Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND SYSTEM FOR DRILLING WITH
REDUCED SURFACE PRESSURE
FIELD
The present invention relates generally to a method and system for
drilling a well.
BACKGROUND
To extract hydrocarbon fluids from reservoirs in earth formations,
wells are drilled into the formations. The development of drilling techniques
has evolved into the possibility of drilling wells in any direction such as to
extract as much as possible of the hydrocarbon fluids present in the
formations drilled. A well may for instance comprise a substantially vertical
section and at least one section deviating from the vertical section, possibly
a
substantially horizontal section. The sections deviating from the
substantially vertical section tend to be long, often extending for thousands
of meters into a formation. To meet increasing demand to add to energy
reserves, hydrocarbon exploration is being pushed into deeper waters, and
the depths of wells are increasing.
Drilling is normally performed by inserting a drill bit on the end of a
drill string into the well. The weight of the drill string is proportional to
the
length of the drill string. When drilling at large water depths, the depth of
the water also influences the pressure conditions in the well and adds to the
weight of the drill string. One does not want the formation fluids to
penetrate
the drilled well during drilling; therefore, the pressure exerted by the
drilling
system on the formation should be higher than the formation pore pressure.
Drilling system should also be understood as including the fluid added
between the drill string and the unlined formation wall. With this one also
has control of the well during drilling and will therefore prevent blowouts of
the well. At the same time, there is also a need to limit the amount of
drilling
fluid that penetrate the unlined formation wall, and also a need to prevent
fracturing the side wall of the drilled bore before production starts. This
gives
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that the pressure exerted by the drilling system must not exceed the
fracturing pressure of the formation. The formation pore pressure is also
dependent on the hydrostatic column, and at larger water depths the
formation pore pressure increases. When the pressure exerted by the drilling
system moves towards the boundaries defined by the formation pore pressure
and formation fracturing pressure, the well has to be cased by casing or liner
before drilling can be continued. It is therefore a need to provide a method
of
drilling where the drilling can proceed for a longer period of time in the
allowed pressure range between the formation pore pressure and the
formation fracturing pressure, i.e., broader drilling window.
The term "drilling" should be understood to refer to establishing a hole
in the ground by the means of a drill string. It particularly applies for
drilling
in the crust of the earth for hydrocarbon recovery, tunnels, canals or for
recovery of geothermal energy, both offshore and onshore.
WO 2010/039043 Al describes a downhole well tool comprising a tool
unit. The tool unit comprises at least one first fluid conduit and a return
fluid
conduit, and the tool unit is arranged to be placed in well bore defining an
annular space between the well unit and the well bore or cased well bore. The
return fluid conduit may be arranged in the first fluid conduit, leaving an
annular space in between the first fluid conduit and the return fluid conduit
for the flow of the first fluid, and wherein the return fluid conduit passes
in
the centrally arranged space of the return fluid conduit.
From document WO 94/13925 Al it is known to drill with dual pipes
arranged next to each other, where one pipe is used for the pumping of fluids
from the surface to the drill bit, and the other pipe serves as a return line
for
the drilled cuttings and fluids from the bit to the surface facility. At a
lower
part of the drill string, above the bit, is arranged a sealing piston. Above
the
piston is a seal closing the space between the pipes and the borehole wall or
casing, defining a volume between said piston and the seal. Pressurized fluid,
such as hydraulic fluid, pumped into this volume, urges the piston, and
thereby the bit, hydraulically against the bottom of the hole. When drilling
in
a formation, trying to reach the potential hydrocarbon reservoir, zones
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having higher formation pore pressures than the surrounding formation may
be encountered. These zones might be pockets or reservoirs of high pressure
gas or water. Drilling through such zones may be difficult, or even
impossible, by conventional drilling technologies as it is very difficult to
keep
control on the pore pressure at the same time as drilling underbalanced
(UBD) or using managed pressure drilling (MPD) and still reaching further
down the high pressure formation zone. UBD is referred to as a drilling state
where the hydrostatic pressure inside the casing or liner is less than the
reservoir pressure, while MPD is a suited method if the difference between
the formation pore pressure and the formation fracturing pressure is low.
MPD is an adaptive drilling method used to more precisely control the
annular pressure profile throughout the wellbore.
To be able to drill, the weight of the drilling system has to be higher
than the formation pore pressure. It might be possible to pump pressurized
fluid, such as hydraulic fluid, above the piston, but then the rotating
control
device (RCD), typically arranged at the top of the well, and defining the
upper boundary of an annular volume between the piston and the top of the
well, would have to withstand the pressures from the pumped pressurized
fluid. As the drill string rotates through the RCD with mud, there will
always be a limited pressure / rotation range for these products.
SUMMARY
In one aspect of the present invention, a drilling method includes
arranging a dual drill string having one inlet fluid conduit and one return
fluid conduit in a well drilled in a formation such that a well annulus is
formed between the dual drill string and a wall of the well. The method
includes arranging a divider element in the well annulus to divide the well
annulus into an upper annular region, which extends above the divider
element to a surface of the well annulus, and a lower annular region, which
extends below the divider element towards a bottom of the well annulus. The
method includes feeding a second fluid having a second density into the
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upper annular region. The feeding includes configuring the second fluid such
that the second density is greater than the first density.
In one embodiment, at least a portion of the first fluid is fluid from the
formation.
In one embodiment, feeding the second fluid includes providing the
second fluid as an unpressurized fluid.
In one embodiment, the method further includes measuring pressure
at or near at least one of an upper surface of the divider element exposed to
the upper annular region and a lower surface of the divider element exposed
to the lower annular region.
In one embodiment, the method further includes using the measured
pressure to selectively adjust the second density such that the second density
remains greater than the first density during drilling of the well.
In one embodiment, the method includes arranging a rotating control
unit near the surface of the well annulus. Pressure acting on the rotating
control unit will be determined by pressure in the well and density of the
fluid in the well annulus.
In another aspect of the present invention, a drilling system includes
a dual drill string having one inlet fluid conduit and one return fluid
conduit.
The dual drill string is arranged in a well formed in a formation such that a
well annulus is formed between the drill string and a wall of the well. A
divider element arranged in the annulus divides the well annulus into an
upper annular region extending above the divider element to a surface of the
well annulus and a lower annular region extending below the divider element
towards the bottom of the well annulus. The lower annular region contains a
first fluid having a first density. A second fluid having a second density is
disposed in the upper annular region, where the second density is greater
than the first density.
In one embodiment, the drilling system further includes a fluid inlet
through which the second fluid is fed into the upper annular region.
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In one embodiment, the drilling system further includes a rotating
control unit arranged near the surface of the well annulus, where the
rotating control unit is in communication with the fluid inlet.
In one embodiment, the drilling system further includes a blowout
5 preventer arranged near the surface of the well annulus in connection
with
the rotating control unit, the blowout preventer forming an upper boundary
of the upper annular region.
In one embodiment, the drilling system further includes means for
measuring pressure at or near a lower surface of the divider element exposed
to the lower annular region.
In one embodiment, the drilling system further includes means for
measuring pressure at or near an upper surface of the divider element
exposed to the upper annular region.
In one embodiment, the divider element is a piston.
In one embodiment, the divider element is fixed to the dual drill
string.
In another embodiment, the divider element is movable relative to the
dual drill string.
In one embodiment, the two fluid conduits of the dual drill string are
concentric.
In one embodiment, at least a portion of the first fluid is fluid from the
formation.
In one embodiment, the second fluid is unpressurized.
It is to be understood that both the foregoing summary and the
following detailed description are exemplary of the invention and are
intended to provide an overview or framework for understanding the nature
and character of the invention as it is claimed. The accompanying drawings
are included to provide a further understanding of the invention and are
incorporated in and constitute a part of this specification. The drawings
illustrate various embodiments of the invention and together with the
description serve to explain the principles and operation of the invention.
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BRIEF DESCRIPTION OF THE DRAWING
The following is a description of the figures in the accompanying
drawings. The figures are not necessarily to scale, and certain features and
certain views of the figures may be shown exaggerated in scale or in
schematic in the interest of clarity and conciseness.
FIG. 1 shows a well with a drilling system according to one
embodiment of the invention.
FIG. 2 is a graph showing pressure as a function of depth in a well
annulus for two different weight fluids.
DETAILED DESCRIPTION
In the following detailed description, numerous specific details may be
set forth in order to provide a thorough understanding of embodiments of the
invention. However, it will be clear to one skilled in the art when
embodiments of the invention may be practiced without some or all of these
specific details. In other instances, well-known features or processes may not
be described in detail so as not to unnecessarily obscure the invention. In
addition, like or identical reference numerals may be used to identify
common or similar elements.
The present invention relates to a method of controlling pressure in a
well annulus such that pressure at a rotating control unit (RCD) near the
well annulus is kept low. The method involves use of a divider element,
typically a piston, arranged in the well annulus. The arrangement is such
that there is fluid below the divider element and fluid above the divider
element, where the fluid above the divider element is a heavy fluid and the
fluid below the divider element is a light fluid, i.e., the fluid above the
divider
element has a higher density than the fluid below the divider element. In
FIG. 2, line A represents the pressure in the well annulus as a function of
depth for the system where light fluid is below the divider element and heavy
fluid is above the divider element. For comparison, line B represents the
pressure in the well annulus as a function of depth if the fluid above and
below the divider element are both light fluids, e.g., having substantially
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equal densities, or if there is no divider element in the well annulus. As
illustrated in FIG. 2, lines A and B have the same bottomhole pressure, as
indicated at P1. Right below the divider element, at P2, the pressures for
lines A and B are also the same. Above the divider element, line A diverges
from line B. The surface pressures of lines A and B are shown at P3 and P4,
respectively. As shown, the surface pressure for line A is lower than the
surface pressure for line P4. This is due to the heavy fluid above the divider
element in the system represented by line A. This means that the RCD in the
system represented by line A will experience a lower surface pressure than
the RCD in the system represented by line B. This reduction in the pressure
acting on the RCD will enable a broader drilling window. If the heavy fluid in
the system represented by line A is unpressurized, the pressure acting on the
RCD can be as low as zero.
FIG. 1 shows a drilling system 1 according to one aspect of the present
invention. The drilling system 1 would exhibit a pressure in the well annulus
similar to line A of FIG. 2, as described above. The drilling system 1 is
shown
in the context of offshore drilling, but it may also be applied to land
drilling.
A well 14 has been drilled through formation 32 and is being drilled through
a high pressure formation 34 overlying a hydrocarbon formation 36. The
upper part of the well 14 is provided with casing 2. The lower part of the
well
14 is not cased. The drilling system 1 consists of a drill string 20 having
dual
pipes. The pipes can be concentric or positioned next to each other. In the
shown embodiment the pipes are concentric. A first pipe 29 has an inlet fluid
path A connected to an inlet fluid conduit 10. A second pipe 30 has a return
fluid path B connected to a return fluid conduit 9. The return fluid conduit 9
is on the inside of the inlet fluid conduit 10, but in an alternative
embodiment the inlet fluid conduit 10 may be on the inside of the return fluid
conduit 9. The lower part of the drill string 20 has a bottom hole assembly
(BHA) 15 and a drill bit 4 having a drilling fluid outlet 18 in its lower end.
The BHA 15 may be provided with a crossover valve 16. A cuttings inlet 17 is
positioned on the upper part of the BHA 15.
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The drill string 20 is arranged in the well 14 such that a well annulus
22 is formed between the drill string 20 and a wall 21 of the well 14. A
divider element 3, such as a piston, plunger or ram 3, is arranged in the well
annulus 22 on the outside of the drill string 20. The divider element 3
divides
the well annulus 22 into an upper annular region 5 above the divider element
3 and a lower annular region 12 below the divider element 3. The upper
annular region 5 extends from an upper surface 3b of the divider element 3 to
the surface of the well annulus 22, while the lower annular region 12 extends
from a lower surface 3a of the divider element 3 towards the bottom of the
well annulus 22. In one embodiment, the lower annular region 12 extends all
the way to the bottom of the well 14. The divider element 3 can be set in an
area of the well 14 with casing 2 or in open hole. In one embodiment, the
divider element 3 is fixed to the drill string 20. In this case, if the force
acting
on the upper surface 3b of the divider element 3 is greater than the force
acting on the lower surface 3a of the divider element 3, the divider element 3
will tend to move downwards. If the net force on the divider element 3
overcomes the weight of the drill string 20, the drill string 20 will be urged
downwards, i.e., towards the bottom of the well 14, by motion of the divider
element 3. In another embodiment, the divider element 3 is not fixed to the
drill string 20 and is free to move relative to the drill string 20.
Near the surface of the well 14 or well annulus 22, in the area close to
the sea floor 13, is arranged a blowout preventer (BOP) 8 and a rotating
control device (RCD) 7. The RCD 7 is in communication with a tank (not
shown) or similar storing facility for storage of fluid, such as drilling
fluid,
through a fluid inlet 6. The fluid inlet 6 also leads to the upper annular
region 5 defined above the divider element 3. A top drive adapter 11, for
rotating or driving the drill string, is arranged at a surface vessel or
platform
(not shown).
When performing drilling operations in the well 14, the drilling system
1 also includes a lower annular region fluid 25 contained in the lower
annular region 12 and an upper annular region fluid 27 disposed in the
upper annular region 5. In one embodiment, at least a portion of the lower
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annular region fluid 25 is from the formation(s) in which the well 14 is
drilled. The remainder of the lower annular region fluid 25 may be from fluid
discharged from the drill string 20 into the bottom of the well 14. The lower
annular region fluid 25 will apply a first pressure on the lower surface 3a of
the divider element 3.
The upper annular region fluid 27 may be fed into the upper annular
region 5 through the fluid inlet 6. The upper annular region fluid 27 will
apply a second pressure on the upper surface 3h of the divider element 3. The
upper annular region 5 may be filled partially or entirely with the upper
annular region fluid 27. The pressure at the surface of the column of fluid in
the upper annular region 5 will determine the pressure at the surface of the
well annulus 22. The pressure acting on the RCD 7 will be determined by the
pressure in the well 14 and the density of fluid in the well annulus 22, which
is related to the pressure in the well annulus 22.
In one embodiment, the density of the upper annular region fluid 27 is
greater than the density of the lower annular region fluid 25. In this case,
the
hydrostatic pressure at the upper surface 3b of the divider element 3 will be
greater than the hydrostatic pressure at the lower surface 3a of the divider
element 3. In one embodiment, the upper annular region fluid 27 is
.`unpressurized," i.e., does not have raised pressure that is produced or
maintained artificially. If the upper annular region fluid 27 is
unpressurized,
the pressure at the surface of the well annulus 22 and acting on the RCD 7
will be essentially zero. The upper annular region fluid 27 may be a drilling
fluid, for example. In general, the upper annular region fluid 27 can be a
liquid, a mixture of one or more liquids, or a mixture of one or more liquids
and one or more types of solid particulates. The composition of the upper
annular region fluid 27 will be selected to achieve a desired density, which
would preferably be greater than that of the lower annular region fluid 25.
Typically, the density of the upper annular region fluid 27 will be greater
than 1.0 kg/litre.
In one embodiment, a pressure sensor 24 is arranged at or near the
upper surface 3b of the divider element 3 to measure pressure at or near the
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upper surface 3h. Alternately, or additionally, a pressure sensor 26 may be
arranged at or near the lower surface 3a of the divider element 3 to measure
pressure at or near the lower surface 3a. As mentioned earlier, the density of
the upper annular region fluid 27 needs to be greater than the density of the
5 lower annular region fluid 25. If the density of the upper annular region
fluid
27 is greater than the density of the lower annular region fluid 25, the
pressure measured at or near the upper surface 3b of the divider element 3
will be greater than the pressure measured at or near the lower surface 3a of
the divider element 3. If the outputs of the sensors 24, 26 indicate that the
10 density of the upper annular region fluid 27 is not greater than the
density of
the lower annular region 25, the density of the upper annular region fluid 27
can be increased. Monitoring of pressure at or near the surfaces 3a, 3b may
be carried out at various times during the drilling process. This is because
the conditions in the lower annular region 12 can change at any time, e.g.,
due to formation fluid influx or change in the composition of the fluid
pumped down the drill string 20. Adjustment of the density of the upper
annular region fluid 27 may be manual or automated.
The method of reducing the pressure acting on the RCD 7 through use
of a heavy fluid above the divider element 3, as described above, can be used
with any drilling mode, such as underbalanced, managed pressure, and
overbalanced drilling modes. This means that selection of the density of the
upper annular region fluid 27 may be influenced by the formation pore
pressure.
Although the present invention is described in terms of some preferred
embodiments, alterations can be made. For example, the fluid flow
directions, and the inlets and outlets of the cuttings and drilling fluid, can
be
swapped. A person skilled in the art will understand that there are other
alterations and modifications that could be made that are within the scope of
the invention as defined in the attached claims.
