SYSTEM AND METHOD FOR CORRECTING ERRORS IN DEPTH FOR MEASUREMENTS MADE WHILE DRILLING

SYSTEME ET PROCEDE POUR CORRIGER DES ERREURS DE PROFONDEUR SUR DES MESURES EFFECTUEES AU COURS D'UN FORAGE

English Abstract

A method and system is disclosed for automatically correcting for depth errors
in measurements taken from a drillstring during the drilling phase of the
construction of a hydrocarbon wellbore. The correction is based on a stress
profile which in turn is based on the states of the drilling rig, drill string
description length spec, borehole description trajectory, friction factor and
weight on bit.

French Abstract

La présente invention concerne un procédé et un système pour corriger automatiquement des erreurs de profondeur sur des mesures effectuées sur un train de forage au cours de la phase de forage de la construction d'un puits de forage d'hydrocarbures. La correction se base sur un profil de contrainte qui se base lui-même sur les états de l'appareil de forage, la spécification de longueur décrite par le train de forage, la trajectoire décrite par le trou de forage, le coefficient de frottement et le poids exercé sur le trépan.

Claims

CLAIMS:
1. A method for automatically correcting for depth errors in measurements
taken from a drillstring comprising the steps of:
receiving data representing measurements taken in a hydrocarbon
wellbore at a plurality of depths within the wellbore from at least one sensor
located
on the drillstring used to drill the wellbore;
automatically calculating corrections for errors in the depths, wherein
said step of calculating corrections is based in part on estimates of stretch
of the
length of the drillstring; and
using the corrected depths to determine a depth of the measured data.
2. A method according to claim 1, wherein the step of automatically
calculating the corrections is based at least in part on the state of a
drilling rig used to
support the drillstring at the times when the measurements are taken.
3. A method according to claim 1 or 2, further comprising the step of
measuring the length of portions of the drillstring prior to insertion into
the wellbore.
4. A method according to claim 3, wherein a time versus depth log is
constructed using at least the measured length of portions of the drillstring.
5. A method according to claim 4, wherein the calculated corrections are
applied to time versus depth log to generate a corrected time versus depth
log, and
wherein the corrected time versus depth log is combined with the data
representing
measurements taken in the wellbore such that a corrected depth can be
attributed to
said measurements.
6. A method according to any one of claims 1 to 5, wherein said step of
calculating corrections includes the step of computing a hookload.
7

7. A method according to claim 6, wherein said step of calculating
corrections includes the step of computing a calculated hookload and varying
the
friction factor or the weight on bit until the hookload and the calculated
hookload
match.
8

Description

CA 02540648 2011-01-26
72424-109
SYSTEM AND METHOD FOR CORRECTING ERRORS IN DEPTH FOR
MEASUREMENTS MADE WHILE DRILLING
FIELD OF THE INVENTION:
The present invention relates to the field of
measurements made during the drilling phase of a
hydrocarbon borehole. In particular, the invention
relates to an automated method for correcting errors in
depth for such measurements.
1.0
BACKGROUND OF THE INVENTION:
During the drilling phase of the construction
of a hydrocarbon wellbore, the length of the drillstring
in the borehole is used to estimate the measured depth
(or along hole length) of a borehole, it is assumed that
the pipe is inelastic and therefore does not stretch.
However, discrepancies in the length of the borehole
estimated at surface during rig operations and the actual
length of the borehole there may cause gaps or lost data,
when the uncorrected depth is used with logs of data
measured with sensors mounted on the drillstring,
such as LWD and MWD logs.
SUMMARY OF THE INVENTION:
According to the invention,a method is provided
for automatically correcting for depth errors in
measurements taken from a drillstring comprising the
steps of receiving data representing measurements taken
in a hydrocarbon wellbore at a plurality of depths within
1

CA 02540648 2011-12-12
72424-109
the wellbore from at least one sensor located on the drillstring used to drill
the
wellbore, automatically calculating corrections for errors in the depths, and
using the
corrected depths to determine a depth of the measured data. The step of
calculating
corrections is based in part on estimates of stretch of the length of the
drill string.
BRIEF DESCRIPTION OF THE DRAWINGS:
Figure 1 shows a scheme for correcting depth for measurements made
from a drillstring according to a preferred embodiment of the invention;
Figure 2 shows an example of data prior to correction according to a
preferred embodiment of the invention; and
Figure 3 shows data that has been corrected according to a preferred
embodiment of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS:
The length of the drillstring in the borehole is used to estimate the
measured depth (or along hole length) of a borehole. According to the
invention, the
depth is corrected. For real drill strings the assumption that the drillstring
is inelastic
is not valid. The length of the drillpipe is a function of several parameters
including
temperature, pressure, and stress. According to the invention, corrections are
calculated based on at least the stress on the drillstring. In particular, a
correction is
calculated based on the un-deformed length of the drillstring and the stress
due to the
buoyant drillstring weight, weight on bit and frictional forces due to contact
with the
borehole acting along the length of the drillstring. Two of these parameters,
friction
factor and weight on bit vary depending on the rig operation and the drillers
input at
surface. According to the invention, a method is provided for correcting the
measurement of depth at
2

CA 02540648 2011-01-26
72424-109
surface for these parameters. The corrected depth is then used to assign
depths
to data measured downhole.
Figure 1 shows a scheme for correcting depth for measurements
made from a drillstring according to a preferred embodiment of the invention.
According to a preferred embodiment of the invention the following steps are
undertaken for each time step:
1) The drillstring description, dimensions pipe weight per unit length
are input, the pipe length as measured at surface is updated from real-time
measurements.
2) The borehole trajectory, inclination and azimuth are input and
updated from downhole measurements in real-time.
3) The rig operation is computed preferably as described in
US Patent No. 7,128,167 entitled "System and Method for Rig State Detection".
4) A model for computing the stress in the drillstring is selected.
5) A friction factor is selected for the given rig state.
6) Weight on bit is either estimated from the hookload and total
hookload or from weight on bit measured downhole.
7) From these inputs the model is used to compute the hookload. If
the hookload is within tolerances equal to the measured hookload the stress
profile is accepted and used to compute the pipe stretch. If it is not then
the
friction factor or the weight on bit are varied until the hookload and the
calculated
hookloads match. The models used here and in step 4 above preferably known
models such as DrillsafeTM.
3

CA 02540648 2011-01-26
72424-109
8) Pipe stretch is then computed using the stress profile.
9) The stretch correction is applied to measured depth to give the
corrected depth and time stamped.
10) Time stamped downhole data is the associated with the
corrected surface measured depths with the same time stamp.
Figure 2 shows an example of data prior to correction according to a
preferred embodiment of the invention. The first frame of Figure 2 shows a
surface time versus depth plot, the first section is drilling without surface
rotation.
As a result all of the friction force is opposing the motion of the
drillstring along the
hole. As a result whilst drilling the direction of the friction force is
towards surface.
The driller then stops the drill, pulls the drillstring off the bottom and
then runs
back to the bottom rotating the drillstring. When rotating, the friction force
opposes the direction of rotation and as a result the frictional force along
the
borehole falls to close to zero. This results in an increase in the tension in
the
pipe and therefore an increase in the pipe stretch. As a result the position
of the
bottom of the hole as measured from the surface appears shallower. In the
second frame the resistivity data are shown plotted against the same time
scale.
In the third frame the resistivity data are plotted against the apparent depth
at
which they were measured. It can be seen that there is a section of data in
lighter
grey that in terms of depths overlaps previously recorded data.
Conventionally,
these data would be discarded. The darker line represents the data that would
be
kept. Thus, failure to compensate for errors in depth results not only in lost
data
but also the thickness of the formation section appearing thinner.
4

CA 02540648 2011-01-26
72424-109
Figure 3 shows data that has been corrected according to a
preferred embodiment of the invention. The stress profile and the pipe stretch
have been calculated according to an appropriate model for the rig operation.
Note that in the first frame, the depth at which drilling resumes is very
close to the
depth at which it stopped. Secondly, the measured resisitivities are properly
allocated to the measured depth. Thus, according this embodiment of the
invention, there is no loss of data or gaps, (the remaining grey points are
recorded
off bottom).
While the invention has been described in conjunction with the
exemplary embodiments described above, many equivalent modifications and
variations will be apparent to those skilled in the art when given this
5

CA 02540648 2006-03-29
WO 2005/033473 PCT/GB2004/004123
disclosure. Accordingly, the exemplary embodiments of
the invention set forth above are considered to.be
illustrative and not limiting. Various changes to the
described embodiments may be made without departing from
the spirit and scope of the invention.
6

Representative Drawing