Correcciones al Norte Verdadero y al Norte de Grilla.pdf

TrueNorthGridNorth.pdf

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Calculating True North and Grid North Subject Matter Experts: Steinar Bakke and Jan J an Daudey.

Sugar Land Learning Center  Center  1998

An asterisk (*) is used throughout this module t o denote a mark of Schlumberger. © Copyright Schlumberger Schlumberger Technology Corp., unpublished work, work, created 1998. The following material includes Confidential, Proprietary Information and is a Trade Secret of Schlumberger Technology Corp. All use, disclosure, and/or reproduction is prohibited unless authorized in writing. All Rights Reserved. This material may also be protected under laws of non-U.S. countries, including copyright copyright and trade secret laws.

Source: TrueNorthGridNorth.doc, Revised 11/11/98SLC

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1 Getting Started

This math module is part three of four and should provide an adequate mathematical background for what is normally required of the directional driller. The level of mathematics, required for  directional drilling work, is fairly basic. However, minimum competence in algebra and trigonometry are required to understand how wells are planned. When doing projections and well planning, the driller is often expected to make quick  calculations at the rig site. While a computer or programmable calculator often performs these calculations, the driller needs to be able to check the results "by hand" using a scientific calculator. In most calculations throughout this module the rules for significant figures are used to calculate the result. The final answer in most cases will be rounded to the nea rest hundredth of  a digit. Getting Started

Before you begin this module you should have already completed or understood the following modules: • • • •

Introduction to Directional Drilling, Introduction to Math, Directional Drilling Math 1: Calculating Coordinates and Directions, and Directional Drilling Math 2: Calculating Directional W ell Profiles.

When you have completed this module, you should be able to: • •

correct survey data to True North, and correct survey data to Grid North.

To facilitate your calculations, you may wish to get • • •

 paper,  pencils, and a calculator.

Study Questions

This module contains study questions. As you come across the questions, try to answer them without looking back at the lesson. Look if you need to. Answers are provided in the last part of  this module.

Source: TrueNorthGridNorth.doc, Revised 11/11/98SLC

TrueNorthGridNorth.pdf

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2 True North

In this lesson you will learn to do the following: •

Given a Magnetic North (MN) direction survey and declination value (DEC), convert the Magnetic North direction to the equivalent True North (TN) direction.

Since the dawn of time, man has had to describe a location in one way or another. The directional driller must be able to use learned skills to read surveys and well proposals in locating the position of a survey point along a well. The well proposal is usually derived from coordinates in a grid system; therefore, directions are referenced to Grid North. However, the well surveys use sensors that reference either Magnetic North or True No rth, and the driller must be able to convert from one to the other.

True North

True North (TN) is the direction of a line from any geographical location on the earth’s surface to the North Geometric Pole. •

Calculate True North with this formula: True North (TN) = Magnetic North (MN) + Magnetic Declination value (DEC).

•

Magnetic North is the direction of a line from any geographical location on the earth’s surface to the North Magnetic Pole.

•

Magnetic Declination value is the angular difference, east or  west, at any geographical location, between True North (Geographic North) and Magnetic North.

Source: TrueNorthGridNorth.doc, Revised 11/11/98SLC

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True North

Values of Magnetic Declination (DEC) change with time and location. As the movement of Magnetic North (MN) is constant and  predictable, Magnetic Declination can be calculated for any given point on the earth at any given time. Charts of the various magnetic declinations and rate of change are widely used.

Easterly Magnetic Declination values are expressed as a positive value and Westerly Magnetic Declination values are expressed as a negative value. Although converting from Grid (GN) to True North (TN) appears a simple task, it requires considerable careful attention, depending on the relative directions of Grid Convergence and the Magnetic Declination value. Grid convergence is the angle between a True North direction and Grid North direction. Before magnetic survey data can be converted to True North, the data at each survey point must indicate the direction of the wellbore at that point. Calculating the wellbore azimuth for each  point provides the direction.

Source: TrueNorthGridNorth.doc, Revised 11/11/98SLC

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2.1 Converting Magnetic North to True North

Work through the example  problem to learn how to calculate True North from a magnetic survey. Example problem: If the observed magnetic survey point is located at N40°E and the Magnetic Declination is 3.4° West, convert the Magnetic  North direction to the equivalent True North (TN) direction.

Use the following steps to solve the problem. •

First: Change the magnetic survey (N40°E) to magnetic azimuth. This is the same as changing a quadrant direction to an azimuth direction as explained in the Directional Drilling Math 1 Module.

•

Second: Calculate True North (MN). (True North = True Azimuth), (Magnetic North = Magnetic Azimuth)

As a result of working through this example problem you should have learned how to convert the Magnetic North direction to the equivalent True North direction. Source: TrueNorthGridNorth.doc, Revised 11/11/98SLC

TrueNorthGridNorth.pdf

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Study Questions 1) If an observed magnetic survey point is located at N38°W, and the Magnetic Declination is 3.4° West, correct the direction to True North.

Select the correct answer. a)  b) c) d)

45° AZM 226.72° AZM 235.3° AZM 12.91° AZM

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3 Grid North

In this lesson you will learn to do the following: •

Given a True North (TN) survey and Grid Convergence (GRID), convert the True North direction to the equivalent Grid North (GN) direction.

The directional driller must be able to use learned skills to read surveys and well proposals to locate the position of a survey point along a wellbore. The well proposal is usually derived from coordinates in a grid system; therefore, directions are referenced to Grid North. However, the well surveys use sensors that reference either Magnetic or True North, and the driller must be able to convert from one to the other. A Local Coordinate System is one requiring ac curate plotting of Grid North. In many cases, government reports require the use of Grid North.

Source: TrueNorthGridNorth.doc, Revised 11/11/98SLC

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3.1 Converting True North to Grid North

Grid North is the direction of a line from any geographical location within a grid system  paralleling the Universal True Meridian as determined by observation of Polaris. •

•

•

Calculate Grid North with this formula: Grid North (GN) = True North (TN) Grid Convergence (GRID). Grid Convergence is the angle between a True North direction and Grid North direction.

A magnetic survey is a directional survey in which the d irection of the wellbore at the survey  point is determined by a magnetic compass detecting the earth’s magnetic field, indicating a Magnetic North direction.

•

Calculate True North with this formula: True North (TN) = Magnetic North (MN) + Magnetic Declination value (DEC).

•

Magnetic North is the direction of a line from any geographical location on the earth’s surface to the North Magnetic Pole.

•

Magnetic Declination value is the angular difference, east or west, at any geographical location, between True North (Geographic North) and Magnetic North.

Source: TrueNorthGridNorth.doc, Revised 11/11/98SLC

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Converting True North to Grid North

Work through the example  problem to learn how to convert a True North direction to a Grid North direction. Example problem: The direction of a wellbore at a magnetic survey point is S23°E, the Magnetic Declination is 3.4° West, and the Grid Convergence is 8° West. Correct the direction to Grid North.

Use the following steps to solve problem. •

•

First: Change the magnetic survey (S23°E) to magnetic azimuth. This is the same as changing a quadrant direction to an azimuth direction as explained in the Directional Drilling Math 1 Module.

Second: Calculate the Grid North (GN) direction. (T rue North = True Azimuth), (Magnetic  North = Magnetic Azimuth)

As a result of working through this example problem you should have learned how to convert the True North direction to the equivalent Grid North direction. Source: TrueNorthGridNorth.doc, Revised 11/11/98SLC

TrueNorthGridNorth.pdf

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Study Questions 2) If the observed magnetic survey direction is N38°W, and the Magnetic Declination is 5° West, and the Grid Convergence is 3° East, correct the direction to Grid North.

Select the correct answer. a)  b) c) d)

326.72° AZM 314° AZM 136° AZM 120° AZM

3) If the observed magnetic survey direction is S40°W and the Magnetic Declination is 8° East, and the Grid Convergence is 10° East, correct the direction to Grid North.

Select the correct answer. a)  b) c) d)

120.91° AZM 218° AZM 136°34’ AZM 26.72° AZM

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4 Summary

Computer programs are widely used to perform the calculations in this module. Even though technological advancements have led to improved and more effective calculation methods, onsite and emergency situations will require that the directional driller be able to perform well  planning calculations independently. In this module you have learned to do the following: •

Given a Magnetic North (MN) survey and declination value (DEC), convert the Magnetic  North direction to the equivalent True North (TN) direction.

•

Given a True North (TN) survey and Grid Convergence (GRID), convert the True No rth direction to the equivalent Grid North (GN) direction.

Source: TrueNorthGridNorth.doc, Revised 11/11/98SLC

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Part 5, Answers to Study Questions Part 2, True North 1) If an observed magnetic survey point is located at N38°W and the Magnetic Declination is 3.4° West, correct the direction to True North.

Select the correct answer. a)  b) c) d)

45° AZM 226.72° AZM 235.3° AZM 318.6° AZM

Part 3, Grid North 2) If the observed magnetic survey direction is N38°W and the Magnetic Declination is 5° West, and the Grid Convergence is 3° East, correct the direction to Grid North.

Select the correct answer. a) b) c) d)

326.72° AZM 314° AZM 136° AZM 120° AZM

3) If the observed magnetic survey direction is S40°W and the Magnetic Declination is 8° East, and the Grid Convergence is 10° East, correct the direction to Grid North.

Select the correct answer  . a) 120.91° AZM b) 218° AZM c) 136°34’ AZM a) 26.72° AZM

Source: TrueNorthGridNorth.doc, Revised 11/11/98SLC

TrueNorthGridNorth.pdf

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TrueNorthGridNorth.pdf

Revised: 11/11/98

Printed: 11/12/98

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