 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates generally to techniques used in geophysical well
logging, and more particularly to new techniques for automatically
processing dipmeter signals or displacement measurements obtained between
these signals to produce more accurate dip and aximuth representations of
subsurface formations.
A common method of measuring the dip angle and direction or azimith of
subsurface formations employs a dipmeter tool passed through a borehole
drilled into the subsurface formations. This tool may apply any of
numerous means to obtain geophysical signals representative of variations
of a particular formation characteristic, such as its resistivity. One
such tool is described in the paper: "The High Resolution Dipmeter Tool",
by L. A. Allaud and J. Ringot, published in the May-June 1969 issue of The
Log Analyst.
Dip and azimuth measurements representing the inclination of a formation
characteristic or feature may be determined from dipmeter signals
containing information representing the intersection of such a feature at
three or more radially spaced points on the borehole surface. The
displacement between two points intersecting a common feature may be
determined, under favorable circumstances, by correlating pairs of the
dipmeter signals, each having a similar response to the common feature.
Two displacements between three related points determine the position of a
plane. The position of the plane is conveniently expressed by its dip
.theta., an angle measured from a reference (usually horizontal) plane and
its azimuth .PHI., an angle measured from a reference direction (usually
true North).
Typically, the dipmeter signals are recorded on computer compatible
magnetic tape at the well site for later processing. The recorded signals
are processed using any of several techniques. Manual, semi-automatic and
fully automatic processing may be used with the automatic processing being
performed with either analog or digital computers. When digital computers
are used, a computer program is also required.
A computer program to perform the digital processing operations is
described in a paper, "Automatic Computation of Dipmeter Logs Digitally
Recorded on Magnetic Tape" by J. H. Moran, et al and published in the
July, 1962 issue of the Journal of Petroleum Technology. An additional
computer program is described in the paper, "Computer Methods of Diplog
Correlation" by L. G. Schoonover, et al, pages 31-38, published in the
February 1973 issue of Society of Petroleum Engineers Journal. Further,
programs to process digitally-taped dipmeter data may be obtained from
digital computer manufacturers, such as IBM.
Results from digital processing are normally presented in tabular listings
as dip and azimuth measurements versus borehole depth. When desired, the
individual displacements found between the correlated curve pairs which
led to the dip and azimuth values may also be presented. Further, most
such programs will provide the ability to vary both the length of the
correlation interval and the step used to move this interval between each
correlation sequence. For the next sequence, the same correlation length
is used, but the actual interval correlated is moved by one correlation
step length.
At each step or depth level, one sequence of displacements between various
pairs of signal combinations may be obtained. A typical sequence includes
at least two displacements but may include a round of up to six
displacements in each sequence when four separate signals are employed,
for example. When a round of more than two displacements in one sequence
is obtained, the displacements may be combined into many more possibly
different combinations, each combination corresponding to perhaps a
different dip and azimuth measurement. Since only two related
displacements are required, it is common practice to utilize only what
appears to be the two best qualified displacements. All others are
discarded without further consideration, thereby producing only one result
per sequence. Further, little is retained as to the accuracy or quality of
either the discarded or the utilized displacements.
When large numbers of measurements result, as from recent high resolution
dipmeter techniques, tabular listings are usually augmented by graphic
presentations of dip and aazimuth representations. The graphic displays
vary with the interpretation objective, depending upon whether the purpose
is for stratigraphic or structural studies. Accordingly, relationships
between the corresponding dip and azimuth measurements and their
continuity with depth are considered in different manners.
Graphic displays used in stratigraphic analysis are typically the azimuth
frequency plot (no dip or depth representation) and the Schmidt net and
the Stereonet (azimuth versus dip but still no depth representation).
These nets and several variations thereof have known statistical
characteristics in that they may enhance either low or high dip
measurement point groupings. Note that in their use, the dip and azimuth
value for each measurement is combined and represented by a point in these
nets. A description of some of these displays and their application is
given in the paper "Stratigraphic Applications of Dipmeter Data in
Mid-Continent" by R. L. Campbell, Jr., published in September 1968 in the
American Association of Petroleum Geologists Bulletin.
Stratigraphic and structural analyses distinguish themselves in the type of
information needed. In stratigraphic analysis, the dipmeter signals
hopefully represent bedding planes within the boundaries of a given
geological unit. These bedding planes have little, if any, regional
extent. In structural analysis, a deliberate attempt may be made to mask
out such sedimentary features in favor of enhancing the boundaries of the
individual strata.
Short lengths (1 to 2 or 3 feet) of dipmeter signals are correlated to
obtain stratigraphic information while long lengths (10 to 20 or 30 feet)
of signals are often correlated to obtain structural information. While
use of long correlation lengths to obtain structrual dip has been standard
practice for some time, there are certain disadvantages associated with
this practice. One is that the use of long correlation lengths masks dip
patterns needed for stratigraphic analysis, thus additional computations
must be made using a short length to obtain stratigraphic information.
Another is that most long correlation length techniques may be influenced
by frequently occurring stratigraphic features having a common dip and
direction, even though each such feature is less pronounced than the
structural feature. Thus, the use of long correlation lengths does not
assure obtaining accurate structural dip information. Yet another
disadvantage is that such correlation techniques tend to ignore possibly
objectionable effects of rotation of the dipmeter tool within the long
correlation interval.
The preferred approach is to obtain the detailed information available only
from short signal intervals and then apply previously mentioned trend
analysis to separate the stratigraphic and structural dips. However, as
the correlation interval is shortened, the probability of obtaining a
completely erroneous displacement increases substantially. The wrong peak
on the correlation function produced in the correlation process may be
used to determine the displacement. Such invalid displacements may be
combined with valid displacements and produce an erroneous dip which add
scatter and confuse valid trends or when systematically erroneous, may
even appear as false trends.
As a compromise, longer correlation intervals than are actually desired are
employed to artificially reduce this scatter to an acceptable level so
that any valid trend which may be present might be found.
It is therefore an object of this invention to provide a technique to
reduce the scatter in dip and azimuth measurements used in determining
structural dip.
One technique which is employed to reduce scatter and find structural dip
is to average long intervals of dip measurements obtained from much
shorter intervals. Unfortunately, valid structural trends present only for
short intervals may be masked completely by such an averaging process.
Further, the resolution, quality and correlogram peak position obtained by
correlating short intervals tends to vary considerably; consequently, the
corresponding displacements may lack accuracy. Certain combinations of
such displacements may compound the variation and introduce unacceptable
inaccuracies in the resulting dip and azimuth measurements.
It is therefore an additional object of the present invention to provide a
technique to improve the accuracy and reduce the scatter of dip and
azimuth measurements without necessitating long interval averaging.
Some of the averaging techniques include a preliminary process of sorting
or discarding apparently stray dips before averaging to prevent their
contributing to the average. This process adds both time delays and
expense to a process which already produces too few dips for many
purposes. Further, some of the apparent strays may actually be part of a
valid trend which was unfortunately just sampled infrequently. Both the
discarding and averaging processes suppress such valid dips. Still
further, the apparently stray dip may have been produced by combining a
valid displacement with an invalid displacement. Unfortunately, discarding
this dip also discards the valid displacement information.
It is therefore a further object of the present invention to provide a
technique to minimize the likelihood of discarding valid displacement
information combined with invalid or inaccurate displacements.
As previously discussed, there are prior art techniques for statistically
analyzing either the dip or azimuth information for long interval trends.
These methods usually employ polar chart representations to classify the
dip and/or azimuth measurements. In these plots, the dip varies with
distance from either the center or the edge of the plots and the azimuth
varies with the radial distribution from the center of the plot.
However, when one considers the type of errors likely to take place in the
correlation processes, particularly in deviated holes, it is desirable
that any analysis not separate the dip from the azimuth values for the
purposes of the analysis. The analysis should be able to detect any
interrelationship between the dip and azimuth for the individual
measurements. More particularly, the analysis should respect the fact that
an erroneous displacement can be concealed when combined with another
displacement and expressed as a dip and azimuth measurement.
It is therefore a further object of the present invention to provide a
technique for analyzing displacements rather than combinations of
displacements or the resulting dip or azimuth measurements.
Prior art methods do attempt to select only the best displacements or
combinations thereof by assigning a quality rating according to the
correlation process which determined the displacement. The best rated
displacements are selected while discarding poor quality displacements.
The best rated displacements may be distorted or exaggerated due to
failure of the signal source to maintain its proper position in the
borehole, while poorer rated displacements may be obtained from sources in
a much better position to produce more accurate displacements.
It is therefore a particular object of the present invention to provide a
technique to retain even apparently poor quality or less accurate
displacements, as they may in fact be valid, until a better basis for
judging the validity of these displacements is available, thereby
preventing premature loss of this information.
There are numerous methods of obtaining displacement measurements between
pairs of geophysical signals. It is well known how to use one of several
different correlation functions to produce a correlogram--a function
representing the correlation factor, likeness or similarity of signal
features in given intervals of two signals versus the displacement
measured between the intervals. The displacement corresponding to the best
correlation or likeness is usually selected as the displacement
measurement and the corresponding correlation factor or likeness used to
express the quality. Also, the shape of the correlogram adjacent to this
best correlation factor is related to the displacement measurement
accuracy.
Another correlation method of obtaining displacements recognizes
characteristic signal features by their patterns and determines which
features on both pairs of signals correspond to one another by comparing
those characteristics. Each comparison yields a quality factor; the best
comparing pattern determines the feature correspondence and displacement,
and the nature of a characteristic of the pattern (for example, the rate
of change in signal amplitude) provides a measure of displacement
accuracy. Thus, with a variety of techniques available to determine
displacement measurements between pairs of signals, the corresponding
quality factors and some measure of displacement accuracy, it is desirable
to have a general technique to process these displacements which is
relatively independent of the technique used to obtain these measurements.
It is therefore an object of the present invention to provide a general
technique to process displacement measurements to determine the dip of a
formation and, particularly, to provide a technique which utilizes to
advantage the quality factor and displacement accuracy information
corresponding to each such displacement when available.
in accordance with these and other objects of the present invention,
apparatus and methods are provided which automatically determine with a
machine the dip of a formation feature reflected on geophysical signals
derived from signal sources located at different positions in a borehole
penetrating subsurface formations. The dip is determined by processing
displacements obtained between pairs of these signals. These displacements
may be obtained by comparing similarities of the signal features in a
given interval of the signals. Each displacement is represented by its
possible dips. At least two displacements obtained between different
intervals of at least one pair of geophysical signals are so represented.
The position of coincidence between these possible dip representations is
determined along with the corresponding dip of a formation feature.
In one embodiment of the invention, the possible dip representations
correspond to a line projected in a plane normal to the borehole or, if
the axis of the borehole varies substantially over the intervals
considered, the mean axis of the borehole. The orientation of the line is
determined from the orientation of the signal sources in the borehole. The
distance between the line and the intersection point of the borehole axis
with the plane represents the range of possible dip values for the
displacement and depends upon the magnitude of the displacement. When two
different displacements corresponding to the same formation but measured
with different orientations relative to features of the formation are so
represented, their line representations intersect. The intersection point
corresponds to the actual dip of the formation and its orientation
corresponds to the azimuth of this dip. Displacements between various
signal pairs within a particular interval and within at least one
additional nearby interval are represented by such lines. When several
intersections at different positions occur, the position of the highest
coincidence of intersections is determined as corresponding to a more
accurate formation dip than might be determined by combining only two
displacements.
In one aspect of the invention, a quality is associated with each
displacement and used to control the intensity or weight of the line
representation. For a given line width, the highest intensity line
corresponds to the best quality displacement. Therefore, the intensity or
weight of the line intersections will vary not only with the number of
lines intersecting at the same position, but also with the quality
associated with displacements represented by these lines. The position of
the most intense intersection determines the most accurate formation dip.
In another aspect of the invention, a displacement accuracy or error factor
is associated with each displacement and used to control the line width
accordingly. Wider lines represent less accurate displacements while
narrower lines represent more accurate displacements. The intensity of the
lines representing the same quality factor is decreased for the wider
lines and increased for the narrower lines; i.e., the intensity is
inversely proportional to the line width. In this embodiment, the
intensity of the line intersections varies not only with the number of
coincident intersections but also with both the accuracy and quality of
the displacements they represent. Again, the most intense intersection
corresponds to the most accurate dip.
Since all the displacements which may be obtained at a given depth level
between the various pair combinations of two or more signals may be
represented as well as the displacements from nearby depth levels, a large
number of displacements may be represented in the same plane. All the
displacements corresponding to the same formation dip intersect at
substantially the same position. Therefore, the position corresponding to
the highest coincidence of line intersections most accurately represents
the dip of this formation.
Further, since invalid and inaccurate displacements will not consistently
intersect at the same position as the valid displacements, no penalty is
imposed by representing all displacements regardless of their apparent
quality or accuracy. Therefore, there is not need to prematurely and
perhaps, somewhat arbitrarily, pass judgment on each displacement in order
to determine only the two best rated displacements required to determine
the formation dip in the prior art techniques. Rather, all displacements
are processed without regard to apparent quality or accuracy except as
mentioned above to vary the line width and intensity. Furher, the
formation dip may be determined and confirmed by substantially more than
two displacements. Still further, these displacements may have been
obtained between unrelated pairs of signals or between the same pair of
signals but at different nearby intervals or depth levels.
In highly deviated holes characteristically employed to exploit offshore
oil and gas producing formations, the ability to obtain accurate formation
dips from apparently inaccurate or poor quality displacement measurements
is an important advantage since signal measurement problems associated
with deviated boreholes often contribute to inaccuracies and misleading
quality factors for displacement measurements between signals obtained in
different sectors of the borehole. For example, the signal obtained from
the signal source in good contact with the borehole wall may not contain
the same signal features as a signal obtained from a signal source not in
contact with the borehole wall, because of mechanical problems associated
with maintaining the desired source position in such highly deviated
holes.
Displacements associated with poorly positioned signal sources are often
distorted. Further, it is difficult to detect when such distortion occurs.
For example, the quality factors associated with such distorted
displacements are often of the best quality factors in a particular
interval. However, in accordance with the features of the present
invention, the displacement representations of such distorted
displacements will not intersect at substantially the same position; i.e.,
they are scattered in accordance with their varying distortion, and
accordingly, do not coincide with the intersections of valid
displacements.
A further advantage of the present invention occurs when only two signal
sources contain reliable signal features, the remaining sources suffering
from signal distortion or attenuation problems. In such cases, the prior
art techniques requiring two displacements over corresponding intervals on
three different signals must attempt to utilize one of the bad signals in
order to produce any results whatsoever. However, in utilizing the present
invention, it is possible to obtain useful results under certain
circumstances with only two signals.
In addition to the above advantages, the techniques of the present
invention may be used to preserve the displacement integrity information
for consideration in the displacement processing in a manner which
enhances the determination of the dip corresponding to the more accurate
and better quality displacements. This is carried out by varying the
effect of each displacement representation in accordance with the
associated quality factor and/or displacement accuracy. For a given
displacement representation, the weight the displacement contributes to
the dip determination is increased for the higher integrity displacements,
while decreased for the lower integrity displacements. This allows each
displacement to possibly contribute in some degree to the dip
determination.
For a better understanding of the present invention, together with other
and further objects thereof, reference is had to the following description
taken in connection with the accompanying drawings, the scope of the
invention being pointed out in the appended claims.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 illustrates a method and apparatus for producing dip-meter signals,
obtaining displacements between pairs of these signals and processing
these displacements in accordance with one form of the invention.
FIG. 2 illustrates how certain references relative to the borehole tool are
measured.
FIG. 3 shows how displacements obtained between similar characteristics on
pairs of geophysical signals derived at spaced positions in a borehole are
related to the plane of a formation feature intersecting the borehole.
FIG. 4 illustrates the general characteristics of a correlogram in terms of
correlation quality, displacement and displacement accuracy determination.
FIG. 5A illustrates possibly corresponding displacements between two
similar signal elements, A and A', on one signal with similar signal
elements, B through D, on various other signals. FIG. 5B shows
correlograms related to the possibly corresponding displacements
illustrated in FIG. 5A.
FIG. 6A illustrates additional possibly corresponding displacements between
two similar characteristic features present on the signal curves in the
same interval.
FIGS. 6B through 6G illustrate, in simplified form, six correlograms and
the corresponding displacements usually selected in correlating various
pairs of the four curves illustrated in FIG. 6A.
FIG. 7A illustrates one technique for representing as a line in Plane P the
many possible dip vectors corresponding to a displacement between Points A
and B.
FIG. 7B illustrates in a plane normal to the borehole and including line
R-R' shown in FIG. 7A certain relationships pertaining to the orientation
and representation of a displacement representation.
FIG. 7C illustrates in a view along line R-R' shown in FIGS. 7A and 7B one
way the displacement between A and B-d.sub.A-B --may be represented in
Plane P.
FIG. 8 illustrates a technique to vary the line with respresenting a
Displacement d.sub.A-B in accordance with a displacement error range of
-e' to +e and therefore, uncertainties in the position of B relative to A
varying between E and F.
FIG. 9 illustrates certain relationships useful in translating vector
components between the Plane P normal to the mean borehole axis and a
horizontal Plane X-Y when deviated boreholes incline Plane P from the
horizontal plane.
FIG. 10 illustrates the representation of several displacements which vary
in displacement accuracy (line width), quality (line weight or intensity)
and orientation (direction of line).
FIG. 11A illustrates the Plane P when represented as an array of individual
counters oriented to the X.sub.1 -Y.sub.1 axis, each counter or cell
having a unique address which may, in one form, be considered as indices I
and J.
FIG. 11B shows a section of FIG. 11A and how a line defined by two line
borders B1 and B2 is regarded in relation to the cells it touches in the
array of cells.
FIG. 12A illustrates how displacements corresponding to different intervals
and different signal pairs within a given region, along with associated
depth, quality and error factors may be produced as input to a subsequent
process.
FIG. 12B illustrates how the input illustrated in FIG. 12A may be produced
by a technique employing correlation functions, correlograms, etc.
FIG. 12C illustrates how the input of FIG. 12A may be produced using a
pattern recognition type of correlation technique.
FIGS. 13 and 14 illustrate a procedure illustrative of the steps of one
form of the displacement processing technique.
FIG. 15 illustrates how different displacements corresponding to the same
formation dip may be obtained between a pair of signals when the borehole
dipmeter tool rotates.
FIG. 16 shows how the different displacements of FIG. 15 may be utilized in
accordance with the techniques of this invention to determine the true
displacement and therefore true formation dip and azimuth.
Referring now to FIG. 1, there is illustrated a method of acquiring and
processing signals obtained from a borehole investigating device commonly
known as a dipmeter. This device is described in one form in U.S. Pat. No.
3,521,154 issued July 21, 1970 to J. J. Maricelli. The purpose of the
dipmeter device is to obtain signals from three or more radially spaced
sources usually in the form of pads which contact the borehole wall.
Signals obtained from such sources reflect formation features at their
intersection with the borehole wall and are useful in determining the
orientation of the formations penetrated by the borehole.
Typical earth formations are represented by the shale formations 13 and 14
shown in FIG. 1, and intervening sand formation 15. Typical formation
features are boundaries 16 and 17 shown between these formations.
As shown in FIG. 1, the borehole apparatus 18 is lowered on cable 30 into a
borehole 10 for investigating the earth's formations. The downhole
investigating device 18 is adapted for movement through the borehole 10
and as illustrated, includes four pads designated 19, 20, 21 and 22 (the
front pad 19 obscures the view of back pad member 21 which is not shown).
The pad members 19 through 22 are adapted to derive measurements at the
wall of the borehole. Each pad includes a survey electrode shown as Ao.
One of the pads, herein designated as pad 19, may contain an additional
survey electrode Ao' useful in determining the speed of the tool. Each
survey electrode is surrounded by an insulating material 48. The
insulating material and thus all the survey electrodes are surrounded by a
main metal portion 45 of the pad. The metal portion 45 of each pad, along
with certain other parts of the apparatus, comprise a focussing system for
confining the survey current emitted from each of the different survey
electrodes into the desired focussed pattern. Survey signals
representative of changes in the formation opposite each pad are obtained
from circuits comprising Ao electrodes, focussing elements, and a current
return electrode B shown in FIG. 1.
The upper end of the borehole tool 18 as shown in FIG. 1 is connected by
means of an armored multiconductor cable 30 to a suitable apparatus at the
surface for raising and lowering the downhole investigating device through
the borehole 10. Mechanical and electrical control of the downhole device
may be accomplished with the multiconductor cable which passes from the
| | |
