The COMPASS Anticollision module is defined by four concepts:
The “Data Structure” section of this manual described how the
Company Properties dialog box is used within the COMPASS software to apply company anticollision policies so that all anticollision results are consistent within the same rules and assumptions defined by the chosen models. It is very important that companies recognize the importance of ensuring that COMPASS data is distributed to all sites with exactly the same company properties, and that it is generally kept locked to prevent the setups from being changed.
This concept... Determines...
Error System How positional uncertainty is calculated Scan Method How wellpath separation is calculated Error Surface How separation factor is calculated Warning Type What criteria is used to issue warnings
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Use the File > Properties > Company > Properties > Anticollision tab to specify the anticollision analysis properties.
Error Systems
Prediction of wellpath location uncertainty is fundamental to safe and cost-effective well design. Wellpath trajectory is only imperfectly represented by survey measurement and trajectory calculations.
Because survey instruments are not 100% accurate, errors can occur in a calculated borehole trajectory. Uncertainty envelopes for wellpath trajectory are calculated based on survey tool error models and provide the minimum standoff distance to prevent wellbore collisions.
Uncertainty estimates range from field-based rules of thumb to strict analytical and statistical methods.
The COMPASS software uses the ISCWSA or Cone of Error survey tool error models.
The error system determines how the
positional uncertainty is calculated. The error surface determines how the separation factor is calculated.
This grid is used to define a number of anticollision
Chapter 6: Anticollision Module
ISCWSA
The ISCWSA committee’s remit was to “produce and maintain standards for the Industry relating to wellbore survey accuracy.” A number of companies participate. The committee recognized that directional drilling requirements have moved on from the 1970s, when the Systematic Ellipse model was constructed. Modern needs require smaller geological targets to be hit, often drilled in mature fields with a large number of nearby wellpaths. The simplistic WdW model could not handle such strict requirements and accurately model additional performance parameters measured from vendor survey tools.
Dynamic Number of Error Sources (Terms), each defined by:
•Value error value for the source of error
•Tie-On determines how an error source is tied onto sources:
•Random
•Systematic
•Well
•Global
Formula weighting for each error term for example, ASX
cos sin sin cos cos tan cot cos τ
τ τ Θ τ
Industry Steering Committee for Wellbore Survey Accuracy
MKsvy mi l k m m
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A number of other factors provided the incentive for an alternative industry model to be developed:
• Risk-based approaches to collision avoidance and target hitting required positional uncertainty to be associated with confidence levels, a term only implied with the WdW model.
• Changed relationships between operators, directional drilling, and survey companies forced all parties to share information on tool performance.
• Drilling and geoscience software enabled more sophisticated tool error models to be incorporated, with results that could be viewed in 3D earth model visualizations.
• Survey program designs to hit smaller drillers targets were dictated by tool error models and smaller geological targets.
As described in the “Survey Tool Editor” section of this manual, the ISCWSA committee designed a dynamic survey instrument error model specifically for solid state magnetic instruments (for example, MWD and EMS). The resultant model is described in a paper published by H.Williamson “Accuracy Prediction for Directional MWD” by Hugh Williamson as SPE56702. Essentially, the model enables an operator or survey contractor to define a dynamic number of parameters or error terms appropriate for a survey instrument.
Cone of Error
This model assumes an error sphere around each survey observation.
The model is empirical and is based on field or test observation comparisons of bottomhole positions computed from various instruments. The size of the sphere is computed as follows:
Radius of sphere around previous observation + MD interval × survey tool error coefficient / 1000.
The starting error around the wellbore is the well error plus the top borehole radius. The survey tool error coefficient depends on the current tool inclination and the values contained in the Inc/Error grid for that survey tool.
Chapter 6: Anticollision Module
Scan Methods
The purpose of an anticollision scan is to calculate the distance from the scanning point on a reference well to the closest point on an offset well.
This distance is known as the center-to-center distance, or wellpath separation. Different scan methods determine different separation distances because each technique uses a different algorithm and may not find the same closest point as another technique.
Four scan methods are available in the COMPASS software:
• Closest Approach 3D
• Traveling Cylinder
• Horizontal Plane
• Trav. Cylinder North
In the following explanations, the reference wellpath is the wellpath being planned, drilled, or surveyed. You check the distance from the reference wellpath to any number of offset wellpaths. The COMPASS software scans down the reference wellpath at intervals that are defined in the Interpolation Interval and computes the distance to the offset wellpaths by using one of the following scan methods.
3D Closest Approach
At each MD interval on the reference wellpath, the COMPASS software computes the distance to the closest point on the offset wellpath. At some scanning depth on your reference wellpath, imagine an expanding spheroid. The minimum separation occurs when the surface of the spheroid initially touches the offset wellpath; separation is the radius of the spheroid. Because the offset wellpath is now at a tangent to spheroid, the line of closest approach is perpendicular to your offset wellpath.
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The following graphics display the 3D closest approach scan method (left) and the traveling cylinder method (right):
Traveling Cylinder
This scan method uses a plane perpendicular to the reference wellpath and intercepting offset wellpaths as they cut through the plane. The surface resembles a cylinder with the size of the maximum scan radius.
The traveling cylinder method computes distance from the offset wellpath stations back to the reference wellpath. The benefit of this method is that intercepts are detected even when the wellpaths are approaching at a perpendicular. In this case, more than one point may be in the traveling cylinder plane for the same depth on the reference.
Offset Well Reference Well
3D
Offset Well Reference Well
Orthogonal
Chapter 6: Anticollision Module
Depths are interpolated on the offset wellpaths, which results in irregular depths on the reference wellpath. Therefore, the 3D anticollision view and traveling cylinders depth slice option are not possible with this method, because they rely on regular depths on the reference.
Trav Cylinder North
This scan method uses the same perpendicular plane as the traveling cylinder scan method, but toolface orientation from reference to offset is added to current wellbore direction. The traveling cylinder plot is oriented to Map North when the reference well is at low angles.
Toolface angle to an offset well is then reported as the angle from the high-side of your current wellbore plus the azimuth of your current wellbore. This method avoids the confusion in the traveling cylinders plot caused by large changes in toolface angle when kicking off from vertical.
Horizontal Plane
The Horizontal Plane scan method calculates the horizontal distance from the reference wellpath to the offset wellpath. It is similar to the traveling cylinder method, except that the cylinder expands horizontally irrespective of the wellbore direction. This method is not recommended for horizontal wells that it might miss and directional wells where it might provide late warnings because, when the well does approach, it does so very quickly. It is in the COMPASS software, but do not use it.
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The following graphic displays the Horizontal Plan scan method:
Offset Well Reference Well
Horizontal
Chapter 6: Anticollision Module
Comparing the Scan Methods
The most important difference in the methods is that they are all capable of determining a different closest point. It is for this reason alone that Scan Method should be defined within a company and locked, so that all anticollision results can be compared on the same basis.
The following diagram highlights the differences by using the preceding example. From the same reference well scan point, the different methods have all found a different closest point, with different values of calculated wellpath separation.
When comparing scan methods, assess the advantages and disadvantages of each technique.
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Traveling Cylinder Scan and Near-perpendicular Intersections
The primary deficiency with the traditional traveling cylinder method is that it can miss near-perpendicular intersections if the scan interpolation interval is large. The following graphic depicts the problem:
On the preceding graph, E4-S0 (right side) is the reference well being scanned down. A2-S0 is the offset well. The graph displays a depth slice that represents the orientation of the traveling cylinder at its scanning point. As the traveling cylinder scans down E4-S0, it misses the nearby A2-S0 well and finds a closest point some distance up A2-S0 and away from the critical area. Even with the interpolation interval set at 25 ft, the A2-S0 well is missed entirely.
E4-S0 Reference
A2-S0 Reference Wellpath
Scanning Point Traveling Cylinder Scan calculated closest point from E4-S0 scan point to A2-S0:
C-C Separation = 4967.40 ft Ratio Factor = 47.57
Chapter 6: Anticollision Module
Warning Types
When you scan a wellpath or plan against other wellpaths, you want the program to report only those wellpaths that pose a collision risk. To include wellpath positional uncertainty in the assessment of collision risk, the COMPASS software can report separation factors or assess against risk-based rules or depth ratios.
Error Ratio
Also known as ratio factor, error ratio is a value that includes center-to-center separation and positional uncertainty. It can be modified to include casing diameters.
The following graphic depicts the error ratio method and example results:
As described in Company Properties, the COMPASS software enables multiple ratio factor warning levels to be defined and a given warning or action to be taken if such a level is exceeded. These warning levels appear in the anticollision report and in some of the anticollision graphs in the form of levels and color-shaded lines.
Error Ratio > 1
Error Ratio = 1
Error Ratio < 1