This invention relates to logging plugs for use in oil, gas and other
To ascertain parameters of well operations such as pressure, temperature
and flow rate one or more logging tools are run down the well on a logging or
slick line cable, which will subsequently be referred to by the further alternative
term wireline. As many of these parameters must be measured while the pressure
in the perforation zone is different from that obtaining in the upper part of
the well down which the logging tool is run, it is necessary for the wireline to
be passed through a logging plug which maintains a good, but not perfect seal,
with the wire, and which is itself sealable within a nipple incorporated in production
tubing, by-pass tubing or the well casing.
Considerations arising from a typical employment of a conventional
logging plug will now be discussed with reference to the accompanying diagrammatic
drawings in which:
- Figure 1 is a vertical section through a length of an oil well including a
perforated zone, and a logging plug, electric submersible pump (ESP) and by-pass
system installed therein with the pump running;
- Figure 2 corresponds to Fig. 1 and shows pressures obtaining after the pump
has been switched off;
- Figure 3 corresponds to Figs. 1 and 2 illustrates a stage in the removal of
the logging plug; and
- Figure 4 is a detail illustrating circulation in the absence of a logging plug.
As shown in Fig. 1 a well casing 10 has a perforated zone 12 through
which oil flows from a reservoir 14 as indicated by the arrows 16. From the bottom
of a production string 18 is suspended a Y-crossover tool 20 from which depends
an electric submersible pump 22 with the interposition of a pump sub 24 containing
a non-return valve 26. From the tool 20 there also depends a length of by-pass
tubing 28 formed with a nipple 30 in which a conventional logging plug 32 is sealed;
slidably passing through the plug 32 but in effective sealing engagement therewith
is a wireline 34 from which a logging tool 36 for measuring well parameters is
In Fig. 1 the pressure obtaining within the well casing is denoted
P&sub2; and the pressure on the outlet side of the pump 22 which is higher than
P&sub2;, as P&sub1;. There is thus a differential pressure P&sub1;-P&sub2;, defined
as ΔP&sub1;, acting downwardly on the logging plug 32. As this differential
pressure holds the plug firmly in place, it must be substantially eliminated to
allow the logging plug to be removed, and the tools withdrawn. However, equalisation
of pressure cannot be achieved simply by switching off the pump 22 due to the action
of the non-return valve 26, as will now be explained with reference to Fig. 2.
When the pump 22 is switched off the pressure below the check valve
26 will increase until it equals the static reservoir pressure P&sub3;. P&sub3;
is greater than running pump intake (flowing well bore) pressure P&sub2; but less
than pump discharge pressure P&sub1; which still obtains above check valve 26.
The pressure differential holding the plug 32 in place will now be P&sub1;-P&sub3;,
defined as ΔP&sub2;, and the force exerted on the plug 32 will be ΔP&sub2;
x (cross-sectional area of plug 32).
In current practice the pressure differential is equalised simply
by relying on leakage past the wireline 34, as indicted by the arrows 38 in Fig.
3, where there is not, as previously mentioned, a perfect seal. However, it can
take considerable time for such equalisation to occur, and P&sub1; to equal P&sub3;,
thus allowing the logging plug 32 to be withdrawn, withdrawal being effected by
abutment with the tool 36 as shown in Fig. 3.
Fig. 4 illustrates the situation if it were attempted to run a logging
tool through a by-pass system without isolation. As before the intake pressure
of pump 22 is designated P&sub2; and the discharge pressure P&sub1;, the pressure
differential P&sub1;-P&sub2; being defined as ΔP. The existence of this differential
tends to cause re-circulation of pumped fluid around the by-pass system in the
direction of the arrows 40.
A logging plug may be used in other situations in which it is required
to suspend a logging tool on a wireline passing through a point of pressure differential,
for example in a standard well completion without an ESP being present in the
US-A-3 180 420 discloses a retrievable bridging plug which may be
locked in a well casing simply to form a seal against flow in either direction.
Removal of the plug is effected by pulling a sleeve upwards, initial movement
of the sleeve relatively to a central stem uncovering ports in the stem which communicate
with a blind central bore therethrough. Flow passages through the bridging plug
are thus established which facilitate the withdrawal of the plug through well fluid,
after it has been unlocked by continued upward pull on the sleeve.
The present invention allows for the first time the use of logging
tools in wells with electric submersible pump by-pass completions, giving to the
well operator additional information which was previously unavailable.
US-A-4 307 783 discloses a piece of surface equipment which allows
well logging operations to be carried out in potential blow-out conditions, and
is essentially a lock which allows the logging tool to be removed. The equipment
of US-A-4 307 783 is designed to contain pressure from below, whereas the plug
of the present invention can be released even though it is being forced downwards
by the action of differential pressure. Moreover, this prior art equipment cannot
be lowered into the well, as can the logging plug of the present invention, to
a predetermined position in order to isolate the production tubing below the plug
from the production tubing above the plug, whilst allowing logging operations
to be carried out below the plug.
It is an aim of the present invention to provide a logging plug which
allows the pressures obtaining above and below the plug to rapidly equalise and
thus allow withdrawal of the plug and logging tools.
According to the present invention there is provided a plug for sealing
a well bore or tubing string, comprising a hollow body provided with sealing means
by which it may be sealed within a nipple included in the well bore or tubing
string, and an equalising member arranged for sliding movement relatively to the
body between a sealing portion in which a path for fluid flow from above said
body across the sealing means to below said body is closed and an open position
in which said flow is allowed, characterised in that the plug is a logging plug
in which the hollow body and equalising member are apertured to allow a logging
tool to be suspended at any desired position below the plug, the hollow body providing
or receiving flow tube means to maintain a substantial seal across the plug.
Preferably the equalising member has at least one port communicating
with an axial bore through which the wireline passes and which in the sealing position
is closed by a sleeve forming part of the hollow body, and in the flow position
communicates with one or more ports formed through the wall of the hollow body.
Preferably again, the equalising member is located at the lower end
of the logging tool so that it may be displaced from the first to the second position
by the upward impact of a logging hammer or of a logging tool attached to the
An embodiment of the invention will now be described, by way of example,
with reference to Figure 5 of the accompanying drawings which is an axial section
through an equalising logging plug according to the invention.
The equalising logging plug 50 shown in Fig. 5 surrounds the wireline
34 and comprises a hollow body 52 fitted at its lower end with seals 54 by which
it may be seated in a nipple, for example the nipple 30 of Fig. 1. Ports 56 are
formed in the wall of the body 52 above the packers 54.
Sealed within the upper part of the body 52 by an O-ring 58 and retained
by a collar 60 is a flow tube 62 having an internal diameter such that the wireline
34 may pass freely therethrough in the presence or absence of a pressure differential
without causing substantial friction or wear, but on the other hand allowing very
little fluid flow around the wireline 34.
Forming the lower part of the body 52 is an insert 64 having an axial
bore 65, the upper portion 65a and the reduced diameter medial portion 65b
of which receive in a close sliding fit lengths of an equalising tube 66 which
itself has an axial bore of a diameter such as to define with the wireline 34 passing
there-through an annular passage 68. The tube 66 is sealed within the bore portion
65a by O-ring 70, and within the bore through the body 52 in which its upper
part is a sliding fit by O-ring 72. Lateral communication through the wall of
the tube 66 with the annular passage 68 is provided by upper ports 74 and lower
ports 76. A snap ring 78 is recessed into the equalising tube 66 in a position
which in the Fig. 5 condition is just below the bore portion 65b.
A logging hammer 80 may be secured to the wireline 34 above or below
the plug 50 to assist in either seating the logging plug in the nipple or removing
it from the nipple.
If the plug 50 is substituted, in the by-bass system operating shown
in Fig. 1, for the plug 32, well fluid discharged from the pump 22 will similarly
be pumped through the Y-crossover tool 20 and production string 18, but a negligible
quantity will flow between the flow tube 62 and wireline 34 and through the annular
passage 68 to be recirculated through the pump 22; a seal with the wireline 34
is thus established in the by-pass tubing 28 while still allowing passage of the
When the plug 50 is in the closed condition shown in Fig. 5 there
is no communication from the ports 56 through the body 52 down across the seal
54 to the bottom of plug 50: as will be seen the O-ring 70 blocks communication
through the bore 65, and the O-ring 72 through the bore in the body 52, blocking
leakage into the annular passage 68 round the top of the tube 66. The plug 50
thus effectively seals across the nipple 30.
As explained in connection with Fig. 3 the plug is still subjected
to a considerable downward force ( ΔP&sub2; x cross-sectional area) after
the pump 22 has been switched off. It is usually operationally impossible to pull
the logging plug 32 out of the nipple 30 by applying a upward force greater than
this downward force, and it is therefore necessary to equalise the pressure differential
from above to below the plug. Such equalisation is readily achieved with the plug
It will be noted that by virtue of the annular passage 68 the pressure
on the tube 66 is the same at all points. No pressure differential therefore has
to be overcome in displacing the tube 66 upwards, which can easily be achieved
by pulling on the wireline 34 and striking the bottom of the tube 66 with logging
hammer 80 or the logging tool. The consequent upward displacement of the tube
66 to a position in which the ports 74 are in register with the ports 56 establishes
fluid communication from above the seals 54, through the ports 56 and 74 and the
annular passage 68, and out into the by-pass tubing 28 below the packers 54 via
both the lower ports 76 and the open end of the annular passage 68. The upward
displacement of the tube 66 compresses the snap ring 78 as it passes through the
bore portion 65b; after the ring 78 passes the shoulder at the top of bore
portion 65b it expands to maintain the tube 66 in its upper, open position.
After fluid flow through the lower part of the plug 50 along the
above-described route has achieved equalisation of the pressures above and below
the packers 54, the plug 50 may easily be withdrawn from the nipple 30 and the
Instead of the snap ring 78 a similar retaining device such as a
shear pin may be used.