Note: Descriptions are shown in the official language in which they were submitted.
CA 02237013 1998-OS-07
- 1 -
TS 6013 PCT
METHOD OF QUALIFYING A BOREHOLE SURVEY
The present invention relates to a method of
qualifying a survey of a borehole formed in an earth
formation. In the field of wellbore drilling, e.g. for
the purpose of hydrocarbon exploitation, it is common
practice to measure the course of the wellbore as
drilling proceeds in order to ensure that the final
target zone in the earth formation is reached. Such
measurements can be conducted by using the earth gravity
field and the earth magnetic field as references, for
which purpose accelerometers and magnetometers are
incorporated in the drill string, at regular mutual
distances. Although these sensors in most cases. provide
reliable results, a second, independent, measurement is
generally considered necessary. The independent
measurement is commonly carried out using a gyroscope
which is lowered into the borehole after setting of -
casing in the borehole. Such procedure is costly and time
consuming, and it would be desirable to provide a method
which obviates the need for conducting independent
gyroscopic measurements.
EP-A-0 384 537 discloses a method for surveying a
borehole whereby directional data of the logged borehole
are computed on the basis of earth field parameters
measured by downhole sensors. To improve accuracy,
expected values of the earth gravitational field
intensity, earth magnetic field intensity and earth
magnetic dip angle are used in the method of Lagrange
multipliers to impose a three constraint fit on
accelerometer and magnetometer reading.
CA 02237013 1998-OS-07
i , _.. ..
- la -
EP-A-0 654"686 discloses a method whereby nominal
magnetic field strength and nominal dip angle are used in
combination with sensor readings to yield the best
estimate of the axial component of the magnetic field,
which best estimate is used for calculating the borehole
azimuth.
It is therefore an object of the invention to provide
a method of qualifying a survey of a borehole formed in
an earth formation, which method obviates the need for
conducting a second, independent, borehole survey.
In accordance with the invention there is provided a
method of qualifying a survey of a borehole formed in an
earth formation, the method comprising:
a) selecting a sensor for measuring an earth field
parameter and a borehole position parameter.i~ said
borehole;
MVM13/TS6013PCT
:.~:
AM~w~cD SHEET
CA 02237013 1998-OS-07
WO 97/19250 PCT/EP96/05170
- 2 -
b} determining theoretical measurement uncertainties of
said parameters when measured with the sensorp
c) operating said sensor so as to measure the position
w
parameter and the earth field parameter at a selected
position in the borehole;
d) determining the difference between the measured earth
field parameter and a known magnitude of said earth
field parameter at said position, and determining the
ratio.fof said difference and the theoretical
measurement uncertainty of the earth field parameter;
and
e) determining the uncertainty of the measured position
parameter from the product of said ratio and the
theoretical measurement uncertainty of the position
I5 parameter.
The earth field parameter can, for example, be the
earth gravity or the earth magnetic field strength, and
the borehole position parameter can, for example, be the
borehole inclination or the borehole azimuth.
The ratio of the difference between the measured
earth field parameter and a known magnitude of said earth
field parameter at said position, and the theoretical
measurement uncertainty of the position parameter, forms
a preliminary check on the quality of the survey. If the
measured earth field parameter is within the measurement
tolerance of this parameter, i.e. if the ratio does not
exceed the magnitude 1, then the survey is at least of
acceptable quality. If the ratio exceeds magnitude 1, the
survey is considered to be of poor quality. Thus the
ratio forms a preliminary measure for the quality of the
survey, and the product of this ratio and the theoretical
measurement uncertainty of the position parameter (as
r
determined in step d} forms the best guess of the survey
quality.
CA 02237013 1998-OS-07
WO 97/19250 PCT/EP96/OS170
- 3 -
The invention will be. illustrated hereinafter in more
detail and by way of example with reference to the
accompanying drawings in which:
' Fig. 1 shows schematically a solid state magnetic
survey tool;
' Fig. 2 shows a diagram of the difference between the
measured and known gravity field strength in an example
borehole, against the along borehole depth;
Fig. 3f shows a diagram of the difference between the
measured and'known magnetic field strength in the example
borehole, against the along borehole depth; and
Fig. 4 shows a diagram of the difference between the
measured and known dip-angle in the example borehole,
against the along borehole depth.
Referring to Fig. 1 there is shown a solid state
magnetic survey tool 1 which is suitable for use in the
method according to the invention. The tool includes a
plurality of sensors in the form of a triad of
accelerometers 3 and a triad of magnetometers 5 whereby
for ease of reference the individual accelerometers and
magnetometers are not indicated, only their respective
mutual orthogonal directions of measurement X, Y and Z
have been indicated. The triad of accelerometers measure
acceleration components and the triad of magnetometers 5
measure magnetic field components in these directions.
The tool 1 has a longitudinal axis 7 which coincides with
the longitudinal axis of a borehole (not shown) in which
the tool 1 has been lowered. The high side direction of
the tool 1 in the borehole is indicated as H.
During normal use of the tool 1, the tool 1 is
incorporated in a drill string (not shown) which is used
to deepen the borehole. At selected intervals in the
borehole, the tool 1 is operated so as to measure the
components in X, Y and Z directions of the earth gravity
field G and the earth magnetic field B. From the-measured
CA 02237013 1998-OS-07
WO 97/19250 . PCT/EP96105170
- 4 -
components of G and B, the magnitudes of the magnetic
field dip-angle D, the borehole inclination I and~the
borehole azimuth A are determined in a manner well-known
in the art. Before further processing these parameters,
the theoretical uncertainties of G, B, D, I and A are
determined on the basis of calibration data representing '
the class of sensors to which the sensors of the tool 1
pertains (i.e. bias, scale factor offset and misalign-
ment), the local earth magnetic field variations, the
planned borehole trajectory and the running conditions of
the sensor such as corrections applied to raw measurement
data. Since the theoretical uncertainties of G, B, D, I
and A depend mainly on the accuracy of the sensors and
the uncertainties of the earth field parameters due to
I5 slight variations thereof, the total theoretical
uncertainty of each one of these parameters can be
determined from the sum of the theoretical uncertainties
due to the sensor and the variation of the earth field
parameter. In this description the following notation is
used:
dGth,s = theoretical uncertainty of gravity field
strength G due to the sensor uncertainty;
dgth,s = theoretical uncertainty of magnetic field
strength B due to the sensor uncertainty;
dDth,s = theoretical uncertainty of dip-angle due to
the sensor uncertainty;
dBth,g = theoretical uncertainty of magnetic field
strength B due to the geomagnetic uncertainty;
dDth,g = theoretical uncertainty of dip-angle due to
the geomagnetic uncertainty;
dlth,s = theoretical uncertainty of borehole
inclination I due to the sensor uncertainty;
~th,s = theoretical uncertainty of borehole azimuth
A due to the sensor uncertainty;
CA 02237013 1998-OS-07
WO 97/19250 PCT/EP96/05I70
- 5 -
~th,g = theoretical uncertainty of borehole azimuth
A due to the geomagnetic uncertainty;
In a next phase the uncorrected gravity and magnetic
' field data obtained from the measurement are corrected
for axial and cross-axial magnetic interference and tool
' face dependent misalignment. A suitable correction method
is disclosed in EP-B-0193230, which correction method
uses as input data the local expected magnetic field
strength end dip-angle, and which provides output data in
the form of corrected gravity field strength, magnetic
field strength and dip-angle. These corrected earth field
parameter values are compared with the known local values
thereof, and for each parameter a difference between the
computed value and the known value is determined.
A preliminary assessment of the quality of the survey
is achieved by comparing the differences between the
corrected measured values and the known values of the
earth field parameters G, B and D with the measurement
uncertainties of G, B and D referred to above. For a
survey to be of acceptable quality, said difference
should not exceed the measurement uncertainty. In
Figs. 2, 3 and 4 example results of a borehole survey are
shown. Fig. 2 shows a diagram of the difference OGm
between the corrected measured value and the known value
of G, against the along borehole depth. Fig. 3 shows a
diagram of the difference ~Bm between the corrected
measured value and the known value of B, against the
along borehole depth. Fig. 4 shows a diagram of the
difference dDm between the corrected measured value and
the known value of D, against the along borehole depth.
The measurement uncertainties of the earth field
parameters in this example are:
uncertainty of G = dG = 0.0023 g (g being the
acceleration of gravity);
uncertainty of B =- dB = 0.25 E.t,T;
CA 02237013 1998-OS-07
WO 97/19250 PG'T/EP96/05170
- 6 -
uncertainty of D = dD.= 0.25 degrees.
These measurement uncertainties are indicated in the
Figs. in the form of upper and lower boundaries 10, 12
for G, upper and lower boundaries 14, 16 for B, and upper
and lower boundaries 18, 20 for D. As shown in the
Figures, all values of OGm, OBm and ODm are within the '
respective measurement uncertainties, and therefore these
values are considered acceptable.
To determine the uncertainty of the position
L0 parameters L and A as derived from the measured earth
field parameters G, B and D, the following ratios are
first determined:
OGm / dGth,s
dgm / dgth,s
~Dm / dDth,s
~,Bm / dBth, g
ODm / dGth,g
wherein
~Gm = difference between the corrected measured value
and the known value of G;
OBm = difference between the corrected measured value
and the known value of B;
ODm = difference between the corrected measured value
and the known value of D;
To compute the measured inclination uncertainty it is
assumed that the above indicated ratio of the gravity
field strength OGm / dGth,s represents the level of all
sources of uncertainties contributing to an inclination
uncertainty. If, for example, at a survey station in the
drill string the ratio eguals 0.85 then it is assumed
that all sensor uncertainties in the drillstring are at a
level of 0_85 times dIth,s. Therefore the measured
inclination uncertainty for all survey stations in the
drillstring is:
DIm = abs [ (~GTn / dGth, s~ filth, s~
CA 02237013 1998-05-07
WO 97/19250 PCT/EP96/05I70
_ 7 _
wherein
~It'Tl = measured inclination uncertainty due to .sensor
uncertainty.
' The measured azimuth uncertainty is determined in a
similar way, however two sources of uncertainty (sensor
and geomagnetic) may have contributed to the azimuth
uncertainty. For each source two ratios i.e. magnetic
field strength and dip-angle are derived, resulting in
four meas~,.red azimuth uncertainties:
DAsB = abs[(OBm / dBths)dAths]
mss, D = abs [ (ODm / dDth, s) With, s]
DAg. B = abs [ (OBm / dBth g) dAth g]
DAg.D = abs[(ODm / dDthg)dAthg]
The measured azimuth uncertainty DAm is taken to be
the maximum of the these values i.e.:
DAm = max[DAsB ; DAsD ; DAg~B ; DAg.D].
From the measured inclination and azimuth
uncertainties, the lateral position and upward position
uncertainties can be derived. These position
uncertainties are usually determined using a covariance
approach. For the sake of simplicity the following more
straightforward method can be applied:
LPUi = LPUi_1 + (AHDi - AHDi_1)(DAim sin Iim + DAi-lm
sin Ii_lm} / 2;
and
UPUi = UPUi_1 + (AHDi - AHDi_1)(DIim + DIi_1m) / 2.
wherein
LPUi = lateral position uncertainty at location i
AHDi = along hole depth at location i
dAim = measured azimuth uncertainty at location i
DIim = measured inclination uncertainty at location i
UPUi = upward position uncertainty at location i.
The lateral position uncertainties and the upward
position uncertainties thus determined are then compared
with the theoretical lateral and upward position
CA 02237013 1998-OS-07
WO 97/19250 PCT/EP96/OSI70
_ g _
uncertainties (derived from the theoretical inclination
and azimuth uncertainties) to provide an indicator of the
quality of the borehole survey.
y
