Answers to questions and responses to feedback gathered during our Beta timeframe.
Q: What are the standard default values for various fields?
A: While we can not presume to know the values you should use for any certain wellbore, the following values are recommendations for defaults from industry experts if the specific values are not known for a wellbore.
Reference Well Hole Size: Correct value along the wellbore or if unknown enter a value of 36 inches.
Offset Well Max Hole Size: The maximum hole size or if unknown enter a value of 36 inches.
Surface Margin: If unknown enter 0.3m or 0.97ft. (definition)
Projection Uncertainty: If unknown enter 0.5m or 1.62ft. (definition)
Sigma: This will generally be given by the drilling engineer or if unknown enter a value of 3.5. (definition)
SF Warning Levels: A value of 1 generally means you’re in a Stop Drilling situation, 1.25 would normally trigger a detailed engineering review and would be considered Proximity Drilling.
Q: Will you default some of the field values?
A: We are exploring a few different ways to allow your workflow to be simplified when setting up anti-collision for your jobs. This includes allowing you to easily default values both on the reference well as well as offset wells.
There are also User Interface changes we will make based on your feedback.
Q: What model is used?
A: This depends on the tool code(s) selected for the survey program. If you use one of the many MWD or EMS tool-codes then you’re making use of the ISCWSA MWD error model. If you’ve selected a gyro then you’re using the gyro model.
We currently include a large number of standard models, but welcome feedback as to any models that are absent.
Q: When the Pedal Curve option is turned off what method is used?
A: The alternative is vector separation. ISCWSA standard rule uses pedal curve, which is the more conservative option.
Q: What is Surface Margin and how is it related to slot uncertainty?
A: Surface margin increases the effective radius of the offset well and is used to accommodate small, unidentified errors. It also defines the minimum allowable slot separation during facility design and ensures the separation rule will prohibit the activity before nominal contact when the uncertainties are zero.
It’s an extra buffer which mainly affects top-hole between wells on the same pad. It’s used when wells are close and positional uncertainties small in order to be extra conservative as there may be un-modelled effects or small user errors that could turn a marginal anti-collision pass into an actual problem.
It’s not directly related to site or slot uncertainty although you could create similar modeling by bumping up the slot uncertainty.
Q: What is Projection Uncertainty and how is it used?
A: It is additional uncertainty that will be added to the projection to bit as well as to a drillers ability to steer to that exact location. It is used to add an additional level of caution to unknown projected wellbore positions.
Q: If the max hole size of an offset well isn’t known what should be used?
A: Use the largest likely diameter in order to be conservative. If you have a completely unknown diameter use 36 inches.
Q: What is Sigma?
A: A dimensionless scaling factor which represents the number of standard deviations at which SF is evaluated. This is related to the confidence level in the SF result. The more confident you are in the result and the actual wellbore positions the lower this number could be. The less confident, the higher this number should be. It is typically in the range from 2 to 3.5.
Q: Are Field Strength and Total Intensity the same thing?
A: Yes, these are interchangeable terms.
Q: How are Site, Slot, and Vertical Uncertainty related between the job and offset well anti-collision setups?
A: Site Uncertainty is the only potentially related value between the job setup and offset well setup. Site Uncertainty is used from the job’s site uncertainty for an offset well IF the offset well is indicated as being on the same pad as the job’s well bore. By indicating the offset well is on the same pad, the Site Uncertainty field for the offset well is disabled in the UI and the job’s Site Uncertainty will be used.
Each well bore should have it’s own Slot Uncertainty and Vertical Uncertainty values.
Q: How are the Magnetic Qualifiers from the job related to the Magnetic Qualifiers for an offset well?
A: There is no current relationship between the Magnetic Qualifiers on the job setup and the setup for an offset well.
The general recommendation is to use the most accurate known values at that location at the time the well bore was drilled (magnetic qualifiers vary by location and over time). This combined with an accurate survey program allows for more accurate anti-collision calculations.
Q: Which geodetic datum (NAD 27, NAD 83, etc.) should be used when importing data and setting up anti-collision?
A: Any datum can be used, as long as the same datum is used consistently for the whole job and all offset well paths. This ensures correct relative spacing of slot locations on surface.
The following references are industry best practice standards for collision avoidance:Introduction to Wellbore Positioning by Angus Jamieson
Collision Avoidance Lexicon by the ISCWSA Collision Avoidance Work Group