The use of compact drill rigs would help make geothermal resources more cost-competitive as a power-generating fuel. Last month, this article covered compact drill requirements relative to rig style, depth capabilities, coring capabilities and the rig's mast. We pick it up here, starting with primary power for the rig.

The use of compact drill rigs would help make geothermal resources more cost-competitive as a power-generating fuel. Additionally, environmental concerns make smaller rigs more attractive. Last month, this article covered compact drill requirements relative to rig style, depth capabilities, coring capabilities and the rig's mast. We pick it up here, starting with primary power for the rig.

Primary Power

Power for the drawworks, the top-head drive (THD) and rig floor needs probably should be supplied by a DC electric motor with a silicon-controlled rectifier (SCR). Some engineers believe that AC motors now are being built that are as versatile as the DC motors; accordingly, some consideration should also be given to use of the less expensive, less complex AC systems. In the long run, weight and size considerations may favor diesel/hydraulic systems over electric motors.

Approximately 1,000 horsepower (HP) should be adequate to run the drawworks, a THD and floor devices. The speed and torque flexibility of DC/SCR is a desirable advantage as is the smaller size of the motor. If, in addition, this prime mover were to be used to run four 400-HP mud pumps, at least 2,600 HP should be planned. If compressors/boosters are to be electric powered, then up to 4,000 HP might be needed. It seems logical to assume that an electrically powered THD would be vulnerable to serious damage or destruction if live steam were to escape the hole and come into contact with the THD. It should be noted that live steam also could seriously damage or ruin hoses used to run a hydraulic THD.

Though SCRs are quite reliable and long-lasting, there is a significant risk that a lot of time would be lost in case of an SCR breakdown. An SCR system is not something that can be fixed by an average mechanic; considerable specialized training is required.

Torque

It should be anticipated that the drilling system will be subjected to severe torque shocks when penetrating the variably hard/soft materials and/or through highly fractured rocks of any type. The THD should therefore have a torque capability rating of at least 20,000 foot-pounds to 30,000 foot-pounds. As a guide, the torque rating of a rig designed to drive a 27.5-inch rotary table can be used. This requirement is now being met and exceeded by hydraulic THDs in use on drills run by a number of contractors.

Overpull vs. Jacking

When drill rod gets stuck, fishing is necessary, casing needs to be pulled out or when other drilling problems occur, the mast, rigging and drawworks must be able to support the weight of the heaviest drill string or casing string. For safety sake, an overpull capability of 25 percent to 33 percent should be provided. Assuming use of 6,000 feet of 47-pound-per-foot 95/8-inch casing having a weight of 282,000 pounds in air, 25 percent to 33 percent overpull capacity would be 352,000 pounds to 375,000 pounds. The mast rating, with a 10 percent safety factor, should therefore be 388,000 pounds to 413,000 pounds.

Some “old school” drillers like to have the ability to “pull their drill pipe in half.” Others like to be able to pull the weight of their heaviest tubular, full of water or drilling mud, out of a dry hole. This is all well and good, but the cost ­ in money and in rig weight ­ of providing such overpull capacity is extraordinary.

An alternative to provision of overpull capacity is to plan to jack stuck tubulars out of trouble. A four-jack system can be designed to put pressure on the substructure bottom beams or on the rig floor (with jacks pointing either up or down). The hydraulic system needed to put hundreds of tons of upward pressure on wedged tubulars is relatively small and inexpensive and the effect can be dramatic. It also is possible to apply some torque to the tubulars while jacking using the THD. Jacking is slow, but it can work, and after the tubulars are loosened, they can be pulled out of the hole rapidly using the drawworks or THD rams. The planned use of jacks can eliminate the need to design a compact rig with a pulling capacity of 400,000 pounds or more.

It should be noted that pulling or jacking alone, without simultaneous rotation, might not free stuck pipe. For this reason, sound arguments can be made against having too much pulling capacity and for drilling carefully to avoid getting stuck.

Rotating Drive

Despite a cost that is greater than that of rotary-table/kelly systems, the use of a single- or double-ram hydraulic THD is recommended. Its main advantages over the conventional rotary table drive are that the drill string does not have to be pulled back while a new joint is added, and that rotation and well control always are possible. Though circulation still is interrupted during addition of a new stand of pipe, use of the THD eliminates the possibility of cave-ins that commonly occur in incompetent rock when the kelly is raised.

Though electrically driven THDs are dominant in the petroleum industry, they may not be the best choice for geothermal applications because of their potential vulnerability to damage by escaping steam. The hoses used in hydraulically driven THDs can also be damaged by steam, but their protection and/or replacement is easier and less expensive than for electric systems. THDs are available from at least eight manufacturers, but TESCO Drilling Technology and VARCO Drilling Systems have geothermal application experience and make both electric and hydraulic units. Both companies are based in Houston, Texas.

Rotary Table

Though, strictly speaking, there is no need for a rotary table when a THD is used, consideration has been given by some geothermists to the merits of having a 27.5-inch table on a compact rig. Their primary reasoning is that it could be used for very minor rotational adjustments.

Such a table would be very heavy and would require a heavy, bulky drive mechanism on the drill. If minor rotational adjustments are required, e.g., during directional drilling operations, they can be accurately made with today's modern THDs or with the rig's power tongs. Accordingly, the inclusion of a rotary table is not recommended.

BOPE Accommodation

It would be prudent to anticipate the need to accommodate a blow-out preventer equipment (BOPE) stack, including ­ from the bottom up ­ a single-gate pipe ram, a double-gate complete shut-off (CSO) ram, a banjo box or flow tee, an annular (bag) preventer and a rotating head. Despite the fact that pipe and CSO rams now are thinner that they were several years ago, at least 18 feet (preferably 20 ft.) of room should be allowed beneath the rig floor for BOPE stack accommodation. With the use of a THD eliminating the drilling kelly, an annular BOP no longer is needed to seal around its square or hexagonal cross-section, but it probably is desirable to keep an annular BOP in the stack in case it has to be used for stripping pipe or casing or to close over large diameter casing, a drill collar or the bottom hole assembly. ND

This article is provided through the courtesy of the United States Department of Energy's Geothermal Technologies Office.