| Title | Tolhuaca Update |
|---|---|
| Authors | Wilmarth, Maxwell; Melosh, Glenn; Sussman, David; Swanson, Robert; Cumming, William; Colvin, Anna; Iriarte, Sergio; Lohmar, Silke; Soto, Elizabeth |
| Year | 2010 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Tolhuaca; Chile; Drilling; Exploration; Discovery |
| Abstract | In April 2009 GeoGlobal Energy LLC (GGE) discovered a shallow geothermal reservoir (160 °C at 150300 m and isothermal) and inferred a deeper high temperature reservoir (290 °C at 1070 m with falling gradient) at Tolhuaca-1 on the northwest flank of Tolhuaca volcano in southern Chile. To discover the deeper reservoir GGE contracted Inaer to heli-lift a larger, custom-retrofitted LF-230 coring rig operated by Geotec to a drilling location approximately 400 m south of Tolhuaca-1 in February 2010, one week before the great M8.8 earthquake. The earthquake was felt strongly at Tolhuaca by the rig crew but caused only minor disturbance and a delay of less than a week while the crew returned home to check on their families. Tolhuaca-2 was spudded on February 6 with a tricone bit and encountered the same shallow reservoir as Tolhuaca-1 at approximately the same depth, but with higher temperature and pressure. Drilling was shut down for approximately two weeks for ThermaSource to train the drilling crew in proper well control techniques and ensure that the well could be drilled safely. The well took kicks repeatedly from the overpressured shallow reservoir but well control was maintained through this zone and HWT casing was successfully cemented by BJ Services at 300 m. The well was rotary drilled steadily with HQ rods at approximately 50 m/day through relatively impermeable interbedded chloritized basaltic/andesitic flows and clastic lacustrine sediments to a depth of approximately 1200 m. Wellbore temperature and pressure were regularly monitored with maximum registering thermometers and a Kuster K10 tool by GGE geologists during drilling. Below the shallow reservoir, the static temperature profile converged from approximately 50 °C above that recorded in Tolhuaca-1 at equal depth to a similar profile below approximately 1000 m. Drilling breaks and lost circulation occurred at approximately 800 m, where a 3 m core confirmed propylitic alteration. Specially perforated HQ slotted liner ordered from the Geotec machine shop in Santiago was set at approximately 1200 m and NQ-sized tools were used to core to the well’s total depth of 1235 m. The static temperature profile 63 hours after the end of circulation indicated three main zones of permeability: the isothermal shallow reservoir with a slight temperature reversal between 200 and 250 m, a nearly isothermal section between 400 and 450 m, and a bottom section from 1000-1150 m paralleling the boiling point vs depth curve with a temperature transient indicating permeability. The maximum recorded temperature was 303 °C at 1190 m. The well was flow tested and geochemical samples were collected by GGE geologists, ThermaSource and Thermochem on May 18. Due to the high temperatures and pressures, the well’s master valve was only partially opened. However, during a 3-hour flow test a flowing temperature of 232 °C at 37.5 bars and 95 l/m was measured. Additional magnetotelluric data collected by Wellfield Services in May indicate a low-resistivity (<10 ?•m) clay cap may extend over an area of approximately 20 km2, but remains unbounded by data to the east. A road was designed and constructed by GGE engineers from the southern access to the concession, which arrived at the wellsite in time to demobilize the rig and camp before the beginning of Chilean winter. Three well pads are currently under construction for production drilling scheduled to begin in November 2010. |