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2.1.1. The driving depth of the conductor pipe which is specified in the Drilling Programme is established with the following formula:


Hi = Minimum driving depth (m) from seabed

E = Elevation (m) distance from bell nipple and sea level H = Water depth (m)

MW = Maximum mud weight (kg/l) to be used GOVhi = integrated density of sediments (kg/dm3/10m)

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2.1.2. Drive the conductor pipe till the final depth or the refusal point of about 1000-1100 blows/meter.

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2.2. Surface Casing

2.2.1. The depth should be great enough to provide a fracture gradient sufficient enough to allow drilling to the next casing setting point and to provide reasonable assurance that broaching to the surface will not occur in the event of BOP closure to contain a kick.

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2.3. Intermediate Casing

2.3.1. In general practice, drilling is allowed until the mud weight is within 50gr/l of the fracture gradient measured by conducting a leak-off test at the previous casing shoe.

P-1-M-6100 3.3


3.1.1. Evaluate ‘kick tolerance volume’ at the end of each hole section. PL.02.01 2.1

Reference List:

‘Drilling Design Manual’ STAP-P-1-M-6100

‘Casing Design Manual’ STAP-P-1-M-6110

‘Drilling Procedures Manual’ STAP-P-1-M-6140





1.1. The SDE will ensure that the Directional Contractor is provided with all data necessary for an initial well profile.

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1.2. The well deviation diagram (plan and vertical section) is included, along with output tables.

In the case of cluster wells, diagrams and tables for vertical wells are also given.

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1.3. Preliminary Specification

1.3.1. The following information will be specified :

• Surface and target co-ordinates - UTM or geographical

• Local reference co-ordinates - platform centre, slot

• Orientation of the wells bay (if applicable)

• Displacement among the slots (if applicable)

• Consider the wells position in the template, cluster, platform slots

• Expected lithology - with a clear indication of subsea or RKB depths

• Total well TVD - with a clear indication of subsea or RKB depths

• Inclination at target

• Shape and size of target(s) - restrictions, if applicable

• Preliminary casing programme

• Type of drilling fluid

• Potential drilling problems which may affect the directional profile.

• Definitive survey data of all well bores which may constitute a collision risk.

P-1-M-6120 3.5

1.4. Topographic References

1.4.1. Will comprise the following information:

• Reference meridian

• Starting latitude (geographic) N/S

• Starting longitude (geographic) E/W

• Latitude at the targets (geographic) N/S

• Longitude at the targets (geographic) E/W

• Starting latitude (metric) N/S

• Starting longitude (metric) E/W

• Latitude at the targets (metric)

• Longitude at the targets (metric)

• Type of projection

• Semi-major axis

• Squared eccentricity (1/F)

• Central meridian

• False East

• False North

• Scale Factor

P-2-N-6001E 7.1.1

2. DEFINITIONS Reference

2.1. UTM (Universal Transverse of Mercator)

2.1.1. The co-ordinates for each UTM grid sector are given in metres with the origins (i.e. the zero value) at a line 500,000m West of the centre meridian to avoid negative values and at the equator. The co-ordinates are given as Eastings and Northings.

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2.2. Convergence angle

2.2.1. The convergence angle is the angle between UTM North (Grid North) and True North (Geographic North). In carrying out the projection there is some distortion of the axes such that UTM North is slightly offset from Geographic (True) North. This small difference is significant over large distance and so must be taken into account when converting co-ordinates from one system to another.

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2.3. Origin Reference Point

2.3.1. Is the origin which will be used for the horizontal co-ordinates e.g.

latitude and departure of the well to be drilled. This will be the zero point on the horizontal well plan used to plot the well while drilling.

P-1-M-6120 3.2.4 Isolated wells:

The initial ORP will be the planned RKB location.

P-1-M-6120 3.4.3 Template wells:

The ORP is the designated template centre.

P-1-M-6120 3.4.3 Platform wells:

The ORP is the slot area centre or a designated slot.

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2.3.2. Onshore cluster wells:

The ORP is a designated slot.

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2.4. Local Magnetic Declination Correction

2.4.1. The magnetic declination will be individually calculated for each new location.

Since it is a time based measurement, the date used for the calculation will be an estimated mid-point for the drilling operation period. Subsequent surveys will require the re-calculation of magnetic declination if taken more than six months after the well is drilled.

It is obtained from actual geomagnetic field maps.

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3.1. Calculation techniques

3.1.1. Eni-Agip standard survey calculation method:

‘minimum radius of curvature method’

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3.2. Source of survey errors

3.2.1. Algorithm used to calculate position.

3.2.2. Survey tool uncertainty. Tab. OP.02.02-3

Tab. OP.02.02-4 P-1-M-6120 4.4.4

3.3. Surveying requirements

3.3.1. General Surveying Requirements:

• All magnetic surveys will have to be reported after being corrected for magnetic declination. Magnetic declination must be specified.

• For other surveys, ensure that magnetic declination is considered while aligning.

• Gyro survey output does not need to be corrected for magnetic declination.

• The depth of a survey is the survey instrument depth not the bit depth. This applies to MWD and survey tools.

• Azimuth will be referenced to true North.

• Bottom hole location will be extrapolated from the last survey.

This will normally not be more than 30m. To confirm the bottom hole location the dipmeter can be used as it can survey down to around 5m from TD if hole conditions allow.

For drilling purposes ‘depth’ will always be quoted as drilled depth and not confused with wireline depth.

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4.1. Approved surveying tools 4.1.1. Magnetic Survey Tool List

MSS Magnetic single shot (film) MMS Magnetic multishot

EMS Electronic magnetic multishot MWD Measurement while drilling HDT High resolution dipmeter Gyroscopic Survey Tool List

GSS Gyro single shot (film) GMS Gyro multishot

SRG Surface reading gyro

NSG North seeking gyro (FINDER) GCT Guidance continuous tool

FINDS Ferranti Inertial Navigation System

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4.1.2. Survey programme for vertical holes:

• TOTCO will be acceptable only in vertical wells for surface holes if inclination is less then 1.5°

• MSS (magnetic multishot) is the standard.

• MWD will be run if economically and technically justified.

• GSS will not be run below 400m

• In cased hole: gyro multi shot (GMS) is the standard.

• If anticollision is a critical concern the NSG/GTC or the FINDS will be used

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4.2. Factors affecting survey tool selection 4.2.1. Maximum inclination.

Survey accuracy requirement will differ between vertical and deviated wells.

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4.2.2. Casing size.

FINDS inertial surveying system can be run only in 133/8" casing or larger.

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4.2.3. Survey depth.

GSS will not be run deeper than 400m due to excessive drift rates.

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4.2.4. Hole inclination.

Maximum inclination for GSS, GMS and SRG is 70° (stability limit).

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4.2.5. Potential drilling problems:

Differential sticking problems precludes the use of wireline based surveys with drillpipe in open hole (GSS, SRG, MSS and EMS).

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4.2.6. High pressure reservoirs:

In an isolated deviated well, GMS or SRG will be run in the previous casing to establish minimum uncertainty before drilling through a high pressure zone (in case of a blow out and a relief well is required). A more accurate tool (NSG/GCT) may be used for accuracy improvement.

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4.2.7. Temperature limitation.

Maximum borehole temperature must be within specification for the survey tools proposed for the programme.

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4.2.8. Well proximity.

Template/platform wells, which are drilled in the neighbourhood of other wells, must maintain a minimum separation with respect to the other wells. This may require additional surveys (e.g. NSG in drill pipe) more often than with individual wells.

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4.2.9. Survey accuracy.

Installations will be most crowded immediately below the platform/template and will require greater survey accuracy to fix well bore locations. The most accurate tools (FINDS or NSG) may be necessary for minimum uncertainty in critical situations.

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4.2.10. Magnetic Influence.

Magnetic based surveying instruments will not be used, in any situation, as the prime source of well location calculations when within 8m of any adjacent casing string.

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4.2.11. Target size and depth:

The accuracy of the surveying tools used on a well will be such that the total horizontal uncertainty at target depth is reasonable compared to the target size. The smaller and the deeper the target, the more stringent the survey requirements.

P-1-M-6120 4.5.3

4.3. Non-magnetic drill collars requirements 4.3.1. See proper charts in reference documents.

Non-magnetic stabilisers will be the only type permitted for use between NMDC's.

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4.4. Quality control

4.4.1. Magnetic Survey Tools:

Magnetic azimuth values will be considered invalid when the survey instrument is within 8m of an adjacent casing shoe:

When magnetic influence is expected from adjacent casing (or when the well is separated less than 8m horizontally from an adjacent casing string), provision will be made to run a gyro based survey tool on top of the MWD.

Survey repeatability should be within 0.5° inclination and 2° azimuth (above 10ο inclination).

MWD: survey repeatability should be within 0.5° inclination and 2°

azimuth (above 10° inclination).

P-1-M-6120 4.6.1