The Drilling Engineer must ensure that the wellhead co-ordinates and the target co-ordinates are given in the same co-ordinate system (origin, azimuth depth reference).
When storing the well proposal, the Drilling Engineer shall specify the horizontal and the vertical wellhead position uncertainty.
Once entered, the position uncertainty numbers are defaults for future storage of that survey file.
Survey quality check
The survey quality check is a Survey Module feature which allows the user to check the consistency of the survey, using the varying curvature scan. It calculates the variation of the bottom hole location when removing survey stations one at the time, and plots this variation as a function of the measured depth. The tool is particularly powerful for detecting typing errors introduced while entering the survey data. In addition, the overall statistics also indicate the noisiness of the survey data.
Survey tool selection
Before computation of borehole position uncertainties of a (quasi) survey, a selection of survey tools, running and environment conditions shall be made. In addition, for a tool a quality level shall be selected. The survey tools selected for use in various sections of the well shall be associated with the appropriate sections.
Planning
For planning the Drilling Engineer may select a Good or Excellent magnetic survey tool during drilling but a verification survey tool, usually a gyro, should be selected at the Acceptable quality level. This is to ensure that no over-optimistic survey programme is designed, for which it is unsure whether the quality and hence accuracy can be met even if the tool performs within the Suco's calibration requirements.
Borehole position uncertainty
The output from survey instruments are directions and distance for most tools and position co-ordinates for Inertial Navigation Systems. Given a number of surveys in a single well using several instruments of a single tool type, no two surveys will be identical.
Differences occur due to:
·statistical uncertainties, such as due to noise, in the sensor readings from which inclination and azimuth values are derived;
·systematic uncertainties, such as due to variations in calibration of linearity, sensitivity, bias and drift.
The effect of these differences is an uncertainty in the measured positions, resulting in an uncertainty around the calculated borehole position. Typically, the uncertainty in lateral direction is larger than the uncertainties in upward and measured depth. Consequently, the resulting uncertainty at a survey station has a ellipsoidal shape. The uncertainty accumulates along the well trajectory, and will be largest at the last survey station.
Anti-collision monitoring (ACM)
In this section, anti-collision monitoring (ACM) procedures as performed by COMPASS-SE are described. These are required to check the proximity of the wellplan or survey under consideration, referred to as the generic well, and the existing well, referred to as the check well. Checking against multiple wells is merely a repetition of the basic ACM procedures.
For anti-collision monitoring purposes three quantities are calculated:
·the distance from the generic well to the check well;
·the minimum distance between the uncertainty envelopes;
·the separation factor.
Distance to a check well
In anti-collision calculations the basic methods used for calculating distance from a point on the generic well to the check well are: the horizontal distance, the travelling cylinder distance and the 3D distance, in order of increasing computational difficulty.
Minimum distance between uncertainty envelopes
The minimum distance between uncertainty envelopes, also referred to as inter-boundary (IB) separation, represents the minimum distance the wells can possibly have, given their nominal well positions and the uncertainty on that position.
Separation factor (SF)
The SF will be:
< 1.0 in case of a potential collision, where the uncertainty envelopes overlap
= 1.0 when the uncertainty envelopes touch
> 1.0 when they have a positive inter-boundary separation.
Typically, the accumulated uncertainty in upward direction is significantly smaller than the uncertainty in lateral direction. Therefore, the benefit from the 3D proximity analysis is largest if the two wells are in a vertical plane with respect to each other. If two wells are in lateral relative position, a wellplan shift up- or downward may be very effective in increasing the Separation Factor.
ACM while planning
The key issue in ACM while planning is to be complete. In this stage there is time to perform a check against all wells of the platform and all relevant offset wells. If the generic well will extend to within the range of wells from another platform then these shall also be included in the check.
While planning, it must be taken into account that a well section is usually drilled with magnetic MWD, which tends to introduce a larger uncertainty than a gyro which may be run afterwards. Therefore, the Drilling Engineer shall check that sufficient clearance is maintained between the generic well and the check wells at all stages of drilling.
ACM while drilling
While drilling, the Wellsite Engineer will be entering survey stations as they are received from the MWD. In case of deviations from the proposal, a project ahead will be made to check the proximity to other wells in the interval to be drilled next.