CA1117791A - Apparatus and method for measuring properties of fluid - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An apparatus and method for determining fluid density and for gel strength comprising a conduit having two isolation diaphragms spaced verti-cally apart therealong, a pump means for passing a fluid through said vertical conduit, and means for alternation flow and static conditions of the fluid in said conduit. The isolation diaphragms are connected through conduits containing noncompressive fluid with a differential pressure transducer. The pressure transducer is connected to a gauge or other reporting and/or recording device such that the true density of the fluid is determined by the pressure differential between the isolation diaphragms when the fluid is static and an estimation of the gel strength of the fluid is determined by the increase in the apparent density of the fluid during flow at low flow rates.

Description

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.

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.

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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.

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.

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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.

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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) 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 ~.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.

Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for measuring the properties of a liquid which comprises:
(a) a nonhorizontal conduit positioned to conduct liquid therethrough, (b) means for measuring the pressure differential between two vertically spaced locations in said conduit, (c) means for flowing liquid through said conduit at a rate where the shear rate of the flowing liquid is in the range of greater than zero to about 20 sec-1, (d) means for cyclically interrupting flow of liquid through said conduit to provide a flow interval and a static interval, and (e) means for measuring the differential pressure between said spaced locations during the static interval.

2. Apparatus as defined in Claim 1 and further comprising means for maintaining a sufficient back pressure on said conduit to reduce substantially the effects of gas entrained in said mud.

3. Apparatus as defined in Claim 1 and further comprising means for maintaining a pressure on liquid flowing through said conduit of at least two atmospheres.

4. Apparatus as defined in Claim 2 wherein said means for maintaining back pressure includes a valve connected to said conduit downstream of said differential pressure measuring means and including a housing and an elastomeric sleeve which in combination define a pressure chamber, and means for delivering a predetermined pressure to said chamber, said sleeve being inwardly deformable by pressure in said chamber to throttle flow therethrough to maintain said back pressure on said conduit.

5. Apparatus as defined in Claim 1 wherein said conduit has an average flow area between said vertically spaced locations of not more than 52 square inches.

6. Apparatus as defined in Claim 1 wherein said means for interrupting flow provides 8 cycling frequency of between about 0.1 cycle and about 15 cycles per minute.

7. Apparatus as defined in Claim 1 wherein the means for flowing liquid through said conduit is operative to flow liquid upwardly through said conduit.

8. An apparatus for measuring fluid properties comprising:
(a) a longitudinal conduit positioned in a nonhorizontal attitude, (b) a first flexible isolation diaphragm having an inner sur-face communicating with said conduit and an outer surface communicating with a chamber containing a noncompressible fluid, (c) a second isolation diaphragm having an inner surface communi-cating with said conduit and an outer surface communicating with a chamber containing a noncompressible fluid, (d) means operably associated with said conduit for moving a fluid upwardly through said conduit at a controlled rate where the shear rate of the flowing column of greater than zero to about 20 sec-1, interrupting the flow of the fluid so that said fluid is alterntely flowed through said conduit and held therein without flow, (e) differential pressure transducer means communicating with said outer chambers for measuring the pressure difference in said sample fluid in the conduit between said first and second isolation diaphragms, (f) means operably associated with said differential pressure transducer for indicating the pressure difference in said sample fluid in the conduit between said first and second isolation diaphragms, (g) means for cycling fluid flow through said conduit at a predetermined frequency, and (h) means for comparing the pressure difference in said sample fluid when said fluid is held in said conduit without flow indicating true density and when the fluid is flowing indicating an apparent density comprising the true density and a frictional pressure gradient density, said frictional pressure gradient density being proportional to the gel strength of said fluid.

9. The apparatus according to Claim 8 wherein said differential pressure transducer is connected to each isolation diaphragm outer chamber by a conduit containing the same, noncompressible fluid as in said outer chamber and for this includes means for dampening the output of said transducer to filter out signal frequencies outside a predetermined range.

10. The apparatus according to Claim 8 having a throttle valve for causing a back pressure in said conduit, located downstream from said isolation diaphragms.

11. The apparatus as defined in Claim 8 wherein said means for cycling flow through said conduit includes means for reversing flow therethrough.

12. An apparatus for measuring the properties of liquid which comprises:
(a) a conduit positioned in a nonhorizontal attitude and adapted to conduct liquid therethrough;
(b) means for measuring pressure differential between two vertically spaced locations positioned along said conduit;
(c) an inlet line connected to one end of said conduit;
(d) a discharge line connected to the other end of said conduit;
(e) a bypass line interconnecting said inlet line and said discharge line;
(f) valve means for alternately directing flow through said conduit and said bypass line;
(g) a back pressure valve connected to said discharge line downstream of said bypass line connection;

(h) pump means connected to said inlet line upstream of said bypass line, said pump means being adapted to pump liquid continuously during measurement of said liquid, said pump means adapted to pump liquid through said conduit at a flow rate where the shear rate of the flowing liquid is in the range of greater than zero to about 20 sec-1, and (i) means for comparing the pressure differential measured when the fluid is flowing through the bypass line indicating a true density and when said fluid is flowing through said conduit indicating an apparent density comprising the true density and a frictional pressure gradient density, said frictional pressure gradient density being proportional to the gel strength of said fluid.

13. A method for measuring the properties of a fluid, com-prising:
(a) measuring the pressure of a static column of said fluid at two vertically spaced points, (b) determining the difference in pressure between said points to thereby indicate the true density of said fluid, (c) measuring the pressure of a flowing column of said fluid at said two points said flowing column having a flow rate where the shear rate of the flowing fluid is in the range of greater than zero to about 20-1, (d) determining the difference in pressure between said points during flow to thereby indicate an apparent density comprising the true density and a frictional pressure gradient density, said frictional pressure gradient density being proportional to the gel strength of said fluid.

14. The method according to Claim 13 wherein said flow is upward and said apparent density is greater than said true density.

15. The method according to Claim 13 wherein a pressure greater than atmospheric pressure is applied to said fluid during said measurements.

16. The method according to Claim 15 wherein said pressure is up to about 10 atmospheres.

17. The method according to Claim 13 wherein said measurements and determinations are cyclically repeated.

18. A method of measuring the properties of a fluid comprising the steps of:
(a) flowing a fluid through a substantially vertical conduit, (b) terminating said flow to retain a static column of fluid in said conduit, (c) measuring the pressure of said static column of fluid at vertically spaced points in said conduit, (d) determining the difference in pressure between said points to indicate the true density of said fluid, (e) restarting said flow of fluid and flowing said fluid at a flow rate where the shear rate of the flowing fluid is in the range of greater than zero to about 20 sec-1, (f) measuring the pressure of said flowing fluid at said points, and (g) determining the difference in pressure between said points during flow to indicate an apparent density comprising the true density and a frictional pressure gradient density, said frictional pressure gradient density being proportional to the gel strength of said fluid.

19. A method of monitoring the gel strength of a drilling fluid:
(a) generating a first signal proportional to the difference in static pressure of a liquid in a conduit at two vertically spaced points to indicate the true density of the liquid, (b) thereafter, generating a second signal proportion to the difference in pressure between said points during flow of the liquid through the conduit at a flow rate where the shear rate of the flowing liquid is in a range of greater than zero to about 20 sec-1 to thereby indicate an apparent density comprising the true density and a frictional pressure gradient density, and (c) comparing the first signal with the second signal to determine the difference therebeween, the difference being indicative of the gel strength of the drilling fluid.

20. An apparatus for measuring the density of a liquid which comprises:
(a) a nonhorizontal conduit positioned to conduct liquid therethrough, (b) means for measuring the pressure differential between two vertically spaced locations in said conduit, (c) means for flowing liquid through said conduit, (d) means for cyclically interrupting flow of liquid through said conduit to provide a flow interval and a static interval, (e) means for measuring the differential pressure between said spaced locations during the static interval, and (f) means for maintaining a sufficient back pressure on said conduit to reduce substantially the effects of entrained gas in said liquid, said mean includes a valve connected to said conduit downstream of said differential pressure measuring means and including a housing and an elastomeric sleeve which in combination define a pressure chamber, and means for delivering a predetermined pressure to said chamber, said sleeve being inwardly deformable by pressure in said chamber to throttle flow therethrough to maintain said back pressure on said conduit.

21. The apparatus as defined in Claim 20 wherein said back pressure means maintains a pressure on liquid flowing through said conduit of at least two atmospheres.

22. An apparatus for measuring the density of a liquid which comprises:
(a) a nonhorizontal conduit positioned to conduct liquid therethrough, (b) an outwardly flared inlet means disposed on the end of said conduit, said inlet means positioned in a generally vertical attitude and sized so that the vertical com-ponent of the liquid velocity in the area defined by the inlet means is less than the minimum liqud velocity through said apparatus.
(c) means for measuring the pressure differential between two vertically spaced locations in said conduit.
(d) means for flowing liquid through said conduit, (e) means for cyclically interrupting flow of liquid through said conduit to provide a flow interval and a static interval, and (f) means for measuring the differential pressure between said spaced locations during the static interval.