This document provides minimum standard and general practices for wellbore survey.
1 GENERALITIES
1. During the well construction process the position of all the shoes (including conductor pipe) and/or the well TD must be known.
2. Survey programmes shall be designed for every well such that the wellbore position is known with sufficient accuracy to:
a. Meet local regulations,
b. Penetrate the geological target specified in the well’s objectives,
c. Eliminate the risk of intersection with any nearby wellbore,
d. Drill a relief well.
3. Wellbore position shall be calculated by the minimum curvature method.
4. A database of planned and actual well trajectories, including all slot locations, target locations and projection data, shall be maintained by COMPANY and the surveying or Directional Drilling Contractor.
5. The contractor in accordance with a quality assurance procedure approved by COMPANY shall carry out well position monitoring. This QA procedure must include as a minimum:
a. Position or uncertainty analysis,
b. Specification of instrument performance models,
c. Last calibration of the survey instruments
d. Survey quality, especially in case of:
o Possibility of magnetic interferences (Top hole, Conductors)
o Poor survey’s quality of historic wells (E.G.Totco measurement or equivalent)
e. In case of magnetic interference survey tools equipped with a gyro shall be used
f. Controls and approval process.
6. The performance specifications of all survey instruments to be used shall be in accordance with standards as agreed with the Surveying contractor.
7. A definitive survey will be collated for every well drilled and this shall be retained by COMPANY and the Contractor. This survey shall be approved by the COMPANY Well Operations Manager.
2 MINIMUM SEPARATION OF WELLS
The rule is as follows:
1. A separation factor shall be used to determine whether risk of collision exists and the required well shut in requirements for production and injection wells. The separation factor is defined as the ratio of centre to centre distance and the sum of error ellipses of the two wellbores.
a. The minimum separation method will be used to determine centre to centre distance.
b. The ellipsoid model of uncertainty will be used to determine the sum of error ellipse.
2. Production or injection wells will be shut-in if the separation factor is less than 1.5. Total wellhead positional uncertainty must be accounted for.
3. If the 1.5 separation factor value is encountered below the Down Hole Safety Valve, additional safety measures must be taken to isolate the reservoir from this depth such as deep set plugs.
4. Drilling must stop if at any time the extrapolated survey (projecting 60 meters (200 feet) ahead of the bit) indicates a separation factor equal to or less than 1.0 between the drilling well and any neighbouring well.
3 WELL PROGRAMME
1. In the Well Programme, the following details shall be found:
a. Well co-ordinate and datum information
b. A directional drilling plan (well proposal plan), spider plot and travelling cylinder plot.
c. Drilling targets details. Targets should be made as large as possible whilst still meeting reservoir objectives.
d. Procedures for assessing acceptable risks of collision, defining and maintaining minimum well separations.
e. Criteria for adjacent well shut-in or avoidance measures.
f. Survey requirements for each hole section, including conductor pipes (hammered or drilled).
g. Details of additional survey requirements and contingency measurement.
h. Recommended BHA’s.
i. Any limits on drilling i.e. dogleg severity, maximum inclation, etc.
4 WELLSITE MONITORING
1. Prior to commencement of drilling, the Well Operations Supervisor shall:
a. Ensure that any well is shut in and depressurised in accordance with the Well Operations Programme.
b. Hold a meeting with all relevant personnel to ensure the Well Operations Programme and procedures for collision avoidance are fully understood.
2. During normal Well Operations, the following shall be carried out:
a. Surveys shall be taken at the intervals as specified in the Well Operations Programme.
b. Surveys shall be taken whenever one of the critical tolerances is approached.
3. 24-hour supervision will be provided at the rig site by the directional well ops supervisor and the Well Operations Supervisor/Engineer.
4. For each survey, the directional drilling supervisor and Well Operations Supervisor (or well ops engineer) shall verify that correct survey data has been received and wellbore positions calculated.
5. The well path shall be projected 60m ahead on both straight and predicted steered course. The separation between wells will be calculated. If forward projections indicate that the criteria as detailed in the Well Operations Programme are being approached then remedial actions shall be initiated. These shall be detailed in the Well Operations Programme and may include:
a. Additional surveys to reduce uncertainties,
b. Setting of deep plugs and depressurisation of the adjacent well.
c. Plug back of well and sidetrack.
6. A survey report shall be submitted every day along with the Daily Drilling Report (DDR).