Method of assessing positional uncertainty in drilling a well

• US 20030046005A1
• Filed: 06/06/2002
• Published: 03/06/2003
• Est. Priority Date: 12/08/1999
• Status: Active Grant

• Current Assignee: Statoil Petroleum As (Asa Equinor)
• Original Assignee: Den Norske Opera & Ballett As (Government of Norway)
• Inventors: Haarstad, Ivar
• Granted Patent: US 6,834,732 B2
• Time in Patent Office: Days
• US Class Current: 702/9
• CPC Class Codes: E21B 47/022 of the borehole, e.g. using geomagnetism

Abstract

A method is provided for estimating the positional uncertainty in drilling a well such as an oil well. A first set of values is supplied representing a first three-dimensional uncertainty of the actual position of a drill bit with respect to the estimated position. A second set of values is supplied representing a second three-dimensional uncertainty of the actual position of a geological feature with respect to the estimated position thereof. For example, the first set of values relates to positional uncertainties because of the drilling procedure whereas the second set of uncertainties is associated with the obtaining and interpretation of seismic data. The first and second sets of values are combined to form a third set of values which represents a third uncertainty of the position of the drill bit with respect to the geological feature. A probability is then calculated from the third uncertainty and gives the probability that the drill bit will reach a predetermined position relative to the geological feature.

Claims

1. A method of estimating positional uncertainty in drilling a well, comprising supplying a first set of values representing a first three-dimensional uncertainty of the actual position of a drill bit with respect to the estimated position thereof, supplying a second set of values representing a second three-dimensional uncertainty of the actual position of a geological feature with respect to the estimated position thereof, combining the first and second sets of values to form a third set of values representing a third uncertainty of the position of the drill bit with respect to the geological feature, and calculating from the third uncertainty the probability that the drill bit reaches a predetermined position relative to the geological feature.

2. A method as claimed in claim 1, in which at least one of the first, second and third sets of values comprises parameters of an error ellipsoid with a predetermined confidence interval referred to a Cartesian coordinate system.

3. A method as claimed in claim 1 or 2, in which at least one of the first, second and third sets of values comprises a covariance matrix referred to a Cartesian coordinate system.

4. A method as claimed in any one of the preceding claims, in which the first and second sets of values are referred to different coordinate systems and the combining step comprises transforming the first and second sets of values to fourth and fifth sets of values, respectively, referred to a common coordinate system and summing corresponding values of the fourth and fifth sets to form the third set of values.

5. A method as claimed in any one of the preceding claims, in which the probability is calculated as a normal distribution.

6. A method as claimed in any one of the preceding claims, comprising defining a geological target as a finite surface and selecting a desired point of intersection of the drill path with the geological target

7. A method as claimed in claim 6, comprising calculating the probability of the drill path intersecting the geological target.

8. A method as claimed in claim 7, in which the geological target is a polygon.

9. A method as claimed in claim 8, in which the geological target is rectangular.

10. A method as claimed in claim 8 or 9, in which each side of the polygon is ascribed a maximum acceptable probability of the drill path missing the geological target on that side.

11. A method as claimed in any one of claims 6 to 10, comprising calculating the probability of the drill bit being at a predetermined distance from the geological target.

12. A method as claimed in any one of claims 6 to 11, comprising using information from a marker point whose relative position including positional uncertainty to the geological target is at least partly known to correct at least one of the first set of values.

13. A method as claimed in claim 12, in which the marker point is the position of the drill bit during drilling when the drill bit penetrates a seismic reflector whose distance from the geological target is at least partly known.

14. A method as claimed in claim 12, in which the geological target is selected to coincide with a predetermined geological structure, the marker point is disposed at the the predetermined geological structure, and the position of the predetermined geological structure is derived from a pilot well.

15. A method as claimed in claim 12, in which the marker point is observed during drilling using means disposed at or adjacent the drill bit.

16. A method as claimed in any one of claims 6 to 15, comprising defining a drill target as a sub-surface within the geological target and calculating the probability that the drill path directed at a point within the drill target will intersect the geological target.

17. A method as claimed in any one of claims 6 to 15, comprising defining a drill target as a sub-surface within the geological target and calculating the lowest probability that the drill path directed within the drill target will interset the geological target.

18. A method as claimed in any one of claims 6 to 15, comprising defining a drill target as a sub-surface within the geological target and calculating the total probability that the drill path directed within the drill target will intersect the geological target.

19. A method as claimed in any one of claims 6 to 15, comprising deriving a drill target as a sub-surface within the geological target whose boundary is defined by a predetermined probability.

20. A method as claimed in any one of claims 6 to 15, comprising defining a plurality of geological targets along an intended drill path, calculating the probability of the drill path intersecting each of the geological targets, and deriving from the calculated probabilities the probability of the drill path staying within a corridor defined by the geological targets.

21. A method of assessing the value of a well, comprising supplying details of a hydrocarbon reservoir, selecting an optimum point of intersection of a drill path with the reservoir, calculating the probabilities of the drill path intersecting the reservoir at a plurality of points using a method as claimed in any one of claims 1 to 20, calculating the probability distribution of the value of recoverable hydrocarbons for each of the points of intersection and deriving from the calculated probabilities and the probability distribution a distribution of the value of the well.

22. A method as claimed in claim 14, in which the drill is partially withdrawn and the direction of drilling is changed if the probability of the drill path intersecting the geological target following correction of the first set of values is less than a predetermined value.