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Improving the Accuracy ofDirectional Wellbore Surveying in

the Norwegian SeaI. Edvardsen, SPE, University of Troms, Baker Hughes; T. L. Hansen, University of Troms;

M. Gjertsen and H. Wilson, SPE, Baker Hughes

Summary

Time-dependent current fluctuations in the Earths ionospherecause inaccuracies in wellbore directional surveying. These inac-curacies increase at higher latitudes, and although monitoring andcorrection are possible, they become less valid as the distancebetween the monitoring site and the rigsite increases, which is aparticular problem for offshore drillsites. The characteristics ofthe ionosphere currents indicate that the most favorable locationfor monitoring stations is on the same geomagnetic latitude as thedrillsite. Such an arrangement has been used to monitor and cor-rect directional surveys at the Haltenbanken area of the Norwe-gian Sea over a period of approximately 2 years. Haltenbanken isapproximately 200 km west of the Norwegian coast at latitude65N, where magnetic-storm activity can have a significant effecton directional surveying. A monitoring station was set up on thecoast at the same geomagnetic latitude as Haltenbanken. To testthe idea that magnetic disturbances are similar along constantmagnetic latitude, an additional monitoring station was estab-lished 200 km east of the main station. The data broadly con-firmed the hypothesis, although isolated events were observedwhen this was not the case. The challenges of surveying at off-shore sites north of 62N latitude are probably greater than the oiland gas industry is accustomed tobut such challenges willbecome more significant if the Arctic Ocean is opened to drillingoperations. The technique described in this paper may contributeto safer and more-productive offshore operations at high latitudes.

Introduction

Magnetic-measurement-while-drilling (MWD) systems, incorpo-rating three orthogonally mounted magnetometers, are widelyused to steer directional wells and define the as-drilled well path.Knowledge of the local-magnetic-field vector is required to oper-ate such tools. Uncertainty in the reference-field values translatesto uncertainty in recorded azimuth and in calculated position. Thebasic estimates of the reference field are usually obtained from aglobal geomagnetic model. The uncertainty associated with themodeled values can be significantly reduced through geomagneticsurveying of the area and subsequent corrections for local crustalanomalies, commonly referred to as in-field referencing (IFR). Itis possible to further reduce uncertainty by monitoring and cor-

recting for the time-dependent external field that is generated byelectrical currents flowing in the Earths upper atmosphere. How-ever, monitoring of the external field at an offshore location is dif-ficult and expensive, and is currently not performed. Instead,onshore monitoring stations are used to predict what is happeningat the rigsite. The position of the stations relative to the rigsite hasa significant bearing on the validity of the prediction. At low andmiddle geomagnetic latitude, a monitoring station will yield use-ful information even when 1000 km away. In the auroral and sub-auroral zone, which is above 50 geomagnetic latitude in theNorth Sea, the critical distance is much smaller. More impor-

tantly, it becomes strongly dependent on the direction. This asym-metry is caused by the so-called auroral electrojet, an intensecurrent system flowing mainly east/west in the ionosphere. Amethod that requires land-based variometers to be placed on thesame geomagnetic latitude as the drilling site is being evaluatedto determine whether it will provide reliable results for distant off-shore locations at high latitudes.

TheEarths Magnetic Field

The importance of the earths magnetic field as a reference fordirectional drilling has been described in several earlier papers

(Zijsling and Wilson 1989; Russell et al. 1995; Williamson et al.1998; Bang et al. 2009). The geomagnetic reference field B iscommonly described as a vector sum of the main field Bm, thecrustal field Bc, and the disturbance field Bd:

B Bm Bc Bd: 1

Williamson et al. (1998) state that the ability to estimate theeffect of the disturbance-field effects at high latitudes is signifi-cantly less than in the UK region. This is explained by the rapidlychanging pattern and strength of the auroral electrojet. From beingable to estimate the external-magnetic-field variations over a fewhundred kilometers in the North Sea area, the distance decreases totens of kilometers in the auroral zone. However, the study by Tor-kildsen et al. (1997), by use of data from 60 to 79 geographical

latitude, indicated that interpolation techniques can be used up to200 km. To select the optimum location for monitoring stations inthe auroral zone, it is helpful to understand the physics behind theelectrojet. The sun emits plasma that spreads out through the solarsystem as the solar wind. When the solar wind reaches the earthsmagnetosphere it interacts with the geomagnetic field, giving riseto electric currents. Variations in solar activity result in fluctua-tions in these currents. During heightened solar activity, magneticdisturbances normally observed in the auroral zone are experi-enced much farther south. On the way from the sun to the Earth thesolar wind carries along some of the suns magnetic field, calledthe interplanetary magnetic field (IMF) (Cowley 2007). The elec-tric conductivity of the solar wind is high, and thus a magnetic fieldappears as being frozen into the plasma. This effect causes thesolar wind to compress the Earths magnetic field on the day sideand drag it out on the night side, thereby shaping the so-calledmagnetosphere. Furthermore, the IMF will couple to the Earthsmagnetic field when they are antiparalleli.e., IMF pointingsouthward (Brekke 1997). The effect of this coupling is a transportof magnetic flux from the dayside to the tail of the magnetosphereat the nightside. This process continues for several hours beforethe magnetic configuration in the tail breaks down and the fieldreturns to its normal shape. This cyclic process was first describedby Dungey (1961) and is illustrated by Fig. 1 (Johnsen 2011).

The solar wind flowing past the Earths magnetic field givesrise to an electric field in the magnetosphere. Because of highconductivity along the magnetic-field lines, the field maps into thepolar regions of the ionosphere, driving a convection pattern ori-ented along the dawn/dusk direction (lower part in Fig. 1). In the

upper ionosphere, this is only plasma convection, but in the lowerionosphere (the E-layer), the electric conductivity is enhanced and

. . . . . . . . . . . . . . . . . . . . . . . . .

CopyrightVC 2013 Society of Petroleum Engineers

This paper (SPE 159679) was accepted for presentation at the SPE Annual TechnicalConference and Exhibition, San Antonio, Texas, USA, 810 October 2012, and revised forpublication. Original manuscript received for review 5 November 2012. Revised manuscriptreceived for review 28 February 2013. Paper peer approved 26 March 2013.

158 June 2013 SPE Drilling & Completion

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an electric current is generated along with the plasma circulation(Prolss 2004). The return flow at lower latitudes gives rise to cur-rent tracks known as convection or auroral electrojets, which flowparallel to the lines of geomagnetic latitude (Fig. 2).

On the dusk side, there is an eastward electrojet, whereas awestward current flows on the dawn side of the Earth (Campbell2003). The directions of the currents are opposite to the convec-tion pattern. During substorms, the electrojet current is consider-

ably larger and typically more concentrated around midnight thanduring quiet times.

TheChainof ExistingMonitoring Stations in

Norwayand Denmark

Troms Geophysical Observatory (TGO) maintains long-termgeophysical observation series started by the Norwegian Institute

1 2 3 4

Solar Wind 6

1 2 34

5

6

Midnight

Dusk

Noon

7

7

5

Magnetopause

Fig. 1Illustration of the Dungey cycle in the Earths noon/midnight meridian plane. In the main illustration, the sun is to the left.The IMF reconnects to the Earths dipole field at (1), and the opened field lines are peeled back (2 through 4) and reconnected inthe magnetotail (5). Closed flux is returned to the dayside (6 and 7). The magnetopause is indicated by a black dashed line. Thenorthern and southern polar caps are north and south of the red lines indicating the open/closed field line boundary (OCB),respectively. In the inset, the resulting two-cell ionospheric convection is shown. The blue dots correspond to numbered field linesin the main illustration. The auroral oval is represented by the green band and the OCB by the red dotted line.

electric field

auroral electrojet

substorm electrojet

magnetic field

convection cell

12.00 hr

24.00 hr

06.00hr,dawn

18.00

hr,dusk

60

70

80

Fig. 2The convection pattern and electrojet system at high latitudes.

June 2013 SPE Drilling & Completion 159

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of Cosmic Physics around 1930. Today, these tasks are the respon-sibility of the University of Troms, which in 2000 establishedTGO to carry them out. There are three geomagnetic observatoriesthat track the field in absolute sense, Bjrnya (BJO), Troms(TRO) and Dombas (DOB) and 11 variometer stations that con-centrate on recording magnetic disturbances. TGO cooperatesclosely with the geomagnetism group at DTU-Space in Denmark.Adding one variometer and one geomagnetic observatory in Den-mark (BFE), there are 16 magnetometers monitoring the mag-netic-field variations between 79N at Svalbard and 55N inDenmark (Fig. 3).

During the mid-1990s, the Norwegian/Danish magnetometerchain was only half the number it is today. Since then, an increasingdemand for magnetic monitoring for offshore directional drillinghas made possible the expansion to todays number of stations. Allvariometer stations are equipped with standard triaxes fluxgate sen-sors mounted on stable ground at magnetically undisturbed sites.

Electronics, data loggers, and communication equipment are placedin separate nearby buildings. Short-term stability and temperature

drift are minimized, but some long-term drift is accepted. The fieldis sampled every 10 seconds and transmitted to Troms with inter-vals of a few minutes. When a magnetometer is first installed, abso-lute measurements of the field at the site are made to determinesensor orientation and other constants necessary to transform itsoutput to declination, dip, and total field. This procedure is repeatedwhen needed, usually with intervals of a few years. Variometer sta-tions are simple and inexpensive compared with traditional geo-magnetic observatories with frequent absolute calibration and highstability. A good variometer is sufficient for monitoring the accu-racy and stability of directional drilling; the high-precision observa-tories mainly serve as regional points of reference.

Drilling operations in the southern part of the North Sea, at lat-itudes between of 52N and 56N, are well covered by the magne-tometers in Denmark along with the magnetic observatories inScotland and northern Germany. The external field seldom createsmajor difficulties for directional-surveying operations in this area,

but monitoring is still important for quality-control purposes. Theeffect of magnetic substorms increases in the north. Fig. 4 shows

Norwegian Stations, TGO

Danish Stations, DTU

Fig. 3The Norwegian/Danish magnetometer chain.


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how the total field is affected along the coast of Norway during astrong geomagnetic substorm from Karmy at 59 latitude toNordkapp at 71. In this region, continuous monitoring and, ifpossible, correction for the disturbances are advisable. In the pres-ent study, we will focus on the area around 66N.

TheProposedTGOCorrectionMethod

Directional surveying with magnetic MWD in northern waterswill generally benefit from being corrected for variations in theEarths external magnetic field. Correction would be straightfor-ward if it were possible to set up a reference magnetometer withina few kilometers of the drillsite. Because the magnetometer can-

not be mounted on the rig and seabed magnetometers are not eas-ily available and tested in the area of interest, we are forced torely on magnetometers onshore. Along the coast of Norway thatmeans a distance of more than 100 km in most cases. The ques-tion of how far away the reference instrument can be while stillproviding useful information must be raised.

British Geological Survey offers a method known as interpola-tion IFR (IIFR) (Shiells and Kerridge 2000). Here, a simple alter-native procedure is described, for the time being called the TGOCorrection Method.

As pointed out previously, the magnetic field at the drillsitecan conveniently be split into three components: the main fieldfrom Earths core, the crustal field, and the external or disturbancefield originating in the ionosphere and magnetosphere:

B Bm Bc Bd:

2

For the present discussion, we merge the main field and crustalfield into a single quasistatic component, BsBmBc, subjectonly to the slow (but not negligible) secular variation:

B Bs Bd: 3

Similarly, the field observed at the reference site is denoted byvectorR:

R Rs Rd: 4

We now assume BdRd. This crucial assumption will be dealtwith in more detail later, but already a rough comparison ofneighboring stations along the coast of Norway (Fig. 4) points toa coherence in the disturbances over at least 200 km. Thus, wecan write

B Bs Rd: 5

The undisturbed field Bs at the drillsite is not subject to directmeasurement; its value must be taken from a model of Earths field,which normally is adjusted with data from a magnetic survey of thearea (IFR). The disturbance field RdR Rs is derived from themagnetograms at the reference site. Being a difference only, thereis no need for precise absolute observations. Thus, in contrast towhat is claimed for IIFR, there is no need for measurements of ob-servatory standard; the approximate calibrated variometers suffice.

Let X0, Y0, and Z0 be the components of Earths static field(Bs or Rs) in the standard Cartesian coordinate system of. . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

50265

1 2 3 4 5 6

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

D H Z I F Real time

kar

sol

rvk

don

jck

and

tro

sor

nor

200 nT

VTC

51032

51910

52375

52552

53218

53402

53096

53842

F component

06.aug 2011nT.

Magnetometer

Fig. 4Total field output from the Norwegian chain of magnetometers from 6 August 2011.


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geomagnetismX as north, Y as east, and Z as vertically downand recall the expressions for

Total field:

F20 X20 Y

20 Z

20 6

Horizontal field:

H20 X20 Y

20 7

Dip (inclination in the parlance of geomagnetism):

tgI0 Z0

H08

Declination:

tgD0 Y0

X0: 9

The disturbance field is always quite smallat most, a fewpercent in strengthcompared with the static field. Then, theeffects of the disturbance field can be treated as perturbations ofthe main field. Differentiating the expressions for F0, I0, and D0,we obtain the changes DF, DI, and DD in the observed field result-ing from the components Xd, Yd, and Zd ofRd:

DF X0

F0Xd

Y0

F0Yd

Z0

F0Zd 10

DIH0

F20Zd

Z0

F20

X0

H0Xd

Z0

F20

Y0

H0Yd 11

DD X0

H20Yd

Y0

H20Xd

:

12

This means that as long as the static field (the subscript 0) isapproximately the same at the drillsite and the reference site, thecorrections DF, DI, and DD found at the reference site can be

applied directly to the field at the drillsite. If that is not the case,we have to calculate Xd, Yd, and Zd and apply them to Eqs. 6 to12, along with the appropriate values of the static fieldcomponents.

The problem is now reduced to determining Rsin the formof F0, I0, D0 or, if needed, X0, Y0, Z0from the recordings at thereference station. At TGO this is achieved by an automatic pro-cess by use of a least-square-root approximation. For each compo-nent a, a quiet level as is determined so that

Xi

ffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffiffiffijai aqj

q min: 13

The sum is taken over a 24-hour period, and the final level isthe average over 10 subsequent days. Fig. 5 shows the output fordeclination at Dnna from March 2011 to February 2013. Theblue dots are the mean values for quiet days, the horizontal redlines are the quiet values, and the sloping red line is the trend linefor the whole period.

The slope reflects mainly the secular variation, but may alsohave a contribution from sensor drift. Whatever the cause, the dif-ference DDD D0 is not affected. Estimated uncertainties (3rstandard deviations) in corrections are 0.04 in declination, 0.02

for dip, and 7 nT in total field.The accuracy of MWD surveys is normally specified by an

uncertainty model that accounts for all significant error sources. Itis common practice to consider measurement errors that exceedthe models 3-standard-deviation confidence interval to be out ofspecification.

It is desirable to identify surveys acquired when the magnetic-field disturbance level exceeds the error-model specification, and,

. . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . .

0.40

mar apr may jun july aug sep oct nov dec jan

2012 2013

feb mar apr may jun july aug sep oct nov dec jan feb

0.30

0.20

0.10

3.7012

+0.10

+0.20

+0.30

+0.40

Deg.Mean Values

QMN Values

MKM 14.3' year

Fig. 5Quiet days at Dnna from 2011 to 2013.


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if possible, correct it to within the models tolerances. However,although the models include terms that describe the uncertaintyassociated with the magnetic reference field, the contribution thatrelates specifically to the external field is not well defined or

widely agreed upon within the drilling industry. The disturbance-limit values given in Table 1 are derived from the Baker HughesMWD model.

With these figures in mind and by use of investigations and ex-perience derived from data from the Norwegian magnetometernetwork, the following recommendations are made for drillingsites between 62 and 72 geographic latitude:

Corrections derived from a reference magnetometer can besafely applied even under intense magnetic storms up to a dis-tance of roughly 100 km (unfortunately, few fields are that closeto the coastline).

At distances more than 100 km, the asymmetry in the distur-bances between geomagnetic east/west and north/south directionsshould be taken into account. Generally, the spatial correlation is

clearly better east/west than north/south. Preferably, the referencemagnetometer and the drillsite should match within 61 in geo-magnetic latitude.

Given that the geomagnetic latitude condition is fulfilled,corrections can be applied satisfactorily out to distances of at least500 km for weak and moderate magnetic storms (i.e., moststorms). During major storms (a few in every solar cycle), the cor-rections cannot be trusted.

Skarv Case Study

The Skarv-Idun development lies in the Norwegian Sea, atapproximately 65.7N latitude and 200 km west of the Norwegiancoastline. The Skarv-Idun development consists of the Skarv oiland gas-condensate field and the neighboring Idun gas field. Skarv

is in the Haltenbanken area, and a total of 24 wells are eitherdrilled or planned to be drilled. From a geophysical perspective,the Skarv field is close to the Arctic auroral zone. The auroraloval expands during severe magnetic substorms, causing anenhanced auroral electrojet in the vicinity of the drilling site at theSkarv field. The external magnetic field is therefore likely toaffect magnetic MWD measurements taken during drilling. Tomanage the disturbance field at Skarv, a new variometer stationwas established onshore in Norway. The locations of available

variometer stations were not optimal with regard to the Skarvfield. In 2008, the station was set up at the island of Dnna. Dnnalies on the same geomagnetic latitude as the Skarv field, and thedistance between the two locations is approximately 220 km. The-oretically, the auroral electrojet flows along lines of geomagneticlatitude. Therefore, the induced magnetic field should be of simi-lar character along a given geomagnetic latitude for hundreds ofkilometers, as long as the conductivity and strength of the electricfield are homogeneous. To verify this simplified assumption, itwas decided to extend the chain of Norwegian calibrated vario-meters into Sweden. An additional geomagnetic control stationwas established at Jackvik in Sweden, approximately 220 km eastof Dnna. Jackvik was chosen because the spatial geomagneticvariations between Jackvik and Dnna should be similar to thosebetween Dnna and Skarv.

The actual correlation between disturbances at onshore andoffshore locations is, in principle, affected by the difference inelectric conductivity between the land and the sea (Williamsonet al. 1998). The higher conductivity of the sea will tend to dampout the higher frequencies in the magnetic field at sea, with thecutoff frequency decreasing with increasing water depth. By useof the formulas and data of Filloux (1987), we find that the 400 mof water at Skarv will damp out periods shorter than approxi-mately 2 seconds. Considering our magnetic data are one-minuteaverages, the induction effect will be negligible in our case.

Fig. 6 illustrates how the disturbance field Bd is created by the

westward electrojet current I, in accordance with Amperes lawwith Maxwells correction. The effect from the disturbance field

Bd on the main field Bm varies with both locality and distancefrom which the electrojet flows. With the right-hand rule, we seethat locations north of the electrojet current I will experience anincrease in total field intensity as the disturbance field Bd andmain field Bm point in the same direction. The effect will be oppo-site at Rrvik, which is south of the current.

VariomometerComparison

In this analysis, data from five selected variometer stations in Nor-way, Sweden, and Finland, acquired in 2011, have been compared.The locations of the control stations are described in Table 2.

When recordings at Dnna exceeded at least one of the error-

model disturbance limits described in Table 1, data from all siteswere analyzed. Data for declination (DD), dip (DI), and total-field-intensity (DF) variations were analyzed separately. A totalof 56 days were recorded when Dnna exceeded any of thesethree disturbance limits, approximately 15% of the days in 2011.The results are presented in Table 3. The assumption of good cor-relation between monitoring sites on the same geomagnetic lati-tude is confirmed regarding dip angle and total field intensity. Theconformity between Dnna and Jackvik is good. However,

Arcticcircle

Jckvik

Bd

Bm

Bd

DnnaRrvik

Skarv

I

Fig. 6Auroral electrojet.

TABLE1DISTURBANCELIMITS

jDDj () jDIj () jDFj (nT)

0.45 0.18 147


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regarding declination, the correlation is approximately half of thedisturbance limit, but slightly poorer than the numbers from Rr-vik, which is south of Dnna. These numbers will be more reli-able when data from the coming years are included in theanalysis.

Figs. 7 through 9 show a comparison between the relative dis-turbances at Dnna and the variometer stations at Jackvik andRrvik. The correlation reported for a given disturbance at Dnnais the average absolute difference recorded whenever Dnnaexceeds that same disturbance. This also means that data quantityused in the analysis is larger at lower disturbances. The red dottedlines show the error-model disturbance limits at Dnna. Differen-

ces greater than the limit values are recognized as out-of-specifi-cation conditions. The green-shaded area indicates that themagnetic disturbance measured at Dnna is within the error-model disturbance limit.

When the declination disturbances measured at Dnna aregreater than 0.45, the average differences of both Rrvik andJackvik to Dnna are approximately 50% of the error-model dis-turbance limit. At disturbances larger than 0.9 measured atDnna, the average difference to Jackvik exceeds the error-modeldisturbance limit. At Rrvik, this occurs at disturbances largerthan 1.7 measured at Dnna.

The dip-variation comparison clearly indicates homogeneityalong geomagnetic latitude. The average difference between Dnnaand Jackvik is only 0.04 when exceeding the error-model disturb-ance limit. Pello, which is more than 500 km east of Dnna, showsan average difference of less than 0.10.

The figure for total intensity also shows a good correlationbetween Dnna and Jackvik. Disturbances at Dnna up to approx-imately 6700 nT can be corrected with the use of Jackvik dataand still be within the error-model disturbance limit of 147 nT.

Fig. 10 shows the variations in total intensity DF, declinationDD, and dip angel DI for the calibrated variometer stations atDnna, Jackvik, and Rrvik from 0100 to 0400 UTC (UniversalCoordinated Time) on 6 August 2011. In that time interval, thevariometer stations recorded disturbances in the geomagnetic fieldcaused by electrojet currents. From 0100 to 0130 UTC, the DFfigure shows that the main field is affected by a positive disturb-ance field. The electrojet current is then moving northward and

the DF changes sign, first at Rrvik and then at Dnna and Jack-vik. In the period from 0130 to 0230 UTC, the current is directlyoverhead with respect to the area of interest, causing a fluctuatingmagnetic field. Regarding DF and DI, there seem to be some timeshifts between the different locations, but the correlation betweenDnna and Jackvik is quite good. However, DD seems to have thebest correlation between Dnna and Rrvik. This can be ex-plained by a discontinuity effect, caused by the substorm electro-

jet that is the dominant current in the border area around magneticmidnight. In this period, the direction of the current is directedfrom north to south before it splits into the westward and eastwardauroral electrojets.

To illustrate how the TGO correction method will work inpractice, the moments for three imaginary survey stations are

shown in Fig. 10. Earlier in this paper, we showed thatBBsRd.

TABLE 2LATITUDEANDLONGITUDE FORSTATIONS

Station

Geomagnetic

Latitude

Geographic

Latitude

Geomagnetic

Longitude

Geographic

Longitude

Skarv 63.3 65.7 91.7 7.6

Dnna 63.4 66.1 95.8 12.5

Jackvik 63.5 66.4 99.5 17.0

Pello 63.6 66.9 105.4 24.1

Rrvik 62.2 64.9 93.2 11.0

Solund 58.5 61.1 86.1 4.8

TABLE 3COMPARISONWITHDNNA DURINGDISTURBED

PERIODS

Station

Average Values

jDDj (degrees) jDIj (degrees) jDFj (nT)

Jackvik 0.23 0.04 41

Pello 0.44 0.08 84

Rrvik 0.19 0.11 63

Solund 0.50 0.30 182

00.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8

D

3.2 3.6 4.0 4.4 4.8 5.2 5.6 6.0 6.4

0.2

0.4

0.6Averagediff[deg]

Disturbance at Dnna [deg]

0.8

1

1.2

1.4

Jckvik Rrvik

Fig. 7Declination-variation comparison.


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The results are presented in Table 4. Note that the referencefield is corrected for all parameters for Survey Station 2, eventhough only the variation for the dip angle is outside the disturb-ance limit. Survey Station 3 remains uncorrected.

Operational Procedure for HandlingDisturbances

in theExternalMagnetic Field

The Dnna variometer will be used manage external field distur-bances at Skarv. Any disturbance outside the error-model limit(Table 1) will involve a correction for all three magnetic-field ele-

ments. For directional surveying at Skarv, this means that drilling

may continue even during disturbed periods. However, in caseswhen the electrojet current is directly overhead with respect toDnna, a reliable correction may not be available. These periodscan be identified from the magnetogram.

Conclusions

These conclusions apply to drilling locations in the Scandinavianauroral and subauroral zones. Analyses of magnetogram data from several locations support

the theory of similar, and nearly simultaneous, magnetic distur-bances occurring along geomagnetic latitude for several hun-

dreds of kilometers.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

I

0.4

Average

diff[deg]

Disturbance at Dnna [deg]

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Jckvik Rrvik

Fig. 8Dip-variation comparison.

00306090

120

150

180

210

240

270

300

330

360

390

420

450

480

510

540

570

600

630

660

690

720

50

100

150

200

F

250

Average

diff[nT]

Jckvik Rrvik

Disturbance at Dnna [nT]

Fig. 9Total-intensity-variation comparison.


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The locations of monitoring stations, relative to the drillsite, areimportant for the effective monitoring of electrojet currents.

Variations in declination are well correlated north/south, up toapproximately 200 km.

Placing the monitoring station on the same geomagnetic lati-tude as the drillsite provides better correlation of total intensityand dip angle, and allows useful correlation of all three mag-netic-field elements at distances up to 500 km from the rigsite.

Corrections for external-field variations on the basis of remotemonitoring may be less valid within two to three hours of mag-netic midnight because of the effect from substorm electrojet.During this period, the distance for good correlation betweenthe rigsite and monitoring station is reduced to 100 km.

In cases when the electrojet current fluctuation is directly over-head with respect to the rigsite and monitoring station, a reli-able correction may not be available.

The method described for the remote monitoring of the Earthsexternal magnetic field can be usefully applied to distant off-shore drillsites at high latitudes, improving directional controland increasing wellbore-survey accuracy.

Acknowledgments

The authors wish to thank BP, the University of Troms, andBaker Hughes for their permission to publish this paper.

600

450

300

150

0

150

300

450

Dnna

Jackvik

Rrvik

0.40

0.20

0.00

0.20

0.40

0.60

0.80

1.00

Dnna

Jackvik

Rrvik

1.20

0.80

0.40

0.00

0.40

0.80

1.20

1.60

2.00

2.40

2.80

3.20

3.60

D[deg]

I[deg]

F[nT]

Dnna

Jackvi

Rrvik

Survey Station 1 Survey Station 2 Survey Station 3

00:00 00:30 01:00 01:30 02:00 02.30 03:00 03:30 04:00 04:30 05:00 05:30

00:00 00:30 01:00 01:30 02:00 02.30 03:00 03:30 04:00 04:30 05:00 05:30

00:00 00:30 01:00 01:30 02:00 02.30 03:00 03:30 04:00 05:00 05:3004:30

Fig. 10Magnetogram comparison from 6 August 2011.

TABLE 4DECLINATION, DIP,ANDTOTAL INTENSITY FORSURVEYSTATIONS

Survey

Station

Time,

UTC

Declination, D (degrees) Dip, I (degrees) Total Intensity, F (nT)

BS RD B BS RD B BS RD B

1 0111 3.52 0.89 4.41 76.13 0.79 76.92 52,383 230 52,613

2 0245 3.52 0.47 3.99 76.13 0.42 76.55 52,383 123 52,260

3 0430 3.52 0.31 3.52 76.13 0.02 76.13 52,383 77 52,383


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References

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Harry Wilson is currently a technical adviser working in thecompanys drilling services technical support group. He joinedBaker Hughes as a directional surveyor in 1981, and since thenhas held a variety of wellbore-positioning-related posts withinoperations, technical support, and marketing. Wilson qualifiedas a survey engineer in the British Army.

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Inge Edvardsen is pursuing a PhD degree in cooperationbetween Baker Hughes, the University of Troms, and the Nor-wegian Research Council. He joined Baker Hughes in 2002 as asurvey-management engineer and is still working part time inthe survey-management department in Norway. Edvardsenholds a Sivilingenir degree in geodesy andcartography at theNorwegian University of Science and Technology in Trondheim.

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