DIPMETER LOG BASICS
In the beginning, there were no dipmeters. Dip magnitude and direction of rock strata was assessed by knowing the subsea elevation of a distinctive rock layer in three or more wells which were closely spaced. The equation of a plane is defined by the X, Y, and Z coordinates of three points, so the elevations and well locations were sufficient data to define dip. Subsurface mapping of clearly defined formation markers is widely used today to estimate both regional and local dips.

Measurement of dip in outcrop is also widely used to assist in mapping overall basin structure. Neither of these methods will find structures located between the control points because there is insufficient data.

Dipmeter logs were developed to resolve the "3-points define a plane" problem in a single well bore by measuring some physical property of the rocks in at least three directions around the borehole. Since the borehole is typically only 8 inches in diameter, considerable precision in measurement was required. Tool design evolution over the years was directed towards improving precision and redundancy to acquire more accurate dips more frequently in the borehole. In order to obtain true dip angle and dip direction, the tool orientation in space had to be monitored. Techniques for doing this also evolved dramatically over the years.

Dipmeters and image logs have evolved from primitive beginnings to become an essential and accurate log for assessing structure and stratigraphy of reservoir rocks. Identification of fracture intensity and fracture orientation are another important use for these logs.

Conventional dipmeters work in conductive muds in open hole and specialized special tools are available for oil based mud systems. They do not work in cased holes.

Tools described below are based on Schlumberger's offerings over the years. Other service companies provided similar or near identical services under various trade names.

THE FIRST DIPMETER LOG
In 1933, attempts were made to evaluate dip by analyzing resistivity anisotropy effects on a modified electrical log. The resistivity of a layer is usually lower parallel to the bed than perpendicular to it. By taking resistivity measurements with suitably arranged electrodes, the dip direction of thick, well-stratified beds could be found.

The dip angle had to be known from cores, and the hole direction had to be measured. This was possible using a device called an electromagnetic teleclinometer, which sent a signal up the logging cable proportional to the tool's deviation from the vertical. From this data, a crude dipmeter survey was presented. It is doubtful that any copies survive in well files. More modern data is often available in any event.

SP DIPMETER LOG and PHOTOCLINOMETER
The anisotropy dipmeter was supplanted in 1943 by a tool using three simultaneous spontaneous potential measurements oriented 120 degrees apart around the circumference of the logging tool. Using the same principle that three points define a plane, the tool provided sufficient data, along with bit size, deviation, and direction, and tool orientation in the hole, to calculate dip. The three points were taken as the bed boundaries defined by the SP curve from each electrode, as shown below.

The balance of the data came from a photoclinometer survey taken at stations near the top and bottom of the recorded intervals of the dipmeter curves. A schematic example of the concept is shown below. The photoclinometer consisted of a magnetic compass, a ball bearing in a graduated curved dish, and a camera that photographed these components on demand.

This sounds simple. However, the magnitude of dip which is of interest in exploration is from a fraction of 1 degree to vertical. This poses serious constraints on tool design and data analysis. For example, a regional dip of about 50 ft/mile is equivalent to 1/2 degree dip. Local structure or drape over deeper erosional surfaces may modify this dip to flat or 1/2 degree in another direction. In some areas this is significant and could define the trapping mechanism. The dipmeter device, the recording process, and the curve correlation methods must have sufficient resolution to enable us to see this small difference.

A bed dipping at 1/2 degree across a 9" borehole will be less than l/10 inch higher on one side of the hole than on the other. The displacement between curves will be less than 0.1 inches if recorded at 12 inches per foot of borehole. If recorded at 5 inches per hundred feet, a normal detail logging scale, this displacement would be only 0.0004 inches on the film. As a result, scales of 60 inches per 100 feet were used. Now the 1/10 inch displacement is represented by 0.005 inches - a measurable distance on the film.

Due to the relatively round shape of the SP curve at most bed boundaries, this level of resolution was not achieved with the SP dipmeter. Moreover, the tool was useless in carbonates where SP does not develop well. The only dips presented were those from major bed boundaries where dip was steep enough to be obvious.

Although the SP dipmeter was abandoned quickly in favour of three resistivity curves, the photoclinometer survived well into the 1960's as a directional survey tool. A sample is shown below. In addition to the photographs of the compass and deviation ball, typed listings of computed results and a plan of the well track were presented. Since the well bore often deviated, without any help from the drilling crew, to keep the bit perpendicular to the formation dip, the directional survey data was sometimes used as a guide to dip.


SP photoclinometer dipmeter log presentation circa 1943


SP photoclinometer dipmeter listing circa 1943


Photoclinometer directional survey presentation circa 1943

RESISTIVITY DIPMETER LOG
The resistivity dipmeter used three laterolog curves instead of SP curves, mounted on the same rubber arms as were used for the SP version. Accuracy was better in hard rock areas. A sample is shown below. Both SP and resistivity dipmeters were only recorded over selected intervals, chosen by observation of the other open hole logs. Only short intervals where there is lots of curve action were suitable for dip computation.


Laterolog photoclinometer dipmeter log presentation

Typical computed results from the SP or resistivity dipmeter are shown below. Although rare, examples can be found in files for wells drilled in the 1940's.


Computed dipmeter results circa mid-1940's


Computed dipmeter results circa mid-1940's

3-ARM CONTINUOUS DIPMETER and POTECLINOMETER (CDM)
In 1950, better accuracy was obtained by a newly designed dipmeter utilizing three microlog resistivity pads. Now a continuous log could be made, and with very detailed resolution from the microlog pads, a fine scale dipmeter was a reality. In 1952, the microlog pads were replaced with microlaterolog pads which measured conductivity instead of resistivity. In the literature, this tool was called the Continuous Dipmeter or CDM for short.

Orientation data was recorded simultaneously and continuously with a device called a poteclinometer. Poteclinometer is a contraction of the word potentiometer (a variable resistor) and inclinometer - this word sounds a lot like the earlier photoclinometer. Directional output from this device is an electrical signal instead of photographs. Data consisted of hole deviation angle, relative bearing (which describes the angle to the high side of the hole from pad number one), and the azimuth (which describes the angle between magnetic north and pad number one). This is sufficient data to orient the dip azimuth and the direction of hole deviation. The algebra is described later in this Chapter.

This eliminated the need to stop the tool to take pictures with the photoclinometer. Directional surveys run with this equipment were also more accurate, but considerably more expensive.

The optical comparator was also developed during this period. This increased dip accuracy further by reducing errors in measuring the offset between traces.

The computed data was presented in the same tabular and graphical fashion as previously , but with considerably higher frequency. However, by 1958, some hardy souls were plotting individual dips as small arrows on a graph of dip magnitude versus depth. The direction of the arrow represented the dip direction relative to a compass rose with north at the top. This was the precursor to the now common arrow plot, sometimes called a tadpole plot, generated by computer. Computer plotting was first seen around 1961.


Computed microlog dipmeter results circa mid-1950's

The first attempts to legitimately use detailed dip data for stratigraphic evaluation occurred around 1955. An example of the difference in data quality and quantity between short interval and continuous data is shown below. The raw data was recorded at 60 inches of log for 100 feet of wellbore, or 1:20 scale, shown half size below. Literally miles of this photographic paper was developed, processed, and sifted through the optical comparator each month. Most of it has deteriorated or been destroyed and is not available for re-computation.


Long and short interval computed dipmeter results circa late-1950's


 Expanded scale paper log of raw dipmeter curves late-1950's

Fortunately, beginning in 1961, dipmeters were recorded on digital magnetic tape, reducing and finally eliminating the need for the detailed paper logs. The offsets between traces were derived by computer correlations, leading to a whole new language: correlation window, step length, search angle, etc.

 

4-ARM HIGH RESOLUTION Dipmeter (HDT)
In 1969, a new four pad high resolution dipmeter was introduced. The electrodes had even finer resolution than the microlaterolog pads and the electronics were improved to transmit data at a higher rate, so that the well could be logged faster and finer bedding features could be recorded. Four pads allowed for calculation of four different sets of 3-point planes as well as a four point curved surface or a "best fit" flat surface. Program logic could compare all results and eliminate bad correlations, or grade the results to show how well the different results matched.

A special "speed button" on one pad provided information to the program to compensate for minor speed differences as the tool moved up the hole. These variations created scatter in the computed results, illustrated below. In addition, a synthetic resistivity curve was generated from the dip curves, to be used as a correlation curve.


 Computed dipmeter results circa 1969 showing effect of speed correction

The geometry of a four pad device is shown schematically at left and the arrangement of tool components below.


 

 

 

 

 


 Arrangement of tool components for 4-pad dipmeter


Typical raw data curves and an answer plot circa 1970.

In 1975, secondary computer processing, called CLUSTER (Schlumberger trademark) or SHIVA (Gearhart trademark), were developed to validate the results from the standard program. Other secondary programs were developed to enhance stratigraphic features, notably GEODIP (Schlumberger trademark). These processes are described later.

About 1980, three axis accelerometers and three axis magnetometers replaced the magnetic compass, relative bearing, and hole azimuth potentiometers. However the log still presented these three curves, derived now from the solid state sensors instead of the more failure prone electromechanical devices.

STRATIGRAPHIC HIGH-RESOLUTION  Dipmeter (SHDT)
A further refinement in 1983 created the stratigraphic high resolution dipmeter. An additional electrode set was added to each pad giving eight dip correlation curves instead of four. With this number of measurements, the results can be presented more often, as many as 10 or 20 per foot if desired, instead of the more usual 1 or 2. Better speed correction is provided by accelerometer data from sensors inside the tool. Typical raw data plot is shown below. A six arm dipmeter has also been developed to meet the need for stratigraphic information, with a lower cost tool.


Raw log curves on stratigraphic high resolution dipmeter (SHDT) circa 1980


Evolution of the Schlumberger dipmeter pad designs from the early days up to just before the invention of the resistivity image log in 1986. See
Resistivity Image Logs .

Three dip computation modes are available from the stratigraphic high resolution dipmeter. The same mathematical algorithms are available on resistivity image logs.

First is the usual pad to pad correlation, which benefits from the extra redundancy of two electrodes per pad. This is called Mean Squares Dip or MSD, and often is used for structural or regional dip analysis. The dip is a weighted average of all pad to pad dips. In strongly parallel beds, the result is very good, but in cross bedded formations with varying dip, the average dip has little significance, except to show overall direction of dip.

Second is called Continuous Side by Side or CSB dip correlation using only the individual electrode pairs on each pad. Dip vectors from adjacent electrode pairs are used to define dip. CSB dips respond to short interval, low contrast changes often characteristic of internal layering in clastics, but also will respond to high contrast structural dips. It is very useful for structural dip analysis in high angle apparent dip, greater than 50 degrees. In finely bedded rocks exhibiting cross bedding, considerable detail can be shown if the correlation length and step distance are kept fairly short.

Third are pad to pad correlations using a pattern recognition rather than cross-correlation system. This is called Local Dip or LOC dip and responds to non-repetitive events such as erosional surfaces or breaks in the depositional sequence. A comparison of the three modes with normal high resolution dipmeter results is shown below. It is now possible to analyze data with a resolution of a few inches and compare it to core data .


MSD, CSB, and LOC dips from same recorded curves


High resolution dips compared to core

Schlumberger introduced a dipmeter for use in oil-based (nonconductive) mud systems in 1988. It uses micro induction resistivity measurements instead of the usual electrical resistivity pads. A knife blade electrode, or scratcher pad, version is available from several suppliers. In 1989, a 4 arm focused acoustic dipmeter was introduced by Atlas Wireline, with a resolution of about 1 cm.

An extension of the SHDT processing provides a core-like image of the borehole, using the LOC dip correlations and the measured resistivity curves. The program is called STRATIM (Schlumberger trademark).

An example is given on the left. The program produces a 360 degree image of the borehole wall by interpolating between the eight resistivity measurements from the eight electrodes on the SHDT pads. Images can be coded in gray scale or colour. Dark gray or dark colour usually represents conductive, often tight shale, beds and light colour resistive, often porous sand, beds. If shales are more resistive than sands (or carbonates), the colour scheme can be reversed to keep shales looking dark.


The dipmeter curves are rotated to their true azimuth but are not adjusted to true dip. The dips seen on the image are as they would appear on the surface of a conventional core. The trace of a plane dipping bed forms a sinusoidal curve when the image of the borehole wall is unwrapped and laid flat, as they are in these images. Bed boundaries, dipping beds, slump features, and fractures are easily seen, if present. Images can be enhanced as in Formation Microscanner processing, but processing is cheaper because much less data is manipulated.

A similar program, called DIPVUE is available from Western Atlas, illustrated below. Here the 3-D image can be rotated in real time to view the artificial "core" from any direction. In addition, most core service companies can provide core photographs and dip logs from core data for comparison with log derived borehole images.

 

 


DIPVUE image created from dipmeter data
 

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