S. Homuth

Lithofacies and depth dependency of thermo- and petrophysical rock parameters of the Upper Jurassic geothermal carbonate reservoirs of the Molasse Basin

In the early stages of hydrothermal reservoir exploration, the thermo-physical characterisation of the reservoir is accomplished by evaluating drilling data and seismic surveys. Especially in carbonate reservoirs, the distinction of different facies zones and heterogeneities in general is very complex. For economic reasons a sufficiently high flow rate toward the production well and an according high fluid temperature is necessary. For reservoir predictions and modelling, geothermal parameters such as permeability, thermal conductivity/diffusivity, and specific heat capacity have to be quantified. The thermophysical parameters are facies related. The application of a thermofacies classification to Upper Jurassic limestones serves to understand the heterogeneities and to identify production zones. Outcrop analogue studies enable the determination and correlation of facies related thermophysical parameters and structural geology data and thus the geothermal exploration concept becomes more precise and quantitative. The analogue outcrops of the Swabian and Franconian Alb represent the target formations of Upper Jurassic carbonate reservoirs in the adjacent Molasse Basin. These limestone formations contain the main flow paths through fractures, faults, and characteristic of limestone formations also through karstification. The type and grade of karstification is also facies related. In general, the matrix permeability has only a minor effect on the reservoir’s sustainability except for some grainstones and dolomitised zones with higher porosities and permeabilities. Permeabilities range from 10-18 to 10-13 m2 (0.001 mD to 100 mD). The permeability range of mudand wackestones is about the same. A high variation of thermophysical parameters is recognised within individual facies zones or stratigraphic units. Mudand wackestones show thermal conductivities around 2 W/(mK), whilst mudstones have lower thermal conductivities than wackestones. The thermal conductivities of massive reefal limestones show values of 1.8 to 3.9 W/(mK). Secondarily silicified reefal limestones and dolomites show the highest values of thermal conductivity. These parameters determined on ovendried samples have to be corrected for water-saturated rocks under the according temperature and pressure conditions using transfer models. A comparison of calculated reservoir properties with measurements from deep drill cores confirms a good correlation. Based on the investigation of the matrix parameters in combination with reservoir transfer models, the reservoir prognosis and numerical simulation can be improved. The facies related characterisation and prediction of reservoir formations is a powerful tool for the exploration, operation, extension and quality management of geothermal reservoirs in the Molasse Basin. Kurzfassung: In der Planungsphase einer hydrothermalen Reservoirerkundung erfolgt die thermophysikalische Charakterisierung des Reservoirs durch die Auswertung von Bohrungsdaten und seismischen Erkundungen. Im Falle von Karbonatreservoiren ist die Differenzierung von verschiedenen Faziesbereichen bzw. Heterogenitaten im Allgemeinen sehr komplex. Aus wirtschaftlichen Grunden ist eine hohe Forderrate der Produktionsbohrung und eine entsprechend hohe Fluidtemperatur notwendig. Fur eine Reservoirprognose und -modellierung mussen die geothermischen Kennwerte wie Permeabilitat, Warmeleitfahigkeit, Temperaturleitfahigkeit und spezifische Warmekapazitat im Reservoir quantifiziert werden. Diese thermound petrophysikalischen Kennwerte sind faziesabhangig. Die Anwendung einer thermofaziellen Klassifikation auf die Karbonate des Oberen Jura dient hierbei zum besseren Verstandnis der Heterogenitaten und zur Identifikation von Produktionszonen. Aufschlussanalogstudien ermoglichen hierbei die Bestimmung faziesabhangiger thermophysikalischer Kennwerte und deren Korrelation mit strukturgeologischen Daten, wodurch die geothermische Erkundung praziser und quantitativer wird. Die analogen Aufschlusse der Schwabischen und Frankischen Alb reprasentieren die Zielformation der

Thermo-Triax: An Apparatus for Testing Petrophysical Properties of Rocks Under Simulated Geothermal Reservoir Conditions

A Thermo-Triax apparatus has been developed to facilitate research on petrophysical properties of rock samples under simulated geothermal reservoir conditions. The apparatus consists of control systems for vertical stress and horizontal confining pressure, a pair of independent pore pressure controllers for applying different upstream, and downstream pore pressures at bottom and top of rock specimens, an external heater and a data logging system. Permeability of rocks is measured using steady state and transient flow methods. The thermal expansion of metallic parts in the triaxial cell and the error introduced into the readings of the extensometers at high temperatures are calibrated via experiments on an aluminum specimen with known coefficient of thermal expansion. The possibilities of studying the effect of stress and temperature on permeability and compressibilities of porous rocks with the Thermo-Triax apparatus are presented with first data. The change of pore volume during the non-isothermal process between adjacent temperature levels as well as along the measurement of permeability at leveled temperatures is interpreted and calibrated. The thermal expansion of mineral grains during heating is verified with the data of pore volume change and the magnitude of thermal expansion of mineral grains is estimated and compared with reported values. The permeability measurements along different heating paths can be used to verify the temperature dependency of stress-dependent rock properties.

BHE logging and cement hydration heat analyses for the determination of thermo-physical parameters

To design an efficient and cost effective borehole heat exchanger (BHE), it is necessary to know the thermo-physical parameters of the subsurface at the project location. The quality management and the examination of existing BHEs are becoming particularly important with the increasing application of ground coupled heat pumps. The analysis of data collected from a BHE-logger (temperature-depth-measuring system) in a water-filled BHE allowed several geothermal parameters to be determined. A comparison of measurements obtained from the BHE-logs, geothermal response tests and laboratory soil tests indicated a good correlation. In addition to detecting hydraulic short circuits due to defects in the pipe system or backfill material, being relatively quick and easy to use, the BHE-logger can be a valuable tool in the long-term monitoring system and quality control of BHEs.Résumé Pour concevoir un dispositif efficace et rentable d’échange de chaleur en forage, il est nécessaire de connaître les paramètres physiques et thermiques du sous-sol à l’emplacement d’un projet. La gestion de la qualité des sondes géothermiques verticales devient particulièrement importante avec le développement croissant des pompes à chaleur couplées à des dispositifs d’échange de chaleur dans les sols. Plusieurs paramètres géothermiques peuvent être obtenus à partir d’enregistrements en forage remplis d’eau avec des BHE-logger donnant une mesure de la température en fonction de la profondeur. Des comparaisons entre des mesures obtenues à partir d’enregistrements en forage, de tests de réponse géothermique et de tests sur échantillons au laboratoire ont montré de bonnes corrélations. En plus de la possibilité de détecter des courts-circuits hydrauliques dus à des défauts dans le système de tuyauterie ou le matériau de remplissage, le BHE-logger, relativement rapide et facile à utiliser, peut être un outil précieux dans un système de surveillance à long terme et de contrôle de qualité des dispositifs d’échange de chaleur en forage.