INTERNATIONAL

Volume 11. No. 2

MARCH 2003

geo

Drillin

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irectional drilling has proved essen-tial to the success of the delineationand exploration of the recently dis-covered mineral resources (SWEX

& SWEX-B) to the southwest of the mainorebody at Tara Mines, Navan in CountyMeath, Ireland. Tara is Europe’s largest zincmine with production in excess of 2.2 Mt perannum. The ore is hosted within Lower Car-boniferous carbonate rocks and occurs as aseries of flat-lying, irregular, strataboundlenses. Delineation of resources had tradi-tionally been carried out by 80 m centred pat-tern drilling from the surface, followed up byfurther closer spaced drilling underground.The discovery in late 1995 of high-grade min-eralisation at depths of over 750 m, 2 kmsouthwest of the mine workings, called for aninnovative approach.

Due to the depth of the targets in theSWEX area, the complex structural settingand the irregular style of the mineralisation,directional drilling was the only way to estab-lish the ore’s continuity, needed to constructreliable geological sections and subsequentlydefine and calculate mineral resources. Otherimportant advantages of the technique overtraditional drilling from surface were the sav-ings in both time and money.

The present exploration-drilling pro-gramme is contracted to Priority Drilling Ltd,formerly a Canadian company, which in theearly 1950s moved to Ireland to cater for themineral exploration explosion, taking place atthat time in the country. Drilling is performed(continually) throughout the year with opera-tions carried out on an 8-10 hour shift, 5-dayweek basis due to local community and noiserestrictions. Up to eight drill rigs, a collectionof customised tractor mounted JKS BoylesBBS 56 and 38 surface drill rigs and one truckmounted JKS Boyles B-25 machine have beenin operation on site at any one time.

Directional drilling was first trialed at Taraduring April 1996. CBC Welnav (a divisionof Christensen Boyles) of Salt Lake City, US,was contracted to carry out an initial phase ofwork using Priority’s drilling equipment andpersonnel. Since June 1996 Priority has itselfbeen operating the directional drilling equip-ment and has successfully completed over 136daughter holes to date.

To successfully drill a fan of holes along sec-tion requires a complete record of the currenthole/holes attributes including positional sur-

veys, casing depths, rock quality, previouswedges etc. A forecast of the number of poten-tial deflections and the approximate coordi-nates for these future targets is also needed. AtTara, it is normal practice to first drill an NQsized vertical “mother” hole through theentire host stratigraphy to establish the geo-logical setting and also the local structure.

On completion the hole is surveyed upfrom bottom to surface to ascertain an accu-rate position and facilitate the calculation ofparameters needed for any subsequent daugh-ter holes. At Tara the non-magnetic characterof the mineralisation and host rocks enabledthe mine to employ the Flexit SmartTool,supplied through Encore Borehole Surveys.Priority also utilises the SmartTool system, inboth single- and multi-shot mode, for all itsorientation and surveying needs; this allows astraightforward exchange of data. The direc-tional drilling methodology employed by Pri-ority at Tara is commonplace in the industry,but modified to suit local conditions. A briefsummary of the practices and equipment usedis described below.

MethodologyAfter the parent hole is completed two

plugs are installed below the kick-off point(KOP) to allow the wedge to be located andthe daughter hole started. The first plug - amechanically expandable metallic “Van Ruth”type - is run into the hole attached to the bot-tom of the rod string. After the metallic plugis in place, a “softwood” timber plug is runinto the hole by means of the rod string andfirmly secured on top of the metallic plug.Once the plugs are set in the hole the wedgekit is assembled and run into the hole. InTara’s case, a non-retrievable Hall-Rowe(Whipstock) type wedge is used. When in thehole the wedge is lowered to approximatelyone meter above the wooden plug and orien-tated, using the SmartTool. This may be donerelative to magnetic North, in vertical holes,or relative to gravity when inclined. Once thewedge is orientated it is securely set in place byinsertion in the wooden plug by means of itssharp blade. Tara’s usual procedure once thewedge has been set is to drill 10-15 m past thewedge using conventional wireline coring toestablish the daughter hole. At this point themagnetic influence of the wedge is eliminatedand the directional drilling equipment can beorientated correctly in the hole. When thenew daughter hole has been established bydrilling past the wedge all the equipment ispulled out of the hole. The motor drillingequipment (1-2 stage, 23/8 Accu-dril with 0.5(bent sub) is assembled and run into the holeattached to the bottom of a BQ rod string.Once on the bottom of the hole the motor isorientated by means of the SmartTool, and in

Directional drilling in mineralexploration - an Irish case study

Tara Mines ExplorationDepartment, Priority Drilling

Ltd and FlexIT SmartTooldrillhole survey system

proved a successfulcombination

D

DIRECTIONAL DRILLING

Schematic geological x-section illustrating drill fangeometry.

Map of Ireland showing location of Tara Mines - thelargest zinc mine in Europe.

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the same way as the wedge. After around every3 m of drilling another orientation measure-ment is taken and if necessary, a correction tothe orientation of the tool is made. Once therequired dip and azimuth are achieved themotor drilling is completed and the BQ rodsand motor drilling assembly are retrieved.The hole is then reamed along the length ofthe ‘navi-run’, which smooths and slightlyenlarges the hole allowing the NQ rods to fol-low the bend. On completion, the reamingequipment is pulled out of the hole and wire-line drilling is resumed, coring the hole to thetarget. Experience has shown that coring afterthe “motor drilling” follows a relativelystraight path in NQ diameter holes. Howeveradditional single-shot surveys are done peri-odically to check the dip and azimuth duringthe conventional drilling stage to ensure thatthe target is reached and no remedial direc-tional drilling is required. On completion amulti-shot survey is run from the bottom upto above the wedge to give an accurate posi-tion and to facilitate the calculations for anysubsequent daughter holes.

At Tara drilling is carried out on sections100-200 m apart with mineralisation inter-sections spaced 40-80 m apart at targetdepth. As this is still exploration drilling,deflections are started comparatively highin the hole, to allow for the possible need tomodify drilling proposals for subsequentdaughter holes due to unpredicted geologi-cal structures or mineralisation. To date themost daughter holes completed from a sin-gle mother hole is 14, with the averagebeing five holes per set-up. The daughterholes are sequentially drilled from verticalto a maximum of 45° at 8-15° increments,which provides a lateral coverage in excessof 500 m from the one set-up. The amountof directional drilling required is min-imised, by drilling successive deflectionsone after another to the “same side” of themother hole.

In addition to the more detailed geologicalpicture attainable by this technique, the timesaving allows the exploration and subsequentsurface delineation of resources to stay com-fortably ahead of mine development. Typi-cally each daughter hole saves four to fiveweeks on conventional wireline drilling fromsurface to a comparable depth. This isachieved by a combination of factors, includ-ing obviously starting the daughter holes 400-500 m below surface and the reduced need forsite moves, which not only alleviates landaccess issues but permits the new daughter-hole to begin immediately the parent-hole issurveyed meaning further time savings.

Since the introduction of directionaldrilling at Tara in 1996, the average depth ofthe KOP (kick off point) for each daughterhole has increased from 450 m to 550 mreflecting a gradational targeting of deeper orefurther to the southwest. In contrast thelength of Navi-run (amount of directionaldrilling) had remained constant at around 60m, but showed an increase in 2001 in anattempt to fan the holes out wider, to givegreater coverage along section. The costs of

directional drilling since inception haveincreased due to new contract agreements anda move to longer navi-runs, but Tara’s annu-alised drilling cost savings remain around13% at between €6,000-€14,000 per hole.

Experience thus farThis article summarises the directional

drilling experience at Tara so far. For Tarathis method was the only way to assess, withsome degree of confidence, the geometry,grades and tonnage of this type of orebody.

The time saving and the ability to accuratelyhit a target at depth, rather than the financialsaving, were the most important advantage ofmotor drilling. However, procedures are regu-larly reviewed in an effort to streamline the sys-tem given there are many variables includinglocal geology, rock conditions, drilling equip-ment and most of all the driller’s experience con-tribute to the success, or failure, of the technique.

Michael McCarthy and Priority Drillingare commended for taking up the challenge ofimplementing directional drilling at Tara, butparticular credit and thanks for the success ofdirectional drilling at Tara must go toMichael Dineen of Priority Drilling Ltd whomore than any other person has made this sys-tem work. In addition past and present TaraMines Exploration staff notably Tom Dono-hue, Mark Holdstock, Nicolas Martinez andBilly O'Keeffe have helped in the adoptionand development of the technique at Tara.

ContactsNicholas Walker, Tara Mines Ltd, Explo-

ration Department: Tel: +353 46 29133, E-mail: nick.walker@outokumpu.com.

Michael McCarthy, Priority Drilling: Tel:+353 905 76117,

michaelmccarthy@prioritydrilling.ieEncore Borehole Surveys: Tel: +44 (0)1546

870676, E-mail: encore@ndirect.co.ukFlexIT AB. Tel: +46 (0)8514 30362, E-mail: info@flexit.se

DIRECTIONAL DRILLING

Running the FlexIT survey tool into the hole, prior tocommencing an end of hole survey.

Tracking the latest drilling with FlexIT's DisplayITsoftware.

Reprinted from geoDrilling International, March 2003 pages 4 & 6

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