Exploration and Development Activities
FREQUENTLY ASKED QUESTIONS

1.  How long does it take to drill a slim observation hole and an exploration well?
2.  Will I see or hear the temporary drilling? 
3.  What will the well sites look like after the drilling is complete?
4.  How big is a drilling crew?
5.  Tell me more about the technical aspects of the slim hole drilling and monitoring.
6.  Tell me more about the technical aspects of the geothermal well drilling testing and monitoring.
7.  What happens once the drilling is completed at a site?

To reduce visibility at night, directional lighting was used during the drilling operations in Basalt Canyon in 2002.

The drilling of each small observation hole is expected to take 25 days, while larger exploration wells generally take 35 days.   Once drilling begins, it is possible that difficulties encountered during drilling may extend the time needed to complete the drilling.   Work is typically performed at the drilling site 24-hours per day, 7-days per week.

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2.  Will I see or hear the temporary drilling? 

Although the drilling area(s) are lighted for safety, special care is taken to avoid the placement or use of lights that may be visible from Highway 395, State Route 203, the town of Mammoth Lakes, or any other area where a substantial number of viewers may be present.  Still, it is expected that the upper-most sections of the drilling rigs will be visible during the temporary drilling periods.  Care is also being taken to minimize noise during drilling, but it is possible that some local residents may hear some distant noise during certain atmospheric conditions during this activity.

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Located on the plant site, this is a production well used to pump geothermal fluid to the power plant.  A finished drilling site will have a canister similar to this one.

Following the completion of drilling activities, all drilling and construction equipment is removed.  Only a small, locked steel canister approximately three feet in diameter and up to six feet high remains on an observation hole site.  The residual geothermal well monitoring facilities consist of a small fenced area and geothermal well-head valves which are generally no higher than 10 feet. 

These are not lighted, are painted to blend in with the landscape and are screened by vegetation to ensure they will not be readily visible from Highway 395, State Route 203, or the town of Mammoth Lakes.

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Mammoth Pacific hires an experienced geothermal drilling company to perform the drilling.  Although as many as a dozen people would work on the job, a typical crew during a shift would be 4-6 people.  The workers generally stay in local hotels during the project.

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5.  Tell me more about the technical aspects of the slim hole drilling and monitoring.

Slim holes are data-gathering holes used to confirm preliminary information concerning the underground geology and temperatures inferred from the geophysical surveys.  Each slim hole drilling site is approximately 120 feet by 120 feet.

Slim Hole Drilling:  Each slim hole is drilled with a small, truck-mounted rotary drill rig or coring rig similar to those used for water well drilling. The rigs are equipped with diesel engines, storage tanks, mud pumps, and other typical auxiliary equipment. During drilling, the top of the drill rig derrick is approximately 30 to 40 feet above the ground surface. Drilling is typically conducted 24-hours per day, 7-days per week.

The drilling program involves a sequence of drilling or “coring” a hole to a selected depth, cementing a steel casing of smaller diameter into the drilled hole, then repeating the process with progressively smaller holes and cemented casings.  This is done at progressively greater depths until the design depth (or the depth selected by the project geologist) is reached.  The initial casing is approximately 8-5/8 inches in diameter.   

The hole is drilled or “cored” using special, non-toxic drilling mud composed of a bentonite clay-water or polymer-water mix. The drilling mud helps to circulate the rock cuttings to the surface where they are removed by the surface equipment. The drilling mud or other drilling fluids would then be re-circulated. Compressed air may be used to reduce the weight of the drilling mud in the hole and assist in carrying the cuttings to the surface in some circumstances. The air, drilling mud, cuttings, and any reservoir fluids brought to the surface are then diverted through a separator/muffler to separate and discharge the air and water vapor to the air and the drilling mud and cuttings to a plastic-lined reserve pit.

Once drilled to the final depth, the drilling mud in the hole is circulated out using water. Steel tubing, typically 2-3/8 inches in diameter and perforated at the bottom, is hung in the hole. The water in the hole is  “bailed” by either lifting with a mechanical bailer (basically a small diameter bucket) or by lifting the water out with air pumped into the hole so that a sample of the geothermal fluid in the reservoir can be obtained for chemical analysis.

Slim Hole Monitoring: Following completion of drilling and bailing, all drilling equipment is removed from the site. The surface facilities remaining on the site consist only of several valves on top of the surface casing, covered by a locked steel canister approximately three feet in diameter and up to six feet high which provides protection for the valves. The surface facilities/wellhead monitoring equipment is not visible from Highway 395 or State Route 203, and is screened by vegetation and painted in a color to blend with the landscape.

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6.  Tell me more about the technical aspects of the geothermal well drilling testing and monitoring.

The geothermal wells are designed to drill into and flow test the geothermal reservoir to confirm the characteristics of the reservoir and determine if the resource is commercially viable. Each geothermal well drilling site is approximately 200 feet by 300 feet.

This core sample came from drilling at well 12-31 in Basalt Canyon.  This section is from 1,730 feet under the earth's surface, and the fractures indicate significant permeability --- an important requirement for successful geothermal production.

Geothermal Well Drilling: Each geothermal well is drilled with a large rotary drill rig essentially identical to those used to drill oil and gas wells. During drilling, the top of the drill rig derrick is as much as 140 feet above the ground surface, and the rig floor could be 20 to 30 feet above the ground surface. The typical drill rig and associated support equipment (rig floor and stands; draw works; derrick; drill pipe; trailers; mud, fuel and water tanks; diesel generators; air compressors; etc.) is brought to the prepared site on trucks. The typical drilling process takes approximately 35 days; however, difficulties encountered during the drilling process, including the need to re-drill the hole, could double the time required to successfully complete a geothermal well.  Drilling is typically conducted 24-hours per day, 7-days per week.

Like the slim hole drilling process, the geothermal well drilling program involves a sequence of drilling a hole to a selected depth, cementing a steel casing of smaller diameter into the drilled hole, then repeating the process with progressively smaller holes and cemented casings to progressively greater depths until the design depth (or the depth selected by the project geologist) is reached.   However, the size of the geothermal well holes and casing are substantially larger than the slim hole and slim hole casing, typically ranging from a 22-inch casing at the surface, to a 13-3/8-inch slotted liner at the bottom of the well.  During drilling operations, a minimum of 10,000 gallons of cool water and 6 tons of inert, nontoxic, non-hazardous barite (barium sulfate) is stored at the well site for use, if required, in preventing well flow.

The hole is drilled using special, non-toxic, temperature-stable, drilling mud composed of a bentonite clay-water or polymer-water mix. Additional non-hazardous and non-toxic additives are added to the drilling mud as needed to prevent corrosion, increase mud weight, and prevent mud loss. The drilling mud is cycled down the drill pipe, out the drill bit, up the outside of the drill pipe, through the drill rig mud system, and back into the drill pipe. The drilling mud helps to cool and lubricate the drill bit, maintain the well bore, prevent the loss of drilling fluids into or out of the rock formations, and circulate the rock cuttings to the surface. The drill rig mud system acts to remove rock cuttings produced by the drill bit from the drilling mud, and discharges them, along with any waste drilling mud, into a plastic-lined reserve pit.  Additional drilling mud is mixed and added to the mud system as needed to maintain the required quantities.

In some circumstances, compressed air may be added to the drilling mud, or used instead of drilling mud, to reduce the weight of the drilling fluids in the hole and assist in carrying the cuttings to the surface. The air, any drilling mud, rock cuttings, and any reservoir fluids brought to the surface are then diverted through a separator/muffler to separate and discharge the air and water vapor to the air and the drilling mud and cuttings to the reserve pit.

Geothermal Well Testing: Once the slotted liner has been set, and while the drill rig is still over the geothermal well, the residual drilling mud and cuttings are flowed from the well bore and discharged to the reserve pit. This may be followed by one or more short-term flow tests, each lasting from two to four hours and also conducted while the drill rig is over the well. Each test consists of flowing the geothermal fluid into portable steel tanks brought onto the well site while monitoring geothermal fluid temperatures, pressures, flow rates, chemistry and other parameters. An “injectivity” test may also be conducted by injecting the produced geothermal fluid from the steel tanks back into the well and the geothermal reservoir. The drill rig is usually moved from the well site following completion of these short-term test(s).

Elizabeth Wilbrecht of Mammoth Pacific investigates preparation activities at the 12-31 site in Basalt Canyon as drilling equipment and supplies arrive.

One or more long-term flow test(s) of each geothermal well drilled would likely be conducted following the short-term flow test(s) to more accurately determine long‑term well and geothermal reservoir productivity. The long‑term flow test(s), each lasting approximately five days or more, are conducted by either pumping the geothermal fluids from the well through onsite test equipment closed to the atmosphere, or allowing the fluid to flow naturally to the surface, where the produced steam and non-condensable gases, separated from the residual geothermal fluid, would be discharged into the atmosphere. In either case, a surface booster pump would then pump the residually produced geothermal fluid through a temporary pipeline to the other geothermal well, where it would be injected back into the geothermal reservoir. The temporary pipeline would be laid on the surface on the disturbed shoulders of the access roads connecting the two geothermal exploration wells.  The onsite test equipment includes standard flow metering, recording, and sampling apparatus.

Geothermal Well Monitoring: Following completion of geothermal well testing, all drilling and testing equipment is removed from the site. The surface facilities remaining on the site consist only of several valves on top of the surface casing, which are chained and locked and surrounded by an approximately 12-foot by 12-foot by 6-foot high fence to prevent access and vandalism. Pressure and temperature sensors may be installed in the hole at fixed depths to monitor any changes in these parameters over time. This monitoring may be continued indefinitely.

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After drilling operations are completed, liquids from the plastic-lined reserve pit(s) are either evaporated, pumped back down the well or hole, or disposed of in accordance with regulatory requirements.

The solid contents remaining in the reserve pit(s), typically consisting of non-hazardous, non-toxic drilling mud and rock cuttings, are tested.  It would then either be dried on the site and then buried in the on-site reserve pit in conformance with the applicable regulatory requirements, or removed and used as either construction material on private lands, or disposed of in an authorized waste disposal facility.

Upon the completion of well drilling and flow testing, a decision would be made by Mammoth Pacific regarding the commercial potential of each well. If a well is believed to have commercial potential, Mammoth Pacific would seek regulatory approvals to place the well into commercial service.   If a well is judged to not have commercial potential, it may continue to be monitored, or it may be abandoned in conformance with the well-abandonment regulatory requirements of the U.S. Bureau of Land Management and the U.S. Forest Service.  Abandonment of either a slim hole or a geothermal well typically involves plugging the well bore (or hole) with cement.  The well head (and any other equipment) is then removed, the casing cut off well below ground surface, and the well site reclaimed.

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