Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROUND OF THE INVENTION
l. Field of the Invention
The present invention relates to a method and apparatus for
measuring properties of fluids, in particular the density and/or gel strength
of drilling fluids.
2. Description of the Prior Art
In the drilling of wells by rotary drilling techniques, a drilling
fluid, referred to as mud, is circulated from the surface through a drill string
and back to the surface. The mud serves several important functions in the
drilling operation, two of the most important of which are maintenance of hydro-
static pressure on subsurface formations and the suspension and removal of drill
solids from the well. To achieve these functions, the mud must be maintained
within a carefully controlled tensity range and gel strength range. The density
and gel strength properties of the mud, therefore, are constantly monitored
- during the drilling operation.
Over the years several instruments for measuring the mud density
and gel strength have evolved. Instruments for measuring density range in
complexity from the slmple mud balance to the rather sophisticated differential
pressure mud weight instrument. A differential pressure mud weight instrument
sold by Samega unter the trade name of Densimeter DMC is disclosed on Page 5719
of the ComPosite CataloR of Oil Field EquiPment and Services, 33rd Revision
(1978-79), published by World Oil. The differential pressure mud weight instru-
ment measures the difference in pressure between two vertically spaced points in
mud flowing through a tube. The pressure differential provides sn indication of
mud density. Although this type of instrument represents a significant improve-
ment over the mud balance, it still has certain disadvantages. The instrument
does not distinguish between hydrostatic pressure and friction pressure; there-
fore, measurements under certain conditions will indicate densities substantially
higher than actual densities.
Another disadvantage of the differential pressure mud weight
2 instrument of ~he prior art is its sensitivity to the presence of gas in
the mud. It is known that gas entrained in the mud has an effect on mud
4 density. At the surface, the gas-cut or aerated mud density would b~ less
than the mud density in the well under pressure. From an operational
6 standpoint, the density of the mud under pressure is more meaningful because it more accurately represents the actual mud density in the well. The
8 effects of air on ~ud density may be even more pronounced when certain lost
circulation materials such as straw or other fibrous matter are present in
the mud because these materials tend to entrain air.
Other instruments for measuring mud weight or density are disclosed
12 in U.S. Patent 2,609,681 and Canadian Well Logging Society Paper No. 706S
presented in Calgary, May 6-8, 1970. Differential pressure measurement
14 instruments are also disclosed in U.S. Patents 3,175,403 and 4,0S9,744.
Gel strength, defined as the property of a mud to develop and
16 retain rigid form, is an important measure of the mud's ability to suspend
drilled solids in quiescent condition. The gel strength should be suffi-
18 ciently high to suspend the solids, but not so high as to retard drillingoperations. The most common instrument for measuring gel strength is the
Fann V-G Meter. (See Page 126 of Composition and Properties of Oil Well
Drilling Fluids, by Walter F. Rogers, Gulf Publishing Company (1963). The
22 Fann V-G meter, however, does not provide a continual record or comparison
of the mud gel strength at frequent time intervals. U.S. Patent 3,069,900
24 discloses apparatus and method for measuring mud properties (viscosities
and gel strength) of non-Newtonian liquids.
26 SUMMARY OF THE INVENTION
The present invention provides method and apparatus for accurately
28 determining the density and/or gel strength of drilling mud. Briefly, the
method comprises the repetitive steps of passing a mud through a conduit
positioned in a nonhorizontal attitude, interrupting the flow of mud to
provide a static column of mud in the conduit, and measuring the differential
32 pressure of the static mud between vertically spaced points. The pressure
differential measurement under static conditions indicates the true density
34 of the mud. An advantageous feature of this method is the determination of the density of the mud independent of the friction of the fluid flowing
36 through the conduit.
Because of its viscosity and gel strength (i.e., thixotropy), the
2 mud under flowing conditions will generate friction, particularly through
relatively small conduits. By measuring the differential pressure of a
4 static column, in accordance with the present invention, friction effects
are eliminated.
6 The invention, in a preferred embodiment, also contemplates
maintaining a back pressure on the mud column to eliminate or substantially
8 reduce the effects of gas entrained in the mud. From an operational point
of view, this is a significant feature because the mud is used in the well
under pressure. Accordingly, the density measurement under pressure more
realisticly represents the mud density in the well.
12 The density measurements may be performed at relatively short
time intervals to provide a virtually continual record of the mud density.
14 In another embodiment of the invention, the differential pressure
between the vertically spaced points in the mud column is measured during
16 the flowing phase of the cycle. It has been found that the difference
between this measurement and the static measurement provides an indication
18 of the gel strength of the mud. The gel strength property of mud is impor- tant because of the necessity of the mud to suspend solids in the well
under quiescent conditions. By recording the flowing and static measure-
ments, cycled at frequent intervals, a record of the mud gel strength as
22 well as mud density may be obtained. The continual record permits instant
recognition of a change in these important properties of the mud.
24 Variations in the method involves reversing the flow through the
conduit at frequent time intervals and measuring the differential pressure
26 at vertically spaced points in the conduit in each flow direction, and
averaging the two pressure differential measurements. The average differen-
8 tial pressure measurement represents the mud density independent of friction.The apparatus of the present invention includes a conduit posi-
tioned in a nonhorizontal, preferably vertical, attitude; means for deter-
mining the pressure differential between vertically spaced locations in the
32 conduit; and means for cycling mud flow through the conduit to create
alternating dynamic and static columns, (or flow reversal) between the
34 spaced locations. In a preferred embodiment of the apparatus, a valve is
employed to maintain the desired back pressure on the mud column. Because
36 of the tendency of mud to cake and plug, the back pressure valve should be
an elastomeric sleeve-type valve which throttles flow by deforming inwardly.
An advantage of the present invention is that it provides a
2 substantially continuous determination of density and/or gel strength of a
drilling mud which determination is substantially unaffected by flowing
4 friction or entrained gas. The apparatus is a simple, compact configurationand readily adapted to field operations. For example, it may be employed
6 to test fluids from a fluid pit or a flowing line. A further feature of
the present invention is its ease of calibration. These and other advan-
8 tages and features will become apparent from the following discussion.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a curve representing the rheological behavior of a
typical drilling fluid.
12 Fig. 2 is a schematic diagram illustrating one embodiment of the
apparatus of the present invention.
14 Fig. 3 is a schematic diagram illustrating another embodiment of
the apparatus of the present invention.
16 Fig. 4 is a schematic diagram illustrating still another embodiment
of the present invention.
18 Figs. 5 and 6 are recordings of mud properties obtained from the
apparatus shown in Fig. 3.
Fig. 7 is an idealized plot illustrating the performance curve of
apparatus shown in Fig. 4.
22 Fig. 8 is a longitudinal, sectional view of a preferred back
pressure valve for the system disclosed in Figs. 2, 3, and 4.
24 DESCRIPTION OF THE PRE~ERRED EMBODIMENTS
The present invention will be described with reference to its
26 application in measuring the properties of a drilling mud. It should beunderstood, however, that it may be also used in measuring the properties
28 of any viscous, non-Newtonian fluid. With reference to Fig. 2, the apparatus
includes a vertical conduit or tube 12 closed at its opposite ends and
having an inlet line 11 and discharge line 22. A pump 10 is adapted to
pump a mud sample from the mud system through inlet line 11, through
32 conduit 12, out discharge line 22, and back to the mud system. The present invention will be described iQ terms of upward flow through conduit 12,
34 which is the preferred direction to minimize settling of solid materials
from the mud. However, downward flow is also possible. Modifications to
36 the following description to accommodate downward flow will be obvious to
one skilled in the art.
~ 7~
The means for measuring the differential pressure at vertically
2 spaced locations within conduit 12 include two flexible isolation dia-
phragms 13 and 14 vertically spaced apart by distance L. The diaphragms
4 separate the interior of the conduit 12 from chambers 15 and 16, respec-
tively, defined by suitable housings. Chambers 15 and 16 are in fluid
6 communication with opposite sides 23 and 24 of a differential pressure
transducer 19 by lines 17 and 18, respectively. The chambers 15 and 16 and
8 the lines leading to the pressure transducer 19 are filled with a noncompress-
ible liquid. It is preferred that a bac~ pressure be maintained on the
fluid column by back pressure valve 25.
The system is operated such that the flow of the mud sample
12 through the conduit 12 is intermittant to provide a flow interval and a
static interval. In the embodiment illustrated in Fig. 2, the intermittant
14 operation may be achieved by intermittantly operating pump 10 or by inter-
mittantly closing valve 25.
16 The measurement of differential pressure between diaphragms 13
and 14 during the static interval of the cycle provides an indication of
18 mud density independent of friction.
The pressures in vertical conduit 12 at isolation diaphragms 13
and 14 are transmitted via the noncompressi~ble fluid in chambers 15 and 16
and lines 17 and 18, respectively, to the differential pressure transduc-
22 er 19, such as the ITT Barton Model 752 Differential Pressure Unit, which
senses the pressure difference between isolation diaphragms 13 and 14 and
24 transmits the information as an electrical, mechanical, hydraulic, or
pneumatic signal via line 21 to a read-out device 20 such as a gauge and/or
26 recorder which may be calibrated in convenient units te-g-, pounds per
gallon).
28 Another embodiment of the invention is illustrated in Fig. 3
where like reference numerals indicate corresponding components of Fig. 2.
The primary difference between the apparatus of Fig. 3 and that of Fig. 2
is in the means for cycling flow through conduit 10. In this embodiment, a
32 bypass line 26 running parallel to conduit 12 interconnects lines 11 and
22. Two valves, 27 and 28, are provided in lines 11 and 26, respectively.
34 Air-operated, elastomeric sleeve ("pinch") valves, such as Mini-Flex
Series 2600 sold by ~ed Valve Company, are preferred for this application.
In operation of the apparatus disclosed in Fig. 3, pump 10 is run
2 continuously and valve 25 maintains a substantially constant back pressure
on the column 12 and line 26. The valves 27 and 28 are sequentially operated
4 between a first condition in which mud flow is direct~d through conduit 12
and out line 22 (valve 27 open and 28 closed), and a second condition in
6 which mud is directed through bypass line 27 (valve 27 closed and 28 open).
The mud cycling through the conduit 12 produces a flow interval and a
8 static interval in conduit 12.
In both the embodiments disclosed in Figs. 2 and 3, the mud flow
is cycled at a predetermined frequency to provide the alternating static
and flowing conditions within conduit 12. The frequency should be suffi-
12 ciently slow to provide a stabilized reading in each period, but not soslow as to cause the mud to gel excessively. Frequencies between about 0.1
14 and lS cycles per minute should be satisfactory for most types of mud.
When fluid in the vertical conduit 12 between the two isolation
16 diaphragms 13 and 14 is static, the pressure at diaphragm 13 will be greater
than that at diaphragm 14 because of the hydrostatic pressure gradient of
18 the fluid. Read-out device 20 will provide an indication of the true
density of the fluid. When mud in the vertical conduit 12 is flowing,
read-out device 20 will provide an indication different from the true
density due to the frictional pressure gradient of the flowing fluid. It
22 has been found that at low flow rates the difference between static and
flowing measurements is proportional to the gel strength. Cycling the
24 flowing and static phases every 20 seconds, for example, produces an accurate
and substantially continuous plot of the density and gel strength of the
26 fluid being sampled similar to those shown in Figs. 5 and 6.
The principle of operations of the present invention is described
28 below. Under static conditions in the apparatus in Fig. 2, and assuming nopressure loss across the isolation diaphragms 13 and 14, the differential
pressure, ~P, across a transducer 19 may be represented by the equation:
~P = (p - pf) gL cos 0 (1)
32 where p = the density of the fluid in conduit 12
pf ~ the density of the noncompressible fluid in lines 17
34 and 18 connecting the isolation diaphragms 13 and 14 to
the differential pressure transducer 19,
36 g = the gravitational constant,
L = the distance between the centers of the isolation
38 diaphragms 13 and 14,
O = the angle between the axis of the conduit 12 containing
the isolation diaphragms 13 and 14 and vertical.
--6--
Hence, in the vertical configuration of the apparatus shown in
2 Fig. 2, cos ~ = 1 and
~P = (p - pf) gL. (2)
4 Since pf~ g and L are constant for any given apparatus, the
output of the differential transducer can be calibrated to read p, the
6 fluid density, directly according to
8 P gL + Pf (3)
For upward flowing conditions through conduit 12, the frictional
pressure gradient results in apparent fluid densities greater than the
actual dPnsity as measured in the static environment. The difference in
12 measured fluid density for fluid flowing vertically upward in a pipe can be shown to be:
14 ~p = 4l (4)
16 where ~p = difference in fluid density measurement,
D = inside diameter of conduit 12,
18 1 = shear stress at wall of conduit 12.
The shear stress versus shear rate behavior of a typical water-
based clay drilling mud is shown in Fig. 1. This behavior can be considered
as having two regions of characteristic behavior depending on the shear
22 rate. Below a shear rate of about 500 sec 1, the behavior is often nonlinear,
tending toward a non-zero value of shear stress with decreasing shear rate.
24 This value is called the initial gel strength G of the mud. Above 500 sec 1,
the behavior is generally linear.
26 Low shear rate behavior is important since this characteristic
governs the ability of the mud to suspend weignting materials and to trans-
28 port drill cuttings from the well. It is known in the art that the gelstrength G is a governing property with regard to low shear rate behavior.
Because it is somewhat difficult to measure, it is common practice in the
drilling industry to determine a parameter called yield point. This is
32 determined by measuring the shear stress at 511 and 1022 sec 1 using a
Fann V.G. Meter and constructing a straight line through these points. As
34 shown in Fig. 1, the intercept of this line on the shear stress axis is
called the yield point YP. It is known in the art that yield point and gel
36 strength are related and that this fictitious parameter (yield point) can
be used to indicate changes in gel strength. However, it does not represent
38 a real property of the mud.
~ 7~
When gel strength is measured, it is common to measure it at a
2 very low value of shear rate, typically 5.11 sec 1. In the present appara-
tus, low flow rates are used so the shear rate in conduit 12 will be low.
4 As a consequence, the shear stress I will approximate the gel strength &.
Therefore, the gel strength is related to the difference in measured density
6 during flowing conditions by
8 G = 4D ~p
By alternately measuring the density of the mud during flowing and static
1~ conditions, the density difference ~p can be determined and related to the
gel strength through Eq. 5.
12 If it is assumed that flow in the vertical conduit is laminar and the fluid is Newtonian, the shear rate y at the wall of the conduit 12
14 (assuming a cylindrical conduit) will be
16 ~ D 3 (6)
where Q is the volumetric flow rate in the conduit 12. Although drilling
18 fluids are not generally Newtonian, Eq. 6 is still a reasonable approximation
for the wall shear rate. As mentioned previously, in the drilling industry
a shear rate of S.ll sec 1 is typically used to measure gel strength. An
approximation of this shear rate will be obtained in the present apparatus
22 by circulating at a rate given by
Q = 0.5 D3 (volume/sec) (7)
24 which is obtained by substituting 5.11 sec 1 for y in Eq. 6 and solving forQ. Using units common in the drilling industry, it is found that the flow
26 rate corresponding to a S.11 sec 1 shear rate in a 4-inch diameter pipe is
8.3 gallons/minute.
28 Generally, flow rates for operation of the present apparatus and
method are preferably adjusted for a shear rate, y, at the wall of the
conduit in the range of from greater than zero to 20 sec 1 using Eq. 6. As
used herein the term shear rate is understood to be at the wall of the
32 conduit 12.
Thus, in operation, a gauge which has been calibrated to report
34 the fluid density reports the true density when the flow is stopped and
reports the true density plus the difference, ~p, when the fluid is moving
36 upward past the isolation diaphragms. Since the density difference is
proportional to the gel strength, information on this property is also
38 available from the apparatus.
~L3~177~
A convenient manner of reporting the density and gel strength of
2 the fluid is to record the output signal from the gauge on a continuous
chart recorder. If the fluid in the gauge is alternated between flowing
4 and static conditions at a relatively rapid rate (for example, 10 seconds
flowing, 10 seconds static) and the chart is moved relatively slowly (for
6 example, one inch per hour), the movement of the recording pen will create
a band (illustrated in Figs. 5 and 6), the lower edge of which is indicative
8 of the true density and the width of which is proportional (according to
Eq. 5) to the gel strength.
Fig. 4 illustrates another embodiment of the invention. The
apparatus disclosed in this figure also includes a conduit and differential
12 pressure measuring means which are represented by li~e reference numerals
as in Figs. 2 and 3. In this embodiment, however, the fluid flow is reversed
14 through the conduit 12.
Pump 10 is capable of continuously pumping fluid to a 4-way, 2-
16 position valve 30. The valve 30 alternately sends the fluid throughlines 11 and 22 i~to the top or bottom of vertical conduit 12. The fluid
18 alternately exits conduits 22 and 11 and passes through valve 30 into
conduit 31 which is provided with back pressure valve 25. The means for
measuring and displaying or recording differential pressures between verti-
cally spaced locations in conduit 12 may be the same as described in the
22 previous embodiments.
By reversing flow through the vertical conduit 12 and by measuring
24 the pressure differential between two vertically spaced points in the
conduit, an accurate and substantially continuous report on the density and
26 gel strength of the fluid being sampled can be determined. When fluid
enters the top of conduit 12 through line 22, the read-out device 20 will
28 provide an indication of a fluid density that is less than the true densityby the amount ~p. When fluid flow is reversed and fluid enters the bottom
of conduit 12 through line 11, the read-out device 20 will provide an
indication of a fluid density that is greater than the true density by the
32 amount ~p.
Fig. 7 shows a schematic plot of the signal emitted by readout
34 device 20 as fluid is alternatively flowed upward and downward in conduit 12.
During the time period between to and tl, pump 10 is shut off and the fluid
36 in conduit 12 is static. At time tl, pump 10 is started and fluid is
pumped through line 11. At time t2, valve 30 is shifted to cause the fluid
38 to flow through line 22 into the ,op of conduit 12. Similarly, at times t3
_g_
and t4, the valve 30 is adjusted to cause upward flow in the conduit 12 and
2 at times t4 and t6, valve 30 is shifted to cause downward flow in the
conduit 12. Between time to and tl, readout device emits a signal Sl.
4 From time tl to t2, fluid flows upward through conduit 12 and read-out
device 20 emits a signal S2. From time t2 to t3, fluid flows downward
6 through conduit 12 and readout device 20 emits a signal S3. The average
value of signals S2 and S3 is indicative of the true density of the fluid.
8 The difference between signals S2 and S3, ~p is twice the value of ~p
obtained from the embodiments of Figs. 2 and 3. It is proportional to the
iO gel strength G according to
G = gD ~p, (8)
12
FIEID TESTS
14 The following field tests demonstrate the operability of the
present invention and its ability to measure both the true density and gel
16 strength of mud. The tests were performed under actual drilling conditionsin which a bentonite water base mud was being circulated in a well. The
18 apparatus used in the tests was similar to Fig. 3, consisting of the fol-
lowing components:
(a) A conduit (12) 72 inches long, 4.0 inches I.D.
(b) ~iaphragms (13 and 14) placed 48 inches apart:
22 Differential Pressure Transducer (19): ITT Barton Model 752
Strip Chart Recorder (20) having a variable speed
24 (c) A Moyno Model lL4 progressive cavity pump
(d) A 314-inch back pressure valve(26): Mini-Flex 2600
26 sold by Red Valve Company
The suction line to pump 10 was placed in the tank feeding the
28 mud pumps. The mud was pumped continuously at a rate of 8.7 gallons per
minute and the valves 27 and 28 were cycled between the first (through)
position and second (bypass) position at 10 seconds to provide a continuous
recording as shown in Fig. 5. As indicated, the mud properties during
32 about the first 1.5 hours of testing had a density of about 9.4 to 9.6 and
the gel strength remained relatively constant at about 3 pounds per 100
34 square feet. After about 1.5 hours of operations, bentonite was added to
increase the gel strength of the mud. As clearly seen in the plot, the
36 thickness of the line increased to a relatively constant value which corres-
ponds to about 9 pounds per 100 square feet. The density also increased
38 which is attributed to the higher viscosity mud picking up barite which had
-10-
~.177Si.~
settled in the tanks. At about 2.8 hours, water was added to dilute the
2 mud and thereby reduce the density. As indicated at about 4 hours the
density returned to about 9.5 pounds per gallon. The gauge calibration was
4 verified at about 4.1 hours by displacing the mud in the conduit 12 with
water. The reading was 8.34 pounds per gallon indicating accurate calibra-
6 tion. Upon resuming operations, the mud properties remained relativelyconstant.
8 As mentioned previously, it has been found that the maintenance
of a back pressure on the metering conduit 12 is desirable to eliminate or
substantially reduce the effects of gas (including air) entrained in the
mud. The gas may be present in the mud as a result of the influx of forma-
12 tion gases, mud agitation when adding weighting materials such as barite,operation of some equipment to remove drilled solids from the ~ud, or
14 entrained in certain types of fluid loss additives. Fig. 6 illustrates theeffect of gas in the mud. The same instrument described above was operated
16 at 45 psi back pressure for about 20 minutes during which the density
leveled off to a value of about 17.1 pounds per gallon. The back pressure
18 was then reduced to zero; the mud density immediately dropped to a value
slightly above 17.0 pounds per gallon. Upon returning the back pressure to
45 and even 30 psi, the effects of gas were substantially reduced.
The back pressure valve 25 must be of construction that prevents
22 plugging or caking by the mud. A particularly useful back pressure valve
is a rubber-sleeve type pinch valve illustrated in Fig. 8. The valve
24 connects to discharge line 22 and includes a housing 35 which has mounted
therein a rubber sleeve 36. The housing 35 and sleeve 36 define an internal
26 annular chamber 37. A port 38 formed in the housing communicates with
chamber and permits the pressurization of chamber 37 by line 39 which is
28 connected to a suitable gas or fluid source. Passage 40 of sleeve 36 is
straight and presents no obstructions. The pressure in chamber 37 determines
the back pressure on the mud. The sleeve 36 pinches inwardly to throttle
flow and thereby maintain the desired pressure.
32 A particular advantage of the present apparatus is its ease of
calibration, which is particularly desirable for field use where access to
34 fluids of accurately known and varying density is restricted. Calibration
requires only that the density, pf~ of the noncompressible fluid be accu-
36 rately known. Since this can be determined at the time of manufacture ofthe apparatus, this value will be considered a known constant.
~17~9~
The ease of field calibration can be seen by considering the
2 differential pressure sensed at the transducer l9 under two specific
conditions.
4 When the apparatus is placed in a horizontal position, cos ~ = 0
and Eq. l becomes
6 QP = O (9)
When the vertical conduit is emptied (p = 0) and turned ver-
8 tically upside down (cos ~ = -l), Eq. l becomes
AP = pf gL (lO)
Substitution of these ~P values into Eq. 2 demonstrates that,
when turned horizontal and upside down, the differential pressures sensed
12 by the transducer correspond to those which would be sensed if the apparatus
were in its operating position and full of static fluids having densities
14 equal to pf and 2pf~ respectively. This provides two known calibration
points without the necessity of filling the apparatus with fluids of known
16 densities. For example, if pf were 9 pounds per gallon which approximates
a water-ethylene glycol mixture, the calibration points would be g and 18
18 pounds per gallon. This approximates the normal range of drilling fluids
quite well.
The principle of the invention and the best mode in which it is
- contemplated to apply that principle have been described. It is to be
22 understood that the foregoing is illustrative only and that other means andtechniques can be employed without departing from the true scope of the
24 invention defined in the claims.
-12-
