the lack of solubility data of anhydrite at high temperatures (>100°C) and salinities (>100 g NaCl / L), a core flooding experiment was performed in order to determine kinetic rate law parameters for reactive flow simulations. A Bentheim sandstone core was flooded with a highly saline NaCl solution oversaturated with respect to CaSO4. A spatially constant thermal gradient was maintained in the core increasing from 70°C at the core inlet to 123°C at the core outlet. Because CaSO4 is less soluble at higher temperatures, anhydrite was expected to precipitate towards the outlet of the sandstone core. From time to time during the experiment the core was scanned by X-ray tomography to investigate the temporal development of crystal growth. Under experimental conditions, precipitation of anhydrite can only occur at high core temperature, where the CaSO4 supersaturation Omega exceeds a threshold value of Omega=1.5. As a result, the anhydrite distribution within the core has aperiodical cementation pattern: The growth of anhydrite crystals reduces the supersaturation of CaSO4 and thus inhibits further growth in the direction of flow, until fluid mixing by diffusion compensates for the concentration deficit again. The core flooding experiment was simulated applying (1) the Pitzer activity-coefficient approach suited for solutions at high ionic strength, (2) a new developed stochastic approach for anhydrite nucleation, which incorporates a relationship between nucleation probability, supersaturation and pore size-distribution, and (3) a reaction rate law which considers a coupling between crystal surface area and reaction rate. The simulation fits the experimental data well. The low nucleation probability and the threshold supersaturation value for precipitation cause a cementation pattern similar to the pattern in the core flooding experiment, but on a much larger spatial scale. Large scale reservoir simulations suggest that minerals such as anhydrite may clog the pore space near of fault zones which had been activated temporarily by seismic events. Without an additional heat event, anhydrite will only precipitate in small amounts where strata-bound flow is directed down-dip into deep and warmer regions of the reservoir. However, the productivity of a massive cemented reservoir can be increased by chemical stimulation, i.e. the forced dissolution of anhydrite around the borehole and on fracture walls in response to the injection of cold brines.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2006

Author dc:creator

• Wagner, Roland

Contributors dc:contributor

• Clauser, Christoph

• info:eu-repo/classification/ddc/550
• Geowissenschaften
• Geothermik
• Hydrogeothermik
• Reservoirsimulation
• reaktive Strömung
• Anhydrit-Zementation
• Geothermische Heizzentrale
• Reservoir Simulation
• reaktive Transport Simulation
• Anhydrite Precipitation
• Heat Mining

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-122912%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:61234