We measured in situ stresses in a Hot Dry Rock geothermal reservoir using hydraulic fracturing of the rock mass as well as the novel technique of differential strain curve analysis (DSCA). We found that the DSCA method gave reliable stress estimates for deep rock masses, as well as provided complete stress tensor information from single core samples. Our results also showed the existence of unexpected and substantial changes of stress state: the stress state rotates from a normal faulting to a strike-slip faulting system within 1 km depth change.
Nous avons mésuré des contraintes in situ dans un réservoir souterrain de roche chaude et sèche en employant Le procédé par fracturation hydraulique du massif rocheux ainsi que la nouvelle méthode d ''analyse differentielle de la courbe de déformation. Nous trouvons que cette méthode donne des évaluations fiables pour les matériaux rocheux prof''onda et qu''elle fournit les informations entières du tenseur contrainte d''un échantillon unique de carotte. Nos résultats montrent aussi l''existence de changement imprévus et importants de l''état de contraintes: I''etát de contraintes se tourne de la formation normale de failles à un systeme de faille à rejet horizontal dans le changement de profondeur dans les limits de 1 km.
Wir haben in-situ Spannungen in einem Erdreservoir von heissem trockenem Tiefengestein gemessen und wir haben die neue Methode der Differentialanalyse von Dehnungskurven benutzt. Wir finden, dass diese Methode zuverlässige Spannungsabschätzungen für Tieigebirgsmassen gibt und vollstandige Angaben über den Spannungstensor von einer einzigen Kernprobe liefert. Unsere Ergebnisse zeigen auch die Existenz von unvorhergesehenen und beträchtlichen Veränderungen des Spannungszustandes: Der Spannungszustand dreht sich von einer normalen Sprungbildung bis zu einem Seitenverschiebeungssystem in einer Tiefenveränderung innerhalb 1 km.
As part of the development of a Hot Dry. Rock geothermal reservoir, we estimated in situ stresses using hydraulic fracturing methods and the novel technique of differential strain curve analysis (DSCA). This geothermal project is being conducted by Los Alamos National Laboratory with funding from the United States Department of Energy, Japan''s New Energy Development Organization and West Germany''s Ministry for Science and Technology. The experimental site is located on the western flank of the Valles Caldera at Fenton Hill, New Mexico, USA. The age of caldera formation is about 1.1 m.y. with volcanism occurring between about 10 m.y. and 0.1 m.y. ago. Two wells, one of which is sidetracked and redrilled into a different geometry, penetrate the geothermal reservoir region which extends from about 3 km depth to about 4.5 km depth. The geology (Laughlin et al. 1983) consists of Precambrian metamorphic and igneous intrusive rocks with temperatures ranging from 200–320°C. The great depth of the reservoir, coupled with the presence of numerous pre-existing fractures, makes conventional hydraulic fracturing stress measurement difficult to carry out. The depth, coupled with the high temperatures, produces conditions too severe to use the borehole tools necessary for finding and isolating unfractured intervals as is conventionally done (e.g. Bredehoeft et aI, 1976). Consequently, an additional method for estimating stresses at these severe conditions was sought, and the method of differential strain curve analysis (DSCA) of core samples was chosen to provide additional information on the state of stress. Ren and Roegiers (1983) used this technique successfully to estimate the state of stress in deep boreholes drilled for petroleum exploration and production.
Based on the theoretical work of Walsh (1965), Simmons et al. (1974) developed an experimental method for measuring total microcrack volumes and orientations in rock samples in the laboratory. Samples are prepared with foil strain gages attached in appropriate orientations. After being jacketed, the sample is loaded hydrostatically to a pressure sufficient to close essentially all the crack porosity: 200 MPa or so. Projecting the asymptotic slope of the strain pressure curves back to zero pressure, as Walsh described, gives the contribution of crack closure to the strain recorded by each gage. With a minimum of six appropriately oriented gages, this crack strain contribution can be resolved into a strain tensor with both the three principal crack strains and their directions determined (e. g. Solkolnikoff 1956). Under suitable conditions, this crack strain tensor can be interpreted to give the in situ stress state. A necessary condition is that the great majority of microcracks present in the sample are due to the relief of the in situ stress during and following cutting of the core sample. A further assumption necessary is that microcrack porosity oriented in any given direction is produced in proportion to the magnitude of the effective compressive stress that was relieved in that direction. In other words, if the in situ stresses in the x and y directions are in the ratio of 1: 2, then the crack strains measured in these directions must also be in the ratio of 1:2.
