Journal of Petroleum Technology December 2012 : Page 30

TECHNOLOGY UPDATE Generating Electricity From Produced Water Robin Dahlheim, Gulf Coast Green Energy, and William J. Pike, SPE, Leonardo Technologies There are 823,000 oil and gas wells in the United States that coproduce hot water with their hydrocarbon output. This equates to approximately 25 billion bbl of water annually that could be used as fuel to produce up to 3 GW of clean electrical power. Not only would electricity gen-erated from produced water add much needed electrical power, the life of many of these wells also would be extended and additional oil and gas produced. A recent project funded by the National Energy Technology Laborato-ry of the United States Department of Energy conducted field demonstrations to determine the potential of generat-ing electricity from hot produced water. Participants included Gulf Coast Green Energy (GCGE), ElectraTherm, Den-bury Resources, the Southern Methodist University (SMU) Geothermal Labora-tory, the Texas A&M University petro-leum engineering department, and Dixie Electric Cooperative. The primary goal of the project was to prove the feasibility of interfacing the ElectraTherm Green Machine, a waste heat-to-power generator, with a produc-ing oil or gas well. The project’s subsid-iary goals were as follows: 1. Demonstrate the ability to produce electricity from waste heat in the produced water. 2. Show that producing electricity from produced water does not interfere with normal well operations. 3. Demonstrate how small oil and gas producers might increase their profitability by adding an income stream from power generation. 4. Determine the economic viability of generating electricity from waste heat in the produced water. Evaporator Expansion of pressurized vapor Heated pressurized vapor produces power. Generator rotation produces electricity Twin screw expander Generator ORC Refrigerant Loop Low-pressure vapor Condenser Pump Preheater Low-pressure liquid Fig. 1—A schematic of the Organic Rankine Cycle (ORC) generator with a twin screw expander. 5. Determine whether the kWh output of electricity from the produced water has practical applications. 6. Identify the environmental impact of generating fuel-free, emission-free electricity from waste heat in the produced water. It was important to hold a field trial to determine the extent of known factors that could not be identified or quanti-fied in laboratory and bench scale runs. In addition, a field trial was necessary to identify potential corrective measures for new equipment designs and future produced water projects. The site cho-sen was a producing oil well, Denbury’s Summerland No. 2 well, near Laurel, Mis-sissippi. In production for 5 years, the well has a high water cut and high pro-duced water temperature. The well pro-duces 100 BOPD and 4,000 BWPD from a depth of 9,500 ft with an electric sub-mersible pump. The temperature of the produced water exiting the “knockout” tank at 120 gal/min is 204°F. The site has an ambient temperature range of 60°F to 105°F. Choosing the Right Technology Organic Rankine Cycle (ORC) genera-tors create pressure by boiling vari-ous chemical working fluids (refrig-erants) into a high-pressure gas. The gas expands in a one-way system and turns an expander or high-speed tur-bine, which drives a generator that gen-erates electricity. Historically, ORC gen-erators incorporating turbo expanders or turbines have not been commercial-ly viable in sizes less than 1 MW. How-ever, the Green Machine uses a patented, robust, low-cost twin screw expander (see Fig. 1 ) that requires much less water vol-ume than the larger ORC generators. The Green Machine can generate between 30 kWh and 65 kWh with hot water flows of 200 gal/min or less, and because most oil and gas wells produce less than 200 gal/min of hot water, it was select-ed for this demonstration. The generator 30 JPT • DECEMBER 2012

Technology Update • Generating Electricity From Produced Water

Robin Dahlheim, Gulf Coast Green Energy, and William J. Pike, SPE, Leonardo Technologies

There are 823,000 oil and gas wells in the United States that coproduce hot water with their hydrocarbon output. This equates to approximately 25 billion bbl of water annually that could be used as fuel to produce up to 3 GW of clean electrical power. Not only would electricity generated from produced water add much needed electrical power, the life of many of these wells also would be extended and additional oil and gas produced.<br /> <br /> A recent project funded by the National Energy Technology Laboratory of the United States Department of Energy conducted field demonstrations to determine the potential of generating electricity from hot produced water. Participants included Gulf Coast Green Energy (GCGE), ElectraTherm, Denbury Resources, the Southern Methodist University (SMU) Geothermal Laboratory, the Texas A&M University petroleum engineering department, and Dixie Electric Cooperative.<br /> <br /> The primary goal of the project was to prove the feasibility of interfacing the ElectraTherm Green Machine, a waste heat-to-power generator, with a producing oil or gas well. The project’s subsidiary goals were as follows: <br /> <br /> 1. Demonstrate the ability to produce electricity from waste heat in the produced water.<br /> <br /> 2. Show that producing electricity from produced water does not interfere with normal well operations.<br /> <br /> 3. Demonstrate how small oil and gas producers might increase their profitability by adding an income stream from power generation.<br /> <br /> 4. Determine the economic viability of generating electricity from waste heat in the produced water.<br /> <br /> 5. Determine whether the kWh output of electricity from the produced water has practical applications.<br /> <br /> 6. Identify the environmental impact of generating fuel-free, emission-free electricity from waste heat in the produced water <br /> <br /> It was important to hold a field trial to determine the extent of known factors that could not be identified or quantified in laboratory and bench scale runs. In addition, a field trial was necessary to identify potential corrective measures for new equipment designs and future produced water projects. The site chosen was a producing oil well, Denbury’s Summerland No. 2 well, near Laurel, Mississippi. In production for 5 years, the well has a high water cut and high produced water temperature. The well produces 100 BOPD and 4,000 BWPD from a depth of 9,500 ft with an electric submersible pump. The temperature of the produced water exiting the “knockout” tank at 120 gal/min is 204°F. The site has an ambient temperature range of 60°F to 105°F. <br /> <br /> Choosing the Right Technology <br /> <br /> Organic Rankine Cycle (ORC) generators create pressure by boiling various chemical working fluids (refrigerants) into a high-pressure gas. The gas expands in a one-way system and turns an expander or high-speed turbine, which drives a generator that generates electricity. Historically, ORC generators incorporating turbo expanders or turbines have not been commercially viable in sizes less than 1 MW. However, the Green Machine uses a patented, robust, low-cost twin screw expander (see Fig. 1) that requires much less water volume than the larger ORC generators. The Green Machine can generate between 30 kWh and 65 kWh with hot water flows of 200 gal/min or less, and because most oil and gas wells produce less than 200 gal/min of hot water, it was selected for this demonstration. The generator was also chosen for its relatively small size and portability. It is skid mounted, can be moved with a small forklift, and has a minimal equipment footprint of 300 ft2.<br /> <br /> While the technology is relatively new, a prototype unit suitable for oil and gas applications was tested and demonstrated in a boiler room application beginning in May 2008 at SMU.<br /> <br /> When the generator is in use, produced water from the well enters a heat exchanger where the hot water excites (pressurizes) the working fluid—an EPA-approved nonhazardous, nontoxic, and nonflammable fluid—which drives the twin screw expander (the power block) to create electricity. The twin screw expander is unique in its configuration, lubrication, and specifications, but uses reliable, proven compressor technology that has existed for more than 20 years. The twin screw expander has a rotational speed of 4,300 to 4,800 rev/min, one-tenth of that of most turbo expanders. The robust screw allows wet vapor to travel through the expander, thereby enabling access to lower temperature resources. A patented process and lubrication scheme simplifies and/or eliminates lubrication reservoirs, oil coolers, pumps, lines, and filters, creating a simple and efficient system with fewer parasitic loads.<br /> <br /> After the working fluid expands across the twin screw expander (spinning a generator) the low-pressure vapor must be condensed to a liquid to begin the cycle again. The condenser for the demonstration generator was air cooled to eliminate the extensive use of fresh water and the maintenance expenses associated with operating a cooling tower. The generator’s control system is fully automated, thus allowing remote control, remote monitoring, and offsite diagnostics and trending. <br /> <br /> The generator (Fig. 2) and condenser were tested at the factory, mounted on a drop-deck flatbed trailer, and trucked to the site, where a test run was completed soon after arrival. A hot water bypass valve was installed by GCGE and Denbury field employees, which allowed the produced water to bypass the generator during downtime. Denbury laid and connected the pipe from the hot water bypass to the trailer and the final connections to the generator were made up with high-pressure hoses.<br /> <br /> Dixie agreed to “net meter” the electricity generated by the unit and credit the electrical production at retail rates, which meant that the generated electricity was allocated directly to the field.<br /> <br /> Lessons Learned <br /> <br /> The 6-month demonstration successfully concluded in November 2011, with 1,136 total runtime hours, and provided excellent insight for future installations. The high summer temperatures reduced the temperature differential between the hot water temperature and the condensing temperature so much that the equipment was programmed to shut down when the ambient temperature was above 92°F. Future shutdowns could be avoided by using larger condensing fan units. The larger condensers could have added up to 40% more KWh output by increasing the heat transfer surface area for the refrigerant, thus allowing the temperature differential to increase. Because the hot water bypass valve became clogged and required a replacement valve to be installed, it was determined that the bypass valve used for produced water applications must have a different design.<br /> <br /> Geothermal Brine Issues <br /> <br /> Water corrosion and mineral buildup in the ORC generator’s heat exchangers were considered a major challenge going into the demonstration. The investigators understood that brazed-plate heat exchangers are not optimally suited for brine as they tend to clog and experience stress corrosion cracks. Thus, the investigators concluded that the chosen heat exchanger design would be insufficient for long-term operation. However, a 6-month-long, 1,000-hour test run using the installed heat exchangers presented no problems. The addition of a gasketed plate-and-frame heat exchanger would allow various metallurgy options and cleaning ability, as well as extend heat exchanger life. The use of a small metering pump to add a scale inhibitor to the produced water before it enters the generator is another potential solution.<br /> <br /> Economics <br /> <br /> A review of the demonstration and subsequent cost analysis confirmed the economic benefits of the application. A postproject analysis concluded that the Green Machine’s power generation offset about 20% of the energy required to run the downhole pump on the oil well. Thus, it provided an attractive payback at oil and gas sites where power costs more than USD 0.08/kWh and where producers see generation of waste heat electricity as a public relations or corporate social responsibility value.<br /> <br /> For wells with increased produced water flow and/or temperature, the internal rate of return (IRR) and net revenue will be substantially greater. For example, a single well that produces 65 kWh of electricity by using the Green Machine would have an IRR of 25% with a 20-year power cost of USD 0.028/kWh and net revenue of USD 1.16 million over the life of the equipment. This will provide the incentive for oil and gas producers to continue operating a well long after it would usually be shut in because of the produced water level. It may also offer an incentive for operators to consider bringing wells onto production that previously would have been uneconomic because of their projected water volumes.<br /> <br /> Conclusions <br /> <br /> The lessons learned from the demonstration at Denbury’s Laurel site provided insight that could help future applications of the ORC generator technology to reduce installation time, increase efficiency, generate additional power, and minimize maintenance. This kind of cogeneration can be effective in reducing the energy costs particularly for pumping geographically remote oil wells, a need increasingly seen in the US.<br /> <br /> However, some hurdles remain in developing opportunities for the use of coproduced fluid to generate electricity. Economics will play a critical role in the growth of this sector. Depending on criteria, there is an attractive return on investment in locations where the cost of power is USD 0.10/kWh or higher. In locations where the cost of power is less than that, more incentives or corporate objectives will be necessary to justify a project.<br /> <br /> Environmental Impact <br /> <br /> The waste heat-to-power generation technology demonstrated contributes directly to the reduction of harmful atmospheric emissions. The total electricity production was 19,180 kWh, which is equivalent to offsetting 172 tons of produced CO2. Applying the lessons learned from the air-cooled condenser, a well with a produced water flow of more than 150 gal/min generating a net output of 38 kWh of electricity would offset 360 tons of CO2 production, according to an online CO2 emissions calculator.<br /> <br /> The process also conserves water, a focal point in major US producing regions that have recently experienced extreme drought. Operators become better resource stewards by generating electricity in the field to offset some of their electrical power use and reduce the likelihood of power shortages in an expanding electricity market.

Previous Page  Next Page


Publication List
Using a screen reader? Click Here