Note: Descriptions are shown in the official language in which they were submitted.
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LIQUID LIFT METHOD FOR DRILLING RISERS
FIELD OF THE INVENTION
The invention relates generally to offshore drilling systems. More
particularly, the invention relates to a dual-gradient offshore drilling
system using
low-density liquid lift for drilling risers.
BACKGROUND OF THE INVENTION
The search for crude oil and natural gas in deep and ultra-deep water has
resulted in greater use of floating drilling vessels. These vessels may be
moored or
dynamically-positioned at the drill site. Deep water drilling typically
involves the use
of marine risers. A riser is formed by joining sections of casing or pipe. The
riser is
deployed between the drilling vessel and wellhead equipment located on the sea
floor
and it is used to guide drill pipe and tubing to the wellhead and to conduct a
drilling
fluid and earth-cuttings from a subsea wellbore back to the floating vessel. A
drill
string is enclosed within the riser pipe. The drill string includes a drilling
assembly
that carries a drill bit.
A suitable drilling fluid (commonly called "drilling mud" or "mud") is
supplied or pumped under pressure from the drilling vessel. This drilling mud
discharges at the bottom of the drill bit. Mud lubricates and cools the bit,
and lifts
drill cuttings out of the wellbore. In conventional offshore drilling,
drilling mud is
circulated down the drill string and up through an annulus between the drill
string and
the welibore below the mudline (sea floor), and from the mudline to the
surface
through the riser/drill string annulus.
Drilling mud is very important in the,drilling process. It serves as: (1) a
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lubrication and heat transfer agent; (2) a medium to carry away and dislodge
pieces of
the formation cut by the drill bit; and (3) a fluid seal for crucial well
control purposes.
To maintain well control, drilling operators attempt to carefully control the
mud
density at the surface of the well to avoid many potential problems. One
potential
problem is "lost circulation" when a colunm of drilling mud exerts excess
hydrostatic
pressure, which propagates a fracture in the formation. Formation fluids may
enter
the wellbore unexpectedly when the hydrostatic pressure falls below the
formation
pressure. Such an event is called "taking a kick." A blowout occurs when the
formation fluid enters the wellbore in an uncontrolled manner. Both of these
problems become even more difficult to overcome in deep water. In a
conventional
drilling system, the relative density of the drilling mud over that of the
seawater,
along the length of the riser in deep water, combined with a low overburden
pressure,
results in excess hydrostatic pressure in the riser/drill string annulus and
the
wellbore/drill string annulus.
Because of the narrow margins between pore pressure (formation fluid
pressure) and fracture pressures (leak-off/lost circulation pressures),
equivalent
circulating density (ECD) is tightly controlled by balancing hole cleaning
requirements and circulation rates. The wellbore is also cased off at frequent
intervals
to maintain well control.
One solution to these problems known in the art is dual-gradient drilling.
Dual-gradient drilling is an area of technology that is primarily used to
overcome the
narrow pore pressure/fracture gradient margins found in abnormally pressured,
ultra-
deepwater wells. As an enabling technology, dual-gradient drilling permits
drilling in
deep and ultra deep water using fewer casing strings than possible using
conventional
drilling systems. Because there are fewer casing strings used, there is
potential for
drilling dual-gradient wells faster than conventionally drilled wells. Dual-
gradient
drilling can also enhance extended-reach drilling by reducing the influence of
circulating pressure losses on bottom-hole pressure. Dual-gradient drilling
can be
used to drill a wellbore with a larger diameter hole at the bottom of the
wellbore,
resulting in lower pressure drop per unit length than a smaller diameter
wellbore.
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Forms of dual-gradient drilling technology being developed include pump-
lifted and gas-lifted drilling risers. Pump-lift systems use pumps positioned
near the
sea floor to pump the heavy mud/drilling returns from the mud line to the
drilling
vessel to reduce the hydrostatic pressure at the mud line, generally to that
which
would result from a sea water gradient. Illustrative of the pump-lift systems
is U.S.
Patent 4,813,495 to Leach that discloses a method and apparatus for drilling
subsea
wells in water depths exceeding 3000 feet (915 meters) (preferably exceeding
4000
feet (1220 meters)) where drilling mud returns are taken at the seafloor and
pumped to
the surface by a centrifugal pump that is powered by a seawater driven
turbine. See
also U.S. Patent No. 4,149,603 to Arnold and published PCT application
W09915758. Limitations with the pump-lift systems include wear and equipment
reliability for the subsea pumps and motors. Also, the ability of the pump-
lift system
to handle dissolved and entrained gas is potentially very poor.
Gas-lift systems use air or nitrogen to "lift" the drilling returns,
effectively
lowering the riser hydrostatic pressure to a seawater pressure gradient. For
example,
U.S. Patent No. 4,099,583 to Maus discloses an offshore drilling method and
apparatus which are useful in preventing formation fracture caused by
excessive
hydrostatic pressure of the drilling fluid in a drilling riser. One or more
flow lines are
used to withdraw drilling fluid from the upper portion of the riser pipe. Gas
injected
into the flow lines substantially reduces the density of the drilling fluid
and helps
provide the lift necessary to return the drilling fluid to the surface. The
rate of gas
injection and drilling fluid withdrawal can be controlled to maintain the
hydrostatic
pressure of the drilling fluid remaining in the riser and wellbore below the
fracture
pressure of the formation. See also U.S. Patent No. 3,815,673 to Bruce, et
al., U.S.
Patent No. 4,063,602 to Howell, et al. and U.S. Patent No. 4,091,881 to Maus.
Limitations with the gas-lift system include inefficient or ineffective
cuttings
transport, dealing with pressurized equipment on the drilling vessel, and
detection of
fluid influx from the formation to the well bore (kick detection).
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SUMMARY OF THE INVENTION
Generally, the invention is a method of drilling a well below a body of water
using a drill string that starts by injecting into the well, at a depth below
the water
surface, a liquid having a lower density than a density of a drilling mud.
This
produces a mixture of drilling mud and low-density liquid in the well. The low-
density liquid may be miscible or immiscible with the drilling mud. The
mixture of
drilling mud and low-density liquid is withdrawn from an upper end of the
well. At
least a portion of the low-density liquid is separated from the mixture of
drilling mud
and low-density liquid, with at least a portion of the separated low-density
liquid
returned to the depth below the water surface and at least a portion of the
drilling mud
depleted of low-density liquid being returned to an upper end of the drill
string.
An embodiment of the invention includes controlling the injection rate of the
liquid. First, the rate of the liquid injected can be selected so the cuttings
within the
riser pipe have an upward velocity in excess of the settling rate of the
cuttings in the
riser pipe. Secondly, the rate of the liquid injected can be selected so the
liquid lift
maintains a bottom-hole pressure that is below the fracture pressure of the
earth
formation and above the pore pressure of the formation.
Other aspects and advantages of the invention will be apparent from the
following description and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates an offshore drilling system configured for dual gradient
riser drilling.
Figure 2 illustrates a liquid lift system for drilling risers in accordance
with
one embodiment of the present invention.
Figure 3 illustrates mud processing in a liquid lift system for drilling
risers in
accordance with one embodiment of the present invention.
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Figure 4 depicts a flowchart of miscible liquid lift in accordance with one
embodiment of the present invention.
5 Figure 5 depicts a flowchart of immiscible liquid lift in accordance with
one
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Specific embodiments of the invention will now be described in detail with
reference to the accompanying figures. Like elements in the various figures
are
denoted by like reference numerals for consistency.
Fig. 1 illustrates one type of offshore drilling system (10) where a drilling
vessel (12) floats on a body of water (14) which overlays a pre-selected earth
formation (17A). A drilling rig (20) is positioned in the middle of the
drilling vessel
(12), above a moon pool (22). The moon pool (22) is a walled opening that
extends
through the drilling vessel (12) and through which drilling tools are lowered
from the
drilling vessel (12) to the sea floor or mudline (17). At the mudline (17), a
structural
pipe (32) extends into a wellbore (30). A conductor housing (33) is attached
to the
upper end of the conductor pipe (32). A guide structure (34) is installed
around the
conductor housing (33) and adjacent a blowout preventor (38) before the
conductor
housing (33) is run to the mudline (17). A wellhead (35) is attached to the
upper end
of a conductor pipe (36) that extends through the structural pipe (32) into
the wellbore
(30). The wellhead (35) is of conventional design and provides a facility for
hanging
additional casing strings in the wellbore (30).
A riser system like the one depicted in Fig. 1 typically includes one or more
auxiliary lines (well-control lines 53 and boost line 68) on the outside of a
riser (52).
Well control lines (53) provide a high-pressure conduit for fluid flow between
a BOP
(38) and a drilling rig (20). A boost line (68) supplies drilling fluid to the
bottom of a
riser (52) to enhance the removal of drill cuttings.
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A drill string (60) extends from a derrick (62) on the drilling rig (20) into
the
wellbore (30) through a riser (52) which extends generally from the blowout
preventor (38) back to the drilling vessel (12). Attached to the end of the
drill string
(60) is a bottom hole assembly (63), which typically includes a drill bit (64)
and one
or more drill collars (65). The bottom hole assembly (63) may also include
stabilizers, mud motor, and other selected components required to drill a
wellbore
(30) along a planned trajectory, as is well known in the art. The end result
is the
creation of a well that extends from above the water surface to below the
mudline (17)
into the earth formation (17A). During conventional drilling operations,
drilling mud
is pumped down the bore of the drill string (60) by a surface pump (not shown)
and is
forced out of the nozzles (not shown) of the drill bit (64) into the bottom of
the
wellbore (30). Cuttings resulting from the drilling become entrained in the
mud at the
bottom of the wellbore (30) and the mud laden with cuttings rises up the
wellbore
annulus (66) and into the riser/drill string annulus (54 in Fig. 3), and to
the surface for
treatment in mud cleaning facilities (not shown). The passage of the mud from
the
bottom of the wellbore to the surface of the body of water may be referred to
as a
return flow system.
The present invention is not limited to any particular return flow system. In
one embodiment, the return flow system may comprise a first annular space
between
the drill string (60) and the wall of the wellbore (30), and a second annular
space
between the drill string (60) and the inner surface of casing (36) positioned
in the
wellbore, and a third annular space between the drill string (60) and the
riser (52)
extending between the cased wellbore and the surface of the body of water
(14).
A liquid-lift drilling riser system, as shown in Fig. 2, uses a lightweight
miscible or immiscible fluid to reduce the density of a drilling mud to as low
as that
of seawater. A surface pump (not shown) pumps a low-density liquid (74)
through a
riser boost line (68). The low-density liquid (74) is directed to the riser
(52)
approximately at the mud line (17) via the riser boost line (68). During
normal
drilling, the low-density liquid (74) will mix with the high-density mud (76)
returning
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from the bottom of the well. This mixture (80) will return to the surface and
flow over shale shakers (not shown). Once through the shale shakers (not
shown), the
mixture (80) will be separated and treated into its original low-density
liquid (74) and
high-density mud (76) components. The high-density mud (76) (preferably
substantially all of the high-density mud which is depleted of low-density
liquid 21)
will again be pumped down the drill string (60) and the low-density liquid
(74)
(preferably substantially all of the separated low-density liquid 74) will
again be
pumped down the riser boost line (68) back to the bottom of the riser (52).
Proper
separation provides a closed loop system with low fluid losses.
Fig. 3 shows an alternative configuration for a liquid lift drilling system. A
lightweight miscible or immiscible fluid is used to reduce the density of a
drilling
mud to as low as that of seawater. A surface pump (not shown) pumps a low-
density
liquid (74) through a fluid injection line (72). The low-density liquid (74)
is directed
to a position below the mud line (17) via a parasite string (71) installed in
the cased
wellbore (37). The parasite string thereby placing the low-density liquid 74
in an
annular space between the drilling string 60 and the inner wall of casing 36 .
During
normal drilling, the low-density liquid (74) will mix with the high-density
mud (76)
retuming from the bottom of the well. This mixture (80) will return to the
surface and
flow over shale shakers (not shown). Once through the shale shakers (not
shown), the
mixture (80) will be substantially separated and treated into its original low-
density
liquid (74) and high-density mud (76) components. The high-density mud (76)
will
again be pumped down the drill string (60) and the low-density liquid (74)
will again
be pumped down the fluid injection line (72) through the parasite string (71)
to the
cased wellbore (37).
In one embodiment, a miscible liquid-lift system uses a miscible liquid such
as
seawater to be injected into a water-based mud. For lifting a water-based
drilling
mud, seawater is injected into the riser boost line (68) to dilute the mud,
effectively
reducing mud density (weight). A portion of a return fluid is discarded at
surface, and
the water-based drilling mud is rebuilt with necessary additives needed to
regain the
desired mud weight.
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For lifting a weighted mud, or if drilling with a synthetic or an oil-based
mud,
it may not be economical or environmentally acceptable to discard diluted
drilling
mud at surface. In such a case, the miscible liquid-lift system can comprise a
base
fluid common to both the low-density liquid (74) and the high-density mud
(76). The
high-density mud (76) generally contains barite, hematite and/or other
suitable
weighting agents and is directed down the drill string (60) as previously
explained.
The low-density liquid (74) may contain one or more density-reducing agents,
such as
low-density particulate materials, including, for example, hollow glass
beads/microspheres or other density-reducing additive. As previously
explained, the
low-density liquid (74) is directed to the riser (52) at the mud line (17) via
the riser
boost line (68 in Fig. 2), or is directed into the wellbore (37 in Fig. 3) via
a parasite
string (71 in Fig. 3). The fluid mixture (80) returning up the riser pipe (52)
contains
both weighting agents and weight-reducing agents (if any).
Referring to Fig. 4, drill solids are removed from the return fluid mixture
(80)
using one or more standard rig solids control devices (116). The resulting
fluid (82)
then travels to one or more separation devices (112), such as mechanical
separators,
gravity separators, centrifuges, or other similar equipment. The one or more
separation devices (112) separate the fluid (82) into the low-density liquid
(74) and
weighting agent (114). The low-density liquid (74) is moved to mud pits (110)
before
being redirected into the riser annulus (54 in Fig. 2) above the BOP (38 in
Fig. 2) or
into wellbore annulus (37 in Fig. 3) below the mud line (17 in Fig. 3). The
high-
density mud (76) is re-formulated at (106) by combining the weighting agent
(114)
and a portion (83) of unprocessed fluid (82). Then, the re-formulated high-
density
mud (76) may be moved to mud pits (111) for temporary storage before being
redirected into the wellbore (30 in Fig. 2). The miscible liquid-lift system
can be used
for any type of drilling fluid, and this embodiment of the liquid-lift system
can be
used to drill part or all of the well.
Another embodiment is an immiscible liquid-lift system. Referring to Fig. 5,
an immiscible system uses a low-density boost liquid (74) that is
substantially
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immiscible with the high-density mud (76) to lighten the returning drill
fluid. An
example of this is to drill with a weighted water-based mud and boost with a
lightweight, immiscible synthetic fluid, such as an ester, olefin or glycol.
The low-
density liquid (74) is introduced into the returning drill fluid at the base
of the riser
(52 in Fig. 2) or down the fluid injection string (72 in Fig. 3) or both the
base of the
riser (52 in Fig. 2) and down the injection string (72 in Fig. 3)
simultaneously. The
resulting fluid (80) is a stable, two-phase fluid of lower density than the
mud (76).
Referring to Fig. 5, one or more conventional separation devices (81), such as
a three-
phase centrifuge, can be used to separate the fluid mixture (80) on the
drilling vessel
(12 in Fig. 1), where the fluids (74, 76) can be re-circulated. First, the
fluid mixture
(80) can be processed using standard solids control equipment (120), such as
course-
screen shakers, to remove part or substantially all of the drill solids. Next,
the
resulting fluid (82) is separated in oil-water separator (81), such as a three-
phase
centrifuge, to produce drill solids (86), low-density liquid (74), and drill
fluid (122).
The drill solids (86) may be discarded in any environmentally suitable manner.
The
low-density liquid (74) may be moved to mud pits (110) for temporary storage.
The
drilling fluid (122) in this embodiment may pass through additional standard
rig solids
control devices (116), and then moved to mud pits (111) for temporary storage
as
high-density mud (76).
Another embodiment of the liquid lift system uses a combination fluid, such as
low-density glass beads (or a density-reducing agent) in a miscible low-
density liquid
slurry. By using miscible low-density liquid slurry instead of the low-density
mud
without the slurry, the volume of low-density liquid needed for producing a
significant mud weight change in the riser (52 in Fig. 2) may be reduced. The
density-reducing agent may be recovered at the surface before discarding the
excess
volume of fluid, if any. The result is a stable, homogeneous fluid of lower
density
than the mud pumped down the drill string (60 in Fig. 1).
Referring to Fig. 2, controlling the rate of the low-density liquid (74)
injected
into the riser (52) at or near the mud line (17) via the riser boost line
(68), or directed
into the cased wellbore (37 in Fig. 3) via the fluid injection string (72 in
Fig. 3) has
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two primary purposes in the liquid-lift system. First, the rate of the liquid
injected can
be controlled so the cuttings within the riser pipe annulus (54) have an
upward
velocity in excess of the settling rate of the cuttings in the riser pipe
(52). Secondly,
the rate of the low-density liquid (74) injected can be controlled to maintain
a bottom-
5 hole pressure that is below the fracture pressure of the earth formation and
above the
pore pressure of the formation.
The liquid-lift system has several advantages over pump-lift and gas-lift
systems. The liquid-lift system can use conventional solids control equipment
and rig
10 pumps to produce a simpler, more reliable dual-gradient drilling system
than a pump-
lift system. Cuttings transport is conventional, kick detection is
conventional,
circulation can be stopped (remain static) without adverse consequences, and
there is
little or no additional subsea equipment to break down, thereby creating a
need for a
riser trip to repair.
The liquid-lift system also allows the switching of drilling from dual-
gradient
to conventional, single-gradient merely by ceasing the injection of the low-
density
boost fluid to the riser (52 in Fig. 2). The liquid-lift system also allows
for additional
injection/lift points than just the mud line. The use of a parasite string (71
in Fig. 3)
to inject lift fluid below the mud line (17 in Fig. 3) increases the
effectiveness of the
liquid-lift system and provides incentive for use of dual-gradient drilling in
shallow
water or on land. Additionally, by using the parasite string to inject the
lift fluid
below the mudline (17 in Fig. 3), the volume of lift fluid necessary to create
lift in the
riser (52 in Fig. 3) can be reduced.
While the invention has been described with respect to a limited number of
embodiments, those skilled in the art will appreciate that other embodiments
can be
devised which do not depart from the scope of the invention as disclosed
herein.
Accordingly, the scope of the invention should be limited only by the attached
claims.
