Note: Descriptions are shown in the official language in which they were submitted.
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PATENT APPLICATION
TITLE OF INVENTION:
DRAIN HOLE DRILLING IN A FRACTURED RESERVOIR
INVENTOR: WILLIAM C. MAURER
BACKGROUND OF INVENTION
1. Field of the Invention
[0001] Disclosed herein are improved methods for recovering fluids
from horizontal
wells that have been hydraulically fractured. More particularly, methods are
disclosed for
drilling drain holes that intersect hydraulic fractures that have been formed
from wells in a
hydrocarbon reservoir.
2. Description of Related Art
[0002] Hydraulic fracturing of vertical wells is a very mature art in
the oil and gas
industry. The method is well-known: inject a fluid at high rate and a pressure
sufficient to
overcome earth stresses at the bottom of the well, then inject a proppant
material in fluid after
a fracture has been opened in the rock surrounding the well. In recent years
hydraulic
fracturing has received a great amount of attention because it, combined with
directional
drilling, has made possible large increases in production of oil and gas from
"unconventional"
reservoirs in the United States. The reservoirs that have been exploited in
recent years have
been primarily in shale, which was previously thought to be too low in
permeability to
produce at commercial rates. The shale has natural fractures, and can be
fractured with
hydraulic fracturing techniques such that in a horizontal well, where tens of
spaced-apart
hydraulic fractures can be formed along a horizontal segment of a wellbore,
very economic
production rates can be obtained. ("Horizontal" will be used herein to
designate segments of
wells or wells that have segments in a hydrocarbon reservoir that are more
than sixty degrees
from vertical.) This revolution in technology has in recent years made the
United States again
a world-class producer of hydrocarbons.
[0003] Hydraulic fracturing in shale introduces new aspects to the
hydraulic fracturing
process that were not known in the classical hydraulic fracturing treatments
in vertical wells,
first performed in 1947 and first commercially used in 1949 in the United
States. In the
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classical hydraulic fracturing process, a single vertical fracture extends in
a productive zone
from a vertical wellbore. The fracture is thought to be formed in two "wings"
extending from
the wellbore into the reservoir, normally of about equal length, extending in
the direction
perpendicular to the direction of the least horizontal stress in the earth.
Amounts of fracturing
fluid and proppant are injected to make possible fractures that are propped to
a thickness
estimated to be about 1/32 inch up to about 1/16 inch. The width of fractures
formed while
pumping fluid into the earth are often calculated to be up to about 1/2 inch
in some cases. In
the matter of hydraulic fracturing of shale reservoirs, the presence of
natural fractures in the
rock creates further uncertainty in the rate of fluid leak off from the
hydraulic fracture while
pumping, and the lack of well-defined strata of rock and other factors make
prediction of
fracture geometry and properties in shale a more difficult proposition. Many
technical papers
have been published in recent years on the process and results of fracturing
of shale reservoirs,
but there is still much to be learned.
[0004] One general characteristic that is consistently observed in
wells that are
hydraulically fractured in shale reservoirs, however, is that the rate of
decline of production in
the wells is generally greater than the rate of decline of production from
wells fractured in
conventional reservoirs. The reasons for this and for other observations in
fracturing shale
wells are complex to explain. A review paper entitled "Examining Our
Assumptions ¨ Have
Over-Simplifications Jeopardized Our Ability to Design Optimal Fractured
Treatments?"
(SPE ¨ 119143-MS, Society of Petroleum Engineers, January, 2009) discusses
many of the
factors that can explain results of hydraulic fracturing in shale wells. A
recent review paper
that discusses results of re-fracturing to address the decline in productivity
of fractured wells
in shale reservoirs is "Beating the decline through refracturing," (World Oil,
June 2015, pages
39 - 43).
[0005] The decline in production rate of a well can be caused by a decline
in fluid
conductivity of the propped hydraulic fractures in the reservoir around the
well. The
conductivity of hydraulic fractures can be decreased by several phenomena that
have been
recognized in the past. One effect is the presence of residue from polymers
used in fracturing
fluid; another is the crushing of proppant particles in response to the
stresses in the earth; and
a third, not widely studied, phenomena is the migration of fine solid
particles in the hydraulic
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fracture. Plugging of fractures because of solid particle migration would be
exacerbated near
a horizontal wellbore, because the fluid velocity flowing into the wellbore
becomes extremely
high as radial flow from the fracture nears the wellbore. The high velocity
increases the
tendency for solid particles to migrate in the fracture and plug the
conductivity or flow
capacity in a fracture. High fluid velocity may also cause erosion or
mechanical failure of
rock abound the fracture or of the proppant.
[0006] For example, if a horizontal well is producing 100 barrels of
liquid per day or
0.385 cu ft per minute from a set of perforations in the horizontal segment of
the well, the
wellbore has a diameter of 8 inches and a single fracture 1/16 inch wide
intersects the
wellbore, the total cross-sectional area of the fracture at the wellbore is
only 8ir x 1/16 = 1.57
sq inch or 0.011 sq ft. 100 barrels = 560 cu ft, so the linear velocity
("Darcy velocity") of the
liquid at the wellbore is 0.385 cu ft/min /0.011 sq ft = 35 ft/min or 7
inch/sec. Eight inches
away from the wellbore, fluid velocity would be half as large¨still a high
value. Over time,
this high velocity fluid (in turbulent flow) will cause erosion and failure of
rock or proppant.
Particles produced from the failure could cause a decrease in conductivity of
the fracture at or
near the wellbore. Well production rate will decrease as fracture conductivity
decreases. It is
believed that these conditions cause the rapid decline of production rate of
fractured horizontal
wells.
[0007] U.S. Pat. No. 8,646,526 discloses methods of improving
production in a
wellbore that includes determining the textural complexity of a formation and
the induced
fractured complexity in creating a plurality of fractures, then drilling a
lateral well originating
from a wellbore to maintain conductivity of the formation. More than one
lateral well may be
drilled. The disclosure also includes drilling a second wellbore in the
formation. Method for
injecting a material that sets to a solid in hydraulic fractures is also
disclosed.
[0008] U.S. Pat. No. 8,333,245 discloses increasing production rate from a
reservoir
by drilling a plurality of branching wellbores through the reservoir.
International Patent
WO/2014/107475 discloses developing high pressure shale in tight rock
formations using a
profusion of sinusoidal open-hole laterals. The object is to drill enough
laterals to achieve
contact surface areas comparable with that of hydraulically fractured wells.
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[0009] Drilling of drain holes in the earth around wells is well-
known. For example,
U.S. Pat. No. 6,668,948 describes jet drilling of small holes from a cased
wellbore. Drain
holes are not cased. This technique may be used to extend the drainage radius
of a well,
somewhat similarly to the hydraulic fracturing technique. "The main body" of a
drain hole is
defined as that part of the drain hole that is outside the influence of the
window in the casing,
or generally more than one to two feet from the casing wall.
[0010] What is needed is a method to utilize hydraulic fracturing in
shale reservoirs
but to decrease, minimize or eliminate the decrease in production rate after
the well has been
fractured and placed on production.
BRIEF SUMMARY OF THE INVENTION
[0011] Drain holes are drilled from selected locations in a main
wellbore to extend
selected distances alongside the main wellbore. The drain holes are drilled,
preferably using
commercial directional drilling technology, after a window is made in the
casing of the main
wellbore at a selected location. The drain holes may be drilled after the
reservoir rock around
the main wellbore has been hydraulically fractured and the well has been
produced.
Alternatively, the drain holes may be drilled before the well has been
fractured or after the
well has been fractured and before the well is produced. The drain holes may
extend across
only one hydraulic fracture formed from the main wellbore or they may extend
across multiple
hydraulic fractures. Drain holes will normally not contain casing and cement.
Mechanical or
diverter means may be used to plug the drain holes, preferably at the casing
in the main
wellbore. Offset horizontal wells may also be drilled with open hole segments
at selected
distances from the horizontal segment of an original horizontal well.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0012] FIG. 1 is a side view of hydraulic fractures at intervals
along a horizontal well
and a drain hole drilled parallel to the horizontal segment of the horizontal
well from the
vertical segment of the horizontal well and a naturally fractured zone.
[0013] FIG. 2 is a side view of hydraulic fractures at intervals
along a horizontal well
and a drain hole drilled from the horizontal segment of the horizontal well to
intersect multiple
fractures.
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[0014] FIG. 3 is a side view of hydraulic fractures at intervals
along a horizontal well
and multiple straight drain holes drilled from the horizontal segment of the
horizontal well to
intersect the multiple fractures.
[0015] FIG. 4 is a side view of hydraulic fractures at intervals
along a horizontal well
and multiple curved drain holes drilled from the horizontal segment of the
horizontal well to
intersect the multiple fractures.
[0016] FIG. 5 is a plan view of hydraulic fractures at intervals
along a first horizontal
well and a drain hole drilled from the horizontal segment of the horizontal
well to intersect the
multiple hydraulic fractures.
[0017] FIG. 6 is a side view of hydraulic fractures at intervals
along a first horizontal
well and a second offset horizontal well drilled near the heel of the first
well so as to intersect
the hydraulic fractures in the first well.
[0018] FIG. 7 is a side view of hydraulic fractures at intervals
along a first horizontal
well and a second offset horizontal well drilled near the toe of the first
well so as to intersect
the hydraulic fractures in the first well.
[0019] FIG. 8 is a side view of hydraulic fractures at intervals
along a horizontal well
and a drain hole drilled parallel to the horizontal segment of the horizontal
well from the
vertical segment of the horizontal well and straddle packers placed across a
hydraulic fracture
from the first well and between hydraulic fractures coming from the first
well.
[0020] FIG. 9 is a side view of equipment that may be used to drill
horizontal wells
and drain holes.
DETAILED DESCRIPTION OF THE INVENTION
[0021] Referring to FIG. 1, well 10 is a horizontal well drilled into
productive
reservoir 19. Reservoir 19 is in rock that has very low permeability - usually
shale -- or it
may be very low permeability sandstone or carbonate. Multiple hydraulic
fractures 14 have
been formed at spaced-apart locations along horizontal segment 12 of the well.
Usually, only
by forming multiple hydraulic fractures 14 can an economic production rate be
obtained from
the well. The number of hydraulic fractures, each one resulting from a -stage"
of the
hydraulic fracturing treatment of the well, commonly is in the range from 3 to
40 or more.
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The location of the hydraulic fractures may be determined by perforations in
the cemented
casing of the well or by packers in open hole outside un-cemented casing that
contains
openings that can be selectively opened between swellable or inflatable
packers. These "well
completion" technologies are very well known in the oil and gas industry. The
horizontal
segment of a horizontal well may also be open hole, with no casing. In this
case, the location
of a hydraulic fracture may be determined by placement of straddle packers on
a work string.
[0022] Damage zones 16 around horizontal segment 12 of well 10, where
fluid enters
the casing, is a lower permeability zone that may be formed in or around each
fractured
interval. This lowered permeability or "damage" zone is attributed to near
wellbore effects
such as migration of fine particles to this zone or deposition of solids from
the flowing fluids
entering the well. It is believed that damage zone 16 is normally caused by
plugging of the
flow channels of fracture 14. The high velocity of the fluid entering the
horizontal well
accounts for the increased mobility in solids or deposition of solids in
damaged zone 16. It is
believed that damaged zone 16 accounts for the rapid decrease in production
rate that is often
observed in fractured horizontal wells in low permeability reservoirs.
[0023] Horizontal drain hole 18 may be drilled at any time, but
usually it will be
drilled after production rate from horizontal well 10 has significantly
decreased. Horizontal
drain hole 18 is drilled such that it generally follows a path at distance "d"
offset from original
horizontal segment 12. The drain hole is drilled through a window formed in
the casing in the
vertical segment of well 10. This distance d is preferably greater than the
diameter of damage
zone 16, which is often estimated to be in the range of 10 to 20 feet. Drain
holes are not cased
and cemented. Drain hole 18 may be drilled such that the distance d is
relatively constant, or
it may be drilled with varying d along the path of the well. The angular
location in a vertical
plane of horizontal drain hole 18 with respect to the center of horizontal
segment 12 may vary
along the trajectory of the drain hole. The effect of horizontal drain hole 18
is that a fraction
of the fluid that would otherwise enter horizontal segment 12 is diverted to
drain hole 18. This
has two positive effects: the fluid velocity into horizontal segment 12 is
decreased, because
fluid is diverted into drain hole 18; and, flow can enter drain hole 18 and
well 10 without
passing through damage zone 16. Preferably, drain hole 18 is beyond the radius
of damaged
zone 16. Since drain holes are not cased and cemented, fluid can enter where
the drain hole
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intersects a fracture. (In other words, the drain hole is an "open hole.") A
preferred diameter
of drain holes is in the range of 3 to 5inches, with 4 3/4 inches being a
common value because
of this diameter being a common bit size, but drain holes may be larger or
smaller. The total
effect is an increase in production rate from well 10.
[0024] It is known that natural fracture zones exist in shale and other
very low
permeability rocks containing hydrocarbons. When such zones are indicated in a
reservoir,
from additional geophysics, microseismic measurements, or production data, for
example,
drain holes may be drilled to intersect indicated natural fracture zones, such
as zone 19f in
FIG. 1.
[0025] Referring to FIG. 2, horizontal well 20 has been drilled into low
permeability
reservoir 29. Hydraulic fractures 24 have been formed at selected intervals
along horizontal
segment 22 of well 20. Damage zones 26 have formed around horizontal segment
22 near the
intersection of fractures 24 with segment 22. Drain hole 28 has been drilled
from a window
formed in the casing of horizontal segment 22 at a selected location. Drain
hole 28 preferably
takas a path alongside original well 20, but far enough away to be outside the
radius of
damage zone 26, preferably more than about 10 to 20 feet.. Drain hole 28 may
be straight or
may undulate or go in selected directions dependent on the geometry of the
rock strata in
reservoir 29.
[0026] In another embodiment, illustrated in FIG. 3, horizontal well
30 is drilled
through low permeability reservoir 39. Hydraulic fractures 34 have been formed
from
horizontal segment 32 of well 30. Damaged zone 36 may exist around horizontal
segment 32
at the intersection of hydraulic fractures with the well. In this embodiment,
drain holes 38A,
38B and 38C are formed at different locations along horizontal segment 32 by
cutting holes in
the casing to form windows and drilling drain holes from the windows so as to
intersect the
hydraulic fractures formed outside damaged zone 36. Drain holes 38A, B and C
may be as
short as about 10' or as long as hundreds of feet, depending upon the geometry
of the reservoir
and well 30.
[0027] In another embodiment, illustrated in FIG. 4, horizontal well
40 is drilled
through low permeability reservoir 40. Hydraulic fractures 44 have been formed
from
horizontal segment 42 of well 40. Damaged zone 46 may exist around horizontal
segment 42
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at the intersection of hydraulic fractures with the well. In this embodiment,
drain holes 48A,
48B and 48C are formed at different locations along horizontal segment 42 by
cutting holes in
the casing to form windows and drilling drain holes from the windows so as to
intersect the
hydraulic fractures formed outside damaged zone 46. Drain holes 48A, B and C
may be as
short as about 10' or as long as hundreds of feet, depending upon the geometry
of the reservoir
and well 40 and the drain holes are drilled with a bent sub, which results in
curved drain holes.
[0028] In another embodiment, illustrated in FIG. 5, a top or plan
view, original
horizontal well 50 has been drilled into reservoir 59. Hydraulic fractures 44
have been formed
at intervals along original well 50. Damaged zones 56 may be present near the
intersection of
hydraulic fractures 54 and original well 50. Drain hole 51 has been drilled
also into reservoir
59. Drain hole 51 has a horizontal segment alongside and displaced from
original well 50 and
also intersecting hydraulic fractures 54 formed from original well 50. Drain
hole 51 allows
production of fluid from fractures 54 without flow through damaged zones 56.
[0029] Referring to FIG. 6, horizontal well 60 has been drilled into
reservoir 69. The
vertical segment of offset well 61 may be in the vicinity of the vertical
section of original well
60 and near the heel of the horizontal section of the well. Fractures 64 and
damaged zones 66
exist around each fracture. Alternatively, rather than near the heel of the
horizontal well, such
as illustrated in FIG. 6, in an alternate embodiment, illustrated in FIG. 7,
offset well 71 is
drilled vertically into reservoir 79 near the toe of the horizontal section of
well 70. Fractures
74 and damaged zones 76, have formed along horizontal segment 70.
[0030] Referring to FIG. 8, well 80 is a horizontal well drilled into
productive
reservoir 89. Reservoir 89 is in rock that has very low permeability. Multiple
hydraulic
fractures 84 have been formed at spaced-apart locations along horizontal
segment 82 of the
well, using the same techniques as described above. Damaged zones 86 exist
near the
intersection of the hydraulic fractures and the horizontal well. Drain hole 88
has been drilled
at a distance d from the horizontal well. A work string (not shown) with
straddle packers may
be placed in drain hole 88 and the work string lowered to place the straddle
packers at
locations 87A and 87B. In this location, hydraulic fracture 84 between the
straddle packers
may be re-fractured. Additional proppant may be injected into the fracture.
The straddle
packers may also be placed so as to place straddle packers at locations 85A
and 85B in the
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drain hole. A new hydraulic fracture 83 may be formed from the drain hole
between straddle
packers 85A and 85B. The straddle packers may be released after each
fracturing treatment
and moved to a new location in the drain hole. Using this procedure, multiple
hydraulic
fractures may be formed from a drain hole. After fracturing in a drain hole is
complete, resin
coated sand may be circulated into the drain hole or a slotted liner, pierced
plastic pipe, sand
screen or uncemented casing may be placed in the drain hole.
[0031] Apparatus generally available in industry for drilling
horizontal wells and drain
holes, such as described above, is illustrated in FIG. 9. Bent motor drilling
assembly 90 is
illustrated in use in reservoir 99. Assembly 90 includes instruments for
measurements-while-
drilling 92, which are placed in non-magnetic collar 93. Drilling motor
segment 94 is attached
to bent sub 95. Bent subs and drilling motors are well known in the drilling
industry. The
bent sub may be at selected angles or may be adjustable at angles a, as shown
in FIG. 9. Bit
96, usually a drag bit but possibly a rotary bit, as shown, is attached to the
bent sub and is
powered by drilling motor 94. The majority of horizontal wells in shale or low
permeability
formations are drilled with this assembly. The bent assemblies drill along
straight trajectories
when the drill pipe is rotating and along curved trajectories when the drill
pipe is not rotating
(i.e., is in the "slide" mode). Thus the trajectory of wells can be controlled
by utilizing the
rotating and slide modes as needed. The curved sections in horizontal wells
are drilled in slide
mode.
[0032] The measurement-while-drilling tool measures the azimuth and
orientation of
the borehole. These measurements are needed to steer the drill bit. Non-
magnetic collar 63
allows the MWD tools to make magnetic measurements needed to steer the bit.
The bits are
normally PDC rotary bits using man-made diamonds in the cutters. These bits
allow drilling at
high rates and have good side-cutting capabilities, which are important when
drilling curved
sections in directional wells.
[0033] To drill drain holes out of the cased segment of a well, it is
necessary to mill a
"window" in the casing and drill through this opening. This is a routine
operation performed
in the drilling industry. Preferably, drain holes are drilled "underbalanced."
The pressure in
the drain hole is preferably maintained below the pressure in the reservoir
being drilled. This
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will minimize or eliminate damage to the fractures and reservoir rock during
the drilling
operation.
[0034]
Wall friction between the drill pipe and the wall of the bore hole increases
as
the length of the horizontal section increases. When the drill pipe is
rotating, wall friction is
not a problem. When the drill pipe is not rotating, or is in the slide mode,
the wall friction can
be high and can limit the length of a horizontal section to one or two miles.
[0035]
Service companies have also developed rotary steerable tools (RST) that allow
the drill pipe to be rotated in both straight and curved sections in a
horizontal well. This
overcomes the wall friction problems and allows the drilling of high angle and
horizontal
sections in excess of eight miles in some wells. Most shale wells are shorter,
with horizontal
sections not more than 6,000 feet, so they are mostly drilled with bent
assemblies because of
the high reliability of the bent assemblies and the high cost of the rotary
steerable tools.
[0036]
The drain holes can be drilled horizontal, slanted, curved or undulating
using
these tools or any combination of the tools described above.
Jet drilling can also be used,
such as disclosed in U.S. Pat. No. 6,668,948.
[0037]
Although the present invention has been described with respect to specific
details,
it is not intended that such details should be regarded as limitations on the
scope of the
invention, except to the extent that they are included in the accompanying
claims.
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