Two formations at NPR-3, the Pennsylvanian Tensleep and Mississippian Madison formations, produce sufficient hot water to generate low-temperature geothermal energy. The current flowing water resource from these formations is 45,000 barrels of water per day (BWPD). The present and potential areas for Tensleep and Madison production are shown in Figure 2 (below). The average production temperature for the Tensleep is 195°F to 200°F and for the Madison is 200°F to 210°F. Currently, hot water in the oil field is a waste stream and is treated through a series of treatment ponds and then discharged into an adjacent stream. Projections suggest that with minor work on existing wells, the rate for the combined Tensleep and Madison produced water would be between 126,000 and 210,000 BWPD.
NPR-3 is not located in an area of known high surface heat flow (Figure 3, below), so the produced water temperatures seen from the relatively moderate depths of 5,500 feet are anomalous for the area. Based on the temperatures observed from Tensleep and Madison production, the local geothermal gradient is 3.0°F per 100 feet of depth (55 C per km). This compares with an average thermal gradient for the southern Powder River Basin of 2.2°F per 100 feet. As the figure suggests, the heat flow at Teapot Dome is more similar to the “Battle Mountain High” of northern Nevada.
The water resource in both the Tensleep and Madison formations is continuously recharged from mountains to the west (Figure 4, below) and the Tensleep reservoir has a strong water drive, resulting in no loss of reservoir pressure (2,350 PSI) over 30 years of production. The hydrologic system in the area must have the groundwater heated by proximity to deep basement rocks prior to entering the Teapot Dome anticline.
The Demonstration Project
In January 2007, Reno, Nev.-based Ormat Nevada Inc., which develops and operates geothermal power plants in Nevada, California and Hawaii, entered into a cooperative research and development agreement (CRADA) with DOE at RMOTC to perform a validation of an Ormat organic Rankine cycle (ORC) power system to generate commercial electricity from hot water produced at a typical oil field. The project is designed to validate the premise that a binary geothermal power generation system that uses the hot water produced by an oil field can reliably generate commercial electricity. For the demonstration, Ormat supplied the ORC power unit while RMOTC installed and is operating the facility for a 12-month period.
Prior to this test, hot water in the oil field was considered a waste stream and treated through a series of treatment ponds and discharged into an adjacent stream. The ORC power unit was connected into the field electrical system and the produced energy is metered and monitored for reliability and quality. The produced electricity from the Tensleep wells is presently being used to power field production equipment.
The 250 kW unit arrived in the field as three skids with associated parts. The three main components were an 8 ft by 40 ft vaporizer skid–which also contained the turbine, generator and instrumentation cabinet–and two 8 ft by 40 ft finned-tube condensers. The assembly was completed in about one month using an oil field roustabout crew and contract welders (Figure 5, below). The unit was wired directly into a 480-volt leg of the field power distribution system. Power from the unit is metered and monitored for reliability and quality. For field safety purposes, the Ormat unit was installed such that the unit will shut down if the main field power is interrupted.
The power generation system was installed in August 2008. The unit’s design was based on a relatively low produced water temperature of 170°F and an average ambient temperature of 50°F, as shown in Table 1 (below). At design conditions, the nominal 250 kW unit would produce a gross power of 180 kW (net 132 kW). However, between initial design and installation, two major changes were made. On the equipment design, the pump for the working fluid–isopentane–was incorporated into the turbine-generator package. By incorporating this feature, the unit’s parasitic electrical load was decreased. On the field side, the Tensleep production facility was upgraded and an insulated, produced water storage tank installed. This upgrade kept the produced water temperature in the 195°F to 198°F range.
The higher inlet water temperature allows the system to operate nearer the maximum net power output of 225 kW. Since the system was put into full-time service in September 2008, the net power output has ranged from 80 kW to 280 kW. The output power fluctuates with the average daily ambient temperature when a constant hot water inlet volume is used (Figure 6, below). Through Feb. 9, 2009, the unit had produced over 586,000 kWh of power from 3 million barrels of hot water.
Until last February, the generation system was online 90 percent of the time. If the downtime caused by shutting down the system as a result of field power loss is removed, then the online percentage is 98 percent. System-related downtime was largely the result of the operator’s learning curve.
In February, the unit was shut down because of operational problems. It was determined that changes in the control system and repairs to the generator/turbine system were needed. The existing control system could not prevent higher than desired heat loads caused by daily ambient temperature fluctuations and a constant setting of hot-water flow rate. The high heat loads damaged the generator’s rear bearing. The unit was removed, repaired and reinstalled with a new control system. Repairs consisted of replacing the generator bearings and the mechanical seal between the turbine and generator. The new control system included installing a hot-water flow control valve, a turbine vibrator sensor and temperature probes on both generator bearings. The startup control for the unit was also changed providing for a smoother, trouble-free startup. The unit was restarted on May 7, 2009.
Evaluation of changes to the system for better control of the inlet hot water to reduce fluctuating output power and the ability to generate power above 250 kW are being made. A newly formed collaborative initiative with the DOE’s Geothermal Technologies Program will continue to operate the existing 250 kW unit (Ormat) for three more years and install a second 250 kW unit at an additional site in the north of the field (this one to be water cooled with associated cooling tower) and operate it for three years. In addition, RMOTC will develop a geothermal testing facility for testing small scale prototype power production systems requiring either air or water cooling.