GB2396697A

GB2396697A - Depth correction of drillstring measurements

Info

Publication number: GB2396697A
Application number: GB0322966A
Authority: GB
Inventors: Walter Aldred; Maurice Ringer
Original assignee: Schlumberger Holdings Ltd
Current assignee: Schlumberger Holdings Ltd
Priority date: 2002-12-27
Filing date: 2003-10-01
Publication date: 2004-06-30
Also published as: GB2412678A; NO337843B1; GB0511693D0; AU2003290305A1; CA2511203A1; AU2003290305A8; WO2004059123A1; GB0322966D0; CA2511203C; GB2412678B; NO20053079L; NO20053079D0

Abstract

A method automatically adjusts for errors in the depth associated with measurements taken from a drillstring during the drilling of a borehole. Downhole measurements taken at a number of depth locations within a well bore are received, and corrections for errors in the depth of these locations are calculated. The errors are due to stretch in the length of the drillstring. The correction is calculated based on a stress model which uses the operational state of the drilling rig, a description of the drillstring, borehole information, weight on bit and a friction factor. The pipe stretch is then computed using the stress model and a stretch correction applied to the measured depth to provide a corrected depth.

Description

SYSTEM AND METHOD FOR CORRECTING ERRORS IN DEPTH FOR

MEASUREMENTS MADE WHILE DRILLING

FIELD OF THE INVENTION:

5 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.

BACKGROUND OF THE INVENTION:

During the drilling phase of the construction of a hydrocarbon wellbore, the length of the drillstring 15 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 20 length of the borehole there may cause gaps or lost data, when the uncorrected depth is used with logs of data measured during with sensors mounted on the drillstring, such as LED and MAD logs.

SUMMARY OF THE INVENTION:

According to the invention a method is provided for automatically correcting for depth errors in 30 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 a drillstring used to drill the wellbore, automatically calculating corrections for errors in the depth of the locations, and making use of the measured data having the 5 depths corrected.

BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1 shows a scheme for correcting depth 10 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 15 Figure 3 shows data that has been corrected according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION:

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 25 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 30 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 5 surface. According to the invention, a method is provided for correcting the measurement of depth at surface for these parameters. The corrected depth is then used to assign depths to data measured downhole.

10 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. 20 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 co-pending US Patent Application Serial No. 25 10/400,125 entitled "System and Method for Rig State Detection,H filed on 26 March 2003, which is a continuationin-part of co-pending US Patent Application Serial No. 10/330,634 filed on 27 December 2002. Both of these applications are hereby incorporated herein by 30 reference.

4) A model for computing the stress in the drillstring is given the operation 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 5 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.

10 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 Drillsafe.

8) Pipe stretch is then computed using the 15 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 20 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 25 invention. The first frame of Figure 2 shows a surface time verse depth plot, the first section is drilling without surface rotation. As a result all of the friction force is opposing the motion of the drillsting along the hole. As a result whilst drilling the direction of the 30 friction force is towards surface. The driller then stops drill pulls the drillstring off bottom and then runs back to 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 5 as a result the position of the bottom of the hole as measure from surface appears shallower. In the second frame the resistivity data are shown plot against the same time scale. In the third frame the resistivity data are plotted against the apparent depth at which they were 10 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 15 errors in depth results not only in lost data but also the thickness of the formation section appearing thinner.

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 20 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 measure depth. Thus, 25 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 30 conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this

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 5 the spirit and scope of the invention.

Claims

What is claimed is:

5 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 10 within the wellbore from at least one sensor located on a drillstring used to drill the wellbore; automatically calculating corrections for errors in the depth of the locations; and making use of the measured data having the 15 depths corrected.
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 20 support the drillstring at the times when the measurements are taken.
3. A method according to claim 1 wherein further comprising the step of measuring the length of portions 25 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 is 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 5 said measurements.
6. A method according to claim 1 wherein said step of calculating corrections is based in part on estimates of stretch of the length of the drillstring.