| Abstract |
As part of the Northwest Geysers Enhanced Geothermal System (EGS) Demonstration Project, Prati State 31 (PS-31) and Prati 32 (P-32) were reopened, deepened, and recompleted for direct injection and stimulation of a high-temperature reservoir (HTR) (up to 750 oF, 400 °C). Two wells nearby the EGS Demonstration Project, Prati 5 Sidetrack 1 (P5-St1) and Prati 38 Sidetrack 2 (P-38 St2), were also reopened, deepened, and recompleted as part of the Caldwell Ranch Exploration & Confirmation project. Rock samples were collected from the HTR in the Caldwell Ranch project area for further petrologic and petrophysical studies. The P-5 St1 core (9940-9945 ft) represents one of the few windows into the hornfelsic metagraywacke HTR. Analysis of core samples indicates that the original illitic matrix of the laminated silty to sandy hornfelsic metagraywacke has been converted to biotite, actinolite, and calcium-rich plagioclase. High-temperature magmatic-hydrothermal veins cut the metagraywacke matrix and are composed of actinolite, biotite, clinopyroxene, quartz, albite, pyrrhotite, and tourmaline. Elemental analyses indicate that the vein albite has a pure sodic composition, and bleached selvages on the veins are composed of calcic feldspar. Fluid inclusions trapped in vein quartz from the P-5 St1 core are vapor-rich and multiple daughter phases include halite and sylvite. Vapor-dominated fluid inclusions dominate and indicate that boiling has occurred in the HTR reservoir and that highly saline fluids were present during formation of the veins. The high temperature minerals that only occur as veins in the P-5 St1 core occur throughout the matrix of the hornfelsic metagraywacke in well cuttings from P-38 St2 (8250-9900 ft). In contrast to P-5 St1, the secondary plagioclase in the matrix of the hornfelsic metagraywacke in P-38 St2 is dominantly sodic (albite composition), and the rock appears to have undergone more extensive sodium metasomatism than in P-5 St1. P-38 St2 apparently has had greater volumes of saline hydrothermal brine, perhaps originating as connate water, moving through the rock matrix than in P-5 St1. Sodium metasomatism has caused extensive albite cementation of the rock matrix. Basic core properties (density, porosity, and permeability) and scratch testing of the P-5 St1 core confirm the low matrix permeabilities and high rock strengths in the hornfelsic metagraywacke in the high temperature reservoir. Analyses of the P-5 St1 core samples indicate less than 1% porosity and 90 microdarcy gas permeability in unfractured samples. Scratch test results at ambient conditions indicate very high rock strengths, with unconfined compressive strength estimates of up to 56,000 psi (390 MPa) for the hornfelsic metagraywacke lithologies. Actual in-situ rock strengths and mechanical properties within the HTR are not known; and the rocks are sufficiently hot (400 oC) to behave in a ductile manner at these depths. However, a steam-bearing fracture near 11,000 feet was encountered while drilling P-32 and the injection of cool water into the HTR as part of the EGS Demonstration apparently has promoted brittle failure to depths 1 km below the bottom of the P-32 well. |