ABSTRACT:

The network of joints in the Coso geothermal field, an enhanced geothermal system 200 km north of Los Angeles, CA, is characterized based on the investigation of fractures with significant aperture [1]. Using the stress tensor reported in [2] and a range of linear and non-linear failure envelopes, the limiting stress ratios for the joints is calculated. It is found that for a friction angle ( ö?) of less than 25º, fractures can be critically stressed. The pore pressure increase necessary to initiate slip and jacking on the joints are calculated. The joints, for example, dipping 60º-80º with ö?=35º require a pore pressure increase of 0.0035-0.005 MPa/m. Upward shear growth is predicted for joints having ö? ¡Ý 25º. The injection rate corresponding to the calculated pore pressure increases are found using an injection/extraction model for a joint. The joint orientation most likely to slip without jacking is found and a critical friction angle is determined in terms of injection rates. The orientations are found to be solely a function of the loading on the joint. Finally, the thermoelastic stresses are estimated using a 1D model. It is found that the magnitudes of thermoelastic stresses are significant and can enhance the potential for slip on a joint.

INTRODUCTION

In stimulation of an enhanced geothermal system (EGS), it is important to consider the fluid pathways between the injection and the production well(s), and the factors controlling them. As shown in [3] the permeability of critically stressed fractures (CSFs) are higher than non-critically stressed fractures. Critically stressed fractures are defined as pre-existing fractures that have slipped or are in the state of incipient slip because of the in-situ stress conditions. For the Coso geothermal field, the primary permeability is very low; therefore the secondary permeability (fractures, joints, etc.) must be considered in determining the heat exchange (fluid pathway from injection well to surface). For this study, well 38C-9 (Fig. 1) is of primary concern. This was to be an injection well in the EGS system. Injection had been proposed at 2316- 2346 m [1]. Well 38C-9 is also of importance because a hydraulic fracturing (HF) test was conducted in this well to determine the in-situ stress.

In assessing the conditions of joints, it is important to include the influence of increased pore pressure due to injection as well as the cooling as they both reduce the effective normal stress on fractures.

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