Dipmeter displacement qualifying technique

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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 azimuth representations of subsurface formations.

A common method of measuring the dip angle and direction or azimuth 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 dip-meter 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 different 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 be also 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 position of the sources or dipmeter pads corresponding to 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 azimuth 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.

For stratigraphic purposes, trends of adjacent dip measurements with depth are usually used to classify the measurements. For example, measurements representing a trend of rapidly increasing dip with depth will be considered separately from measurements representing a trend of rapidly decreasing dip with depth.

In the stratigraphic analysis, it is important that the azimuth of these dips must remain substantially constant and thereby represent the general direction of sediment transport or perhaps the probable direction of down dip thickening. Also, dipmeter results are combined in a given analysis from intervals corresponding to a given depositional or stratigraphic unit.

Graphic displays used for stratigraphic analysis often ignore the actual depths once the above dip versus depth trend for a given azimuth range qualifies a group of measurements. Further, since in many cases the actual dip angle is not important and only the dip azimuth is significant, the dip angle may be completely ignored in the graphic display. Such displays are designed to statistically determine the azimuth corresponding to a primary and perhaps a secondary direction of transport or deposition.

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 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 structural 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 current correlation techniques tend to ignore possibly objectionable effects of rotation of the dip-meter tool within the long correlation interval.

The preferred approach is to obtain the detailed information available only from short correlation 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 determined from short correlation intervals.

One technique which is employed to reduce scatter and find dip and azimuth trends is to average long intervals of dip measurements obtained from much shorter intervals. Unfortunately, the valid trends present only as short intervals may be masked completely by such an averaging process. Further, the resolution and position of the correct peak obtained by correlating short intervals tends to vary considerably, consequently, the corresponding displacements 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.

It is therefore a further object of the present invention to provide an automatic technique to improve the accuracy of dip and azimuth determinations without reducing the number of valid dips or discarding dips because they do not comply with some long interval trend.

When such averaging techniques are employed, the intervals to be averaged are often chosen arbitrarily such as every 100 feet or the like. Yet such zoning or sample grouping is an important factor in most statistical analysis. In some techniques, independent geological information is examined (usually manually) to select specific zones to be averaged. This latter process requires considerable time as well as accurate coordination of the depths of the geological information and the dipmeter information. This depth coordination may be a problem in deviating holes where the dipmeter information might not correspond to true depths. It would therefore be advantageous to have the determination of zones be made from the dipmeter data itself.

It is therefore a further object of the present invention to provide a technique for automatically zoning dipmeter information by analyzing the dipmeter information itself.

As previously discussed, these 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 erroneous displacements can be concealed when expressed only as the resulting dip and azimuth measurements.

It is therefore a further object of the present invention to provide a technique for analyzing displacements and combinations of displacements rather than computing and analyzing 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. Yet the best 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.

Therefore, it is a particular object of the present invention to consider the relative position of the signal sources in selecting the most valid displacements.

In accordance with these and other objects of the present invention, apparatus and methods are provided for automatically determining with a machine the most valid combination of displacements from a plurality of displacements and combinations thereof. These displacements may be obtained between pairs of geophysical signals derived from separate signal sources located on a dipmeter apparatus passed through a borehole penetrating subsurface earth formations. Displacements between pairs of geophysical signals may be produced by comparing the similarity of signal features for various displacements on said signals. When it is determined that these displacements are substantially devoid of closure error and thereby correspond to the same formation feature, the signal source most likely not to be in the proper position in the borehole is located and those displacements common to said source are nullified from determining the position of the formation features reflected in the signal features of said geophysical signals.

It has been discovered that for many types of dipmeter apparatus, the signal source or pad located nearest the top side of the borehole when the borehole is deviated substantially from the vertical, tends to lose its proper position in respect to the borehole wall. Further, it has been discovered that the type of focussing normally associated with these dipmeters electrically extends the effect of this pad, overcoming to a large extent the lack of contact with the borehole wall, and in effect, repositioning the pad on the borehole wall. However, the corresponding diameter measurement does not reflect the effective position and thereby produces displacements which are distorted or exaggerated. When no considerations for the above are made, the dips computed from a displacement combination which includes displacements between signals obtained from such floating pads are also exaggerated. By locating the signal source most likely not to be in the proper position and disqualifying or nullifying those displacements associated with this source, particularly when planarity errors are known to exist only those displacements remaining may be selected as the most valid displacements.

In one form of the invention, a closure error is computed and if a substantial closure error is found, it is assumed that one or more displacements correspond to different formation features and the degree of distortion or exaggeration from a planar formation feature cannot be determined. However, if little closure error is found it is assumed that all the displacements used in the closure computation correspond to substantially the same formation features; therefore, planarity error, distortion, or exaggeration may be evaluated.

In one aspect of the invention, the actual position of the formation feature is compared with the expected position of the formation feature on a signal derived from a given source. These positions are determined from the given relationships combining related displacements. The displacements corresponding to the largest difference between the expected and actual positions are considered to be the most exaggerated or distorted and therefore disqualified as valid displacements.

In another aspect of the invention, the source most likely not to be in the proper position is located. The displacement relationships specific to that source may then be used to determine the degree of distortion or exaggeration. If this degree exceeds a given range it implies that the most likely source not to be in the proper position was in fact out of position. The displacements associated with a signal obtained from this source may on one hand be disqualified from further consideration as valid displacements or, on the other hand, corrected to eliminate the distortion. When the above technique is applied in highly deviated holes to determine those displacements which are valid and may therefore be combined as possible corresponding displacements, and these possibly corresponding displacements are used in a further technique, a substantial improvement in dips determined from the combination of techniques is obtained.

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 dipmeter 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. 4A illustrates in a view looking down the borehole one position a borehole tool may take in a deviated borehole.

FIG. 4B illustrates the side view corresponding to FIG. 4A and shows how the measured diameter D.sub.1-3 may not correspond to the effective diameter De.

FIG. 5A illustrates possibly corresponding displacements between two similar characteristics, A and A', on one signal and similar characteristics, 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 characteristics present on the signal curves in the same correlation interval.

FIGS. 6A 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 the general characteristics of a correlogram in terms of correlation quality and displacement determination.

FIGS. 7B and 7C illustrate how spurious peaks in a correlogram may cause temporary changes in what otherwise might be an essentially contiguous sequence of stable displacements.

FIG. 7D illustrates how a valid change in the correlograms and displacements corresponding to a new formation dip may result in a break in the continuity of substantially stable displacements.

FIG. 8A illustrates in simplified form how displacement distances Z determined between an electrode plane on the borehole tool and features on curves or signals derived at the electrodes may be treated algebraically to determine displacements.

FIG. 8B illustrates in relation to diagonal pairs of electrodes the displacement distances depicted in FIG. 8A.

FIG. 8C illustrates how these distances or displacements appear in a plane parallel to the borehole axis and passing through diagonal electrodes 1 and 3.

FIG. 8D illustrates, like FIG. 8C, distances or displacements but now in the plane passing through diagonal electrodes 2 and 4.

FIG. 9A illustrates the displacement relationships corresponding to the ideal case of good closure and planarity.

FIG. 9B illustrates the case corresponding to poor closure between four adjacent displacements.

FIGS. 9C and 9D illustrates the case where good closure but a lack of planarity may result in several possible combinations of displacements and corresponding planes.

FIG. 9E illustrates the relationship between four possible combinations of displacements and their corresponding planes.

FIG. 9F illustrates some additional planes which may result when diagonal displacements are combined as possibly corresponding displacements.

FIG. 10A illustrates how three-dimensional projections of vectors relative to the four pads of the borehole tool may be transformed into two dimensions such that the density of vectors in a given area in the three-dimensional projection is not changed in the two-dimensional transformation.

FIG. 10B illustrates one method of dividing the two-dimensional transformation shown in FIG. 10A into a classification system oriented relative to the position of the pads on the borehole tool, the position of magnetic North and the top side of the borehole.

FIG. 11A illustrates the two-dimensional transformation when represented as an array of individual counters oriented to the tool, each counter or cell having a unique address which may, in one form, be considered as indices I and J.

FIGS. 11B and 11C show how a finite vector falling in one counter or cell may be smeared from its particular counter into adjacent counters.

FIG. 12 illustrates a two-dimensional graphic display corresponding to the two-dimensional transform which may be produced as one feature of the invention.

FIG. 13A shows a sequence of displacements obtained from correlations between various pairs of four signals and illustrates zones containing sequences of displacements indicated to be stable for various possible combinations of displacements.

FIG. 13B illustrates how diagonal displacements and their corresponding diameters can be combined and resolved as tangent vectors.

FIG. 14 illustrates in numerical form sequences of displacements containing zones of stable sequences and adjacent stable sequences which have been determined from displacements obtained by correlating pairs of signals derived from adjacent sources.

FIG. 15A illustrates some preliminary steps in the procedure used to obtain sequences of displacements and corresponding quality diameter and inclinometer information.

FIG. 15B illustrates detailed steps in the procedure to determine substantially stable sequences of displacements.

FIG. 15C illustrates a procedure to determine pairs of adjacent stable sequences.

FIG. 16 describes a step in a procedure to automatically determine the starting and ending boundaries of zones of adjacent pairs of substantially stable sequences.

FIG. 17 illustrates the steps of a program corresponding to the procedure illustrated in FIG. 16.

FIG. 18 illustrates in detail the steps of the procedure to combine corresponding displacements to produce functions representing the angular relationship between the displacements.

FIG. 19 illustrates the detailed steps in a procedure to classify combinations of displacements produced in the process illustrated in FIG. 18.

FIG. 20 illustrates the detailed steps of a procedure to analyze classified combinations of displacements to determine the positions of various classes and the relative position of the dominant class.

FIG. 21 illustrates the detailed steps of a procedure to determine the relative position of features in each signal interval corresponding to the dominant class determined as illustrated in FIG. 20.

FIG. 22 illustrates the improvement obtained in the determination of formation dip and azimuth measurements when techniques of the described invention are applied.

FIG. 23 illustrates certain relationships useful in determing when a source of a signal is displaced from the borehole wall.

FIGS. 24A and 24B illustrate the determination of a meaningful diameter in non-circular holes.

TABLE I illustrates certain relationships between known displacements and equivalent orthogonal displacements.

TABLE II illustrates displacement ratios corresponding to various signal sources or pads.

TABLE IIIA shows by example tabulations of stored cell addresses, cell contents and other entries which may be produced by the process illustrated in FIGS. 19 and 20.

TABLE IIIB shows by example the results of the processing illustrated in FIG. 20 when applied to the example of TABLE IIIA.

TABLE IV shows by example the cell addresses corresponding to the position of clusters of cells or classes of varying rank.

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 downhole tool 18 through the borehole to a sheave wheel 31 at the surface and then to a suitable drum and winch mechanism 32.

Electrical connections between various conductors of the multiconductor cable, which are connected downhole to the previously described electrodes, and various electrical circuits at the surface of the earth are accomplished by means of a suitable multi-element slipring and brush contact assembly 34. In this manner, the signals which originate from the downhole investigating device are supplied to the signal processing circuits 39 which in turn supply the signals to a signal conditioner 40 and recorder 41. A suitable signal generator 42 supplies current to the downhole tool via transformer 50 and to signal processing circuits located at the surface. More details of such circuits are described in the aforementioned Maricelli patent.

Signals obtained from the downhole device may be recorded graphically by a film recorder 41. One such recorder is described in U.S. Pat. No. 3,453,530 issued to G. E. Attali on July 1, 1969. In addition, the signals may be processed to obtain discrete samples and recorded on digital tape. A suitable digital tape recorder is described in U.S. Pat. No. 3,648,278 issued to G. K. Miller, et al on Mar. 7, 1972.

The signals may be sampled by driving sampling devices, such as those described in the above-mentioned digital tape recorder, by the cable motion as measured at the surface. For example, the cable length measuring wheel shown as 34A in FIG. 1 may be used in controlling the signal processing, sampling and recording subcycles as indicated by signal line 34B. Therefore, each sample of a measured signal corresponds to one increment in depth and displacements determined between such sample signals are indicative of depth displacements.

The dipmeter signals or samples thereof may also be transmitted directly to a computer. The computer may be located at the well site or the signals may be transmitted via a transmission system to a remote computer location. One transmission system which may be used is described in U.S. Pat. No. 3,599,156 issued to G. K. Miller, et al on Aug. 10, 1971.

The recorded or transmitted signals may be processed as digital measurements by general purpose digital computing apparatus properly programmed in a manner to perform the processes described herein or by special purpose computing apparatus composed of modules arranged to accomplish the described steps to accomplish the same process.

Alternatively, as shown in FIG. 1, the signals may be processed directly at the well site, using conventional digital computing apparatus 60 when properly programmed and interfaced to the signal conversion means 52. One such computing apparatus is the Model PDP-11/45 obtainable from the Digital Equipment Corporation Suppliers of such equipment may also supply signal conditioning circuits 40 and signal conversion means 52 suitable for conditioning and converting analog signals to digital samples for subsequent digital storage and processing. Further, such computing apparatus ordinarily includes a memory for storing data and information such as parameters, coefficients and controls used and generated by the processing steps.

A brief description of one process which may be performed at the well site by such a computer 60 when properly programmed is illustrated by Blocks 62 through 102 of FIG. 1. Other processes will be described in detail in relation to additional FIGS. 15A through 21.

Blocks 62 through 102 of FIG. 1 illustrate the steps of correlating the dipmeter signals in pairs to obtain sequences of displacements between similar features on the signals, determining a zone of displacement sequences which is suitable for subsequent combination and analysis, combining and classifying all the possible corresponding displacements in this zone and perhaps, as indicated in Block 102, outputting these classifications at this time. However, these classifications may be automatically analyzed to locate the dominant mode for these classifications and, if found, the location of this mode may also be output as indicated by Block 98. This location is indicative of the dip and azimuth of the formation features in the zone. Processing may then continue with more correlations if needed and the determination of more displacement zones to be processed.

When performed at the well site, it may be desirable to record and/or display the results of such processing on recorder 110 connected to the programmed digital computer 60. Recorder 110 may be a digital tape recorder or have display capabilities such as a printer, plotter or CRT recorder. The nature and use of these devices is well known and will not be described herein.

Referring again to FIG. 1, a more detailed description will now be provided for the process shown there in the form of the steps of a process flow diagram, and which may be performed with the aid of the digital computer 60 programmed in accordance with this invention.

After signal conversion 52 and storage in the memory of the computer 60, signals may be read from memory and correlated by pairs to determine possible displacements between corresponding features on the signals as indicated in Block 62. The correlation process is well known, but in review, includes successively comparing identical length intervals equal to the correlation length 64 of two signals. At each comparison, the interval on one of the signals is displaced by a given displacement from the corresponding interval on the other signal. Successive comparisons and displacements produce a series of correlation values known as a correlogram. These values are compared to determine the displacement indicating the position at which similar features present on both signals correspond.

The above-described process is again repeated for a different interval of both signals using the same correlation length. This interval is located one correlation step 66 from the previously correlated interval. Another correlogram and displacement is obtained for this interval and similarly for a sequence of intervals spaced apart by one corre