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Energy Minerals Division

A Division of the American Association of Petroleum Geologists

Geothermal Energy

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Introduction

A simple definition for geothermal energy exists by taking the word “geothermal” and splitting it into its components, geo meaning ‘earth’ and thermal meaning ‘heat’, making geothermal the heat within the Earth. This heat has many uses, from heating a home, to maintaining a constant temperature for aquiculture and nurseries, to many other industries where a constant heat supply is needed. In fact the shallow part of the earth (200-300 feet) can be used for taking heat out of a building for air conditioning and storing it for future retrieval through the use of a ground source heat pump. However, the focus for this geothermal discussion is on the use of geothermal heat for electrical power generation.

The source of geothermal heat is the result of the natural decay of various radioactive elements found deep within the Earth that were locked in the planet when it was first formed. This heat is the driver for volcanic and plate tectonic activity world wide. But the heat does not remain at these great depths. Rather it is transported outward from the center of the planet to the surface of the Earth through the natural processes of convection, advection, conduction, and radiation. All four of these mechanisms are active within the Earth, though some are more important than others when talking about geothermal energy production for power generation.

Non-Oil & Gas Geothermal–Conventional Geothermal

Traditional development of geothermal electric power generation has focused on regions where the geothermal gradient is anomalously high and where hot water or steam can be produced from relatively shallow depths (10,000 feet or less). Sources of the heat include active surface volcanism or near surface heat sources from magmatic plutons that heat the surrounding rock and associated water. The produced heat is from either natural steam (dry or wet) or hot water (hydrothermal) resources acquired by drilling into the subsurface. Thus most nations developing geothermal energy fall within the region of the world known as the “ring of fire”, or within other areas where local volcanic activity is apparent (i.e. Italy, Iceland).

These resources are generally at temperatures of 350°F or above, with some production temperatures being over 600°F. Heat from these high temperatures resources is captured and converted to electricity using two different conversion technologies: dry steam or flash systems. Dry steam power plants were the first type of geothermal power generation plants built. they use the steam from the geothermal reservoir as it comes from wells, and route it directly through turbine/ generator units to produced electricity. Flash systems use water at temperatures greater than 350oF that is pumped under high pressure to the generation equipment at the surface. The water pressure is allowed to drop suddenly in a chamber which flashes some of the water to vapor that then turns a turbine/generator unit. Several flash systems can be linked in series to acquire additional energy from the remaining water.

Another approach to geothermal energy production is the attempt to generate a subsurface reservoir in a hot rock such as granite where permeability may not exist and water is absent. In this approach, called “Enhanced Geothermal Systems” (EGS), reservoirs are developed by drilling wells into hot rock and fracturing the rock sufficiently to enable a fluid (water) to flow between the wells. The fluid flows along permeable pathways, acquiring in situ heat, and then exiting the reservoir through production wells. At the surface, the fluid passes through a power plant where electricity is generated. After leaving the power plant, the fluid is returned to the reservoir through injection wells to complete the circulation loop. Several countries, especially Australia, have been investigating this process with experimental wells with the goal of generating electrical power in this manner. A successful completion and demonstration of this form of geothermal production could open up a nearly unlimited supply of geothermal energy anywhere in the world so long as the drill bit can reach the depth and the economics can be made to work.

Oil & Gas Geothermal - Unconventional Geothermal

If any of the older geologists or drillers that have been active in the oil and gas industry are questioned about subsurface temperatures, stories abound regarding high subsurface temperatures.  Thus comments such as““the logging tools started to melt” or “we could not keep the drill bit sufficiently cool” or “the water was so hot we could boil eggs” were not uncommon. 

Thus a nontraditional approach to geothermal energy was investigated in the mid-1970s into the 1980s by the U.S. Department of Energy (DOE).  This investigation focused on the potential of geopressured geothermal energy along the Gulf Coastal region of Texas and Louisiana.  In a broader sense, this approach has led to the potential of geothermal energy being developed within sedimentary basins where the oil and gas industry has been active with energy production for over 100 years. 

In the Gulf Coast and in many other areas of the world, the influx of sediments into basins was of ample volume and at a sufficiently high rate that during the burial process large amounts of the brine water became trapped within these sediments during the deposition process and could not escape as the sediments turned to rock.  As burial continued these sediments became overpressured such that when drilling into these formation, the pressures are so high that any fluid encountered flows under high pressure out of the well on its own in artesian fashion.  In some of these areas, such as the Gulf Coast, natural gas is also dissolved within the water usually in several 10’s of cubic feet per barrel of water produced. 

The DOE investigated these resources over a 17-year period with wells donated by the O&G industry along with drilling of several experimental wells of their own.  These studies demonstrated that it was possible to develop geothermal energy from the heat found within these geopressured waters (~250°F and above) and that the dissolved methane could also be extracted and either burned on site to boost the amount of electrical production or transported via pipeline as a byproduct of the produced hot water. The proof and viability of this form of geothermal energy production was finally confirmed by the building and operating of a geothermal power plant in Brazoria County, Texas at Pleasant Bayou #2.

The technology used to convert the heat to electricity was through a binary power plant where the hot water (or produced steam) never comes in contact with the turbine/generator unit. At temperatures below 400°F, heat energy can be extracted from the produced fluid by passing the brine through a heat exchanger where the heat is transferred to a secondary fluid that has a much lower boiling point than water. This secondary fluid can be organic in nature (i.e. isobutene, pentane), hence the name Organic Rankin Cycle (ORC) unit or turbine, or it can be one of the various refrigerants presently available in industry (i.e. R134a). The secondary fluid is vaporized and the high pressure of this vapor turns the turbine/generator unit to produce electricity. Because this is a closed-loop system, virtually nothing is emitted to the atmosphere. The secondary fluid can then be cooled back to a liquid and pass back through the heat exchanger to become a high pressure vapor again. The produced water can be injected into the producing formation or into a shallower formation for water disposal.

In addition to the geopressured-geothermal energy resource, the potential also exists for the oil and gas industry to capitalize on coproduced hot water where a binary power plant can extract heat for electrical production that can be used either by the company for onsite application, or sold to a local utility as a new profit stream. In either way this type of production results in the O&G company becoming a renewable energy producer and allows the company to begin to benefit from the various incentives that have been developed for renewable energy production. The coproduced concept is presently in operation at the Rocky Mountain Oilfield Test Center (RMOTC) in Wyoming where a binary unit is producing electricity from produced water. Several O&G companies and professionals with O&G expertise are in various stages of investigating or developing geothermal energy within traditional O&G environments (AAPG Explorer, EMD Column, August 2007 and November 2010).

Summary

Geothermal electric power generation is not new. It has a history going back to Larderello, Italy where an experimental dynamo produced electricity in 1904 and commercial quantities of electricity by 1931. The United States started producing commercial geothermal electric power in 1960. As of 2005, the International Geothermal Association reported that the U.S. was the highest producer of geothermal electrical energy at 2,544,000 kW of power. By April 2011, the Geothermal Energy Association reported that the U.S. leads in production with 3,102,000 kW of electrical production in 9 states: Alaska, California, Hawaii, Idaho, Nevada, New Mexico, Oregon, Utah, and Nevada. California and Nevada are the present leaders in geothermal power production. Additionally, new projects were identified as under development in 15 states: Nevada, California, Utah, Idaho, Oregon, Alaska, Louisiana, Hawaii, New Mexico, Arizona, Colorado, Mississippi, Texas, Washington, and Wyoming.

With all of the O&G wells drilled within the U.S. and the world, great opportunities exist for companies to take advantage of existing produced hot water or by reentering formations and fields that are known to harbor large quantities of hot water that can be produced for geothermal energy. This does not mean leaving oil and gas production behind. Rather this provides the opportunity to produce additional O&G reserves that are of smaller quantity and lesser value upfront with the ability to produce hot water for a longer term geothermal resource cash flow, a resource that provides steady baseload power. Thus a new profit stream is waiting to be fully investigated and developed by those with vision to the future.

For a complete version of the above, see the Committee’s Annual Report (May 2013) on the EMD Members Only page (log-in required).

If you would like to learn more about geothermal energy or to receive information on geothermal energy, or on activities of the EMD Geothermal Energy Committee, join the EMD http://emd.aapg.org/emdApplication.pdf . If you are already an EMD Member, see “Members Only Pagehttp://emd.aapg.org/members_only/geothermal/index.cfm for updates on geothermal energy, for links to technical information on geothermal energy, and for related environmental information that may impact geothermal energy.

For further information on this committee’s activities, go to the Members’ Only Web page or contact:

Richard J. Erdlac, Jr., Ph.D., P.G.
Acting Chair
Email
432-699-5288

AAPG Explorer

June 2011 Geothermal Potential Touted
Explorer Correspondent, Barry Friedman
November 2010: Tapping Into Geothermal a Hot Idea
EMD Column by Richard J. Erdlac
August 2007: Non-Traditional Interest Increasing, Interest in Geothermal Heats Up
EMD Column by J. J. Renner

AAPG Hedberg Conference: Enhanced Geothermal Systems