There are two major reasons for pursuing the concept of slimholes for geothermal projects: exploration and small power plants.

Geothermal Exploration

Drilling costs associated with exploration and reservoir assessment are a major barrier limiting expansion of proven geothermal reserves. The geothermal industry (utilities and geothermal operators) needs to reduce exploration and reservoir assessment costs to be competitive in meeting the growing requirements for environmentally benign, alternative energy sources. Typical geothermal exploration comprises drilling a large-diameter, production-size well and, if it shows the presence of fluid and high temperature, producing steam or brine from it while measuring the fluid temperature and, ideally, downhole pressure. These flow tests, which usually last for days to weeks, directly evaluate the energy or enthalpy output of the well and indicate whether the reservoir is drawn down significantly over the course of the test. From this information, potential power-plant output and reservoir life can be estimated.

This method has major disadvantages. It is very expensive and there is significant environmental impact from the roads, large drill sites and fluid-handling requirements. And if the operator hopes to turn an exploration well into a production well, it may be located at the fringe of the resource, where it is not convenient for eventual construction of the power plant.

Drilling production-size holes for geothermal exploration also puts a large expense at the beginning of the project, and thus requires a long period of debt service before those costs can be recaptured from power sales. If a reservoir is defined and proved with slimholes, production well drilling can be delayed until the power plant is under construction, saving years of interest payments. High exploration costs also limit the number of poten-tial new resource areas that can be evaluated, preventing significant expansion in the nat-ion's proven geothermal reserves.

Tradi-tionally, diamond-cored "slimholes" (usually 3 inches to 4 inches in diameter) have been used to measure temperature gradients while selecting sites for production-size exploratory geothermal wells. If it can be shown, however, that improved testing in slimholes reliably predicts productivity and identifies a useful geothermal resource, this could eliminate or greatly reduce the need for large-diameter exploration wells. The cost savings and reduced environmental impact of this approach, compared to drilling production-size holes, are compelling incentives to use slimhole technology.

Drilling is cheaper for slimholes than for production wells because the rigs, casing and cementing, crews, locations, and drilling fluid requirements are all smaller because site preparation and road construction in remote areas is significantly reduced, up to and including the use of helicopter-portable rigs; and because it isn't necessary to repair lost-circulation zones before drilling ahead. For comparison, a 3.9-inch slimhole drilled at one project (including all testing and overhead), cost approximately $150/ft., while a neighboring production well (12 1?in. production diameter) cost $377/ft. to its total depth of 740 feet. Although the slimhole's greater total depth reduced its overall cost per foot, the intermediate cost of drilling the slimhole to the same depth as the large well was less than 60 percent of the large well's total cost. Similarly, another slimhole drilled to the same depth as, and within two miles of, a rotary-drilled exploration hole showed that the rotary hole cost approximately 40 percent more.

Sandia National Laboratories began an investigation to establish the basic feasibility of slimhole exploration with in-house analyses and, later, field experiments on existing geothermal coreholes. At the same time, there was an extensive survey of the geothermal industry to define its needs and priorities. Drilling costs associated with exploration and reservoir assessment are a major factor affecting future geothermal development. Industry contacts specified lower-cost exploration as a high priority, and they generally were enthusiastic over the slimhole idea. For this to be a valid exploration method, however, it is necessary to demonstrate that slimholes produce data that provide a prediction of reservoir productivity equivalent to results from full-size wells. Because of the different flow characteristics in slimholes, there was some doubt among industry representatives about the validity of slimhole testing, or even that slimholes could be made to discharge. Experience in the United States and Japan shows conclusively that slimhole flow tests are practical, but early in the program their predictive value was less clear.

In considering the flow of fluid through the formation and into the wellbore, there is friction loss both in the formation and in the wellbore. In a low-permeability reservoir, the mass-flow rate is limited by the friction loss in the formation, so wellbore size has little effect on the total flow. In high-permeability reservoirs, which are of the most interest, the restriction of the wellbore may be the factor that limits flow from the well. Slimholes have a higher circumference/area ratio than larger wells, so friction losses are more significant in slimholes. Demonstrating - through analysis, modeling and field data - that flow in a slimhole can be accurately extrapolated to predict production in a large well has been one of the principal thrusts of Sandia's slimhole research and development.

Although the vast majority of drilling technology used in the geothermal industry is derived from the oil and gas industry, geothermal requirements are qualitatively different. There are hard, abrasive and fractured rocks; high temperatures; and underpressured formations, frequently containing corrosive fluids. All these factors create a harsher environment than normally found in oil and gas drilling. The service and drilling tool industries have little incentive to address these problems, since the number of geothermal wells drilled in a year is about 0.1 percent of the corresponding number for oil and gas. This lack of commercial research and development is the primary rationale for Department of Energy's support of geothermal technology development. Once demonstrated, slimhole drilling technology will have application to geothermal exploration and reservoir assessment in both the domestic and international markets.

Small Power Plants

A huge shift in world energy demand is under way. According to at least one scenario, the growth in energy consumption in development countries between 2000 and 2010 will be greater than today's consumption in Western Europe, and more electrical generating capacity will be built in the next 25 years than was built in the previous century. Geothermal resources, indigenous and environmentally benign, now meet a part of this growing demand, but it is primarily through large power plants that do not serve the market niche considered here.

The conventional notion of geothermally-produced electricity involves large, multi-megawatt (MW) power plants. The World Bank currently considers 5MW plants as "mini-geothermal," but there is a significant market niche for much smaller units - down to the 100- to 1000-kW range. Analyses indicate that holes as small as 3 inches in diameter can drive a 100-kW generator, and somewhat larger holes (but still "slimholes" because they are not greater than our arbitrary limit of 6 inches diameter) can produce well over 1,000 kW. In remote areas, far from the utility grid, third-world villages and facilities such as hospitals, pumps for potable water and long-term mining operations can replace their diesel generators with small-scale geothermal power plants (SGPP) drawing hot fluids from slimhole wells. The cost of getting fuel to remote diesel generators drives those electricity prices above $.50/kWh, but it is estimated that an SGPP - for example, a 300-kW unit using 120 degrees C brine - can produce electricity for about $.11/kWh, even without the cost savings from slimhole drilling. In addition to the advantage of price, SGPP are far more environmentally benign than fossil-burning plants, which is crucial in view of current climate-change concerns and burgeoning electricity demand in the less-developed countries. It thus appears that a significant market opportunity exists for slimhole drilling in support of small geothermal power plants.

Dynatec Drilling Services Division, headquartered in Salt Lake City, has drilled geothermal wells throughout the world, to depths in excess of 8,250 feet and to temperatures in excess of 570 degrees F.

Typical Concerns

Problems described here are not unique to slimholes, but their frequency and relative importance may be different from conventional drilling. Three general situations are covered: fluid-related effects, unplanned reduction in hole size and pressure control.

Fluid-related effects - Drilling fluids are especially important in slimholes, specifically wireline cored holes, for two principle reasons: the percentage of very fine cuttings in diamond drilling is much higher than in rotary drilling, and the small hole/annulus means that both flow rate and total circulating volume of fluid are much smaller than with big rigs. Because of the fine cuttings and small volume, it is critical to keep close control over the solids content of the mud. Failure to do so can lead to differential sticking, mud rings in the drill pipe and accelerated wear in the pumps.

The drilling fluid also must provide enough lubricity to the drill string to avoid high friction and the consequent downhole vibration. This vibration not only is detrimental to the drilling assembly, but often requires the drill string to be turned at abnormally low rotary speeds, thereby reducing the rate of penetration.

Finally, because cement often is the treatment of choice for lost circulation, the mud can quickly become loaded with drilled cement, which thickens the fluid. In many cases, the only remedy for this is to replace the mud with a fresh batch.

Unexpected reduction in hole size - The casing program for any borehole typically is designed from the bottom up. That is, the final hole diameter is chosen and the upper casing sizes are then determined from the number of strings that have to be set to reach target depth. The number of strings usually is based on: 1) the best estimate of setting depths that can be reached with assumed conditions of hole stability at that location, 2) aquifers that need to be sealed and 3) regulatory requirements.

In planning a conventional rotary drilling project, many engineers routinely make the upper hole large enough for a "contingency" casing string, so that if hole problems require setting an unplanned string, the required final hole diameter still can be achieved. If it turns out that the contingency string isn't needed, casing and cement costs are higher than they should have been because the upper casing strings are bigger than they would have been otherwise, but some operators feel that this is an acceptable risk.

In most slimholes, there is much less chance of a contingency string, because the range of possible casing sizes is smaller. In wireline coring, drill rods and bits are sized so that a given size rod will just pass the next smaller size bit. (For example an "H" size drilling assembly has a 3.85-inch bit with rods that are 3.5 inches OD by 3.060 inches ID. The next smaller size is "N", which has a 2.97-inch bit.) For a typical geothermal exploratory slimhole, hole size at target depth usually is designed to be "H," so if there is a problem that requires leaving a string of H-rods in the hole as casing, drilling can continue with "N" size (or composite N/CHD76) equipment. At that point, the hole still is usable because injection tests are possible and it will pass most logging tools that would have fit into H-rod. Further reduction, however, usually is not practical because the hole is becoming too small to be useful.

It is not clear, however, that inability to reduce size is a common problem. In oil and gas exploration, it is estimated that a contingency string is only used in about 5 percent of the wells, so if slimholes reduce cost by significant amounts, they will readily pay for the occasional times when lack of the size-reduction option results in the loss of a borehole.

Pressure control - An essential part of any drilling operation is control of wellbore pressure. There is a major difference, however, between exploring for geothermal resources and for oil and gas. With hydrocarbons, high downhole pressures almost always are the result of formation geometries that trap fluids under some portion of the lithostatic burden. These pressures are controlled either by closing the annulus with a rotating head (a rotary seal around the drill pipe) or by increasing the mud weight to raise the static head in the wellbore higher than the fluid's pore pressure.

In geothermal drilling, formations normally are under-pressured (pore pressure is less than hydrostatic) and high downhole pressures are the result of high temperature causing either formation fluids or drilling fluids to flash into steam. If the hole is in a production zone that shouldn't be cooled, drilling can continue with heavier fluid (usually brine, because small annulus isn't good for barite-weighted mud) or a rotating head, but higher in the hole, a simpler and more common practice is to use cooler drilling fluid or to "kill" the well by pumping cold water into it. These methods usually serve to control pressure, but the driller must always have available geothermal blow-out prevention equipment (BOPE), which essentially is identical to conventional oil and gas equipment, but with high-temperature elastomers and seals in case control of the well is lost. BOPE for geothermal slimholes also is smaller and less expensive than in conventional drilling.

Detection of a "kick" (formation fluid influx to the wellbore) has been detected in large-diameter wells in the past by monitoring pit volume for gain. Although this still is done, flow measurements on the inflow and outflow lines are much more accurate and give much quicker identification of the kick - because of the smaller wellbore volumes, this is even more important in slimholes. Sandia has developed in- and out-flow meters for immediate detection of lost circulation, but they serve just as well for the identification of kicks.

If uncontrolled flow up the wellbore does begin, the narrow annulus has both negative and positive effects. It means that, for equal volumes of gas or steam introduced into the hole, the effect is seen at the surface more quickly than with conventional drilling, but the smaller clearance does have a self-choking effect on the flow. An analysis of slimhole well-control describes a dynamic process of pumping more fluid to increase the pressure drop in the annulus and thus raise the bottomhole pressure. This may not be a viable option in geothermal drilling, but it is still true that flush-joint drillpipe which has a diameter close to the hole diameter will retard flow.

Another aspect of pressure control is potential flow inside the drillpipe. Because the area inside the pipe is greater than the annulus, this actually is a more important problem. The times of greatest risk are those when the hole is "swabbed," that is, either the drill pipe or the core barrel is being rapidly withdrawn from the hole, which tends to reduce the weight of the fluid column and, thus, the bottomhole pressure. In addition to controlling the speed with which the pipe or barrel is withdrawn, the core driller can use a wireline pack-off and a loading chamber (similar to a lubricator) when retrieving the core barrel, and can use a pump-down latch-head with inverted check valve (which fits in the top of the core barrel and lets fluid flow down, draining the pipe, but not up) when tripping pipe.

In summary, even though a slimhole's small annulus means that pressure effects are felt more quickly at the surface than in conventional drilling, currently available technology, including flow-measuring equipment, is completely adequate to handle pressure control for geothermal slimholes.

Sidebar: Benefits of Geothermal

  • Provides clean and safe energy using little land

  • Is renewable and sustainable

  • Generates continuous, reliable and baseload power

  • Conserves fossil fuels and contributes to diversity ies

  • Offers modular, incremental development and village power to remote sites.

Basic diagram of a typical closed loop system.

Sidebar: Types of Geothermal Systems

Geothermal systems use the earth as a heat source and heat sink. A series of pipes, commonly called a "loop," carry a fluid used to connect the geothermal system's heat pump to the earth.

Loop Types

There are two basic types of loops - closed and open. Open loop systems are the simplest. Used successfully for decades, ground water is drawn from an aquifer through one well, passes through the heat pump's heat exchanger, and is discharged to the same aquifer through a second well at a distance from the first. Generally, 2 to 3 gallons per minute per ton of capacity are necessary for effective heat exchange. Since the temperature of ground water is nearly constant throughout the year, open loops are a popular option in areas where they are permitted. Open loop systems do have some associated challenges:

1. Some local ground water chemical conditions can lead to fouling the heat pump's heat exchanger. Such situations may require precautions to keep carbon dioxide and other gases in solution in the water. Other options include the use of cupronickel heat exchangers and heat exchangers that can be cleaned without introducing chemicals into the ground water.

2. Increasing environmental concerns mean that local officials must be consulted to assure compliance with regulations concerning water use and acceptable water discharge methods. For example, discharge to a sanitary sewer system is rarely acceptable.

Closed loop systems are becoming the most common. When properly installed, they are economical, efficient and reliable. Water (or a water and antifreeze solution) is circulated through a continuous buried pipe. The length of loop piping varies depending on ground temperature, thermal conductivity of the ground, soil moisture and system design. (Some heat pumps work well with larger inlet temperature variations, which allows marginally smaller loops).

Horizontal Loops

Horizontal closed loop installations generally are most cost-effective for small installations, particularly for new construction where sufficient land area is available. These installations involve burying pipe in trenches dug with back-hoes or chain trenchers. Up to six pipes, usually in parallel connections, are buried in each trench, with minimum separations of 1 foot between pipes and 10 feet to 15 feet between trenches.

Vertical Loops

Vertical closed loops are preferred in many situations. For example, most large commercial buildings and schools use vertical loops because the land area required for horizontal loops would be prohibitive. Vertical loops also are used where the soil is too shallow for trenching. Vertical loops also minimize the disturbance to existing landscaping. For vertical closed loop systems, a U-tube (more rarely, two U-tubes) is installed in a well drilled 100 feet to 400 feet deep. Because conditions in the ground may vary greatly, loop lengths can range from 130 feet to 300 feet per ton of heat exchange. Multiple drill holes are required for most installations, where the pipes are generally joined in parallel or series-parallel configurations.

Slinky Loops

Increasingly, "slinky" coils - overlapping coils of polyethylene pipe - are used to increase the heat exchange per foot of trench, but require more pipe per ton of capacity. Two-pipe systems may require 200 feet to 300 feet of trench per ton of nominal heat exchange capacity. The trench length decreases as the number of pipes in the trench increases - or as slinky coil overlap increases.

Pond Loops

Pond closed loops are a special kind of closed loop system. Where there is a pond or stream that is deep enough and with enough flow, closed loop coils can be placed on the pond bottom. Fluid is pumped just as for a conventional closed loop ground system where conditions are suitable, the economics are very attractive and no aquatic system impacts have been shown.

Proven System

Geothermal heating and cooling systems can be connected to the earth in a variety of ways - all thoroughly field proven. However, high performance requires the use of experienced professionals who understand local conditions.

This sidebar is provided through the courtesy of the Geothermal Heat Pump Consortium Inc. A wealth of geothermal information can be found at