Introduction |
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Induced tensile fractures are the most common and easily visible induced fractures observed in image logs and core. Induced fractures are easily distinguished from natural fractures in core by visually examining fracture surface morphology and the geometric relationships between the core and the fracture shape, origin flaw and propagation path (Kulander et al; 1979, 1990). Individual fractures cannot be identified positively as natural or induced based solely on the fracture trace in an image log. However, the origin of a group of fractures is determinate from image log data because natural fractures and induced fractures have different geometries relative to the borehole. If breakouts are present and if the natural fractures formed in a stressfield significantly different from the present-day stressfield, then the orientation of the fractures relative to the breakouts provides an additional (though less rigorous) criteria for distinguishing the two types of fractures. This webpage provides some simple rules for using image logs to differentiate natural fractures of all types from induced tensile fractures and to distinguish induced fractures that form after passage of the bit from petal and petal-centerline fractures which form ahead of the bit. These other pages on this site provide useful background material for this page:Breakout and induced fracture basics Regional fracture orientations |
The Rules |
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Rule 1: The stacking rule. Induced fractures that do not
completely cut the wellbore have a consistent orientation
and tend to appear at the same azimuth in the image but
natural fractures with a consistent orientation that do
not completely cut the wellbore appear at different
azimuths in the image. Rule 2: The aperture rule. Induced tensile fractures are always open; natural fractures may be open or partially to completely mineralized or gouge-filled. Rule 3: The continuity rule. The continuity of a fracture trace is not an indication of its origin. Rule 4: The orientation rule. The orientation of a fracture relative to the in-situ (present day) stress is not an indication of its origin. Rule 5: The breakout rule. Hydraulically induced fractures form in, and tend to be restricted to, the tensile quadrants of the wellbore wall, which are 90° from the breakouts. Petal fractures, another common type of tensile induced fracture, form ahead of the bit in what will become the compressive quadrants (where breakouts develop); poorly developed petal fractures tend to be restricted to the compressive quadrants. Rule 6: The symmetry rule. Individual natural fractures are often symmetrically developed on opposite sides of the borehole; petal and centerline fractures are nearly always symmetrical; but hydraulic fractures are usually asymmetrically developed. Note:
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Orientations of breakouts and induced fractures |
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Schematic cross-section of a wellbore showing the orientation of breakouts and induced hydraulic and centerline fractures relative to the borehole perpendicular in-situ earth stress components. Broken-out (missing) material shown in dark gray. In most of the world one of the three principal stresses is oriented vertically, which requires the other two to be oriented horizontally. However, inclined stressfields do occur, especially in tectonically active areas. Breakouts form in response to the minimum and maximum stress components that are oriented perpendicular to the wellbore. Note that these components may or may not be principal stresses depending on the orientation of the wellbore relative to the in-situ stressfield. Induced fractures tend to form perpendicular to the least principal stress, so that well-developed, borehole-parallel induced fractures form when 3 (the minimum principal stress) is oriented perpendicularly to the borehole. Hydraulic induced fractures tend to be inclined to the wellbore when 3 is inclined to the wellbore, although they may not form perpendicular to 3 in this case. Imagine that Figure 1 is a cross-section of a vertical well. In a normal faulting stress regime, min = 3 and max = 2. In a strike-slip faulting stress regime, min = 3 and max = 1. In a reverse faulting stress regime, min = 2 and max = 1. |
Petal fractures, centerline fractures, and petal-centerline fractures |
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Petal, centerline, and petal-centerline fractures form ahead
of the bit during both coring and normal drilling operations.
They normally extend beyond the final borehole diameter so
that they can usually be correlated between core and image
logs. The direction of fracture propagation is easily
determined in core and is always downhole (towards the bottom
of the screen in the images shown here). Petal fractures form
just ahead of the bit and are due to excessive bit weight.
Centerline fractures propagate ahead of the bit but probably
within approximately 1/2 meter of the bottom of the hole.
Centerline fractures are driven by a combination of mud
pressure and bit-induced stresses.
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Schematic log images and cross-sections illustrating identification criteria |
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Natural fractures are distinguished from induced tensile
fractures in image logs using gross geometric criteria. These
criteria are expressed succinctly as the six rules for
distinguishing natural from induced fractures in image logs. This
section provides more detailed explanations and illustrations of
these geometric criteria and provides criteria for distinguishing
pre-drill from post-drill fractures. The principals are illustrated
with schematic image logs and cross-sections similar
to Figure 3.
1. The most fundamental principle for discriminating between natural fractures and all types of induced fractures, tensile or shear: Induced fractures are geometrically related to the wellbore but natural fractures are not. This relationship causes the traces of natural fractures that do not cross the entire wellbore to appear at different azimuthal positions in image logs even if the fractures have similar orientations. However, the traces of induced fractures that do not cross the entire wellbore tend to stack in depth. 2. The extent of a fracture is not an indication of its origin because:
A. Symmetrical fractures are usually pre-drill:
Schematic llustrations | |
STOP! |
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DON'T READ THE ANSWER UNDER THE IMAGE! | |
Look at this image and try to decide on your own: Are they natural or are they induced?
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ANSWER: Image shows two natural fractures (A) and well-developed induced fractures arranged in parallel rows on opposite sides of the wellbore (B). Breakouts are absent. Fracture C is less easy to interpret because it crosses the entire borehole and is oriented parallel to the smaller induced fractures above and below it. The induced fracture immediately above C crosses into the compressive quadrant of the borehole, indicating (but not proving) that fracture C is also induced. These induced fractures appear to be post-drill, probably hydraulic, fractures rather than petal fractures because they are asymmetrically developed and nucleated in two parallel rows on opposite sides of the borehole rather than nucleating on one side of the well and propagating towards the centerline. Note that induced fractures abut both natural fractures, which shows their relative ages. |
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