Making the Most of Borehole Surveying

Transcription

1 Making the Most of Borehole Surveying Prof Angus Jamieson University of the Highlands and Islands Video presentation available at

2 This Presentation Covers Why survey 2. Coordinate Systems 3. North References 4. Survey Tools 5. Error Models 6. Correction Techniques 7. Common Pitfalls

3 Section 1 Why Survey? Prof Angus Jamieson University of the Highlands and Islands Video presentation available at

4 Don t be in the wrong place at the wrong time!!

5 Why do we survey at all? Ensure a safe well path to the target Ensure you hit the target Ensure you don t hit another well Provide good log positions to G&G Provide good reserves estimates Report data to the regulators Conduct forensics investigations afterwards Prepared for relief well if necessary

6 Business Case? A shorter gyro run $10,000 + A proximity shut in $100,000 + A plug back sidetrack $1 million + A dry well or Dead Zone $10 million + A deep landing $100 million + A minor collision blowout $1 billion + A major collision blowout $10 billion +

7 Poor Surveying costs Production

8 Poor Surveying costs Production

9 Poor Surveying costs Production 10% production lost but we saved the cost of a gyro!

10 How serious is a blowout?

11 Blow out with no fire

12 Very High Pressures

13 Add Fire and we have disaster

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19 Low probability High Impact

20 In Summary Saving money on surveying is a high stakes gamble which, if lost, will make you famous

21 Section 2 Coordinate Systems Prof Angus Jamieson University of the Highlands and Islands Video presentation available at

22 Mapping the World

23 Any Projection distorts the World

24 Gerardus Mercator 1512

25 Project from the centre of the Earth

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27 Mercator Projection

28 Greenland is actually only 10% of the size of Africa

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32 The Equator

33 The Centre of the World

34 The Centre of the World

35 The Worlds Time Zones

36 UTM Zones

37 UTM Zones

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41 Section 3 North Reference Prof Angus Jamieson University of the Highlands and Islands Video presentation available at

42 The World

43 The Greenwich Meridian

44 The Equator

45 Latitude & Longitude

46 Transverse Mercator

47 Transverse Mercator

48 For Any Point on the Earths Surface True North is towards the North Pole

49 If a TM cylinder is wrapped at another longitude, Map North follows the cylinder

50 So unless you re at the centreline of the map, True and Grid DON T line up

51 The True Direction of Grid North is called the CONVERGENCE

52 Universal Transverse Mercator

53 Grid Convergence

54 Grid Convergence

55 The True Direction of Magnetic North is called the DECLINATION

56 With three Norths it is easy to get confused

57 MWD measures from Magnetic North

58 Gyros usually measure from True

59 But most surveys are finally reported in Grid

60 For Example if Declination was -6 degs and Convergence was +2 degs

61 Section 4 Survey Tools Prof Angus Jamieson University of the Highlands and Islands Video presentation available at

62 MWD

63 MWD

64 Modern Gravity and Magnetic Sensors (1) Accelerometers use gravity field vector (0 degrees inclination reference) Several designs are available Exciter / pick-offs Torquer Single axis permanent magnet restoring coil pendulous arm hinge Dual axes G (2) Magnetometers use magnetic field vector (magnetic North reference) N N S N Primary coil N S S S cores Two identical cores with primary winding around (in opposite directions). Secondary coil around all. Primary current produces magnetic field in each core, equal and opposite so no voltage induced in secondary winding. When placed in an external magnetic field, an unbalance occurs and a voltage is produced in the secondary coil, this is directly proportional to the external magnetic field. Secondary coil

65 Photo-Mechanical Multishot

66 A Magnetic Drop Tool

67 The Magnetometer has no moving parts but requires three orthogonal instruments to measure the magnetic field. Accelerometers measure Inclination from vertical. Compass v Magnetometer The Compass Measures Both Inclination and Direction but is less accurate and less robust

68 Gyroscopic Effects A gyro does not want to change the orientation of the spin axis. Conventional Gyros are lined up on a reference azimuth and remain facing that way for azimuth measurement down hole.

69 Gyroscopic Principles Inertia: when the spinning portion of a gyroscope (called a rotor) is set in motion it will attempt to keep its axis of rotation continuously pointing in the same direction images 2002 Encyclopædia Britannica, Inc. Precession: when a force is applied to a spinning rotor, it will attempt to compensate by rotating around an axis that is perpendicular to the applied force

70 Conventional Gyro

71 Gyroscopic Effects A gyro forced to torque around its X axis when spinning around the Y axis will start to rotate around the Z axis. This is known as Precession and can be used to measure rate of change of orientation against time. Continuous Gyro surveying integrates rate of azimuth change against time to measure its current direction.

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73 Continuous Gyro Horizontal Earth Rotation Vector Vertical Earth Rotation Vector Gyro Sensor Wellbore Direction

74 North Seeking Gyros A North Seeking Gyro is simply a highly sensitive rate gyro which measures the earths rotation and senses the direction to the polar axis. This usually takes about 1 2 minutes of stationary sensing so is often only used in top hole (up to 15 degrees) after which the survey is run in continuous mode.

75 Gyroscopic Tools

76 Section 5 Error Models Prof Angus Jamieson University of the Highlands and Islands Video presentation available at

77 A Normal Distribution

78 A Normal Distribution

79 A Normal Distribution

80 A Normal Distribution

81 Probability in two dimensions

82 The full distribution in 3D

83 Co-variance determines the orientation

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86 Standard Deviations in 3D 2 SDs in 1D cover 95% BUT SDs in 3D cover approx 95% 2.79 SDs in 3D cover approx 98.5%

87 The Magic Formula?

88 The Magic Formula

89 The Magic Formula

90 The Magic Formula

91 Building a detailed error model Find all error sources affecting Md, Inc & Az Find 1 sd values for each coefficient Work out affects on Md,inc and az Decide whether Random or Systematic Covert to errors in North, East & Vertical Build a Co Variance Matrix Work out Ellipse dimensions and orientation

92 SPE Paper # by Hugh Williamson of BP. Accuracy Prediction for Directional Measurement While Drilling Processes & procedures are followed Tools are properly calibrated Survey intervals no greater than 100 ft Non-magnetic spacing as recommended Individual surveys pass QC checks

93 Propagation of Errors The model recognises 4 modes of error propagation: Random uncorrelated from one measurement to the next Systematic correlated from one measurement to the next within one single tool run Well-by-well correlated from one measurement to the next within an entire well Global always correlated, including well to well

94 The ISCWSA MWD Model

95 Convert Observation Error to Position Error

96 Effect of Inclination Error

97 Azimuth Error only affects horizontal position

98 Building the Covariance For systematic errors dn = dn1 + dn2 + dn3... For random errors dn = sqrt(dn1^2 + dn2^2 + dn3^2...) For each error source Add up all effects in a survey station Add up all survey stations in a leg Add up all legs in a survey

99 The Co-Variance Matrix

100 In the North East Vertical Reference there may be covariances

101 Imagine a different set of orthogonal axes that don t see any covariances

102 Rotate to a new Covariance Matrix

103 Covariances are zero

104 The viewing vectors are Eigen Vectors The ellipse dimensions are Eigen Values

105 Collision Risk

106 High Collision Risk

107 Section 6 Correction Techniques Prof Angus Jamieson University of the Highlands and Islands Video presentation available at

108 Major Corrections Depth Correction There are many sources of error affecting both drill pipe and wireline length downhole but depth correction will usually address the mechanical and thermal stretch. These account for the major depth errors and can be as much as 0.2% Sag Correction This is an inclination correction to allow for the natural bending of the BHA under its own weight. It increases with inclination and can be 0.5 degrees or more. IFR Correction This usually refers to the local correction of magnetic declination and is derived from an In-Field Referencing survey of the oilfield. This can be up to 1 degree in places Magnetic Interference This applies to azimuth only and corrects for the magnetic influence of the BHA itself. It is particularly important when using short non-mag collars.

109 5 Sources of Depth Error Mechanical Stretch Survey Resolution Tool Misalignment Temperature Effects BHA Deflection (Sag)

110 Mechanical Stretch

111 Survey Resolution

112 Normal Minimum Curvature

113 Tool Misalignment

114 Temperature Effects Steel will stretch by 1.3m / 1000 / 100 degs C

115 Sag Correction

116 IFR Correction

117 The Earth s Magnetic Vector

118 A Powerful Force but subject to three Variations 1. Secular Variation Long slow changes in the earths magnetic core. Typical Size: Fractions of a deg/year Cured By: BGGM or HDGM magnetic model 2. Diurnal Variation Rapid daily variations caused by solar wind and earth rotation. Typical Size: 0.2 degs (Randomized) Cured By: Interpolated In Field Referencing (IIFR) 3. Crustal Variation Permanent local effects caused by deep, magnetic basement rock Typical Size: 1 degree Cured By: In Field Referencing (described later) IFR

119 Declination is on the Move

120 Rapidly in Geological Time!

121 Diurnal Variation

122 Crustal Variation

123 Crustal Variation

124 Interactive IFR Map

125 Magnetic Interference The interference created by the collars in the BHA can influence the observed by several degrees. Short Collar solutions only use the X and Y mags to calculate the azimuth Multi Station Analysis uses the fact that as the BHA changes toolface and attitude, the background magnetic field is unchanged but interference components rotate with the BHA. We can therefore back out the interference components over several survey stations

126 Summary Reducing error is nearly always possible Sag is usually the biggest benefit in Vertical IFR is usually the biggest benefit in Horizontal Short Collar should only be used with caution MSA is only reliable in an accurate mag field SC and MSA do not work well when Bz small For high accuracy work nothing beats gyros

127 Section 7 Common Pitfalls Prof Angus Jamieson University of the Highlands and Islands Video presentation available at

128 Top 10 List of what can go wrong Units and conversion factors TVD Referencing Failure to use sag correction Uncertain Connection to Map Misapplied Convergence Old Declination Values Bad Computer Data Unchallenged Use of GPS on wrong Datum Not Enough Surveys Home made software

129 Conclusion If we don t get the message out that wellbore positioning is worth spending money on, we will continue to waste reserves and occasionally risk lives.