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RESISTIVITY IMAGE LOGS


IMAGE LOGS FROM DIPMETERS
Dipmeters and image logs are an essential for assessing structure and stratigraphy of reservoir rocks and for Identification of fracture intensity and fracture porosity. Tool design has improved considerably since its introduction in 1986.

A resistivity image log has about 10 times the spatial resolution of an acoustic image log and 500 times the amplitude resolution, due to the difference in contrast between the resistivity and acoustic impedance ranges measured by the respective tools.

The standard tool works only in conductive mud in open hole, and a specialized tool is available for non-conductive mud. It does not work in cased holes.

An extension of the stratigraphic high resolution dipmeter (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). This image predates the resistivity image log by a few years.

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.

The colour convention is to show low resistivity in black and high resistivity in white (or yellow). This makes shale beds black and clean sands white, with shades of grey or brown representing shaly sands. In carbonates, the same rules are used, but white may now mean tight streaks with grey representing porosity. The colour scale can be stretched and squeezed to enhance the image for a particular situation.

Note that a planar, dipping, bedding plane will trace a sine wave on a circumferential image, such as those shown above


DIPVUE image created from dipmeter data

 

RESISTIVITY IMAGE LOGS
In 1986, the "ultimate" dipmeter was developed by Schlumberger, called the formation microscanner (FMS). Later versions were called formation micro-imager (FMI), there are now similar services available from other suppliers. The tool is generically known as a resistivity image log.

Using an additional 27 electrodes on each of two dipmeter pads of the dipmeter; each pad records 27 microresistivity curves spaced 1/10 inch apart on the borehole surface. Each pad covers a 2.8 inch wide portion of the circumference of the well bore. Several passes over the interval will often provide virtually complete coverage of the rock face.

The resistivity traces are translated into images based on their relative resistivity values, in either black and white or colour. The gray scale or colour spectrum can be stretched or squeezed in the computer to enhance certain features, such as porosity, fractures, or shale laminations. Images can be plotted at the same scale as the core photographs for comparison. A sample is given below.


Resistivity image log showing calculates dips, raw curves, and image log from a two pad device.

A resistivity image logging tool with fewer (sixteen) electrodes per pad, but with four or eight imaging pads, is now available, and provides better coverage of the wellbore wall than the two pad version. The electrodes are smaller, allowing for higher vertical resolution, but are spaced to provide the same wall face coverage, about 2.5 inches per pad. In an 8 inch diameter hole, electrode coverage is about 80% and in a 6 inch hole is greater than 100%. This overcomes one of the major complaints about the FMS, namely the number of passes needed to obtain a complete image of the well bore. An example is shown below.


Four pad FMI log in fractured granite reservoir showing computed dip angle and direction. North is at centerline of image.

The primary use of the tool is for identification of irregular features, such as vugs and fractures, for accurate sand counts in thin bedded zones, and for identifying stratigraphic features. If sufficient rock face is imaged, dips can be found by digitizing the bedding planes visible on the microscanner image, or by automatic computation using all valid image traces.

The dips found by FMI dip processing are superior to CSB or LOC dips because a larger number of resistivity traces can be used in the calculation. They can be computed automatically and displayed on the FMS image. In addition, calculated dips can be edited or removed, and new bed boundary correlations picked with a mouse on an interactive CRT image. Thus dips that pass or fail preconceived processing criteria can be deleted or added as the analyst desires. An example of this technique is shown as a case history in Chapter Seven.

The resistivity data provided by this tool are of very high resolution, in the order of a millimeter. Thus, a substantially large data array must be handled, and it is an obvious challenge to process and display this information in a way which facilitates its interpretation. This is resolved through a point to point mapping of the resistivity traces into a spatial image, each pixel in the image display having a gray scale value associated with a particular current level. Subsequent interpretation benefits from the close relationship between this image and core photography. Samples of six different uses of the images are shown below.

RESISTIVITY IMAGE EXAMPLES


Formation microscanner images in various environments


Formation microscanner images in various environments


Expanded colour scale image of tight streak (light brown) on resistivity image shows detail of porosity laminations - light colour is tight, darker colours are more porous, black is shale.

 


Deep resistivity image from a logging while drilling tool called Resistivity at Bit (RAB). As for all image logs, black represents low resistivity, white is high resistivity. North is at centerline of each image. Since this is a deep resistivity, light colours could mean hydrocarbons or low porosity. Comparison to an open hole FMI or an LWD density image log could resolve the ambiguity.


LWD density image log,  black is low density (shale or porous), white is high density (tight). Comparison to RAB can identify hydrocarbons: light colour on RAB + dark colour on density image = hydrocarbon. This image is from a horizontal well; bottom side of hole is at the centerline of the image.


 

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