This invention relates generally to the field of geothermal steam production and more particularly to methods for shutting-in completed, but temporarily unused, geothermal steam wells.
Large subterranean reservoirs of naturally-occurring steam and/or hot aqueous liquid can be found in many regions of the world. Such reservoirs of geothermal stream and water or brine are particularly prevalent in regions where the thermal gradient near the earth's surface is abnormally high, as in regions of volcanic, geyser and fumarole activity, for example, along the rim of the Pacific Ocean.
In some areas, where readily available and conveniently located, geothermal steam and water or brine have for some time been advantageously used for therapeutic purposes, industrial processes and direct heating. Although there is current interest in further developing geothermal resources for these purposes, the principal effort has more recently been directed towards developing geothermal resources for producing electric power, the use of which is far less site-restricted than is the more direct use of geothermal fluids for the above-mentioned purposes. In particular, increases in hydrocarbon fuel costs and actual or threatened shortages of hydrocarbon fuels have greatly heightened interest in developing alternative fuel sources, including the use of geothermal fluids for electric power generation.
General techniques are known whereby geothermal fluids can be used to generate electric power. For example, geothermal steam can be used in substantially the same manner as boiler-generated steam to drive a steam turbine/electric generator combination. Pressurized geothermal water or brine, at a temperature above about 400.degree. F., can be flashed to a lower pressure to extract steam which is then used to drive a steam turbine and generator. Lower temperature geothermal water or brine can be used in a closed loop binary system to vaporize a working fluid, the resulting vapor being used to drive a gas turbine and generator.
Use of geothermal steam for production of electric power is the most direct geothermal application and is therefore preferred, being generally easier and less costly than geothermal water or brine for power generation. Consequently, although commercially usable sources of geothermal steam are estimated to be only about one fifth as prevalent as those of geothermal water or brine, considerable effort has been and is being directed towards developing new, or expanding existing, geothermal steam power plants. As a point of reference, an estimated five percent of the electric power generated in California is now being geothermally generated at The Geysers.
Continued development of geothermal steam for electric power production, in such locations as The Geysers, requires the building of new power plants and annual drilling of many geothermal steam wells for providing steam to these new power plants. By way of illustration, since about 20 pounds of geothermal steam is, on the average, required for each kilowatt-hour of electric power produced, a typical 100 megawatt geothermal steam power plant requires about two million pounds of geothermal steam per hour. As good geothermal steam wells usually produce between about 150,000 and 200,000 pounds of steam per hour, each such typical geothermal steam power plant requires between about 10 and 15 geothermal steam wells for supplying steam.
Most geothermal steam wells require extensive drilling times and relatively high costs before they can be put into production. The high well drilling cost and comparatively long drilling time reflect the severe problems often encountered in drilling geothermal steam wells. The problems include the penetration of difficult geological formations, high well temperatures (typically about or above 500.degree. F.), corrosive and abrasive characteristics of the air drilling process normally used in combination with the hot steam encountered, and the frequently remote and poorly accessible drill site locations.
Because 10 to 15 geothermal steam wells are typically required for each new geothermal steam power plant involving high drilling costs and long drilling times, the drilling operations are usually spread over several years, for example, over the 3 to 5 year construction time of the related power plant. Although a protracted well drilling operation of this nature is advantageous from standpoints of capital outlay and optimum utilization of drilling equipment, completed geothermal steam wells must stand idle for long periods of time awaiting completion of the full complement necessary to make the power plant, typically at least about a year and sometimes as long as four years. Problems are thereby created, particularly in keeping the wells in operational condition without substantial steam loss or violation of air pollution standards that arise from inherent geothermal steam characteristics.
Typically geothermal steam contains noxious gases that contribute to air pollution if vented to the atmosphere. Especially, the carbon dioxide, hydrogen sulfide and ammonia present in geothermal steam cause pollution when vented to the atmosphere and corrode the wellbore when steam condensate forms under conditions of shut-in.
Moreover, unvented static wells, especially those shut-in for more than twelve hours, develop high levels of hydrogen sulfide so that all static, unvented wells must be considered hazardous to work crews charged with the responsibility of initiating work upon the wells. Any cold wellhead is, therefore, to be considered extremely dangerous if opened up, or even if a fitting is accidentally broken off.
The bottom 2,000 to 3,000 feet of most geothermal steam wells in the steam-producing zone are ordinarily uncased, or "barefoot," to enable high steam extraction rates necessary for efficient energy production. When geothermal steam wells of this type are shut-in after completion and before use, so as to conserve steam and prevent air pollution, steam entering the lower, uncased well region from the surrounding formation rises in the borehole and condenses in cooler, upper borehole regions. As the resulting condensate flows back down the borehole, rocks and other debris along the uncased well region are fractured, loosened, and washed down into the bottom, steam production zone. These fallen rocks and debris, as well as the condensate itself, soon fill the steam-producing zone and "kill" the well. Before being later operatively connected to a power plant, the well requires reworking with a drilling rig at a typical cost of about $150,000 or more per well.
An additional problem results when carbon dioxide gas present in geothermal steam dissolves in the condensate that accumulates in a shut-in well. During normal production, acidic carbon-dioxide-enriched condensate cannot form due to the relatively low partial pressure of carbon dioxide in the steam, the low solubility of carbon dioxide at high temperature, and the pH buffering action of the ammonia present. However, carbon dioxide is readily soluble in hot water so that the head of condensate which forms in a totally static well becomes sufficiently acidic to result in damaged wellhead piping and equipment.
To avoid the high costs associated with reconditioning steam wells, most completed, but idle, geothermal steam wells have heretofore continuously vented an amount of geothermal steam sufficient to prevent well damage by steam condensation in the well. That is, sufficient steam has been vented from the wellhead of idle steam wells to prevent accumulation of condensate and non-condensable vapors in the wellbores. The amount of geothermal steam required for this purpose, of course, varies from well to well and according to the quality of the steam, but has been found to be typically between about 200 and 30,000 pounds per hour.
Venting of steam from geothermal wells to prevent condensation damage, although usually satisfactory for its intended purpose, not only wastes steam but, more importantly, causes air pollution problems which in many areas threaten its continued practice. Hydrogen sulfide is virtually always present in geothermal steam due, at least in part, it is believed, to action of anaerobic bacteria on sulfides naturally present in the ground. The hydrogen sulfide concentration of the vented geothermal steam is typically in a range of between about 40 and 1,000 parts per million, which is usually higher than the point source hydrogen sulfide emission standards of between about 1 and 4.4 pounds per hour per vent applicable in many locations.
Although such strict hydrogen sulfide emission standards have not been uniformly enforced in the past, as the number of geothermal steam wells drilled increases and their intrusion into populated and/or environmentally protected localities grows, more rigorous enforcement of these emission standards is virtually certain. The expected result is that venting of geothermal steam wells to prevent condensation damage may soon be prohibited in many areas unless costly hydrogen sulfide abatement processes are provided.
Similar strict hydrogen sulfide emission standards are also usually applied to "used" steam discharged into the atmosphere from operational geothermal power plants and to the large scale venting, or "stacking," of geothermal steam during brief periods of power plant shutdown or slowdown. However, because of the large amounts of steam and hydrogen sulfide involved and the high cost of the power plant, expensive and complete hydrogen sulfide removal facilities of a permanent nature are feasible and are normally provided.
Unfortunately, facilities of the type used for treating large volumes of steam discharged from geothermal steam power plants, and which may, for example, utilize a hydrogen sulfide removal process such as that disclosed in U.S. Pat. No. 4,283,379 to Fenton et al., are not economically adaptable to removing hydrogen sulfide from the relatively much smaller quantities of steam vented in numerous, isolated locations from idle steam wells to prevent condensation damage.
The strict emission standards are usually also applied to hydrogen sulfide emissions in escaping drilling gas and steam during actual geothermal steam well drilling operations. Because processes and apparatus used for power plant hydrogen sulfide abatement have also not been found economically adaptable for well drilling operations, other hydrogen sulfide abatement processes have been developed for this purpose. One such hydrogen sulfide abatement process particularly useful for geothermal steam well drilling operations is disclosed in U.S. Pat. No. 4,151,760 to Woertz. Although the process disclosed by Woertz has been determined to be effective for removing hydrogen sulfide from emissions during steam well drilling operations and to be comparatively economical for this purpose, it is not economically attractive for abating hydrogen sulfide emissions from vented, idle steam wells.
Another method especially directed towards capping or shutting in completed geothermal steam wells during periods of well nonuse, for example, between well completion and connection of the well to an operational geothermal steam electric power plant, is disclosed by Lieffers, et al. in U.S. Pat. 4,407,366. The well is sealed off, preferably at the wellhead, and a gas other than steam is injected into the well at a rate that substantially prevents condensation of geothermal steam in the well during the period of nonuse. The capping gas may be a gas having a molecular weight lower than geothermal steam such as inert helium or one having a higher molecular weight such as inert nitrogen. The disadvantages of this method are that the capping gas is expensive and inconvenient to administer.
In essence, then, the prior techniques for shutting in a geothermal well present no viable options. Either one bears the costs and inconvenience of inert gas capping or one risks carbon dioxide corrosion and/or condensation in the wellbore "killing" the well. Alternatively, one can vent the well if costly methods for removing hydrogen sulfide and other noxious gases from effluents are instituted.
It is, therefore, a major object of this invention to provide an economical, non-hazardous, and non-corrosive method for eliminating venting of geothermal wells under static conditions without causing condensation in the wellbore and subsequent "killing" of the wellbore. Other objects and advantages will become apparent in view of the following description.