Method of assessing positional uncertainty in drilling a well

Details Method of assessing positional uncertainty in drilling a well Method of assessing positional uncertainty in drilling a well

2002-06-06

2004-12-28

Dang, Hoang (Department: 3672)

Boring or penetrating the earth

With signaling, indicating, testing or measuring

Tool position direction or inclination measuring or...

C175S061000, C702S009000

Reexamination Certificate

active

06834732

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of assessing positional uncertainty in drilling a well.
BACKGROUND OF THE INVENTION
In order to drill a well, it is necessary to define a geological target for the placement of the well. The geological target is a surface which is bounded by geological factors such as the position of geological faults and the extension of an oil-water contact. The geological target is defined by a geophysicist and is based on data about geological structures. Such data may be obtained, for example, in the form of seismic data or as data from nearby existing wells.
Some geological target boundaries are more important than others in the sense that it is more important to be inside some boundaries than others. For example, if a drill bit misses an oil zone, it will never be possible to produce oil. The geophysicist thus defines a reduced geological target whose boundaries are judged to be sufficiently remote from the boundaries of the geological target to ensure that there is a very good chance that the wellbore will not stray outside the geological target.
FIG. 1
of the accompany drawings illustrates such a conventional geological target
1
in the form of a rectangular surface having boundaries
2
to
5
. Each of the boundaries
2
to
5
is associated with a risk in the form of a percentage associated with the drill bore straying outside the boundary. Thus, the risk of straying outside the boundary
2
should be no greater than 1% whereas the risks of straying outside the boundaries
3
to
5
should be no greater than 2.5%.
Within the conventional geological target
1
shown in
FIG. 1
, various geological structures are illustrated by way of example. A conventional reduced geological target
6
is also illustrated and this is defined by the geophysicist on the basis of experience.
Thus, the geophysicist judges how far the boundaries of the conventional reduced geological target
6
should be spaced from the boundaries of the conventional geological target
1
. Because of the higher risk associated with the boundary
2
, which corresponds to a geological fault, the corresponding boundary
7
of the conventional reduced geological target
6
is more remote than the boundary
8
with respect to the corresponding boundary
4
.
The “risk values” shown in
FIG. 1
as percentages are effectively the inverse of the acceptable probabilities of straying outside the respective boundaries. These values are generally referred to as “hardline values” and risks or probabilities are conventionally only assigned to boundaries which must not be crossed.
The geological data about the nature and location of structures beneath the surface of the earth are not precise; if such data were precise, then there would be no need for the conventional reduced geological target. There is a degree of uncertainty in the actual position of geological structures compared with the positions indicated by seismic and other data. This results in the need for the reduced target, whose purpose is to set an actual target for a driller to aim for during drilling of the well. The actual uncertainty in position varies from situation to situation but it is possible to provide some measure of the inaccuracy of the geological data. The geophysicist uses judgement in deciding the size and location of the conventional reduced geological target
6
within the conventional geological target
1
.
Drilling of a well is also not a precise process. The geophysicist supplies the conventional reduced geological target
6
to a drilling engineer who must then define a drillers target within the conventional reduced geological target
6
. The actual position of a drill bit compared with the measured or estimated position is also subject to inaccuracies. Such inaccuracies depend, for example, on the well trajectory geometry and the accuracy of drill position measuring equipment located behind the drill bit. The position measuring equipment can provide measurements of different accuracies depending on the type of measuring equipment and, in particular, on the cost thereof. A typical drillers target is shown at
9
.
The drilling engineer has to define the drillers target such that, if the position of the drill bit is measured to be inside the drillers target, there is a predetermined likelihood that the well will actually be within the conventional reduced geological target
6
and hence, allowing for the inaccuracies in the geological data, the actual positioning of the well will be acceptable. The drilling engineer must judge whether more money should be spent on the drill position measuring equipment in order to improve the chances of drilling the well in the correct place.
SUMMARY OF THE INVENTION
The present invention may be characterized as a method of assessing positional uncertainty in drilling a well. Such a method may be used, for example, at the planning stage in order to direct the drilling operation and to assess whether it is worth while to drill a particular well. The method may also be used in real time to control the drilling of a well.
According to a first aspect of the invention, there is provided 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.
At least one of the first, second and third sets of values may comprise parameters of an error ellipsoid with a predetermined confidence interval referred to a Cartesian coordinate system.
At least one of the first, second and third sets of values may comprise a covariance matrix referred to a Cartesian coordinate system.
The first and second sets of values may be referred to different coordinate systems and the combining step may comprise transforming the first and second sets of values to fourth and fifth sets of values, respectively, referred to a common coordinate system and summing the corresponding values of the fourth and fifth sets to form the third set of values.
The probability may be calculated as a normal distribution.
The method may comprise defining a geological target as a finite surface and selecting a desired point of intersection of the drill path with the geological target. The method may comprise calculating the probability of the drill path intersecting the geological target. The geological target may be a polygon. The geological target may be rectangular. Each side of the polygon may be ascribed a maximum acceptable probability of the drill path missing the geological target on that side.
The method may comprise calculating the probability of the drill bit being at a predetermined distance from the geological target.
The method may comprise 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. The marker point may be 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. The geological target may be selected to coincide with a predetermined geological structure, the marker point may be disposed at the predetermined geological structure, and the position of the predetermined geological structure may be derived from a pilot well. The marker point may be observed during drilling using means disposed at or adjacent the drill bit. Such means may, for example, comprise se

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