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25
Totals VV
ABX VV
ABY VV
ABZ VV
20
AMIC VV
AMID VV
ASX VV
ASY VV
ASZ VV
15
DBHG VV
DECG VV
DRFR VV
DSFS VV
10
DSTG VV
MBX VV
MBY VV
MBZ VV
MSX VV
5
MSY VV
MSZ VV
SAG VV
XYM1 VV
XYM2 VV
0
0
1000
2000
3000
4000
5000
6000
7000
8000
Figure 86c: Vertical Axis Errors.
From these graphs it is easy to see that as expected, the biggest error contribution in the vertical is from SAG. For this well the north and east errors are dominated by axial interference, followed by uncertainty in the magnetic model (i.e. declination uncertainty). Revisions to the MWD Error Model The reprinted MWD paper [1] by Hugh Williamson covers three distinct areas. It lays out the framework of the ISCWSA error model as discussed in the previous section, it defines the error sources applicable to MWD tools and it provides error magnitudes for these values, complete with a technical justification. Ap1.3.6 However, work on the ISCWSA error models is on-going and to date several revisions have been made to the basic model as defined in [1]. A summary of the revisions is presented here. More details can be found in [4,5,6].
The revisions to the MWD Error Model are:
Rev 0 Rev 1 Rev 2 Rev 3 Rev 4
As per SPE 67616 together with a number of typographical corrections [4] Changed to the gyro style misalignment with 4 terms and calculation options [4] Changes to the parameter values for the depth scale and stretch terms [4]
Replacement of all toolface dependant terms. [5] Look up table for BGGM uncertainties. [6]
Revisions 0 and 2 make some relatively minor corrections and changes to the original paper. However, revisions 1 and 3 are more major and replace all if the toolface dependant terms misalignments. Combined these constitute a large change to the model. Between them these two revisions replace 18 of the original 33 weighting functions, including the misalignment terms, the x and y axis sensor biases and scalefactors. 24 new sources are added to model these.
Revision 0 consists of some relatively minor typographical and parameters corrections.
Revision 1 introduces a new method of calculating the effect of tool misalignment to the borehole axis. This avoids the complication of toolface dependency in the misalignments and is considered to handle certain geometries, such helix- shaped, vertical boreholes better than the original MWD terms. This replaces the two existing misalignment terms MX and MY and introduces four new terms and three possible calculation options which are handled via two weight parameters.
The calculation options are:
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