U.S. patent application number 10/466990 was filed with the patent office on 2004-04-15 for pressure pulse generator.
Invention is credited to Knight, John, Prain, Kenneth.
Application Number | 20040069530 10/466990 |
Document ID | / |
Family ID | 9907398 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040069530 |
Kind Code |
A1 |
Prain, Kenneth ; et
al. |
April 15, 2004 |
Pressure pulse generator
Abstract
A pressure pulse generator for use in MWD operations in a
drilling installation having a drillstring, a drilling bit, means
for supply drilling fluid via the interior of the drillstring to
the drilling bit, and an annulus between the drillstring and the
wall of the bore hole being formed. The pressure generator
generates a pressure pulse signal in the drilling fluid which
transmits such signal to pressure monitoring equipment at the
surface. The pressure pulse generator includes an outer housing
which can be mounted in a drillstring component, and in which the
operating components of the pulse generator are housed; a main
valve having a valve operating chamber which allows drilling fluid
to pass from the interior of the drillstring to the exterior when
the valve is opened, and thereby to generator a pressure pulse
signal that travels to the surface. A first pilot valve allows
fluid in the operating chamber of the main valve to communicate
with the drilling fluid in the annulus, and a second pilot valve
controls flow of drilling fluid between the inside of the
drillstring and the operating chamber of the main valve.
Inventors: |
Prain, Kenneth; (Aberdeen,
GB) ; Knight, John; (Kincardineshire, GB) |
Correspondence
Address: |
MADSON & METCALF
GATEWAY TOWER WEST
SUITE 900
15 WEST SOUTH TEMPLE
SALT LAKE CITY
UT
84101
|
Family ID: |
9907398 |
Appl. No.: |
10/466990 |
Filed: |
July 23, 2003 |
PCT Filed: |
January 23, 2002 |
PCT NO: |
PCT/GB02/00302 |
Current U.S.
Class: |
175/40 |
Current CPC
Class: |
E21B 47/22 20200501 |
Class at
Publication: |
175/040 |
International
Class: |
E21B 047/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2001 |
GB |
0101806.8 |
Claims
1. A pressure pulse generator for use in MWD operations in a
drilling installation (10) having a drillstring (11), a drilling
bit (19), means (13, 14) for supplying drilling fluid via the
interior of the drillstring (11) and to the drilling bit (19), and
an annulus (16) between the drillstring (11) and the wall (12) of
the borehole which is being formed, said pressure generator being
operative to generate a pressure pulse signal in the drilling
fluid, and to transmit such signal to pressure monitoring equipment
(20, 21) at the surface, and in which the pressure pulse generator
comprises: an outer housing (30) which can be mounted in a
drillstring component, and in which the operating components of the
pulse generator are housed; a main valve (256, 257) having a valve
operating chamber which, when the valve is opened, allows drilling
fluid to pass from the interior of the drillstring to the exterior,
and thereby to generate a pressure pulse signal that will travel to
surface; a first pilot valve (116, 120) which is normally open, to
allow fluid in the operating chamber of the main valve to
communicate with the drilling fluid in the annular; and, a second
pilot valve (88, 89, 90) which is normally closed, to control flow
of drilling fluid between the inside of the drillstring and the
operating chamber of the main valve.
2. A pressure pulse generator according to claim 1, in which the
first and second pilot valves are electrically actuated valves.
3. A pressure pulse generator according to claim 2, in which the
actuators for the first and second pilot valves are arranged to be
immersed in hydraulic oil to prevent access of the particulate
drilling fluid to the sensitive actuator parts.
4. A method of generating pressure pulse signals in a drilling
fluid which is being supplied to a drilling installation (10)
having a drillstring (11), a drilling bit (19), means (13, 14) for
supplying drilling fluid via the interior of the drillstring (11)
and to the drilling bit (19), an annulus (16) between the
drillstring (11) and the wall (12) of he borehole which is being
formed, and a pressure pulse generator installed in the
drillstring; in which the pressure generator comprises an outer
housing (30) which is mounted in a drillstring component, and in
which the operating components of the pressure pulse generator are
housed; a main valve (256, 257) having a valve operating chamber
which, when the valve is opened, allows drilling fluid to pass from
the interior of the drillstring to the exterior, thereby to
generate a pressure pulse signal that will travel to surface; a
first pilot valve (116, 120) which is normally open, to allow fluid
in the operating chamber of the main valve to communicate with the
drilling fluid in the annulus; and, a second pilot valve (88, 89,
90) which is normally closed, to control flow of drilling fluid
between the inside of the drillstring and the operating chamber of
the main valve; in which a pressure pulse signal is generated in
the drilling fluid by the following steps: actuating the main valve
(256, 257) through a sequence of events at successive time
intervals; closing the first pilot valve and disconnecting the
piston of the main valve from the flow pressure drilling fluid in
the annulus; opening the second pilot valve, allowing access for
high pressure fluid from the drillstring to the main valve piston,
whereby the main valve consequently opens; closing the second pilot
valve, leaving the main valve position unchanged; after a selected
time interval reopening the first pilot valve, allowing the main
valve operating chamber to vent to the lower pressure region of the
annular outside the drillstring, and consequently the main valve
reclosing and the system being restored to its original state.
Description
[0001] This invention relates to a system of communication employed
during the drilling of boreholes in the earth for purposes such as
oil or gas exploration and production, the preparation of
subterranean services ducts, and in other civil engineering
applications.
[0002] Taking the drilling of oil and gas wells as an example, it
is highly desirable both for economic and for engineering reasons,
to obtain information about the progress of the borehole and the
strata which the drilling bit is penetrating from instruments
positioned near the drilling bit, and to transmit such information
back to the surface of the earth without interruption to the
drilling of the borehole. The generic name associated with such
techniques is "Measurement-while-Drilling" (MWD). Substantial
developments have taken place in MWD technology during the last
thirty years.
[0003] One of the principal problems in MWD technology is that of
reliably telemetering data from the bottom of a borehole, which may
lie several thousand metres below the earth's surface. There are
several established methods for overcoming this problem, one of
which is to transmit the data, suitably encoded, as a series of
pressure pulses in the drilling fluid; this method is known as "mud
pulse telemetry".
[0004] In one means of generating pressure pulses at a downhole
location, the fluid flowpath through the drill string is
transiently restricted by the operation of a valve. This creates a
pulse, the leading edge of which is a rise in pressure; hence the
method is colloquially, although rather loosely, known as "positive
mud pulse telemetry". In contradistinction the term "negative mud
pulse telemetry" is used to describe those systems in which a valve
transiently opens a passage to the lower pressure environment
outside the drill string, thus generating a pulse having a falling
leading edge. In a third basic method a continuous pressure wave is
generated downhole and modulated with the information to be
transmitted to the surface. It is to the second of the above
methods, namely "negative pulse telemetry", that the present
invention relates.
[0005] Good practice in the drilling of boreholes in the earth aims
at keeping the diameter of the borehole as small as possible
consistent with the mechanical strength and stability of the
drilling system and with the availability of ancillary equipment
(such as MWD systems, motors, orienting tools, wireline logging
tools, perforating guns and so on). The advantages gained in the
saving of costs of drilling equipment, energy and materials, by
drilling the smallest diameter borehole possible in any given set
of conditions, are obvious. It will be understood that in the
typical operation of drilling an oilwell, a series of coaxial
boreholes, are drilled, each being lined with steel tubing before
the drilling continues at a smaller borehole diameter. By making
the diameter of the final section as small as is practicable, the
diameters of the previous hole sections can also be reduced.
[0006] Continuing introduction of improved materials and equipment
has led to a steady reduction in the typical diameter of borehole
sections over time. It is nowadays relatively common to drill
borehole sections of 3.5"-5.0" diameter whereas only a decade or so
ago those same hole sections would have been in the diameter range
6.0"-8.5". Furthermore it is now common practice to drill high
angle or horizontal extensions from existing boreholes in oil
reservoirs using equipment which will pass through the tubing in
the existing well: this operation requires small diameter drilling
equipment. The continuing demand for the drilling of smaller
diameter boreholes has produced a corresponding demand for ever
slimmer ancillary equipment such as MWD tools.
[0007] Because the present invention extends the applicability of
"negative pulse" systems to very small diameter drilling equipment
(drill strings down to 2.875" diameter), it is appropriate to
comment briefly on the relative merits of "positive" and "negative"
pulse systems. Although this terminology is inexact, as was
explained above, it is well recognised in the drilling industry.
Strictly speaking, the primary distinction is between "throttling"
and "bypass" systems. In throttling systems a valve operates to
contract or enlarge a restriction through which some or all of the
drilling fluid passes on its way to the drill bit. In bypass
systems a valve operates to allow a portion of the drilling fluid
to pass from a relatively high pressure region inside the drill
string to a relatively lower pressure region in the annular space
between the drill string and the wall of the borehole. The pressure
difference between the interior and exterior of the drill string is
created by the dynamic pressure losses as the drilling fluid passes
through equipment situated below the MWD tool, such as drilling
motors and the jets in the drill bit itself.
[0008] In the case of throttling control, the quiescent position of
the throttling valve is naturally configured to be such as to
minimise the restriction offered to the flowing drilling fluid:
when the valve moves towards a closed position and returns to its
starting point, a "positive" pulse is generated. In the case of
bypass control the quiescent position of the bypass valve is
naturally configured to be such as to minimise the loss of drilling
fluid to the drill bit, namely the position in which the bypass
orifice is closed. When the valve is opened and returned to its
closed position, a "negative" pulse is generated.
[0009] On the basis of the above definition, the third type of
pulse generator mentioned above, the continuous wave generator, is
a throttling valve, since it operates wholly within the drill
string and no fluid is bypassed. U.S. Pat. No. 4,641,289 discloses
a method which generates both positive leading-edge and negative
leading-edge pulses by causing a valve in the drill string to move
in a controlled fashion from a mean position in either direction
from its valve seat. Again because no fluid is bypassed, this is a
throttling system as defined above.
[0010] The principal operational distinctions between throttling
and bypass valves can be drawn as follows:
[0011] Throttling pulse generators require that the main valve
parts are exposed to a continuous flow of drilling fluid.
Relatively high forces are required to displace the moving part of
the valve in the mud stream. Because all, or at least a large
proportion, of the total drilling fluid flow has to pass the main
valve, the performance limits of a throttling pulse generator are
largely determined by the flow rate. The operational flow rate is
determined in any particular case according to the drilling
engineering requirements. Drilling fluid flow rates typically range
from 200 litres/minute to 6000 litres/minute and it is necessary to
design throttling pulse generators in a substantial number of
different sizes to cover all possible drilling operations.
[0012] In bypass pulse generators the valve that generates the main
pulse remains fully closed except when generating a pulse and is
not required to support continuous flow. This is a substantial
advantage because the valve parts are protected from erosion by the
solids-bearing drilling fluid for the majority of the operating
period of the equipment. The rise time and eventual amplitude of
the pulse are dependent not on flow rate but on the pressure drop
between the inside and the outside of the drill string. This
pressure drop is dependent on drilling engineering requirements and
can vary from 2 MPa to 200 MPa. It is relatively easy to design a
single bypass pulse generator that will handle this range of
pressure drops, with only minor changes, such as altering the size
of an external orifice to control the pulse amplitude. Because
there is no direct performance dependence on the main fluid flow
rate, a single design of pulse generator--with simple mechanical
adaptation--can be used right across the size range of the drill
string. The eventual lower size limit is reached when the space
available in the drill string element around the pulse generator
becomes too small to handle the required flow: but the flow rate
required to support the drilling operation reduces roughly in
proportion to the size of the hole being drilled, so that this
factor does not becoming limiting until very small drill string
diameters are reached. A further advantage of bypass pulse
generators is that the valve parts may be small and of relatively
low mass, so that fast valve operation can be achieved with little
energy expenditure.
[0013] The present invention discloses a particularly efficient and
flexible method of driving a main mud valve of the bypass type
using twin pilot valves in which the working fluid is drilling the
drilling fluid. The general principle of using pilot valves is of
course well known, and existing applications of this technique in
the MWD field fall into two main classes.
[0014] In the first class energy derived from the mud stream is
used to maintain a source of clean working fluid, such as hydraulic
oil at a suitable pressure to operate, under control of a small
valve, a piston actuator driving the main mud valve. Examples of
this type of device are described in U.S. Pat. Nos. 2,964,116,
4,184,545 and 4,535,429.
[0015] In the second class the working fluid is the drilling mud
itself. Examples of this type of device are described in U.S. Pat.
Nos. 3,958,217, 4,120,097, 4,742,948, 5,040,155, 5,333,686,
5,586,084 and 6,016,228.
[0016] The two main classes of actuation can be categorised in
general as follows. Those that use a separate working fluid are
relatively complex, with many parts co-operating in the processes
of supplying and maintaining the pressure of the working fluid.
Those that use the drilling fluid as the working fluid are
relatively simpler in concept--since there is an unlimited supply
of pressurised fluid available--but the relevant parts of the pilot
mechanism must be capable of withstanding the aggressive and highly
variable properties of the drilling fluid.
[0017] It is an objective of the present invention to provide a MWD
pulse generator operated by pilot valves and in which the pilot
working fluid is drilling fluid.
[0018] It is a particular advantage of the invention that it can
provide a so-called "negative-pulse" MWD in tools of extremely
small diameter: for example the entire pulse generator can be
constructed for installation in a 2.875" diameter drill pipe. The
ability to use negative-pulse MWD has certain advantages, mentioned
in the comparative discussion above, which have not hitherto been
realised in tools of this diameter.
[0019] It is another advantage of this invention that long working
life between service operations can be obtained while retaining the
simplicity of using the drilling fluid to operate the pilot
system.
[0020] It is another advantage of this invention that the pilot
valves may be constructed in such a way that their operations are
hydraulically similar to each other and to that of the main valve.
Thus all the design criteria including those of materials selection
for handling the abrasive drilling fluid may be applied equally to
the valves. The pilot valves are miniaturised versions of the main
valve.
[0021] It is a further advantage of this invention that the energy
required to provide each MWD pulse to surface is extremely small,
making battery operation, with its attendant simplicity, feasible
for long periods of time.
[0022] Yet another advantage of the invention is that the same
central pulse generator may be utilised across the entire range of
drill collar diameters from the smallest practical size
upwards--without limit--as needed, simply by the addition of
appropriate spacers and mechanical adapters. Because several,
usually at least three, different diameters of drill bit and
associated drill collars are sequentially used in drilling a single
borehole this adaptability of the pulse generator can substantially
reduce the operational costs of the MWD equipment.
[0023] The general principle of the operation of the invention is
as follows. A main valve chamber contains a piston-operated,
spring-return poppet valve and seat. When this valve is opened,
drilling fluid can pass from the interior of the drill string to
the exterior, generating the signal pressure pulse that will travel
to surface.
[0024] Two much smaller, electrically actuated pilot valves
co-operate in supplying operating fluid, in this case drilling
fluid, to the main valve piston. A first pilot valve is normally
open, and allows fluid in the operating chamber of the main valve
piston to communicate with the drilling fluid in the annulus. A
second pilot valve is normally closed and controls operating fluid
flow between the inside of the drill string and the operating
chamber of the main valve piston.
[0025] The actuators for the pilot valves are immersed in hydraulic
oil to prevent access of the particulate drilling fluid to the
sensitive actuator parts. The hydraulic oil pressure is equalised
by well-known means to the pressure of the drilling fluid in the
borehole.
[0026] To actuate the main valve the following sequence of events
is caused to take place at successive time intervals.
[0027] The first pilot valve is closed and disconnects the main
valve piston from the low pressure drilling fluid in the
annulus.
[0028] The second pilot valve is opened, allowing the access for
high pressure fluid from the drill string to the main valve piston;
the main valve consequently opens.
[0029] The second pilot valve is closed, leaving the main valve
position unchanged.
[0030] After a selected time interval the first pilot valve is
reopened, allowing the main valve operating chamber to vent to the
lower pressure region of the annulus outside the drill string;
consequently the main valve re-closes and the system is restored to
its original state.
[0031] The principal advantages of this invention arise because
each pilot valve can be specifically designed for unidirectional
flow, and the force needed to operate the pilot valves can be
minimised. The closure of each pilot valve is performed under
no-flow conditions. The opening of each pilot valve vents fluid in
the preferred direction across the valve again allowing the
operating force to be minimised. The small size of the valves and
the hydraulic balancing mentioned above cause the required
operating forces to be very low. The low operating forces and, as
will be seen later, short operating travel of the valves not only
lead to low energy consumption but also allow the operating
actuators to be of very small diameter. This in turn permits the
system to be constructed inside very small diameter drill pipe, as
mentioned earlier.
[0032] Alternative operating means for similar systems are
described in the prior art. For example, U.S. Pat. No. 4,401,134
discloses a means of low-energy operation in which the working
fluid is hydraulic oil: two pilot valves, one electrically and one
hydraulically operated co-operate in driving the main valve piston.
But in this system the high-pressure working fluid supply has to be
replenished by means of a regenerative pump. U.S. Pat. No.
5,586,084 describes a system in which the working fluid is the
drilling fluid, controlled by an electrically operated pilot valve,
but represents the latter by a conventional symbol without any
disclosure of how such a valve may be made to perform reliably when
controlling the flow of a highly particulate fluid such as drilling
mud, nor of the energy requirements of the valve. Both of the above
references describe systems which control a throttling valve, not a
bypass valve such as is the subject of the present invention.
[0033] DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
[0034] FIG. 1 is a schematic representation of a drilling rig and
MWD system to which the invention may be applied;
[0035] FIGS. 2a, 2b and 2c show a cross-sectional view of the pulse
generator divided into three longitudinal sections;
[0036] FIGS. 3a and 3b show enlarged views of the part numbered 120
in FIG. 2b;
[0037] FIGS. 4a, 4b and 4c show details of a fluid filter designed
to protect the pilot valves from particulate matter in the drilling
fluid;
[0038] FIG. 5 is a timing diagram illustrating the sequence of
pilot valve operations required to cause the pulse generator to
provide one pulse;
[0039] FIG. 6 shows an example of how the total current changes
with time as the valve operating sequence progresses;
[0040] FIG. 7 shows an example of how the cumulative energy
consumed changes with time as the valve operation progresses.
[0041] A typical arrangement of a mud pulse MWD system is shown
schematically in FIG. 1. A drilling rig 10 supports a cylindrical
drill string 11 in the borehole 12. Drilling fluid, which has
several important functions in the drilling operation, is drawn
from a tank 13 and pumped, by pump 14, down the centre of the drill
string, shown cutaway at 15, and returning by way of the annular
space 16 between the drill string and the borehole wall 12. The
part of the drill string 18 near the drill bit 19 houses MWD
equipment that includes a means for generating pressure pulses in
the drilling fluid. The pressure pulses travel up the centre of the
drill string, and are received at the earth's surface by a pressure
transducer 20. Processing equipment 21 decodes the pulses and
recovers the data that was transmitted from downhole.
[0042] Referring to FIGS. 2a to 2c, the MWD pulse generator
consists of the contents of the generally cylindrical housing 30,
mounted in the cylindrical drill string element 18. Ribs at 31 and
32 provide a means to support the housing 30 in the drill string.
These ribs are permanently secured to, or an integral part of, the
housing 30, and extend longitudinally as illustrated. There are
several ribs disposed circumferentially around the housing at these
points.
[0043] Drilling fluid flows past the pulse generator through the
space 34--shown in FIG. 2 with horizontal-dash hatching--which is
generally annularly disposed around the housing 30 except in the
regions where the ribs at 31 and 32 contact the drill string
element 18. The drill string element 18 is of course wholly
surrounded by the annular space 16 by way of which the drilling
fluid returns to surface: but for clarity of the drawing only a
small section of the borehole wall 12 is shown, in FIG. 2c.
[0044] The orientation of the pulse generator in the drill string
is not relevant to its operation. Solely for the purposes of
illustration and description it will be assumed that the part at
the top of FIG. 2a is uppermost and the part at the foot of FIG. 2c
is lowermost.
[0045] It will be understood that the other parts of the MWD
system, including a supply of electrical energy, instruments for
measurement of the parameters to be transmitted to surface and
electronic equipment for conversion and encoding of the data for
transmission are also mounted inside the drill string and may be
connected to the pulse generator housing 30 at either or both of
the ends 35 and 36: but because such equipment is well-known and is
not the subject of the present disclosure, it will not be described
in further detail. It should be noted however that a number of
electrical connections may be provided between the housing ends 35
and 36, one of which is shown at 37. These connections may be run
in insulated wires in long bores in the outer housing 30.
[0046] The ribs at 31 and 32 provide physical centralisation for
the housing and also permit access between the interior of the
pulse generator and the drilling fluid in the annular space 16 by
way of the ports 40 and 41 which are formed in the locking bolts 42
and 43 respectively and sealed from the interior of the housing 30
by O-rings 44 and 45 respectively. Each of the several ribs in each
of the circumferential locations indicated by 31, 32, carries a
separate bolt and port as described above. The housing 30 also has
milled recesses 50 and 51 in several circumferentially disposed
places at each location, into which are fitted filter elements 52
and 53 that allow access between the drilling fluid flowing through
the drill string and the interior of the pulse generator. The
filter elements at 52 and 53 are indicated merely symbolically on
FIG. 2; the actual details of their construction will be described
later.
[0047] The access ports, filters and stabilising ribs described
above may of course be disposed around the circumference of the
housing 30 in any suitable number and at any angular separation:
but it is preferred to have three of each set at 120.degree.
intervals. This arrangement is convenient for manufacturing and
ensures that when the housing lies in an inclined borehole there is
free access for fluid to at least two of the sets of ports even if
the third is partially obstructed by contact with the earth
formation being drilled.
[0048] It will be apparent that the radial length of the ribs 31,
32 and of the bolts 42, 43 may be varied to enable the pulse
transmitter to be housed in drill collars of diameters other than
that shown. The ribs may be extended simply by attaching spacers to
them with screws or other fasteners, thus permitting a single
transmitter to be used over a wide range of drill collar internal
diameter.
[0049] An additional separate port 60, formed in plug 61 allows
access between the drilling fluid in the annular space between the
drill string and the borehole and the interior of the pressure
pulse generator for ancillary purposes to be described later. As
will be seen, this port is essentially required only to be
maintained at the borehole pressure and therefore it is not
replicated elsewhere on the circumference of the housing 30.
[0050] Within the pressure pulse generator there are three
principal regions, which have been denoted by letters in FIGS. 2a
to 2c. They are: the pressure switch region A-B, the pilot valve
region B-C and the main valve region C-D.
[0051] In the pilot valve region there are four inner housings: the
upper pilot valve housing 80; the pilot chamber housing 110; the
lower pilot valve housing 140 and the balancing piston housing 170.
These four housings are interconnected sealably each with the next
and for the purposes of understanding the operation of the overall
system they can be regarded as a single inner housing, sliding into
the main housing 30 and carrying a number of internal bores,
cross-bores, seals and fluid access passages. Their division into
the separate parts enumerated above is necessary to make assembly
and maintenance practical.
[0052] Within housing 80 there is an electromagnetic solenoid
actuator consisting of yoke 81, armature 82, coil 83 and casing 84.
Armature 82 carries a valve stem 85 with ball poppet head 86. The
spring 87 biases the armature 82 to be at its maximum distance from
yoke 81 when coil 83 is not carrying current. The metal parts 81,
82 and 84 are made from any suitable magnetically soft material.
The valve parts 85 and 86 extend downwards into the valve chamber
housing 110.
[0053] Within housing 140 there is an electromagnetic solenoid
actuator consisting of yoke 111, armature 112, coil 113 and casing
114. Armature 112 carries a valve stem 115 with ball poppet head
116. The spring 117 biases the armature 112 to be at its maximum
distance from yoke 111 when coil 113 is not carrying current. There
is a further spring 118 in this assembly, the purpose of which will
be described later. Valve parts 115 and 116 extend upwards into the
valve chamber housing 110.
[0054] Within housing 110 there are also the remaining parts of the
upper and lower pilot valve assemblies. Valve guide 88, sleeve 89
and seat 90 complete the upper pilot valve: there is a seal 91
dividing the valve region from the actuator region. Valve guide 119
and seat 120 complete the lower pilot valve: there is a seal 92
dividing the valve region from the actuator region.
[0055] It will be noted that the valve head 116 and seat 120 form a
valve which is open to fluid flow when the actuator coil 113 is not
carrying current. When coil 113 is energised, valve head 116 moves
to close against seat 120. However if valve stem 115 and valve head
116 were rigidly attached to armature 112, it would be impossible
in practice to ensure that the valve head 116 could meet the seat
120 and simultaneously that the contact face of armature 112 could
meet the corresponding face of yoke 111. If the first only of these
conditions was satisfied there would be a residual gap between yoke
111 and armature 112, leading to a requirement for greater current
in coil 113 to hold the valve closed than would be the case if the
actuator yoke and armature were in contact. If the second only of
the conditions was satisfied there would be a residual gap between
the valve head 116 and seat 120. To overcome this difficulty the
spring 118, partially precompressed to exert a force greater than
that exerted by spring 117 in its compressed state, is fitted to
provide compliance between the armature 112 and the valve stem 115.
The assembly is built so that the valve and seat will always
contact each other before the gap between the yoke and armature
closes. Then spring 118 is moved into further compression and the
faces of the yoke and armature meet. The spring 118 also acts to
take up any slight wear which may take place in the poppet head and
valve seat during operation.
[0056] Continuing the description of operation, drilling fluid from
the region within the drill string communicates with upper pilot
valve 86/90 via filter 52, ports 121, 122, 123 and valve chamber
130. Drilling fluid at the lower annular pressure communicates with
lower pilot valve 116/120 via port 40 in the bolts 42, ports 124,
125 and valve chamber 126.
[0057] It should be noted in relation to the ports 121-125
inclusive mentioned above, that they form a communication path for
fluid through several concentric elements of the tool, namely the
main housing 30, the valve chamber housing 110 and the parts of the
upper and lower valve bodies 89 and 120 respectively. Although, as
will be explained later, the quantity of fluid which traverses
these passages at each valve operation is very small, it is
nonetheless important to minimise the restriction to the flow of
the fluid. It is preferable to provide several radial ports,
suitably spaced circumferentially in each of these locations.
[0058] Additionally, to avoid the need for exact lining up of the
various parts which constitute each port, a semicircular groove is
milled circumferentially in each of the concentric parts through
which the ports pass. The groove 127 is an example, and it will be
apparent from the drawing where the other similar grooves are
located. FIG. 3a shows a perspective view, and FIG. 3b a sectional
view, of the valve guide 120, with groove 127 and radial ports 129
indicated, to illustrate this concept.
[0059] It is necessary that the zones in the concentric parts
through which the ports 121-125 pass, and many other mutually
adjacent zones of the entire assembly, must be sealed one from
another. The solidly blocked regions of FIG. 2, for example 128,
all indicate sealing elements. Such seals are typically O-rings
made of Viton material that may also have support rings on the low
pressure side made from Teflon or a derivative thereof: or for
seals which are required to withstand severe pressure differentials
they may be square-section rings with appropriate supports. Such
seals and the techniques for applying them are very well known, and
they will not be described further.
[0060] A chamber 141 is in communication with both the upper and
the lower pilot valves. The chamber 141 also communicates via
several cross-bores 145, of which only one is shown, by dotted
lines, with an annular space 146 formed between the housing 30 and
the housings 140 and 170, the latter two parts being coaxial and
sealably interconnected. This annular space in turn communicates
through bores 171 with the volume 172.
[0061] Connected to the volume 172 are two further elements of the
system. One of these is piston 200, the function of which will be
described later. The other is the main valve stem 250, which,
together with a sleeve 251, also forms a piston running in guide
252. It will be noted that the effective diameter of this piston is
the diameter of the sleeve 251. Ancillary parts 253, 254 and spring
255 cause the poppet head at the end of valve stem 250 to be held
closed against the seat 257 in quiescent conditions. A fluid
deflector 258 with a nib 270 both serves to protect the valve stem
250 from erosive wear where it emerges into fluid chamber 259 and
has a function in controlling the pressure regime in the downstream
part 271 of chamber 259.
[0062] Drilling fluid in the annular space between the drill collar
18 and the main housing 30 has access to the poppet valve 256/257
through filters 53 and chamber 259. The discharge from the poppet
valve, when it is actuated, passes through valve chamber 260 and
ports 41 to the annular space between the drill collar 18 and the
borehole wall 12.
[0063] As mentioned above, volume 172 also communicates with piston
200. Volume 172 is connected to pilot valve chamber 141 by the
cross-bores 145 and annular space 146, as described earlier; thus
the pressure in volume 141 follows that in volume 172, with a
slight time lag. The purpose of piston 200 is to provide pressure
equalisation, whilst separating the fluids concerned, between the
drilling fluid in volume 172 and hydraulic fluid, which may be any
low-viscosity mineral oil suitable for the temperature range
required, contained in volume 300 and in all other spaces connected
to that volume. The regions containing hydraulic oil are shown in
dotted shading in the drawing, and communication between these
spaces is provided by a series of bores and passages. The selection
of the configuration of these bores is a matter of convenience and
will not be described in detail, however it should be noted in
particular that the hydraulic oil surrounds all the elements of the
electromagnetic actuators described earlier and is also provided to
the chamber 290, the purpose of which will be described later.
[0064] The provision of hydraulic oil at the drill-pipe pressure to
the actuators ensures defined pressure conditions on the stems of
the poppets 85 and 115, and the oil acts as lubricant for the
high-tolerance machined parts of the actuators. The length of
housing 170 is made sufficient to maintain piston 200 clear of the
ends of the housing over the anticipated range of change in the
volume of hydraulic oil caused by changes in temperature and
pressure, as is well known in this type of downhole tool. The oil
fill is introduced into the system prior to use by evacuation and
filling through the port 295, which is subsequently closed by plug
296: this again is a well-known technique.
[0065] It was mentioned above that hydraulic oil at drill-pipe
pressure is also provided to chamber 290. This chamber communicates
with one face of piston 291. The other face of piston 291
communicates with the drilling fluid in the borehole via port 60.
The purpose of piston 291 and the associated unlabelled parts is to
close electrical contacts 292 when the differential pressure
between the fluid in the drill string and that in the borehole
reaches a predetermined level. This switch is used for control
purposes in the other parts of the MWD system and has no direct
relevance to the present invention, being briefly described here
for completeness. Other well-known methods are available to provide
this control function.
[0066] Turning in more detail to the operation of the invention, it
can be seen that the pressure in the chamber 141, which is under
the control of the pilot valves consisting of poppets and seats
86/90 and 116/120 respectively is communicated to the piston
chamber 172 of the main valve 256/257, and that when the pressure
in this chamber is sufficiently raised the valve stem 250, together
with its sleeve 251 will move forward, opening a gap between poppet
head 256 and seat 257. Drilling fluid now flows from the drill
string into the pulse generator through the filter 53 and out to
the borehole through ports 41. This causes the pressure in the
drill string in the region of the pulse generator to fall. When the
valve is allowed to reclose after a short time interval, the
pressure in the drill string in the region of the pulse generator
is re-established to its original level. The pressure changes are
propagated up the drill pipe to the equipment at the surface of the
earth, where they are detected as a pulse with a negative-going
leading edge. This procedure, and the details of encoding and
decoding of the data for transmission to surface are not of direct
relevance here, and will not be further described.
[0067] To create the conditions necessary for the valve 256/257 to
open, a sequenced operation of the pilot valves is caused to take
place. The sequence is initiated according to the requirements for
the transmission of coded data and controlled by a processor or
other electronic logic devices using conventional methods.
[0068] Firstly, the coil 113 is energised and the lower valve
poppet 116 moves against the seat 120. During this operation a very
small quantity of hydraulic oil is displaced by the movement of
armature 112 into the general volume of oil surrounding the
actuator, via the oilway 281. Suitable dimensions for the parts
involved are that the diameter of the poppet valve stem may be 2.5
mm and the stroke length may be between 2.0 and 2.5 mm: thus the
maximum oil displacement during the motion of the valve stem
amounts to about 12 cubic millimetres. The piston 200 may have a
diameter of 19 mm and it can readily be calculated that the
displacement of 12 cubic millimetres of oil will cause a linear
movement of the piston of about 0.04 mm. In practice, because this
movement is so small, the piston simply "rocks" on the seal. The
point is important, because the differential pressure required to
move the piston and its seal together, relative to the bore of
cylinder 170, against frictional forces would substantially and
adversely affect the force required from the actuator to move the
valve stem 115. This principle obtains for each subsequent movement
of both of the pilot valves and will not be described
repeatedly.
[0069] At this stage in the sequence, the chamber 141 has been
isolated from the fluid in the borehole.
[0070] Next the coil 83 is energised, causing the upper valve
poppet 86 to move away from the seat 90. Now drilling fluid can
flow from the interior of the drill string 18 through the filter
52, the ports described earlier and the valve seat 90 into the
chamber 141. The rise in pressure in chamber 141 is communicated to
the main valve piston chamber 172 through the ports previously
described, and the main valve opens.
[0071] Next the coil 83 is de-energised. At this point no flow is
taking place through the valve seat 90, and the poppet head 86
returns to its original position under the influence of spring 87.
The chamber 141, and hence the chamber 172, are once more isolated
from the drilling fluid, and the main valve will remain open for as
long as this condition continues. During this phase of the pulse
the only energy being consumed is that required by the coil 113 to
maintain the lower pilot valve poppet 116 against its seat.
[0072] Finally the coil 113 is de-energised and the valve head 116
return to its original position under the influence of spring 117.
Fluid leaves chamber 141 and returns to the borehole via the valve
seat 120, the ports described earlier and exit ports 40. The main
valve piston now returns to its original closed position under the
influence of spring 255.
[0073] The sequence described above is illustrated in FIG. 5 in the
form of a valve timing diagram. The actual times shown are chosen
for illustrative purposes only. Changes in timing over a wide range
are possible and other timings may be used to suit specific
operational circumstances.
[0074] The timing diagram shows an operational sequence for a main
valve operation in which it is assumed that current is applied to
the actuator for valve 116 starting at t=20 milliseconds. Valve 116
closes. No change takes place in the condition of the system
overall, other than the displacement of a very small volume of the
hydraulic oil fill as the valve stem 115 moves.
[0075] After a period long enough to ensure that valve 116 is fully
closed, current is applied to the actuator for valve 86. Valve 86
opens and drilling fluid flows through it to open main valve 256 as
described earlier.
[0076] In principle, valve 86 may be re-closed as soon as main
valve 256 is fully open. In practice it is desirable to leave a
safety margin to ensure that pressure conditions a fully settled
and that there will be no residual differential pressure between
the drilling fluid in the drillstring and that in the main valve
operating chamber. By way of example only, a period of
approximately 140 milliseconds is shown.
[0077] At a later time, shown in this example as t=420
milliseconds, valve 116 is reopened, allowing the fluid from
chamber 172 to return to the annulus and the main valve 256 to
re-close. The system is now once again in the quiescent state,
having been open for a period of approximately 400
milliseconds.
[0078] During the generation of each pressure pulse, as described
above, some drilling fluid flows into the pilot system from the
drill string and returns to the borehole. There is a potential
problem when using drilling fluid as the hydraulic working fluid,
which is that the abrasive and solids-bearing nature of typical
drilling fluids might have an adverse effect when used in a system
with small clearances and passages, such as the present invention.
The practical means of dealing with this potential difficulty are
important to the overall function of the invention and will now be
described.
[0079] Firstly, the volume of drilling fluid handled by the pilot
valve system for each pulse is extremely small. It is defined by
the diameter of the sleeve 251 of the main valve stem and the
stroke of the valve stem. Practical dimensions can be for example
that the valve sleeve diameter is 12 mm and the stroke 3 mm. The
volume of fluid which traverses the pilot valves for each pulse
generated may be calculated as being less than 0.5 ml. In practice
the volume is a little larger because of the compliance of the
fluid itself and of the various seals, but can be maintained at
under 1 ml without difficulty. A typical MWD system may be required
to generate of the order of 10.sup.5 pulses in the course of a
single downhole trip lasting for several days. In such conditions
the total volume of drilling fluid which the pilot system is
required to handle is only 100 litres.
[0080] Secondly it may be noted that the volume of the main piston
chamber 172 and its associated feed ports is very much greater than
the volume of drilling fluid handled at each pulse. Drilling fluid
never has to pass through the main valve chamber: it is in effect
shuttled into and out of the chamber 141 and a part of the
communicating ports 145. The fluid in chamber 140 is subjected only
to pressure changes, not to the passage of abrasive fluid, and it
may therefore be filled with a benign fluid (such as a high
temperature silicone grease, or a high viscosity silicone oil)
prior to operations.
[0081] Thirdly, the role of the filters 52 is clearly important in
ensuring that particulate matter larger than the internal
clearances cannot enter the system. To this end the filters may be
configured as shown in FIGS. 4a, 4b and 4c. FIG. 4a shows a
transverse cross-section of the filter element 52. FIG. 4b is a
view of the inlet side of the filter and FIG. 4c shows the outlet
side, that is, the side presented to the interior of the pulse
generator. Very narrow longitudinal slots 400 are cut in such a way
that any material passing the slot will assuredly exit through the
wider passages 401. A suitable width for these slots is 0.25 mm. It
is re-emphasised that these filters, in a typical situation, have
the repetitive task of handling only 1 ml of entering drilling
fluid at a time, and the total task of handling a flow which may
average less than 1 litre per hour.
[0082] A possibility exists that material such as irregularly
shaped sand particles may be trapped on the inlet side of the
filter slots. Under these circumstances there is an increased
erosion rate of the filter material in the neighbourhood, due to
the effect known colloquially as "washing", in which a scouring
effect caused by irregular turbulence downstream of a flow
obstruction causes locally intense erosion. This "washing" tends
first to widen the entry slot immediately downstream of the
obstruction, which is thereby released into the main flow. Tests
conducted with sand-bearing fluid have shown that this mechanism
operates effectively. After a period of operation under these
conditions inspection of the filter elements reveals some trapped
particles and some slightly widened regions whence previously
trapped particles have escaped. The other safeguard which is
employed is that the total area of the filter slots is such as to
keep the fluid velocity through them extremely small and thus
minimise the occurrence of trapped particles.
[0083] The filter 53 may be similarly constructed, with its
dimensions suitably scaled to be compatible with the clearances in
the main valve and its associated ports.
[0084] Filters 52 and 53 are both considered to be wear parts, to
be replaced as determined by the results of a visual check carried
out in between downhole operational periods.
[0085] It was mentioned earlier that the similar nature and
configuration of the main valve and the two pilot valves allows
them all to be designed using common principles and materials. As
is well known in this art, the parts of valves that handle abrasive
drilling fluid, in this case the stems and poppets 250/257, 115/116
and 85/86 together with the valve seats 257, 90 and 120 may
conveniently be made from tungsten carbide or other similar hard
material.
[0086] The methods by which the energy consumption of this
embodiment of the invention are minimised will now be
described.
[0087] Firstly it will be noted that piston 200 essentially
equalises the pressures in piston chamber 172 and the hydraulic oil
which surrounds the pilot valve actuators. The pressure in chamber
172 is therefore communicated to the region of the valve stems 85
and 115 on the oil side of their respective seals 91 and 92. Taking
each of the valve operations described above in turn:
[0088] At the time that the valve head 116 is required to move to
the closed position, the pressures on both sides of the seal 92 are
equal: provided that the areas of the shaft at the seal and of the
valve seat are equal there is no net hydraulic force on the valve
stem. The actuator only has to do work against the force exerted by
the spring 117, the frictional force due to the valve seal and,
just prior to closure, against force created by the wear take-up
spring 118, the function of which was described earlier. Once the
armature of the actuator has completed its movement, the current
passing through coil 113 may be reduced to a much lower value,
because the two faces of the actuator yoke and armature are now in
contact with each other. This current reduction may be provided by
well-known electronic means, either after the passage of a certain
time or by detection that the movement of the armature is complete,
using known methods such as that described by Scherbatskoy in
Canadian Patent 1, 177, 948. Once the valve is closed, the pressure
feedback provided by piston 200 ensures again that the net
hydraulic force across the valve remains substantially at zero.
[0089] Before the valve head 86 moves from the closed to the open
position the hydraulic pressures across the valve are equal,
because chamber 141 remains at the same pressure as the hydraulic
oil on the actuator side of the seal 91. The actuator initially
only has to work against the spring and then against the flow force
(tending in a direction to re-close the valve) as the small volume
of drilling fluid passes the valve as described earlier. Now the
pressure in chamber 141 rises to that of the fluid in the drill
pipe, that pressure again being communicated to the actuator side
of valve 86/90 and reducing the net hydraulic force across that
valve to zero. As in the case of the valve 116/120, described
above, the current in coil 83 may now be reduced to the level that
is required just to maintain armature 82 in contact with yoke 81
against the force of spring 87.
[0090] It should be readily apparent that the sequence described
above occurs again during the events which lead to the closure of
the main valve. When coil 83 is de-energised, valve head 86 closes
under no-flow conditions under the influence of spring 87, and the
pressures across the valve remain equal. Finally when coil 113 is
de-energised, valve head 116 reopens by the influence of spring 117
under conditions essentially identical to
[0091] those that prevailed during the opening of valve 86, as
described above, the pressures on both sides of the valve remaining
substantially equal throughout.
[0092] FIG. 6 shows, by way of example only, how the current
flowing through the actuator coils changes with time during an
operating sequence timed in the same manner as that described
above. The time taken for each actuator to operate, measured from
the time of application of current, depends on a number of factors
including the current capability of the power supply, the
inductance and resistance of the winding, the temperature of the
winding and the total mechanical load on the actuator shaft. In
this example it is assumed that the actuator is driven from a
capacitor-discharge circuit such as that described by Scherbatskoy
in U. S. Pat. No. 4,839,370. With this type of drive the system is
generally designed so that operation is completed well inside a
period of 20 milliseconds, after which the main current is cut and
a much lower current is sufficient to maintain the actuator in the
energised condition. In FIG. 6 the two current peaks represent the
current supplied for the initial operation of each actuator. During
the period when both actuators are in the energised condition (from
t=80 to 200 milliseconds in FIG. 6) the hold-in current is doubled.
Wide variations of design are practical, but in the example shown
in FIG. 6 the coil has a resistance of around 12 ohms and a
de-energised inductance of around 30 mH. Each actuator is driven
from a 1000 microfarad capacitor pre-charged to 34 volts and under
these conditions the current reaches a maximum of around 2 amperes
in each actuator coil. Actuation is normally complete within 10
milliseconds. The current required to maintain each actuator in the
energised condition is only about 50 millamperes if an efficient
switch-mode power supply is used.
[0093] FIG. 7 is a representation of the cumulative energy
consumption during the course of a single operating cycle. With
parameters as indicated above, the operation of the two actuators
consumes about 1.2 joules. During the period when both actuators
are held in a further 0.4 joule is used. The change in slope at
t=200 milliseconds occurs when the first actuator is released. At
the end of the cycle the total energy consumption is just over 2
joules per generated pulse. In battery powered MWD systems using,
as is commonplace, lithium/thionyl-chloride batteries, it is
straightforward, without using a disproportionate amount of space
in a downhole system, to make 10.sup.6 joules available to drive a
pulse generator. Thus the required downhole lifetime of 10.sup.5
pulses can readily be achieved.
[0094] The above description applies to the case where the
effective areas of the valve seats 90, 120 are the same as those of
their respective valve stems 85, 115 where they cross the seals 91,
92 respectively. In an advantageous variant of the embodiment
described above, additional small imbalances in the forces across
certain of the valves created by inequalities in the relationship
between the diameter of the valve seats and the diameters of their
respective valve shafts may be used to improve the efficiency of
operation. Consider the quiescent (closed) state of valve 86 and
seat 90. Let the seat area be A.sub.1 and the shaft area A.sub.2.
Let the pressure of the fluid in the drill string be Pd and that of
the fluid in the annulus of the borehole Pa. The pressure Pa
appears in the chamber 141 and at the actuator side of the shaft
85. The pressure Pd appears in the chamber 130. The hydraulic force
on shaft 85 in the direction towards seat 90 is
(Pd-Pa)(A.sub.1-A.sub.2). In any circumstances in which the
transmitter is capable of generating a pulse, (Pd-Pa) is positive.
Thus the direction of the force on shaft 85 due to differential
pressure will be in a direction tending to keep the valve closed if
A.sub.1>A.sub.2 and in a direction tending to open the valve if
A.sub.1<A.sub.2. There are of course other static forces,
principally that due to the spring 87, but only the hydraulic
forces are immediately relevant here. Thus by a suitable choice of
the relative diameters of shaft 85 and seat 90 the net force
required to open the valve may be made to increase or decrease, or
remain unchanged, as differential pressure rises.
[0095] When the valve head 86 is initially lifted from its seat it
is immediately acted on by a flow force in a direction tending to
re-close the valve. The initial flow rate, and hence the force,
will increase as the differential pressure increases. If the
combination of the actuator, the spring and other elements of the
valve is designed to minimise electrical energy consumption at low
differential pressures, then at some higher differential pressure,
the valve may fail to open properly because of the increased flow
force tending to re-close it. This is a transient effect which
arises when the valve is "cracked" open and the actuator is near
the beginning of its stroke, where the operating force is much
reduced. If the actuator is designed to deal with the maximum
differential pressure anticipated, then energy will be wasted at
the lower differential pressures. However by taking advantage of
the mechanism described above, the valve can be designed so that
the increasing differential pressure acts to compensate
hydraulically the effect of the increased flow forces by slightly
reducing the net force keeping the valve closed. The forces
involved are small, but significant. Small electromagnetic
actuators are intrinsically limited in capability by constraints on
iron cross-section and copper volume, and further constraints are
caused by the need to operate at the high temperatures prevailing
in many boreholes. For example in this preferred embodiment, the
actuator is 25 mm in overall diameter, and the areas of the shaft
and seat differ by only about 0.5 mm.sup.2. This is sufficient to
provide a change in operating force of the order of 1 kgf when the
differential pressure changes by 20 MPa, representing about 20% of
the initial force capability of this particular actuator.
[0096] During an operational period of the mud pulse transmitter it
is inevitable that there will be small changes in operating forces
caused for example by the bedding-in of seals and temperature
changes. In the area of the valve heads and seats small dimensional
changes occur caused by the erosive effect of fluid flow and by the
impacts between the operating parts of the valves. It is customary,
when such changes in operating characteristics are expected, to
build in a safety margin, which might perhaps be to design the
electromagnetic actuator and its associated circuitry to provide
50% more initial force than is actually needed. Clearly some
operating margin is required: but by using the hydraulic
compensation described above that margin, and hence the quantity of
electrical energy required to deliver each pulse, may be
minimised.
[0097] The same hydraulic compensation principle may be applied to
the pilot valve 116/120. But conditions are now slightly different,
in that the valve closes under no-flow conditions, and re-opens by
the action of the spring 117. The available force from the spring
is of course at a maximum at the beginning of the stroke, and the
need for compensation is less. In practice it has been found that
the saving in energy achieved by compensating this valve is
negligible in comparison with the normal minor variations from unit
to unit.
[0098] Turning to the main valve 256/257, the objective is to
maintain the valve operating conditions stable over a wide range of
conditions.
[0099] When the main valve is closed, the pressures in the chambers
172 and 260 are equal and at Pa. The pressure in chamber 259 is Pd.
If the areas at the seat 257 and the shaft 250 are equal, there is
no net hydraulic force on the valve stem, and the valve is held
closed by force exerted by the spring 255. The valve is opened by
the pressure in chamber 172 rising to Pd, as described earlier. For
a brief period of time, the entire force developed by the
differential pressure (Pd-Pa) across the area of the sleeve 251 is
available to overcome friction and initial flow forces as the valve
opens.
[0100] Thereafter, the pressure regime is as follows. The pressure
in the chamber 172 remains at Pd. The pressure in the region 259
becomes slightly less than Pd because of the pressure drop across
filter element 53. The pressure in region 271 becomes substantially
lower than Pd because of the pressure drop created across the nib
270 by the fluid flowing from chamber 259 past the valve head 256
and out through ports 41. The pressure in region 271 will be
referred to as Pd(-). Forces caused by fluid flow are exerted on
the valve head 256, but are substantially cancelled out because of
the quasi-spherical shape of the valve head. The actual flow rate
which occurs for the two pressures Pd and Pa may be controlled by
appropriate selection of the diameter of the ports 41, this
selection being made to cover a broad range of expected values of
(Pd-Pa) before the equipment is placed in the wellbore.
* * * * *