SAG Quick Start Guide

A guide to the SAG survey correction module in Innova Engineering

1.0 - Introduction

SAG occurs when a BHA bends under its own weight when positioned between two points of stabilisation. The degree of bending is affected by the hole size, stabiliser size, distance between points of stabilisation, and tubular OD & ID. SAG causes MWD sensor misalignment in relation to the borehole, and accounts for up to 80% of wellbore TVD position uncertainty.
The purpose of this document is to guide the user through the setup, calculation and the interpretation of the results of the SAG module in Innova Engineering.

2.0 – Setup

Before inputting data, the user should configure the relevant options in the Units menu and the Options menu:
  • Units: Ensure that all of the units are correctly selected, based on your inputs.
  • Options – Include Motor Bend in BHA Analysis: This lets you toggle whether the program factors in the bend angle in your motor into the SAG calculation. See Section 7.2 for additional considerations when using this option.
  • Options – SAG Calculation Precision: This will let you set the number of decimal places that the SAG results will be calculated to. Default is 3.

3.0 – Required Inputs

In order to run SAG, the user is required to enter data into the Drill String, Well Geometry and Fluids Tab and the Surveys Tab.

3.1 - Drill String, Well Geometry and Fluids Tab

In order to run SAG, the following parts of the Drill String, Well Geometry and Fluids Tab require data entered:
  • Well Geometry: The wellbore geometry tells the program the size of hole that the assembly is in.
  • Mud Weight: The mud weight is used to calculate the buoyancy of the drill string.
  • Drill string – All components: Enter your drill string into the Drill String section. This does not need to include any more than ~30m (100ft) of assembly beyond the last point of stabilisation. Beyond this point the remainder of the drill string will be lying on the low side of the hole and is not considered in the calculation. All components entered into the main Drill String table must have a component type, length, OD and ID. This includes items that don’t normally come with an ID, such as the motor or the bit.
  • Drill String – Stabilizers: As well as the above requirements, stabilizers require the following information entered into their component properties:
    • Stab OD: The outside diameter of the stabilizer.
    • Stab Blade Length: The length of the stabilizer blade that will be in contact with the side of the hole – Watch your units, this will be in inches or mm, depending on your diameter unit.
    • Stab Distance from Bottom: The distance from the Pin end shoulder to the bottom of the stab blade
    • Note: There is also space to enter stabilizer information into the component details for RSS, MWD/LWD, Motor, Turbine and Reamer component types. If you have stabilizer sleeves on any of these items in your BHA, don’t forget to enter the details for them as well.
  • Drill String – Motor Bend: If you have a motor in your BHA, you can enter the motor bend angle in the component details section. This is only required if you have the Include Motor Bend in BHA Analysis option toggled on.

3.2 – Surveys Tab

In order to run SAG, the following parts of the Surveys Tab require data entered:
  • Survey Offset: The distance between the cutting surface of the bit and the MWD inclination sensor.

4.0 – Calculation Types

Select the type of SAG calculation that you want to run from the drop down box in the Survey Connections section. Users have a choice of Single, Inclination Range or Surveys. Single and Inclination Range are configured in the Survey Corrections section.

4.1– Single

Selecting Single allows the user to calculate a SAG correction based on a single inclination:
  • Inc: Inclination that the user requires SAG results for.
  • DLS: Dogleg (°/30m or °/100ft depending on units selected). The program will use the dogleg to simulate hole curvature which affects the results. Zero represents a tangent.
  • TF: The Toolface of the motor bend during the survey. If the Include Motor Bend in BHA Analysis option toggled is toggled off, this will have no effect.

4.2 – Inclination Range

Selecting Inc Range allows the user to calculate SAG over a range of inclinations:
  • Inc Start: Initial Inclination.
  • Inc Stop: Final Inclination.
  • Inc Step: Step change in inclination between the initial and final. A step change of 2.5° will calculate SAG every 2.5° between the selected range.
  • DLS: Dogleg (°/30m or °/100ft depending on units selected). The program will use the dogleg to simulate hole curvature which affects the results. Zero represents a tangent.
  • TF: The Toolface of the motor bend during the survey. If the Include Motor Bend in BHA Analysis option toggled is toggled off, this will have no effect.

4.3 – Surveys

Selecting Surveys allows the user to calculate SAG on Actual Surveys, Well Plan Points or RAW Surveys, depending on what is selected. As well as the offset, the toolface of the motor is the only value in the Survey Corrections section that effects this calculation type. If the Include Motor Bend in BHA Analysis option toggled is toggled off, the TF will have no effect.
Select your survey type from the drop down box in the Survey Selection section, and then enter your surveys into the table. SAG can be run on Raw Surveys, Actual Surveys or Well Plan points. Whichever is selected in the Surveys tab when the calculation is run will be used.
Actual Surveys and Well Plan requires you to enter surveys using Measured Depth, Inclination and Azimuth. Raw Surveys require Measured Depth, raw Accelerometer Values (G) and raw Magnetometer Values (H). Make sure that you are using the correct units for your accelerometers and magnetometers.

5.0 – Running the Calculation

To run the calculation, open the Calculate menu and select BHA Analysis and SAG Correction. The calculation will run and the results window will appear. You may also receive a warning. The most common warnings are:
  • WOB greater than zero, SAG correction value may be incorrect: The WOB parameter entered into the Engineering Parameters tab has an effect on the SAG result. Depending on your circumstances this may not be an error, but as surveys are usually taken when the bit is off bottom, the program will warn you if a value other than zero is entered.
  • No stabilizers: You have not entered any information in the component details for stabilizers, or components with stabilizer sleeves.
  • BHA tangential point found before sensor for 1 or more surveys: The tangential point is the point above the last stabilizer where the assembly touches the side of the hole. Anything above this point will be lying against the side of the hole and will therefore not be subjected to SAG. It is important to note that the tangential point changes depending on the curvature of the hole, and in some instances when running the calculation there will be some stations that will require correction and others that will not. Engineering runs through all of these calculations and assigns a 0° correction to those that do not. If you think you are getting this message in error, it is usually because you have set your survey offset too far back from the last stabilizer.

6.0 – Interpreting Results

The following outputs are displayed in the SAG Results window:
Deflected Shape: Shows how far the BHA is from the centre line of the well bore. The X-Axis shows the distance from the bit along the BHA. The Y-Axis shows the deflection. The blue dot represents the sensor position. Note that the displayed plot always represents the final survey station.
BHA Slope: Shows the angle of the BHA with respect to the centre line of the well bore. The X-Axis shows the distance from the bit along the BHA. The Y-Axis shows the slope. The blue dot represents the sensor position, and the slope at this position is the error that is being corrected by the SAG correction. The displayed plot always represents the final station.
Below the charts is the results table, which contains the following information:
  • MD: Measured Depth of each survey station.
  • Inc (Org): The original uncorrected inclination of the survey.
  • Inc (Corr): The SAG corrected inclination.
  • Sag (Corr): The SAG correction to be applied to the uncorrected inclination to get the corrected inclination. Note that SAG correction is always added.

7.0 - Reporting Results

After running the calculation, the SAG column will be populated in the Survey tab and in the MSA window. Select SAG corrected Inc from the Inclination to Use drop down to use the corrected inclination when calculating the TVD, NS, EW and VS values.
A SAG report can be created with the following steps:
  1. 1.
    Close the results window. In the main Engineering interface, open the File menu and select Print Reports.
  2. 2.
    Configure your format and other options in the Options section.
  3. 3.
    Select the BHA Analysis and SAG type to create a SAG report. You can customize the report using the below options:
    • BHA Overview and Calculation Parameters: Displays the BHA you created in the Drill String tab as well as the parameters you entered in the fluid properties and survey corrections sections.
    • BHA Trend and SAG Correction Data: Displays the results table from the SAG results.
    • BHA Deflected Shape and Slope Charts: Displays the two charts from the results window.

8.0 – SAG Correction Considerations

Before running the SAG calculation there are a couple of things to consider:

8.1 – Minimum Inclination

From 0-3° inclination the potential correction will likely be invalid due to the difficulty with finding the point at which the top of the BHA contacts the wellbore, this is due to the hole not having a defined highside.
Due to this, most companies have a policy that SAG should not be applied until the inclination is >3°. However, >5° is also acceptable in certain circumstances.

8.2 – Motor Bend, Toolface and Surveys

This consideration applies when using the Surveys calculation type.
If the motor bend is not being modelled in Engineering (Bend angle is 0° in the motor component details, or Include Motor Bend in BHA Analysis is turned off) then it is possible to SAG correct all required survey stations in one go. If however, the motor bend is being modelled, then each station has to be SAG corrected individually, assigning the relevant Toolface for each survey. This is more labour intensive and therefore more susceptible to errors.
When the bend is not being modelled, the addition of new stations has no effect on previous corrections. This is not the case when modelling the bend. When a new survey station is added and the correction run, the program runs the correction against all of the previous stations using the assigned toolface. Since every station will likely have been measured on a different toolface, it is possible that the program may change some of the previous corrections based on the new toolface. For this reason it is important to carefully note the correction at each new station as it is calculated, and understand why you may see a difference when looking at earlier corrections.
It is recommended that you try toggling the option on/off and comparing results. If there is a negligible difference, it may be simpler to leave the option off.