U.S. patent number 3,859,850 [Application Number 05/343,081] was granted by the patent office on 1975-01-14 for methods and apparatus for testing earth formations.
This patent grant is currently assigned to Schlumberger Technology Corporation. Invention is credited to Joachim A. Hoppe, Frank R. Whitten.
United States Patent |
3,859,850 |
Whitten , et al. |
January 14, 1975 |
METHODS AND APPARATUS FOR TESTING EARTH FORMATIONS
Abstract
In the representative embodiments of the new and improved
methods and apparatus for testing earth formations disclosed
herein, fluid-admitting means are placed into sealing engagement
with a potentially producible earth formation. Selectively operable
valve means are opened to place the fluid-admitting means into
communication with sample-collecting means comprised of an
initially empty first collection chamber that is tandemly coupled
to a vacant accessible portion of a second sample-collection
chamber that is itself divided by a piston member movably disposed
therein and normally biased toward the entrance to the second
chamber by a charge of compressed gas confined in an enclosed
portion of the second chamber. In this manner, as connate fluids
enter the sample-collecting means, the first sample chamber will
initially be filled before sufficient presure is built up in the
first chamber to begin moving the piston member so as to allow
connate fluids to begin filling the second chamber. By observing
the time required for filling the first chamber, the flow rate of
entering connate fluids can be at least estimated. Once the first
chamber is filled and the pressure of connate fluids therein equals
the pressure of the compressed gas, movement of the piston into the
gas-filled portion of the second chamber will further compress the
gas charge so as to impose a proportionally increasing back
pressure on the connate fluids which may be measured to obtain a
second measurement that is representative of the rate at which
connate fluids, if any, are entering the second sample chamber.
Inventors: |
Whitten; Frank R. (Houston,
TX), Hoppe; Joachim A. (Spring, TX) |
Assignee: |
Schlumberger Technology
Corporation (New York, NY)
|
Family
ID: |
23344621 |
Appl.
No.: |
05/343,081 |
Filed: |
March 20, 1973 |
Current U.S.
Class: |
73/152.24;
73/152.29; 73/152.51 |
Current CPC
Class: |
E21B
49/10 (20130101) |
Current International
Class: |
E21B
49/00 (20060101); E21B 49/10 (20060101); E21b
049/00 () |
Field of
Search: |
;73/151,152,155
;166/264,100 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Myracle; Jerry W.
Attorney, Agent or Firm: Archambeau, Jr.; Ernest R. Sherman;
William R. Moore; Stewart F.
Claims
What is claimed is:
1. A method for investigating earth formations traversed by a well
bore and comprising the steps of:
isolating a wall surface of said well bore adjacent to an earth
formation believed to contain producible connate fluids from well
bore fluids;
communicating a first empty sample chamber with said isolated wall
surface for inducting a preliminary sample of connate fluids from
said earth formation into said first sample chamber;
blocking the discharge of said preliminary connate fluid sample
into a second empty sample chamber until said first sample chamber
is filled with said preliminary connate fluid sample and the
pressure thereof has increased to a predetermined pressure
level;
monitoring the pressure of said preliminary connate fluid sample
for obtaining a first pressure measurement indicative of said
predetermined pressure level to determine when said first sample
chamber is filled; and
once said first sample chamber is filled with said preliminary
connate fluid sample, opening communication between said sample
chambers for collecting said preliminary connate fluid sample as
well as an additional sample of said connate fluids.
2. The method of claim 1 further including the additional step
of:
correlating said first pressure measurement as a function of time
for determining the rate at which said preliminary connate fluid
sample entered said first sample chamber.
3. The method of claim 1 further including the additional steps
of:
regulating the flow of said connate fluid samples as they are
admitted into said second sample chamber by imposing a
progressively-increasing restraining force on said connate fluid
samples for correspondingly increasing the pressure of said connate
fluid samples as they continue to enter said second sample chamber;
and
monitoring the pressure of said connate fluid samples for obtaining
at least a second pressure measurement indicative of the admission
of said connate fluid samples into said second sample chamber.
4. The method of claim 3 further including the additional step
of:
obtaining at least a third measurement of the pressure of said
connate fluid samples at a later time for determining from said
second and third pressure measurements a function representative of
the average flow rate at which said connate fluid samples are
entering said second sample chamber.
5. The method of claim 3 wherein said restraining force has a
predetermined initial magnitude selected for increasing the
pressure of said preliminary connate fluid sample to its said
predetermined pressure level before said preliminary connate fluid
sample first begins to enter said second chamber so that when said
first pressure measurement attains said predetermined pressure
level it will indicate the initial entrance of said preliminary
connate fluid sample into said second sample chamber.
6. The method of claim 5 further including the additional step
of:
obtaining at least two successive additional measurements of the
pressure of said connate fluid samples at different times following
the initial entrance of said preliminary connate fluid sample into
said second sample chamber for determining a function
representative of the average flow rate at which said connate fluid
samples are entering said second sample chamber.
7. The method of claim 1 wherein said restraining force has a
predetermined initial magnitude selected for increasing the
pressure of said preliminary connate fluid sample to its said
predetermined pressure level before said preliminary connate fluid
sample first begins to enter said second sample chamber so that
subsequent increases in the pressure of said connate fluid samples
will be at progressively-greater pressure levels than said
predetermined pressure level as said connate fluid samples continue
to enter said second sample chamber and further including the
additional step of:
re-monitoring the pressure of said connate fluid samples for
subsequently obtaining at least a second pressure measurement
greater than said predetermined pressure level indicative of the
continuing entrance of said connate fluid samples into said second
sample chamber.
8. The method of claim 7 further including the additional step
of:
correlating said first and second pressure measurements as a
function of time for determining the average flow rate at which
said connate fluid samples are then entering said second sample
chamber.
9. A method for investigating earth formations traversed by a well
bore and comprising the steps of:
isolating a wall surface of said well bore adjacent to an earth
formation believed to contain producible connate fluids from well
bore fluids;
communicating said isolated wall surface with a first empty sample
chamber having a selected fixed capacity for inducting a first
sample of connate fluids from said earth formation into said first
sample chamber;
imposing a restraining back pressure of a selected initial
magnitude within said first sample chamber for preventing the
release of said first sample of connate fluids from said first
chamber into an expandable second empty sample chamber adapted to
be expanded from an initial limited capacity to a selected maximum
capacity until said first sample of connate fluids has filled said
first sample chamber and is at a selected pressure sufficient to
overcome said initial restraining back pressure;
monitoring the pressure of said first sample of connate fluids for
determining when said first sample of connate fluids reaches said
selected pressure to provide a first indication at the surface that
said first sample of connate fluids has been released into said
second sample chamber and a second sample of connate fluids is
beginning to enter said first sample chamber;
progressively increasing said restraining back pressure after said
first sample of connate fluids has been released into said second
sample chamber for correspondingly increasing the pressure of said
connate fluid samples as said second sample chamber is expanded
from its said initial capacity toward its said maximum capacity;
and
re-monitoring the pressure of said connate fluid samples for
determining when said connate fluid samples exceed said selected
pressure to provide a second indication at the surface that said
connate fluid samples are continuing to enter said first and second
sample chambers.
10. The method of claim 9 wherein said initial capacity of said
second sample chamber is negligible in relation to its said maximum
capacity.
11. The method of claim 9 wherein said fixed capacity of said first
sample chamber is less than said maximum capacity of said second
sample chamber and said initial capacity of said second sample
chamber is negligible in relation to its said maximum capacity.
12. The method of claim 9 further including the additional steps
of:
after said connate fluid sample reaches said selected pressure,
obtaining at least two successive measurements of the pressure of
said connate fluid samples at spaced time intervals; and
correlating said successive pressure measurements for determining a
function representative of the average flow rate at which said
connate fluid samples are then entering said first and second
sample chambers.
13. The method of claim 9 wherein said restraining back pressure is
imposed within said first sample chamber by disposing a movable
piston member in said second sample chamber for dividing said
second sample chamber into an accessible sample-receiving portion
adapted to be expanded from said initial capacity to said maximum
capacity upon movement of said piston member into said sample
chamber as well as an enclosed portion adapted to be contracted
upon movement of said piston member into said sample chamber, and
filling said enclosed chamber portion with a pressured compressible
gas for imposing said restraining back pressure against said piston
member.
14. The method of claim 13 wherein said compressible gas has a
predetermined initial pressure which will increase in accordance
with the general gas laws as said first sample of connate fluids
first begins to enter said enclosed chamber portion so that the
continued entrance therein of said samples of connate fluids will
progressively increase the pressure of said samples of connate
fluids as said compressible gas is further compressed above said
restraining back pressure by movement of said piston member into
said second sample chamber.
15. Formation-testing apparatus adapted for suspension in a well
bore traversing earth formations and comprising:
a body having a first fluid passage adapted to receive connate
fluids;
fluid-admitting means on said body and adapted to be selectively
engaged with a well bore wall for isolating a portion thereof from
well bore fluids;
means on said body and selectively operable for positioning said
fluid-admitting means against a well bore wall to establish
communication with connate fluids in earth formations
therebeyond;
sample-collecting means on said body and including a first sample
chamber, means selectively operable for coupling said fluid passage
to said first sample chamber to receive connate fluids entering
said fluid-admitting means, a second sample chamber, and a second
fluid passage adapted for communicating said first sample chamber
with said second sample chamber;
pressure-measuring means including a pressure transducer on said
body adapted for providing indications at the surface
representative of the pressure of connate fluids entering said
sample chambers; and
flow-regulating means cooperatively associated with said sample
chambers and adapted for initially blocking passage of connate
fluids from said first sample chamber through said second fluid
passage into said second sample chamber until connate fluids in
said first sample chamber attain an initial predetermined back
pressure and thereafter imposing a progressively increasing back
pressure on such connate fluids being released from said first
sample chamber into said second sample chamber which is
proportional to the volume of connate fluids contained in said
second sample chamber.
16. The formation-testing apparatus of claim 15 wherein said
flow-regulating means are comprised of:
a piston member movably disposed within said second sample chamber
for dividing said second sample chamber into a first
sample-receiving portion adapted to be expanded from an initial
minimum capacity to a maximum capacity upon movement of said piston
member into said second sample chamber as well as a second enclosed
portion adapted to be contracted upon movement of said piston
chamber; and
a pressured compressible gas in said second chamber portion
imposing an initial selected restraining pressure against said
piston member for restraining movement thereof until connate fluids
in said first sample chamber attain said initial back pressure and
thereafter imposing a progressively increasing restraining pressure
against said piston member in keeping with the general gas laws as
connate fluids enter said first chamber portion of said second
sample chamber.
Description
One of the most successful techniques for determining the
production capabilities of earth formations has been to place a
wireline formation-testing tool into fluid communication with a
selected formation interval and, if possible, obtain a sample of
connate fluids. During the sampling operation it is also customary
to obtain one or more measurements which are at least indicative of
the formation pressures in the interval being sampled.
Those skilled in the art will, of course, appreciate that many
different arrangements of formation testers have been employed
through the past several years. In general, these tools include a
fluid entry port or tubular probe which is cooperatively arranged
within a wall-engaging packer for isolating the port or probe from
the well bore fluids during the test. To collect fluid samples,
these tools further include a sample chamber which is coupled to
the fluid entry by a flow line having one or more control valves
arranged therein. A suitable pressure transducer is usually
arranged in the flow line for transmitting pressure measurements to
the surface by way of the cable supporting the tool.
Heretofore, however, no satisfactory arrangement has been provided
for reliably determining during the course of a testing operation
whether a fluid sample is actually being obtained; or, if a sample
is entering the tool, how fast the sample is actually being
admitted to the sample chamber. Some indications are, of course,
provided by the pressure transducer but these indications can be
misleading or false in certain situations. Thus, with the various
formation testers of the prior art, it is usually impossible for
the operator to know with absolute certainty whether a sample is
even being obtained until a considerable time has elapsed. As a
result, it is not at all uncommon for the operator to needlessly
leave the tool in position over extended periods to hopefully
obtain a sample from what is actually a non-producible
formation.
Accordingly, it is an object of the present invention to provide
new and improved methods and apparatus for reliably and quickly
determining when a fluid sample is being obtained during a
formation-testing operation and, if a sample is being admitted,
obtaining a plurality of measurements at the surface which are
representative of the rate at which connate fluids are entering the
tool.
This and other objects of the present invention are attained by
admitting a sample of connate fluids into a formation-testing tool
having fluid-collecting means including an initially empty first
sample chamber which is communicated to the inlet of a second
sample chamber having a piston movably arranged therein to define
an enclosed portion on the opposite side of the piston for
containing a compressed gas at a predetermined elevated pressure
for normally biasing the piston toward the inlet of the second
chamber. In this manner, when connate fluids are admitted to the
sample-collecting means, the pressure of the connate fluids will
increase at a rapid rate until the first chamber is filled and the
pressure of connate fluids has equaled the initial pressure of the
compressed gas. Thereafter, the pressure of the connate fluids will
continue to rise at a slower rate as the connate fluids enter the
second chamber and begin moving the piston into the second chamber
to further compress the trapped gas charge as the second chamber is
filled. By observing the rates of pressure increase, a series of
determinations can be made at the surface of the average rate at
which the connate fluids are entering the sample-collecting
means.
The novel features of the present invention are set forth with
particularity in the appended claims. The invention, together with
further objects and advantages thereof, may be best understood by
way of the following description of exemplary apparatus and methods
employing the principles of the invention as illustrated in the
accompanying drawings, in which:
FIG. 1 schematically depicts a preferred embodiment of new and
improved formation-testing apparatus as it will appear in a well
bore as the methods of the present invention are being
practiced;
FIG. 2 graphically illustrates representative pressure measurements
as might be obtained with typical prior-art formation-testing tools
and without the benefits of the new and improved methods and
apparatus of the present invention;
FIGS. 3 and 4 are graphical representations of typical pressure
measurements which might be obtained while employing the new and
improved formation tester of FIG. 1 to practice the methods of the
present invention; and
FIG. 5 illustrates a representative correlation chart by which the
practice of the present invention will enable an operator to
approximate the rates at which connate fluids are entering the new
and improved formation-testing apparatus of FIG. 1 during a typical
sampling operation.
Turning now to FIG. 1, a preferred embodiment of a new and improved
sampling and measuring tool 10 incorporating the principles of the
present invention is schematically shown as it will appear during
the course of a typical measuring and sampling operation in a well
bore such as a borehole 11 penetrating one or more earth formations
as at 12 and 13. As illustrated, the tool 10 is suspended in the
borehole 11 from the lower end of a typical multiconductor cable 14
that is spooled in the usual fashion on a suitable winch (not
shown) at the surface and coupled to the surface portion of a
tool-control system 15 as well as typical pressure
recording-and-indicating apparatus 16 and a power supply 17. In its
preferred embodiment, the tool 10 includes an elongated body 18
which encloses the downhole portion of the tool control system 15
and carries selectively-extendible tool-anchoring means 19 and
fluid-admitting means 20 arranged on opposite sides of the body as
well as new and improved fluid-collecting means 21 arranged in
accordance with the principles of the present invention and coupled
to the lower end of the tool body.
It should be recognized at the outset that except for the new and
improved sample-collecting means 21, the particular design of the
other elements of the formation-testing tool 10 is incidental as
far as achieving the objects of the present invention are
concerned. Thus, except for the unique arrangement of the
sample-collecting means 21, the tool-control system 15, the
tool-anchoring means 19, and the fluid-admitting means 20 of the
tool 10 can be arranged as has been done with any of the formation
testers which have been successfully employed heretofore. For
example, the tool 10 could incorporate the controls, the tool
anchor, or the sample admitter from any of the tools disclosed in
U.S. Pat. No. 3,011,554, U.S. Pat. No. 3,104,712, U.S. Pat. No.
3,352,361, U.S. Pat. No. 3,385,364, U.S. Pat. No. 3,653,436 or in
U.S. Pat. application Ser. No. 313,235, filed Dec. 8, 1972.
Accordingly, as far as is necessary to understand the principles of
the present invention, the formation tester 10 is illustrated
schematically in FIG. 1 to show only the essential elements of the
tool. As depicted at 22, the fluid-admitting means 20 may
alternatively include either a fluid entry port (e.g., as shown
generally at "57" in U.S. Pat. No. 3,104,712 or at "106" in U.S.
Pat. No. 3,396,796) or a tubular sampling probe (e.g., as shown at
"74" in U.S. Pat. No. 3,352,361 or at "45" in U.S. Pat. No.
3,653,436). In either case, the fluid-admitting probe or port, as
at 22, is coupled to the sample-collecting means 21 by a sample
conduit or flow passage 23 which is communicated with a suitable
pressure-measuring device or transducer 24 such as shown in FIG. 9
of U.S. Pat. No. 3,011,554.
In the preferred embodiment of the new and improved tool 10
illustrated in FIG. 1, fluid communication between the
fluid-admitting means 20 and the sample-collecting means 21 is
controlled by means such as a normally closed valve 25 and a
normally open valve 26 which are cooperatively arranged in the flow
line 23 and respectively adapted for selective actuation by the
tool-control system 15. It will, of course, be appreciated that the
control valves 25 and 26 can be arranged as shown in various ones
of the aforementioned patents for selective operation from the
surface by suitable electrical, explosive, or hydraulic actuating
means on the tool body 18.
As is typical, the sample-admitting means 20 further include a
packing element, as at 27, which is cooperatively arranged on the
tool body 18 around the fluid entry 22 for selective movement
outwardly into sealing engagement with the adjacent wall of the
well bore or borehole 11 so as to isolate the fluid entry from well
bore fluids during a testing operation. Inasmuch as most -- if not
all -- of the aforementioned patents fully disclose various types
of suitable packing elements, it is unnecessary to describe the
packer 27 further.
Similarly, various types of tool-anchoring means 19 are well
described in the aforementioned patents and no useful purpose would
be served by redescribing these known arrangements. It should be
recognized, of course, that since the tool-anchoring means 19 are
provided solely to restrain the tool 10 against longitudinal
displacement in the borehole 11 as well as to insure the sealing
engagement of the packing element 27 against the borehole wall
during a testing operation, an extendible wall-engaging anchor
member, as at 28, is not absolutely essential. For example, where
the sample-admitting member alone is extendible over a substantial
lateral distance as shown in U.S. Pat. No. 3,385,364, extension of
the sample-admitting member will be effective for pressing the rear
of the tool against the opposite wall of the well bore with
sufficient force to anchor the tool as well as to sealingly engage
the sample-admitting member. Conversely, both the sample-admitting
member and an anchor member can be arranged to respectively extend
in opposite directions as in either U.S. Pat. No. 3,295,615 or in
U.S. Pat. No. 3,653,436 where minimum lateral extension of the
individual members is preferred. Thus, as far as the objects of the
present invention are involved, it is necessary only to provide
selectively-operable means of a suitable nature for anchoring the
tool 10 and placing the packer 27 into sealing engagement with the
wall of the borehole 11 to isolate the fluid entry 22.
To appreciate the significance of the present invention, the
prior-art techniques of collecting fluid samples should be first
considered. First of all, most, if not all, of the earlier
commercially-successful formation testers have employed a so-called
"water cushion" arrangement for regulating the rate at which
connate fluids are admitted into the sample chamber. As fully
explained in U.S. Pat. No. 3,011,554, for example, this arrangement
includes a piston member which is movably disposed in an enclosed
sample chamber so as to define upper and lower spaces in the
chamber. Where the entrance to the sample chamber is above the
piston, the upper space is initially at atmospheric pressure and
the lower space is filled with a suitable incompressible fluid such
as water. A second chamber or liquid reservoir which is also
initially empty and having a volume equal to or greater than the
lower space is communicated with this lower water-filled space by a
suitable flow restriction such as an orifice. Thus, as connate
fluids enter the empty upper portion of the sample chamber, the
piston will be progressively moved downwardly from its initial
elevated position to displace the water from the lower portion of
the sample chamber through the orifice and into the initially empty
liquid reservoir. In this manner, as highly-pressured connate
fluids are admitted to the sample chamber, the rate at which these
fluids can enter the chamber will be held at a substantially
constant value as determined by the sizing of the orifice.
Those skilled in the art will, therefore, appreciate that since the
flow rate is constant with this arrangement, the pressure in the
flow line will also remain substantially constant for almost the
entire time required for the connate fluids to fill the sample
chamber. Thus, as seen at 29 in FIG. 2, the measurements from the
pressure transducer in the flow line will provide only an unvarying
reading from the time that the sample chamber is first opened to
just before the sample chamber is completely filled. As a result,
the operator has no unequivocable indication whether a sample is
entering the sample chamber or, if so, how fast the sample is being
admitted during a major portion of the testing operation. To
further complicate the situation, if the orifice unknowingly
becomes wholly plugged during the sampling operation, the pressure
will rapidly rise to the formation pressure, P.sub.f (as shown at
30) even though the piston has prematurely halted and connate
fluids are no longer entering the sample chamber. In many cases,
this may give the operator a false indication that a complete
sample has been obtained. A partial blockage of the orifice can
also give erroneously-high measurements which falsely indicate a
high rate of fluid admission.
An analogous situation will occur where no water cushion is
employed and flow regulation is instead accomplished by
sample-admitting means such as shown generally at "19" in U.S. Pat.
No. 3,653,436. As described there, that sample-admitting means
reliably regulate the flow rate of connate fluids entering the tool
so that only an empty sample chamber which is initially at
atmospheric pressure is necessary. With this arrangement, as
connate fluids are admitted into the initially empty sample
chamber, the pressure in the flow line will imperceptibly rise at
an extremely-slow rate; and, as shown at 31 in FIG. 2, it will not
be until the sample chamber is almost filled that any substantial
increase in this measured pressure will occur. Thus, hereagain, the
operator will have no reliable indication of the rate of fluid
entry during the sampling operation.
Accordingly, with either of these prior-art sample chamber
arrangements there will be no completely reliable surface
indication showing either that a sample is being obtained or -- if
one is being obtained -- how fast the sample is entering the sample
chamber throughout the test. As a result, it is often necessary to
leave the test tool in position for extended periods to be certain
that a complete test is achieved. In addition to needlessly
prolonging a sampling operation of a formation which is ultimately
determined to be of no commercial interest, there is always an
ever-increasing risk that the tool or its supporting cable may
become stuck if the test is continued for extended periods.
Accordingly, as previously considered, the new and improved tool 10
of the present invention is cooperatively arranged to continuously
provide both positive indications that connate fluids are entering
the fluid-collecting means 21 as well as measurements
representative of the rate of fluid entrance into the
fluid-collecting means. To achieve this, the new and improved
fluid-collecting means 21 are coupled to the fluid passage 23 and
are preferably arranged to include first and second enclosed sample
chambers 32 and 33 which are coupled to one another by an
intermediate flow passage 34. As illustrated, the second sample
chamber 33 carries fluid-isolating means such as a piston 35
movably arranged therein for isolating the upper and lower portions
of the second chamber from one another. In the preferred embodiment
of the new and improved tool 10 illustrated in FIG. 1, controlled
access to the two sample chambers 32 and 33 is provided by normally
closed, manually actuated valves 36 and 37 which are respectively
mounted in conduits 38 and 39 in the tool body 18 for communicating
the exterior of the tool with the sample-collecting means 21 when
the tool is at the surface. A compressible gas, as at 40, is
introduced into the lower portion of the second sample chamber 33
by way of the valve 37 and the conduit 39 and elevated in pressure
to a predetermined level, P.sub.i. This will, of course, urge the
piston 35 to an initial elevated position as illustrated in FIG. 1
where the piston will divide the second sample chamber 33 into an
initially empty upper portion of minimum volume and a gas-filled
lower portion of maximum volume. The first sample chamber 32 is
initially empty.
Accordingly, in the practice of the methods of the present
invention, the new and improved tool 10 is positioned in the
borehole 11 and operated as required for engaging the anchor means
19 and the fluid-admitting means 20 against the opposite walls of
the borehole. Then, as is typical, the flow line control valve 25
is opened upon command from the surface so as to place the first
sample chamber 32 as well as the upper portion of the second sample
chamber 33 into fluid communication with the now-isolated portion
of the formation 13. As is typical, since the first sample chamber
32 as well as the upper portion of the second sample chamber 33
above the piston 35 are at or near atmospheric pressure, if there
are producible connate fluids in the formation 13 the formation
pressure will be effective for displacing these fluids through the
flow line 23 and into the first chamber 32, the flow passage 34,
and the upper portion of the second sample chamber. The
pressure-responsive transducer 24 will, of course, be effective for
providing indications on the surface indicator or recorder 16 which
are representative of the pressure of the connate fluids flowing
through the passage 23 into the first sample chamber 32.
In contrast, however, to the results obtained with prior-art
testing tools as graphically depicted in FIG. 2, in the practice of
the present invention with the new and improved tool 10, the
selectively-pressured gas charge 40 will be effective for
restraining the piston 35 against movement until the pressure of
the connate fluids in the flow line 23 and the first chamber 32 has
at least equaled the initial pressure, P.sub.i, of the pressured
gas. In other words, disregarding the frictional restraint on the
piston 35, the piston cannot be moved downwardly into the second
sample chamber until the flow line 23, the first sample chamber 32,
the flow passage 34, and the upper portion of the second sample
chamber 33 have been filled with connate fluids and the pressure of
these fluids rises to the initial pressure, P.sub.i, of the
pressured gas charge 40. Thus, as shown at 41 on the curve 42 in
FIG. 3, the entrance of connate fluids into the first sample
chamber 32 will be immediately reflected by a rapid and
easily-detected increase of the measured flow line pressure to a
value equal to the initial pressure of the gas charge 40. Since the
void space in the chambers 32 and 33 and the flow passage 34 above
the piston 35 is relatively small in relation to the initial volume
of the gas-filled space in the second chamber, only a small volume
of connate fluids is required to achieve a substantial increase in
the pressure in the flow line 23 as measured by the transducer 24.
Accordingly, as one aspect of the present invention, a rapid rise
of the pressure in the flow line 23 reliably signifies that at
least some connate fluids have entered the first sample chamber 32.
Moreover, since the volume of the first chamber 32 is known and the
pressure in the flow line 23 cannot equal the initial gas pressure,
P.sub.i, until the first chamber is filled, by measuring the time
required for the flow line pressure to reach the initial gas
pressure a first indication is obtained of the average flow rate of
connate fluids entering the new and improved tool 10. On the other
hand, the lack of such a sharp pressure rise (as at 41), of course,
will be a clear indication that connate fluids are not entering the
first sample chamber 32 at a very rapid rate.
Once connate fluids have filled the flow lines 23 and 34 as well as
the first chamber 32 and the upper portion of the second sample
chamber 33 to the extent that the pressure of these fluids
approaches the initial pressure, P.sub.i, of the gas charge 40, the
further entrance of connate fluids into the upper end of the second
sample chamber will, of course, be accomplished by an increase in
the pressure of the connate fluids. This will, therefore, cause the
piston 35 to be displaced further into the sample chamber 33 to
accommodate the increase in sample volume. As a further aspect of
the present invention, however, displacement of the piston 35 will
be accompanied by a proportional increase in the pressure of the
gas charge 40. This increase in pressure will, of course, be in
keeping with the general gas laws. Thus, as the gas charge 40 is
further compressed, a greater restraining force will be imposed on
the piston 35 so that the pressure of the connate fluids filling
the upper end of the second sample chamber 33 must also
correspondingly increase. As seen at 43 on the curve 42, this will,
of course, cause corresponding increases in the pressure
measurements provided by the transducer 24. As a result, a series
of progressively-rising pressure indications will then be
successively provided at the surface reliably signifying that
connate fluids are now entering the sample chamber 33. This is, of
course, a clear distinction from the prior art as shown in FIG. 2
where only a substantially constant or unvarying pressure
measurement is obtained over a long period of time.
It should be noted that the pressure measurements provided by
practicing the present invention will be related to the initial
pressure, P.sub.i, of the gas charge 40. Thus, if the gas charge 40
has an initial pressure, P.sub.i, of 100-psig, for example,
movement of the piston 35 to its mid-point in the second sample
chamber 33 will compress the gas charge to 200-psig. Similarly,
further movement of the piston 35 to the "three-quarters" point in
the second sample chamber 33 will redouble the pressure of the
trapped gas charge 40 to 400-psig.
Although pressure changes of this order of magnitude will be
readily detectable with typical pressure-measuring transducers, as
at 24, it will be appreciated that more-discernible measurements
can be obtained by increasing the initial pressure, P.sub.i, of the
gas charge 40. Thus, as illustrated by the curve 44 in FIG. 4, if
the initial pressure of the gas charge 40 is doubled to, for
example, 200-psig, movement of the piston 35 to the mid-point of
the sample chamber 33 will raise the pressure of the gas charge to
400-psig and movement of the piston to the "three-quarters" point
will redouble this pressure to 800-psig. These significant
increases in the pressure measurements provided by the transducer
24 will, of course, be readily indicated on the recorder 16.
It should be appreciated, moreover, that since the gas charge 40
will be responding in keeping with the general gas laws,
calculations can be readily made to determine the volume of connate
fluids in the second sample chamber 33 at any given time. For
example, as previously described, doubling of the initial pressure,
P.sub.i, of the gas charge 40 will signify that the second sample
chamber 33 is half-full and redoubling of this doubled pressure
will indicate that the sample chamber is now three-fourths full.
Since the volume of the sample chamber 33 is known, it can,
therefore, be readily determined that a given quantity of connate
fluids has entered the sample chamber when the initial pressure,
P.sub.i, of the gas charge 40 has increased by a known amount.
Thus, by observing how long it has taken for the pressure of the
gas charge 40 to increase from one selected value to another, a
reasonably-accurate approximation can be made of the average flow
rate of connate fluids entering the sample chamber 33.
To facilitate these flow rate determinations, constant flow rate
curves such as shown at 45-47 in FIG. 5 can be readily developed
for given values of the initial charge pressure, P.sub.i. By use of
curves such as these, a measured pressure at a given elapsed time
can be readily converted to a corresponding average flow rate of
connate fluids. A typical curve, as at 45, can be computed for a
given pre-charge pressure, P.sub.i, by calculating the volume of
the upper accessible portion of the second sample chamber 33 at
each of several incremental positions of the piston 33. Then, by
use of the general gas laws, the pressure of the pre-charged gas 40
at each of the incremental positions of the piston 35 can be
readily determined. These figures can then be easily employed to
determine how long it will take to fill the upper portion of the
second chamber 33 to each incremental volume at a selected rate of
flow, as at v, of connate fluids. Plotting these results as
function of time versus pressure will, therefore, provide a useful
family of constant flow rate curves as at 45-47.
It will be seen, therefore, that by observing the rapid changes in
pressure indications provided by the pressure transducer 24, the
operator will quickly learn if connate fluids are indeed entering
the sample chambers 32 and 33. Moreover, as described above, a
fairly accurate estimation can be made in short order as to whether
the rate of fluid entrance justifies a continuation of the sampling
operation.
Once a fluid sample is obtained, the tool control system 15 is
operated for closing the control valve 26 so as to trap the
collected sample in the chambers 32 and 33. Then, the anchor 28 and
the packer 27 are retracted and the tool 10 is returned to the
surface. To remove the sample from the chambers 32 and 33, a
suitable collection chamber (not shown) is then coupled to the
passage 38 and the valve 36 is opened. It will be appreciated, of
course, that once the valve 36 is opened, the gas charge 40 will
return the piston 35 to its initial position and displace the
collected sample from the chambers 32 and 33. Once the sample is
removed, the gas charge 40 will be reduced to its initial pressure,
P.sub.i, and the piston 35 will again be in the position shown in
FIG. 1. Thus, there is rarely any reason to disturb the gas charge
40 so long as it remains at its initial pressure, P.sub.i.
Accordingly, it will be appreciated that the present invention has
provided new and improved methods and apparatus for obtaining
samples of connate fluids from earth formations. By arranging first
and second interconnected sample chambers in an otherwise typical
formation-testing tool and biasing a fluid-isolating piston to an
initial position in the second sample chamber with a charge of
compressed gas at an initial elevated pressure, displacement of the
piston further into the second sample chamber will be accomplished
only by first filling the first chamber and then further
compressing the trapped gas charge. A first measurement can then be
made of the time required to fill the first sample chamber for
determining the initial average flow rate of the connate fluids
entering the tool. Since the charge of compressed gas will be
further compressed in keeping with the general gas laws and the
pressure of the gas charge must at all times equal the pressure of
connate fluids entering the sample chamber, the resulting changes
in the measured sample pressure will be representative of the
movement or position of the isolating piston. Then, by using these
pressure measurements, a second determination can be made of the
average rate at which connate fluids, if any, are entering the
sample chambers.
While only a particular embodiment of the present invention and one
mode of practicing the invention have been shown and described, it
is apparent that changes and modifications may be made without
departing from this invention in its broader aspects; and,
therefore, the aim in the appended claims is to cover all such
changes and modifications as fall within the true spirit and scope
of this invention.
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