An experimental study of fracture initiation mechanisms during hydraulic fracturing
An experimental study of fracture initiation mechanisms during hydraulic fracturing
Yan Tie 0
Li Wei 0
Bi Xueliang 0
0 Key Laboratory of Enhanced Oil & Gas Recovery, Ministry of Education, Northeast Petroleum University , Daqing, Heilongjiang, 163318 , China
The mechanism of fracture initiation is the basic issue for hydraulic fracture technology. Because of the huge differences in fracture initiation mechanisms for different reservoirs, some successful fracturing techniques applied to porosity reservoirs are ineffectual for fractured reservoirs. Laboratory tests using a process simulation device were performed to confirm the characteristics of fracture initiation and propagation in different reservoirs. The influences of crustal stress field, confining pressure, and natural fractures on the fracture initiation and propagation are discussed. Experimental results demonstrate that stress concentration around the hole would significantly increase the fracture pressure of the rock. At the same time, natural fractures in the borehole wall would eliminate the stress concentration, which leads to a decrease in the fracture initiation pressure.
Hydraulic fracturing; porosity reservoir; fractured reservoir; fracture initiation; fracture propagation; simulation experiment
Since its introduction into the Hugoton gas reservoir
of Kansas in U.S.A. in 1947, hydraulic fracturing has been
and will remain one of the important tools for improving
well productivity. In order to improve the success rate
and fracturing effect, several researchers investigated
fracture initiation and propagation under different reservoir
Hubbert and Willis (1957)
proposed that the
fracture would initiate and propagate in a particular direction,
perpendicular to the minimum principal stress, and then
a macro-crack would be created. They presented the first
formula to calculate the fracture pressure.
) used a high-speed camera to record fracture
propagation and quantitatively measure the fracture geometry.
Their research showed that the fracture propagated following
an elliptical shape in homogeneous materials. This conclusion
was not applicable to heterogeneous interlayers with different
put forward that the
hydraulic fractures initiated and propagated following the rule
Hubbert and Willis (1957)
, but natural fractures
in the formations would affect the propagation of hydraulic
thought that the differential stress
in the horizontal direction and included angle between the
hydraulic fracture and the natural fracture were the major
factors affecting the fracture strike and shape.
and Teufel (1987)
conducted scaled laboratory experiments
to investigate the interaction between a hydraulic fracture
and a natural fracture. They observed that shear failure of
natural fractures happened easily when interference occurred
between hydraulic fractures and natural fractures.
et al (2000
) believed that the horizontal stress difference and
the tectonic stress difference might influence the fracture
propagation and its geometries, and the effect of the latter
was more notable. Yew and Li
(Yew, 1992; Yew and Li, 1987)
studied the fracturing of a deviated well.
Chen et al (1995
Zhang and Chen (2005)
performed a series of
large-scale tri-axial experiments to investigate the influence
of natural fractures and earth stress on the induced fractures.
Taking into account the influence of the natural fracture strike
Jin et al (2005
) proposed three initiation modes of
hydraulic fractures on the borehole wall and corresponding
computational models were built. Laboratory research into
the mechanism of fracture propagation in fractured reservoirs
showed that the horizontal stress difference and the angle of
approach were the macro-parameters affecting the hydraulic
fracture propagation in a normal stress regime, and the
interfacial friction coefficient and dimensionless net pressure
were the micro-parameters
(Zhou et al, 2007)
. The influence
of regional tectonic stress regime on the hydraulic fracture
propagation was greater than that of the natural fractures.
et al (1994
) performed detailed research into mechanisms of
deviated well fracturing. In this study, the fracture initiation
mechanisms in different reservoirs were studied using
hydraulic fracturing tests, and the influences of the crustal
stress field, the confining pressure, and natural fractures on
the fracture initiation and propagation are discussed.
Large size synthetic rock specimens were prepared in a
mold and the pre-fractures were simulated by placing one
piece of A4 paper in the specimens. A hollow tube was put
in the center of specimen to simulate the wellbore. The
specimen was set up on hydraulic equipment to load triaxial
stress. Water with red dye was injected into the specimen
constantly through the hollow tube using a constant pressure
pump, which was used to simulate gradually increasing
pressure in the borehole. The rock would break when the
fracture pressure was reached.
The reservoirs, in which hydraulic fracturing is an
effective method for increasing well productivity, can be
divided macroscopically into porous reservoirs and fractured
reservoirs. A series of 8 hydraulic fracture tests was conducted
to study the fracture initiation and propagation in these two
reservoir types, and to analyze the fracturing azimuth and the
influence of natural fractures on the initiation and propagation
of hydraulically-induced fractures.
E i g h t c o n c r e t e b l o c k s w e r e p r e p a r e d i n a m o l d
0.3m×0.3m×0.3m to simulate porous and fractured reservoir
rocks. Each concrete block contained a cylindrical cavity (50
mm long) and a steel tube (10 mm×150 mm), as shown in
Fig. 1. The steel tube was cast in the concrete block, and 50
mm orifice was kept underneath the pipe to simulate the open
hole section. A summary of their physical properties is given
in Table 1.
Four concrete blocks (specimens 1#-4#) had no fractures
and were used to simulate porous reservoir rocks and the
remaining blocks containing pre-existing fractures (specimens
5#-8#) were used to simulate fractured reservoir rocks. The
pre-fractures in specimens 5#-6# were parallel to the central
injection tube and 5 cm apart. The pre-fracture in specimen 7#
was parallel to and intersecting the central injection tube (Fig.
2), while the pre-fracture in specimen 8# was perpendicular to
and intersecting the central injection tube, as shown in Fig. 3.
Open hole section
3 Results and discussion
3.1 Specimens containing no fractures
Hydraulic fracturing tests were conducted on specimens
1#-4#, which had no fractures, to study the initiation and
propagation of hydraulic fracture in porosity reservoirs. The
The fracture pressure pf can be calculated by the Hubbert
and Willis formula
(Hubbert and Willis, 1957)
where σh is the minimum horizontal stress, MPa; σH is the
maximum horizontal stress, MPa; pP is the pore pressure,
MPa; St is the tensile strength of the concrete specimens.
Due to the low porosity of the concrete specimens, no
fluid can flow in the specimens, so the pore pressure can be
regarded as zero.
When the hydraulic fracture extends beyond the vicinity
of the wellbore, the fracture propagation is dominated by
the far-field stress. The fracture extension pressure can be
calculated by the following equation:
The fracture pressure and extension calculated from Eqs.
(1) and (2) are listed in Table 3.
pressure response in the borehole is illustrated in Fig. 4. The
pressure response curve can be described as four stages:
a fluid injection stage (AB), pressure holding stage (BC),
pressure releasing stage (CD), and the fracture propagation
stage (DF). The peak (point C) represents the fracture
initiation point, i.e., the creation of a fracture. The borehole
pressure drops instantaneously at this point duo to the fluid
leak-off through the induced fracture face and finally reaches
the lowest value (point D). The highest value of extension
pressure reached is point E.
Four specimens were all fractured, and their borehole
pressure responses are similar to that shown in Fig. 4. The
induced fractures initiated in the direction normal to the
smallest horizontal principal stress. As Fig. 4 shows, the
fracture pressure was apparently higher than the fracture
extension pressure. This indicates that the stress was
concentrated around the wellbore, in the range of 3-6 times
the wellbore diameter, i.e. 1/10-1/5 of the specimen size. Out
of this range, it is considered that the far-field stress played a
major part. The data of four specimens is given in Table 2.
The fracture pressure calculated from Eq. (1) was lower
than the experimental value. The average relative deviation
was 24%, which indicates that Eq. (1) is not applicable to the
analysis of hydraulic fracturing tests. The average deviation
between the extension pressure calculated from Eq. (2) and
the experimental value was 4.2%, which indicates that the
resistance force required to be overcome to drive fracture
propagation in the porous specimens is very close to the value
in deep formations.
The fractured section of specimen 3# is shown in
Fig. 5. As can be seen from the figure, the fracture face is
symmetrical and planar, but locally uneven. For the color
distribution, the shade of red varied. The red color was
darker near the fluid injection tube and on the right side of
the specimen. The color of the left side became lighter. The
distribution of injection fluid was not uniform. Based on the
principle that the longer time the fracture face contacted with
the fracture fluid, the darker the color of the fracture face we
observed, the initiation and propagation of fractures could be
Hydraulic fracture initiation is dependent on the injection
pressure, which has to overcome stress concentration around
the borehole, and the fracture fluid is forced into the fractures.
The specimen begins to fracture. However, the specimen did
not break into two parts at a time by the hydraulically-induced
fracture, instead a fracture was formed near the borehole.
was significantly low. This can be explained as the
preexisting fracture could reduce and eliminate the original
stress concentration around the open hole. The stress state
of surrounding rock is the main factor influencing fracture
initiation near the borehole.
Fig. 7 shows the fracture morphology in specimen 8#.
The hydraulic fracture initiated in the direction of the
preexisting fracture, i.e. perpendicular to the maximum principal
stress, and gradually turned to the direction perpendicular
to the minimum principal stress. It is very different from the
fracture geometry in specimen 7#.
Both specimens 7# and 8# had a fracture in the borehole
wall. The pre-existing fracture in specimen 7# was parallel
to the maximum principal stress, and then formed included
angles with the intermediate principal stress and the minimum
principal stress, respectively. The fracture in specimen 8#
was perpendicular to the maximum principal stress. The
fracture pressure for these two tests was very low, about two
thirds that of the specimens without fractures. The fracture
pressure of specimen 8# was relatively higher than that of the
specimen 7#. The experimental results indicate that the
preexisting fracture (on the borehole wall) played a great part in
the fracture initiation. The fracture in the specimens reduced
and eliminated the effect of the stress state of the surrounding
1) The distribution of crustal stress, the stress state around
the wellbore, and reservoir types are the important factors
influencing fracture initiation. Crustal stress acts mainly
in the area far from the wellhole and affects the fracture
initiation. The stress state of the surrounding rock acts mainly
in the area near the hole and affects the fracture propagation.
Reservoir types may affect the stress state of surrounding
rock and the fracture initiation and propagation.
2) Natural fractures in reservoirs have a significant effect
on the fracture initiation. They influence the fracture pressure
and the fracture geometry. The natural fractures would reduce
the rock fracture pressure and make the fracture geometry
3) Natural fractures in the borehole wall would eliminate
the stress concentration around the borehole and reduce the
fracture initiation pressure drastically. Here, the in-situ stress
(tectonic stress) became a dominant factor in influencing the
4) Hydraulically-induced fractures tended to propagate
along the fracture occurrence direction. When the hydraulic
fracture intersected with the natural fracture, the hydraulic
fracture continued to extend along the direction without
natural fractures. When the natural fractures existed in the
wall, the hydraulic fracture initiated along the direction of the
natural fracture and gradually turned to the direction normal
to the minimum principal stress.
This study was supported by the National Natural
Science Foundation of China (No. 50974029), the Doctoral
Program of the Ministry of Education (No. 20070220001),
and Province Natural Science Foundation of Heilongjiang of
China (No. E200816).
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