borehole imaging dipmeter
1 | Introduction
Borehole Image Logs, available in logging oil industry since 1980s are very valuable data able to bring further intelligence to characterize reservoirs. The electric high resolution imager tools such as FMI (Schlumberger), STAR (Baker Hughes) and XRMI (Halliburton & CNLC), CMI (Compact Micro Imager) & HMI (High Resolution Micro Imager) of Weatherford are the appropriate ones to acquire data.
2 | Borehole Map
First of all, it is important to get used to the Boreholemap.
Borehole map is the result of a projection onto a plane preserving angles and distances of objects lying on the surface of the core or/and the surface of the borehole wall.
The boreholemap is generated by cutting, the cylinder modelling the borehole, along a line paralleling the borehole axis, such as for example the borehole high side, Fig. 11.
Basic Elements related to Orientation
The borehole is modelled by an axis and so its position is determined by two parameters: Deviation and Borehole Azimuth.
In addition to the geographic coordinates (X, Y) and the depth of every point of the well trajectory, there are two curves allowing positioning a well track in a volume: Deviation and Azimuth, Fig. 21.
Deviation (DEV, DEVI) is the angle in degree measured from the vertical to the borehole axis, in a vertical plane; this angle is 0o for a vertical well, up to 90o for a deviated well and 90o for a horizontal one.
This notion describes how much the well is moved away from the vertical which is the reference in the early stage of the drilling industry where wells were drilled vertically only.
Azimuth of the deviation (HAZI) describes to what direction this deviation is occurring. It is the azimuth angle 0o-360o, measured on the horizontal plane between the vertical projection of the borehole axis and N (0o).
The attitude of a plane in space is defined by its dip (magnitude/azimuth), angular values in degrees.
It is standard way to represent the dip by two (2) angular values: xxo/xxxo; the first digit set refers to the dip-magnitude (0o-90o) and the second set of three (3) digits expresses the dip-azimuth; e.g. 45o/180o is a plane dipping 45o to S.
MD: Measured Depth
Depth is measured while drilling is ongoing starting from 0 of the Rotary Table and going along the well track, whatever is the borehole vertical, deviated or horizontal.
This depth is called MD (Measured Depth).
It is a standard way to specify the distance between the rotary table and a referential datum, this information is called KB (Kelly Bushing), or RKB (Rotary Kelly Bushing).
The Datum is generally the MSL (Mean Sea Level) or it may be SB (Sea Bed).
TVD: True Vertical Depth
The vertical depth is the projection of the MD onto the vertical axis.
Sometimes, it is about TVDSS (True Vertical Depth Sub Sea), based on SB used as datum; not the MSL.
The Fig. 22 illustrates these notions.
Projecting the trace of a plane intersecting a borehole onto the boreholemap, is represented by a sinecurve. Note that the positions of the bottom and top of the sinecurve are related to the dip of the plane: in fact the abscissa (X) of the sinecurve bottom is related to the dip-azimuth, as it is depicted in the Fig. 23, and the the dip-magnitude, is related to the distance bottom-top of the sinecurve, the sinecurve amplitude, as it is illustrated in the Fig. 25.
For instance, perpendicular and parallel planes to the axis of the borehole are represented by straight lines, Fig. 24.
It is worth to notice, the parallel planes to the axis project into two segments at 180o each other; which is important to keep in mind when trying to identify borehole “breakout” and drilling induced tensile fractures mainly from borehole images to infer the direction of Maximum Horizontal Stress (SHmax) characterizing the present day stress-field orientation in the vicinity of the well.
Therefore, the rule of thumb well known among specialists of borehole images, when looking at first time to images in a field print and wondering if the bedding is dipping at high/low angle is to examine the shape of the sinecurves. More they are expressed and stretched more the surface of interest is dipping at high angle; and more the sinecurves are close to a straight line and more the surface is close to the horizontal…in a vertical well!
That’s important to keep in mind when, at the first, you are looking at the field print (before any further processing and interpretation of the raw data) of Borehole Images delivered by a logging company at well site!
3 | Introduction to Dipmeter and Borehole Imaging
Dipmeter was the first logging tool introduced in the drilling industry; it has evolved into borehole imaging technology.
Nowadays, even if the borehole imaging is dominating the oil industry, older data are there and need to be processed and interpreted by new tools and news ideas. Some oil companies are still acquiring dipmeter logs.
In addition to the borehole map projection, these both technologies have in common two major methodologies widely implemented: tadpole representation of a dip and the use of the stereographic projection to analyse the dip populations.
3.1 | Tadpole Representation
3.2 | Stereographic Projection: Overview
Basics and techniques about the projection
Once data is collected, measurement of geological planes and axes, on the field or from dipmeter/image logs (as it is said when interpreting images: picking is finalised) one of the first following steps in the workflow is the statistical analyses of the planar and axis populations.
We face a population of several hundreds and sometimes several thousands of measured geological planes, different bedding surfaces, minor faults and different fracture types. It needs to study them statistically to highlight the main sets.
One way to deal with populations of orientation data is the spherical projection, Fig. 32. It is described in details in several books, most of them about structural geology such as Phillips, 1971.
It has to be stressed out that what does matter is the angular relationship between geometrical features, i.e. planes and axes, not their position in a cross section or a map views.
For example NS horizontal line represents an axis striking NS, without any parameter enabling to locate it in the 3-D space, means it is located everywhere and in no place at all! But every time lying horizontally and striking NS.
What matters is the direction not the position.
The interest of such a projection is to help determine a trend in a population of measurements of geological features, or to highlight a geometric pattern in spatial variation of the same structural feature, i.e. bedding, fracture type, faults or particular fold axes.
Basically, both projections (Schmidt and Wulff) are the same.
The geometric features (planes or axes) are considered passing through a centre of a sphere: the projection sphere representing the whole space. Just imagine every single geometrical feature moving paralleling itself from its real spatial position to the centre of the projection sphere, as one can see it magically done by drawing software, Fig. 32.
The projection sphere could be easily oriented, referencing it to the Earth Globe, with a vertical axis, an uppermost pole, a lowermost pole, and upper and lower hemispheres separated in the middle by the horizontal plane, i.e. the equatorial plane, Fig. 32.
The equatorial plane, by reference to a map, is easily oriented too, with N and S at the extremities of the sagittal diameter, and E & W of the orthogonal diameter. Actually, the equatorial plane is bounded by a circle (primitive circle) resulting from the intersection of the projection sphere with the equatorial horizontal plane, Fig. 32.
What is important to focus on, at this stage, is the intersection of any geometric feature of interest with the sphere surface. The line (or plane) intersects the upper hemisphere and the lower one too.
Only one of both matters in the spherical projection.
Therefore, there are two ways to project, i.e. by using the upper or lower hemisphere. Both projections are symmetrically opposite and we will see the geometrical relationships between them. That is one parameter, among several, which divided geologists into two worlds: partisans of the lower hemisphere and those backing up the upper hemisphere. Of course, both geologist-populations are spread all around the world. Petroleum geologists are generally using the Upper Hemisphere projection.
So, let us keep on presenting the basis of the projection by using the Upper Hemisphere. An axis first and latter we will see about the projection of a plane.
An axis XX’, passing through the centre of the sphere, as always, intersects the surface of the sphere at a point-within-upper-sphere: Pus. Stretching a line from this point Pus to the lowermost of the projection sphere will intersect inevitably the equatorial plane: the intersection P onto the equatorial plane is the stereographic projection of the axis XX’ of interest, Fig. 33.
That equatorial plane is the projection plane that is the representation plane of the spherical projection. That is the plane we will use to plot and to see, through it, the 3-D representation of orientation data, of populations of geometrical features as planes and lines.
Therefore, an axis is represented/plotted by a point, and its position onto the projection plane is a function of its orientation. As we already mentioned it, the orientation of a line is defined by its dip and azimuth.
A horizontal axis intersects the sphere surface at the circle (primitive circle) and so projects onto the circle bounding the projection plane, Fig. 34.
A vertical line projects onto the centre, Fig. 34, and between these two spatial attitudes (vertical & horizontal), lines with a dip included in the range 00o-90o, intersect between the centre and the peripheral circle, Fig. 34 & Fig. 35.
About a plane. There are two manners to project it:
-2) by implementing the normal to the plane; so it is plotted as a point: the pole to plane, that is a projection of an axis we described previously, Fig. 35.
Poles of different vertical and horizontal planes are illustrated in Fig. 39.
The cyclographic trace of a plane, dipping Easterly for example, is located in the western part of the projection plane, and the axis WE intersects the bow in its middle.
As a result and a thumb rule one can state that a plane (cyclographic projection) and an axis (point) project in the opposite directions they are dipping to.
Using the normal of a plane to project a plane needs further brain exercises to 3-D picture the plane through its pole. One has just to keep in mind that a pole to plane represents a plane dipping in the same azimuth of the projection plane where it is plotted.
These are basically the main important features we have to understand when dealing with planes and axes implementing the upper hemisphere of the spherical projection. Before concluding and going further, it is useful to note that implementing the lower hemisphere reverses the results as seen by using the upper hemisphere: -1) an axis projects in the same azimuth it is dipping to, -2) a plane projects a cyclographic trace in the same azimuth it is dipping towards, -3) and a plane projects its pole in the opposite dip-azimuth.
Keep in mind…
It is usual in petroleum geology to use the Upper Hemisphere of the Schmidt projection
A point represents a pole of a plane (dipping in the same azimuth)
A point represents also an axis (dipping in the opposite azimuth)
A cyclographic trace represents a plane dipping in the opposite azimuth
It is sometimes useful to recall…
Implementing the Lower Hemisphere is the symmetrical opposite of the results issued by using the Upper Hemisphere.
Some typical examples of population distributions
How the pole populations are distributed?
Through some typical examples, we illustrate some population distributions and their interpretations.
A systematic way is to make a contouring first to highlight shapes and number of concentrations. To make it short, one can say that population distributes as
One pole : Unimodal population
The projections of the plane poles are concentrated in one circular area, defining statistically a mean plane at its centre Fig. 310 & 311. The concentration rate can be defined too: it is the number of poles inside the centre vs. the whole population, value up to 1 for a perfect concentration. A value close to zero characterises a poor concentration, or a wide spread population, yet all around a particular orientation.
The interpretation is easy because revealing only one option: all planes parallel only one direction, they are horizontal, vertical or with a particular dip-magnitude/dip-azimuth Fig. 311.
The rate of concentration might be related to the quality of measurements or to another parameter, e.g. geological one.
Two poles: bimodal populations
Two concentrations revealing two means and consequently two planar-attitudes defined by their respective dip-magnitude/dip-azimuth, Fig. 312. The angular relationships between the two plans can be determined.
According to the rate of concentration it is possible to differentiate between them and also to classify them as major and minor or both as majors (or minors) population-sets.
The number of concentrations is implemented to define sub-populations. When there are three (3) or more, it might be useful to use a handy-projection (by plotting the mean-planes as individual planes…if it is not handled by the software) to decrypt any possible geometrical relationship between them.
Are they located into the same plane?
That is the first easy question to ask for and to answer. A quick look at the plot, bearing in minds a cyclographic projection, help detect a possible great circle including all concentration-centres (or the projected mean-planes). There are software able to help highlight a great circle, or one can do it by a handy projection as already said.
Actually, dealing with concentrations will bring you back to the first step of the interpretation of the Schmidt plots: consider concentrations as planes (mean-planes) and try to detect how they distribute in space.
Plane-pole population defines a (great) circle the interpreter tries to fit with a cyclographic projection of a plane.
If it is the case, that means that all the poles (axes) belong (or project orthogonally) to a plane defined by the cyclographic trace. Consequently, the plotted planes are perpendicular to the plane defined by the cyclographic trace. Subsequently, the plotted planes parallel one axis: the perpendicular axis to the plane defined by the cyclographic trace.
The very simple girdle is a straight line, as it is shown in Fig. 313, where one can see that all the planes are paralleling the WE axis, starting from a horizontal position and dipping increasingly to South until to be vertical; they all strike WE.
This geometrical pattern, well known in structural geology, models a fold with its fold-axis and the perpendicular-plane to the axial-plane of the fold, Fig. 314.
If the plane-pole population project as a cluster, often the interpreter looks for a combination of concentrations or girdles, or both of them.
In any case, it is often necessary to figure out the geometrical pattern and therefore to consider several options in structural geology, taking into account the geological setting. By including other data, it might be possible to eliminate/keep option(s) for the final interpretation.
Keep in mind…
A concentration of poles to planes refers to a sub-population of planes that parallels a plane defined by the pole-plane at the centre of the concentration (also called mean-plane).
A girdle of pole to planes defines a cyclographic trace of a plane perpendicular to the plotted planes
From the girdle, it is possible to determine the direction that parallels the projected planes, equivalent to the fold-axis in a model of a fold.
It is sometimes useful to recall…
Considering concentrations as planes helps to figure out angular relationships relating sub-populations.
4 | Dipmeter
4.1 | Introduction
4.1 | Tools
Dipmeter tools are rarely used in oil industry nowadays, borehole imaging tools took over since 1990s, but geoscientists are still dealing with data recorded previously by dipmeter tools, so it is worth to get an overview of these types of devices.
Dipmeter is a pad-based microresistivity tool, containing microresistivity electrodes on the pads, Fig. 42.
While logging, generally from bottom to top of an open hole, the pads are applied against the borehole wall, and therefore the microresistivity buttons are able to register a part of the current flow emitted from the lower part of the device and registered back in the upper part of the tool. This allows, later, a computation of the raw measurements to retrieve dip (magnitude and azimuth) of the planar geological features of the logged formations.
At the same time, the inclinometry part of the device records the tri-axial accelerometer and the tri-axial magnetometers Fig. 43, basic data that will be used during the processing to compute the orientation data.
Because this type of tool is measuring electric current, the mud is a water-base one, a conductive medium.
New electric tools able to log into oil-base mud were developed and are available in the oil industry nowadays.
A brief presentation of the main dipmeter tools of the main logging companies in oil industry is following. We advise the readers to go the respective websites of these companies to get more information and updates.
4.1 | New challenges for Geoscientists
The introduction of these new tools, at this time in the oil industry, makes geoscientists and engineers to face new challenges; most of them are related to the following facts:
These important points will be discussed once the borehole imaging tools will be presented in the next chapter (Borehole Imaging) with what is presented as the state of the art to study dipmeter and borehole image data.
5 | Borehole Imaging
Borehole imaging was an important breakthrough in the logging industry.
A major step compared to the dipmeter: it is far beyond the measurement of the formation dip, but mapping the borehole wall. Real improvement in technology allows retrieving a huge amount of data: it is about 100,000 measurements per meter MD.
Modern tools and the digital age make it possible to manage such amount of data, and in the same time constrain geoscientists to define new methodologies to process and interpret them.
That’s digital geoscience, going from a few measurements to be interpreted into a coherent model illustrated mainly by hard copies of maps and cross sections, to thousands of measurements that have to be interpreted and integrated with thousands of other data from other sources to a 3D model in a powerful laptop.
A dynamic petroleum world constraining geoscientists to be efficient, multidisciplinary skills, working in a team, meaning able to communicate with others colleagues, and in the same time, having each one particular speciality and Working Fast!
Borehole Imaging Tools
All started effectively with the FMS (Formation MicroScanner tool of Schlumberger) introduced in 1986, even if there was several attempts by several companies, registered earlier, such as the first attempt by a photographic device introduced by Birdwell (borehole photography using a 16-mm lens), in 1958 (Ekstrom et al., 1987; Prensky, 1999; Cheung et al., 2002).
The FMS was an improved SHDT (Stratigraphic High Resolution Dipmeter Tool) tool, by adding two orthogonal pads: pad 3 and 4 containing 27 microresistivity electrodes (“buttons”) of 0.5 cm diameter on each one.
Two years later (1988) the FMS was improved: 4 pads with a total of 64 buttons (16 ones per pads); the sampling rate is 0.1 in (0.25 cm) vertically and horizontally. The borehole coverage is 40%, (Boyeldieu and Jeffreys, 1988); Fig. 51.
The FMI (Fullbore Formation MicroImager) tool was the new one of Schlumberger introduced in oil industry in 1991, a real improvement with 4 pads and 4 flaps totalising 192 microresistivity electrodes, still in use nowadays (2014) known as the best electric imaging tool.
Other logging companies followed with the EMI (Electrical Micro Imaging) of Halliburton in 1994 (Fam et al., 1995), the STAR of Western Atlas (now Baker Hughes), in 1995 and HMI (High-resolution Micro Imager) and CMI (Compact Micro Imager) of Weatherford recently in 2009.
Nowadays, the imaging tools are the heart of the logging routine for any oil companies, because the data retrieved for these tools are so valuable, and not running these tools means missing a lot of information.
6 | Processing and QCing data
Once the data loading and in the same time the QC (Quality Check) is carried out, the processing of the data starts. The raw data of both dipmeter and images, is carried out to deliver from the raw data the interpretable outputs, meantime there are several corrections that have to be carried out, such as the speed correction, the magnetic declination correction, depth shift offset if necessary, generating computed dips (dipmeter data) Fig. 61, and images (imaging data), Fig. 62 and the final QC of the outputs to start the interpretation, .
The speed correction converts data vs time into data vs depth (MD), and correct for the erratic tool motion, such as the slip-stick behaviour of the device while running.
The magnetic declination correction is applied to the inclinometric measurements recorded by the tool (relatively to the magnetic North) to convert them into geographic North.
The depth of the log has to be check out, by correlating the GR from image (dipmeter) log, to the GR of the master log; sometimes it is important to use some particular features from other runs in the correlation too.
The static and dynamic images are generated at the end and then the final QC is carried out. The static images, in fact called static normalised images, the computation is carried out in a window covering the whole logged section. The dynamic images (dynamically normalised images), the computation is carried out using a sliding window, generally of 5ft, Fig. 62.
The values of the conductivity are scaled in the range from white, yellow, brown and dark, from minimum conductivity to maximum conductivity, in both images.
The QC needs to be carried out from starting loading data, during the processing and to the end of the interpretation, with an important question in mind, “is this making sense?” The key data are the orientation curves, the matching depth, the correction of the magnetic declination and the measurements.
7 | Basic Interpretation
Once data is processed, QC-ed, the interpretable output images generated, the basic study should be carried out to determine the standard interpretation outputs that will be implemented for further studies and in depth analyses. In general three steps are distinguished: 1) Collecting geologic data, 2) Analysing dip populations and 3) Correlating geologic features.
Collecting geologic data: A first overview of the images aims to scroll up and down all the main and repeat sections to determine the best intervals, to be interpreted first. A diptype listing of geological surfaces needs at start to be finalized to be used for the picking of the sections of the well. The surfaces are of sedimentologic, structural and in situ stress origin, Fig. 71 & 72.
Once picking of surfaces is finalized, it is necessary to analyse the populations, implementing the stereographic projection. This task aims to highlight the existence of the main geometric patterns, already described (§ 3), i.e. concentration (unimodal, bimodal, multimodal), cluster and girdle; and to infer a sound geological interpretation.
8 | In depth analyses
In my opinion, based on several studied cases, there are three major outputs that need to be added to the standard study of borehole image data: 1) Paleohorizontal & Structural dips, 2) Determination of SHmax & its shearing directions, and 3) Zonation based on image fabric
8.1 | Paleohorizontal & Structural Dips
These two major outputs have to be differentiated, even if there was a confusion at start.
At start there was this first definition of structural dip: “…“…Dips with constant magnitude and azimuthin a low energy environment can be selected. They correspond to the groups of beds, whose bedding planes have not undergone any biogenic or tectonic alteration. It can reasonably be assumed that these beds were deposited on nearly horizontal surfaces and that their present dips are the result of tectonic stresses. …” Tiré de Serra, 1985. I highlighted what is important to think about. In my mind this is what I name the Paleohorizontal dip.
Almost a decade later, Rider published the following structural dip definition: “… By Structural dip is intended the “general attitude of beds”. It is the dip that would be measured at outcrop. It is usually the dip seen on seismic reflectors, themselves a generalisation. It avoids any sedimentary structures of any size and is generally considered to represent the depositional surface which also is considered to be horizontal. …” Tiré de Rider, 1996. As above, I stressed out what is significant in my opinion that is the Structural dip.
Therefore I suggest the following definitions I used during several years in my studies to several Co.
Paleo-Horizontal Dip, as is suggested by the name, is the dip of bedding planes that were originally deposited horizontally. Low energy sediments such as shale, planktonic sediments and coals, in specific conditions, can be assumed to be deposited horizontally, Fig. 81.
Such bedding planes may be used to infer tectonic events such as uplift, tilting or fault block rotation.
Therefore one needs to find out the right interval, Fig. 81 of the logged section to determine the paleohorizontal dip, to be implemented to rotate back to the paleohorizontal at the time of the deposition to pin point the true paleocurrent directions and senses at the time of the deposition of the cross bedded sandstone, Fig. 82. And to highlight rotation of the fault block, Fig 83.
Structural dip is restricted to the mean dip of a lithological formation that can be used in geological (structural) cross-section, or related to a specific marker that can be correlated to a seismic one, avoiding detailed sedimentological structures at small scale. The dip and associated dip-azimuth can be used to infer the geometry of the units, Fig. 87 for structural purposes at rather bigger scale, no matter what its genetic origin, or what events the unit has previously undergone.
8.2 | SHmax & its shear directions
8.3 | Zonation of the image fabric
A zonation based on image fabric to highlight some sedimentology/lithology features, and some deformation facies.
Such a method aims to differentiate image fabric related to
1) sedimentologic attributes such as interbedded intervals, stratified, homogeneous, presence of vugs, or bioturbation, different types of vuggy matrix, …
2) Another zonation of image fabric related to alteration (Karstic Zones) or and deformation such as Fracture Zones, Fault Zones, Vertical Fractures, Fractured/Cataclastic Zones, Fracture Network, Stylolite Associated Fractures
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