A Classic Example
The deep resistivity curve has a clear conductive anomaly showing that at least some of the fractures appear to be sub-horizontal with respect to the well. The shallow resistivity is affected the same way. The dual laterolog and Rxo log curves are also affected. The Rxo reading is very low. Because the shallow resistivity is lower than the deep, fractures are indicated. Mud resistivity is too fresh for this to be a salt mud invasion phenomenon.
The density and neutron logs show a high porosity zone while the density correction is very hashy.
The sonic log is strongly affected by cycle skipping. The waveforms on the sonic variable intensity display practically disappear. The sonic amplitude curve is very low. The caliper may be suggesting some mud cake, while the GR log indicates some radioactivity, probably due to uranium salt accumulation in the fractures.
Finally, the dipmeter fracture identification log clearly shows the fractures as individual anomalies. Six different anomalies can be defined; some probably are sub-horizontal. Very short vertical fractures are also present. There was a serious loss of circulation opposite this zone when the well was drilled.
Austin Chalk Example
Dipmeter curves presented in Fracture Identification Log (FIL) format show fractured intervals. The well on the right has far fewer fractures than the well shown on the left.
Austin Chalk fractures can be oriented by using the dipmeter azimuth to determine the direction of hole diameter elongation. A frequency plot of fracture orientations from wells in the Pearsall Field area, have an average strike of N 39 E with a range from N 13 E to N 57 E.
Reservoir development proceeded by orienting large fractures in the good wells or in offset wells where a dipmeter was run. New locations were drilled along these joint lineations. Where an offsetting good well had no available dipmeter data, an average orientation value was used appropriate for that area. Well potentials and production records substantiate the success of this method.
Fractured Shale Example
The SP, gamma ray, microlog, dual induction focused log, density, neutron, gamma ray spectral log, and sidewall acoustic log from this well are shown above. The resistivity measurements show low resistivity and straight line character. The SP did develop and has the same approximate character as the gamma ray.
The gamma ray spectral log provides the most character through the section. The method of analysis of the spectral log curves is to look for intervals which have low values of potassium and thorium. These are zones with less clay minerals, possibly less plastic and more receptive to fracturing. In these zones, present or past permeability is indicated by a higher uranium content. Several such intervals exist and correlate with zones on which mudcake formed indicating present permeability. Intervals which show the higher uranium with lower potassium and thorium have been marked with black bars. They are the most likely to produce.
The sidewall acoustic variable density log is typical of a high porosity sequence. Intervals on the log show high compressional amplitude and reduced shear amplitude, indicating low angle fractures. A few intervals illustrating this response are circled. Chevron patterns are faintly.
A portion of un-fractured log is shown opposite a fractured section for comparison in the bottom half of the above illustration. Notice the different character between the compressional amplitude and the variable intensity display. The chevron patterns are quite distinct.
The formation micro-scanner image of a similar fractured shale is shown above. Notice the steep dips, fine bedding, and the fracture. The sand/shale ratio can be determined easily by computer processing of the image.
Vertical Fracture in Vertical Hole
Notice the enlarged borehole in some of the thin shale beds. The fracture plane is far from smooth and it wanders from one side of the borehole to the other. A dipmeter or older FMS might miss this fracture, or indicate discontinuous vertical fractures. Light colors are higher acoustic impedance, probably dolomite versus darker colored limestone and limey shales. Shale beds are black and washed out.
Vertical Fracture in Horizontal Hole
televiewer images and uranium precipitation shown on the spectral
gamma ray log indicate fractures clearly.
This allows the operator to position completion hardware, such
as centralizers and external inflatable casing packers correctly.
In this example, the hole was designed to run close to the top
of the chalk, and it penetrated the marly zone above in a few
places, shown by the dark bands. It sometimes helps to look at
these images horizontally when analyzing horizontal wells. Both
acoustic amplitude and acoustic travel time images are presented
side by side. The black sinusoidal patterns are wider than the
actual fractures as this is a pretty old version of the logging
tool - a more modern log would do a better job than this.
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