How to apply QA/QC to surveying.
Quality assurance and quality control is applied to MWD surreys using the process shown below.
There is some variation between service companies, but the process stated above is followed in all cases. This process represents the underlying basis for all position error modeling, anti-collision scanning, drilling target sizing and other decisions that are made as a result of determining the well’s position. Generally, the quality control criteria and the field acceptance criteria for surveys provide sufficient checks and balances so confidence in the field data is high and this process gives a clear indication when additional checks are required.
MWD Field Acceptance Criteria Field acceptance criteria for SLB MWD surveys:
|G| earth’s gravitational field = Reference +/- 2.5 mg |B| magnetic field = Reference +/- 300nT Magnetic Dip = Reference +/- 0.45°
Good Survey Practice Regular tool calibration Distance between surveys should follow the well plan. The inclination should be within 4˚of the last survey. The azimuth should be within 5˚of last survey (in vertical holes where a
180 degree azimuth change is normal). Non-Mag spacing (EDI azimuth error under 0.5°) No Mag interference from offset wells Checks on the raw data (Gx, Gy, Gz, Bx, By & Bz)
Pre-job and post-job calibration checks,Excessive sensor uncertainties are eliminated by proper calibration & calibration checks. The calibration check consists of measuring the Earth's magnetic field strength, dip angle, gravity field strength & comparing the results against known
local values. When the MWD tools are returned onshore the post-job calibration check is preformed (this check is also the pre-job calibration for its next run).
Rig site checks made on tools Function tests and setting up checks. The rig calibration checks are per-
formed but will not replace the onshore calibration check. All of the necessary reference data to identify the survey location Check the field acceptance criteria is set-up correctly in the software. Well tie-in location, latitude, longitude, grid convergence, declination, tool-
face offset, elevation reference and other relevant data Rig calibration checks are performed but do not replace a proper calibra-
Non-Mag Drill Collar (NMDC), Inspections for Magnetic HotspotsNMDC and NM components are inspected regularly for magnetic hotspots and the inspection certificates should be available offshore. Collars must be cleaned thoroughly to prevent corrosion and subsequent magnetic hotspots. Any NM equipment that is repaired; welding, threads re-cut or body re-cut, require re-in-spected and recertified for magnetic hotspots.
Shallow Hole Tests, Checkshots and BenchmarksA shallow hole test only ensure tool functionality, as the survey is affected by ex-ternal magnetic interference. The reason for the shallow hole test is to detected a tool failure before a full trip is conducted.
It is desirable to make a quality check of the survey sensors at the earliest oppor-tunity when clear of external interference and when the BHA is changed (and therefore the drillstring magnetic signature has changed). In addition, if the open hole section is long, it is good to take a off-bottom checkshot against the previous run prior to drilling ahead, to re-confirm the hole direction and satisfying the re-dundant surveying principle.
Correlation on the way out of hole will check that the tool has been working cor-rectly, this is limiting in the sense that the hole section has already been drilled and may be off course if any errors have existed during the run.
A benchmark is established when MWD surveys at the same depth using two dif-ferent tools are compared, some estimate is made to weight the tolerance criteria between those two surveys based on geomagnetic location, orientation, BHA configuration and sensor errors. It is preferable to obtain benchmarks at areas of lowest DLS so a fair comparison can be made. The criterion for a good bench-mark survey is +/- 0.1˚ inclination & +/- 0.5° azimuth.
A roll test is to check that the survey results do not exhibit an unexpectedly large dependence on toolface. The roll test consists of four surveys taken at the same depth, 90˚ out of phase and each survey must meet the field acceptance criteria. The test must be conducted at least 150ft away from the last casing shoe or any potential sources of external interference. The roll test takes at least 20 minutes to complete, so due to rig time limitations is not normally conducted.
Single Station Drill String Interference CorrectionSingle-station correction methods attempt to correct magnetic survey data for crossaxial and axial magnetic drillstring interference, bias of the cross-axial sensors and toolface dependent alignment errors. This is broadly achieved by applying the processed results from a rotation shot, or series of rotation shots, to each individual survey station measurement in addition to the application of the main field model data from the BGGM reference model.
The main mathematical algorithms in use are Magcore correction – (BHI) DMAG – (SLB) Shell Survey Platform (SSP, used to be SUCOP) – (Shell/Stat Oil are the
The major advantage of this technique is that prior to its introduction there was not any other practical method available to deal with this problem. The major dis-advantages of this technique are that when rotation shots are used, they have the effect of weighting all of the single surveys by the results from a very small dataset obtained somewhere else in the well. In addition, the residual uncertainty as a result of applying this type of correction degrades very quickly with hole ori-entation approaching east/west and high angle. This means that in practice there is a very large no-go zone (stipulated by Shell as greater than 50° inclination and within 40° of east/west) where the uncertainty left over after correction is larger than not applying the correction and it therefore cannot be used when drilling in these orientations.
The objective of this survey category is to provide an accurate well bore position, from which a ‘benchmark’ survey can be established for the comparison of all other directional surveys taken. Redundant surveys can provide an extra measure of reliability in critical well operations, such as multi-well platforms or relief well drilling. Types of position verification surveys are:
QC surveys - Known values of magnetic dip angle & field strength are used as a ‘check’ to confirm survey data collected.
Overlap surveys - Taken going into and coming out of hole.Reference surveys - Used to establish the ‘benchmark’ against which all subsequent survey data is compared.Verification surveys - Run after MWD to confirm results of initial survey.Final Position surveys - Determine the actual final position of the wellbore
Magnetic Survey Errors
Magnetic Field – Magnetic Dip• The Magnetic Field at every location has a specific Strength and Direction • Horizontal component = H x Cos (Dip)
• Magnetic field strength is the horizontal component that supplies the direction to Magnetic North.
• Magnetic field strength varies with the cosine of the Dip Angle.– North Sea: 49.28 μT (microtesla) X Cos (72.3°) = 14.57 μT– Alaska: 57.51 μT X Cos (80.6°) = 9.39 μT– Gulf of Mexico: 50,450 μT X Cos (59.7°) = 25.25 μT
• Where the magnetic dip is high the horizontal component is low.• Areas where the horizontal component is low, accuracy is low.• Reference Values -
– Wall Plot (if time frame is valid)– Survey from specialist survey
Drill String Interference The instrument must be run inside sufficient length of non-magnetic drill collars, to isolate it from the magnetic influence of the drill string
Two components:– Axial Interference– Cross-axial interference–
Magnetic Variance • Caused by solar activity• Spectacular but limited effect
Crustal Field The Earth’s gravitational field is different throughout planet and information on its strength comes from the BGS Global Geomagnetic Model (BGGM) is a mathematical model of the Earth's magnetic field in its undisturbed state and is revised every year.
The Earth's magnetic field is itself a superposition of the field generated by the geo-dynamo in the liquid outer core (main field) and the field of magnetized rocks in the crust and upper mantle. The main field dominates the long wavelengths, down to about 3000 km, whereas the crustal field dominates at wave-lengths smaller than 2500 km.
The three Geomagnetic declination maps show the varying level of detail from the standard approach (left), that does not account for crustal anomalies and therefore is unrealistically smooth. The advanced approach (middle) shows some detail of the crustal distortion. And the highest resolution (right) that has used additional aero-magnetic survey to map the area for local distortion of the geomagnetic field
Additional Magnetic Effects That Can’t Be Corrected For• Formations effects• Drilling Environment
– mud properties– Casing– batch setting conductors– Fish– magnetized mud/cuttings
• Hot spot in the BHA.
Magnetic Declination• Magnetic surveys need to be corrected to True North• True north is the Geographic North or Map North• Magnetic North is Compass North• Magnetic North changes over time as the location of Magnetic North pole
changes.• Definition: The angle between True North and Magnetic North as
measured from True.• Easterly Declination (clockwise) is positive.• Westerly Declination (anticlockwise) is negative.• Declination is added to Magnetic Azimuth
Grid Convergence• Definition: The angle between True North and Grid North as measured
from True North• Easterly Convergence (clockwise) is positive• Westerly Convergence (anticlockwise) is negative• Convergence is subtracted from Corrected Azimuth
Magnetic Tool Face• Looking downhole • Used when nearly vertical• The angle which describes the orientation sub
relative to north.• Zero degrees MTF means that the steerable
tendency of the BHA is to the north.• 180 Degrees MTF means that the steerable
tendency of the BHA is to the south.
High Side tool face (Gravity Tool Face)• Angle which describes the orientation relative to the
direction of gravity.• 0˚ GTF, steering tendency of the bha is up.• 180˚ GTF, steering down.• 90˚ GTF, steering to the right (270˚ left)