Dynamic fluid testing apparatus and method

A fluid loss test can be performed using a housing having a substantially cylindrical core disposed therein and having a cage paddle structure disposed therein substantially coaxially around the core thereby permitting continuous agitation and even fluid shear rates to be maintained during test periods. The core has an axial opening defined therein through which lost fluid permeating the core can be collected. The core is secured in the housing so that reverse flow permeability tests can be conducted thereon. Although secured in the housing during tests, the core is removable therefrom so that filter cakes applied to the core can be analyzed. The housing is maintained within a system which permits successive fluids to be introduced into the housing while maintaining substantially constant system pressure. Additionally, measured volumes of fluid can be introduced into the housing.

This invention relates generally to apparatus and methods for dynamically 
testing a fluid and more particularly, but not by way of limitation, to 
apparatus and methods for conducting fluid loss tests, fluid flush and 
wash tests, fluid spacer tests, and reverse flow permeability tests on 
fluids used or found in oil or gas wells. 
In drilling and completing a hole for an oil or gas well, drilling mud is 
pumped into the hole during the drilling operation. This mud adheres to 
the wall of the well and forms a filter cake. Once the hole has been 
drilled, a casing tubing is lowered into the hole. To set the casing and 
bond it to the formation, a cement slurry is pumped into the annulus 
between the tubing and the wall of the hole. To insure that the cement 
adequately bonds to the wall of the hole, it is desirable to remove from 
the annulus and formation wall as much of the filter cake and mud as is 
possible prior to pumping the cement slurry into the annulus. This 
flushing of the mud and filter cake is achieved by introducing suitable 
flush and/or wash fluids into the annulus. 
So that the various fluids (such as the flush and/or wash fluids) which are 
to be pumped into the well can be pumped successively without interrupting 
the pumping process, spacer fluids are used between the different types of 
fluids to separate them in the fluid column as it is pumped into the well. 
Because the downhole conditions in a well vary from one well to the next, 
it is desirable to test particular types of drilling mud, flush fluids, 
wash fluids, spacer fluids, and cement slurries or the like to determine 
their suitability under the particular relevant downhole conditions (such 
as temperature, pressure and agitation). For example, wash fluids are sold 
and used on the basis of their compatability with particular types of 
drilling muds and cement slurries and on the basis of their ability to 
remove mud and filter cake from the annulus and wall of the well. Thus, it 
is desirable to test whether a particular wash fluid is suitable for use 
with the particular mud and slurry to be used in a specific well and to 
test whether the wash can remove a sufficient amount of the mud and filter 
cake from the specific well. 
Additionally, when a cement slurry is pumped into the annulus of a well, a 
portion of the slurry is lost into the adjacent formation of the well 
hole. The amount of fluid loss to be expected with a particular slurry in 
a specific well is an important consideration for one designing the actual 
cementing slurry which is most desirable for that well and for one 
analyzing downhole conditions for predicting annular gas flow or gas 
leakage. The amount of fluid loss to expect from a particular slurry is 
also an important consideration for one designing a squeeze job in which 
the slurry will be forced into a fracture or fissure within the formation. 
Therefore, there is the need for a compact and economical apparatus and 
method for conducting tests on such fluids which are to be prepared for 
use in downhole environments. 
There is also the need for such apparatus and method to provide for 
simulating the downhole structure so that its reverse flow permeability 
can be tested to simulate oil and gas production and to determine 
formation damage from filtrates. Additionally, the actual downhole 
structure often is saturated with brine or other fluid, and thus it is 
desirable to be able to simulate the structure to more closely approximate 
the actual downhole conditions during a test. 
There is currently test equipment which can be used for conducting fluid 
loss tests such as the standard American Petroleum Institute (API) fluid 
loss test and the high temperature high pressure (HTHP) API fluid loss 
test. However, such test equipment and the test results obtained therefrom 
have several shortcomings. One shortcoming is that such equipment 
generally does not provide a close simulation of the actual downhole 
conditions, and the test results are not reliable for predicting annular 
gas flow or for determining the amount of fluid loss additive needed. For 
example, such equipment often is unable to agitate the test fluid during 
the period a fluid loss test is being conducted. Where such agitation is 
provided, it is frequently provided with uneven shear rates between the 
test fluid and the medium used to simulate the downhole structure. 
Furthermore, such equipment cannot successively introduce fluids into the 
test location while maintaining substantially constant pressure. It is 
important to be able to introduce successive fluids while maintaining 
pressure so that the sequence of fluids which will actually be contacting 
the downhole formation can be duplicated during the test. 
Further shortcomings include such equipment often being large and 
cumbersome and requiring several hours to conduct a single test. 
These test equipment also generally provide no pre-conditioning of the 
simulation structural material representing the downhole structure. Such 
simulation material is also often difficult to remove from the test 
equipment and thus is hard to analyze. Still further, the simulation 
material is not secured against reverse flow so that reverse flow 
permeability tests of the simulation material cannot be conducted. 
Another shortcoming of the previous equipment is the lack of means for 
conducting flush, wash and spacer fluid tests. 
By way of specific examples of previous equipment, simple HTHP fluid loss 
cells known in the art provide no means for stirring the test fluid either 
before or during a fluid loss test. Fluid loss cells which do provide for 
stirring only provide such stirring before the fluid loss test is 
cnducted. Modifications of stirred fluid loss cells can provide stirring 
during the actual test; however, such stirring is achieved by means which 
affords poor duplication of the shear rate from test to test and which 
creates uneven shear rates across the simulation structure surface. Large 
pump-through test equipment known in the art are cumbersome to use and 
seldom provide the range of shear rates needed for a complete study of 
fluid loss. Neither the stirred fluid loss cells nor the large 
pump-through equipment are satisfactory for evaluating wash fluids. 
Neither the HTHP fluid loss cells nor the pump-through models provide easy 
removal of the simulation structure without disturbing filter cakes 
deposited thereon. Furthermore, the simulation structures in the typical 
HTHP fluid loss cells are not secured against reverse flow thereby 
preventing reverse flow permeability tests to be run. 
The present invention overcomes the above-noted and other shortcomings of 
the prior art by providing a novel and improved method and apparatus for 
dynamically testing fluids. The present invention provides an apparatus 
and method for determining the fluid loss from an oil or gas well 
cementing slurry or the like under conditions such as high temperature, 
high pressure, and fluid agitation occurring both before and during the 
actual fluid loss test. The agitation occurs at a controllable shear rate 
which is substantially the same across the entire interface area between 
the test fluid and a filter medium representing the downhole structure. 
The present invention is also useful for evaluating flush, wash and spacer 
fluids used ahead of the primary cement slurry to remove mud and filter 
cake from the well bore and for conducting reverse flow permeability 
tests. 
Broadly, the apparatus of the present invention includes a housing, filter 
means disposed in the housing for providing a path through which a test 
fluid can pass from an exterior surface of the filter means to the 
interior of the filter means, and movement means for moving the fluid at a 
controllable fluid shear rate relative to the filter means along an 
interface between the fluid and the filter means. More particularly, the 
device includes a cylindrical filter core as at least a portion of the 
filter means. The core is surrounded by a rotating cage paddle forming the 
movement means. The core and cage are contained in a high pressure vessel 
forming the housing. The core has a hollow center which is vented to the 
outside of the vessel, and the core is mounted in the vessel so that it 
can be disturbing without distubing a filter cake which can be deposited 
thereon as a part of the filter means. This latter feature permits direct 
observation of the effectiveness of flush, wash and spacer fluids for 
removing the filter cake. 
The pressure vessel has an inlet and an outlet which can be connected into 
a system which permits fluids to be successively flowed through the vessel 
without relieving pressure in the vessel. This permits successive fluids 
to be flowed through the vessel in a manner simulating the flow of fluids 
into and through a well. 
The core can be pre-conditioned with brine or other fluids to insure the 
core closely simulates the downhole structure. The core is also mounted in 
the vessel so that it is secured against reverse flow thereby permitting 
reverse flow permeability tests to be conducted on the core. 
The present invention permits relatively quick fluid loss tests to be 
conducted with relatively compact equipment which can be carefully 
controlled to closely simulate downhole conditions. The apparatus can be 
controlled so that agitation creating even shear rates across the 
interface between the filter core and the test fluid are maintained. 
Additionally, flush, wash and spacer fluid evaluation tests and reverse 
flow permeability tests can be conducted.

With reference to FIG. 1 a system embodying the present invention will be 
described. The present invention includes a test chamber 2 having an inlet 
4, a first outlet 6, a second outlet 8, and a vent 10. Associated with the 
test chamber 2 are a drive means 12 and a collecting means 14 which will 
be more particularly described hereinbelow with reference to FIG. 2. 
Connected to the inlet 4 of the test chamber 2 is a test fluid injector 
means 16. This connection is effected by means of a conduit 18 connecting 
an outlet of the test fluid injector means 16 with the inlet 4. Disposed 
in the conduit 18 are valves 20 and 22. The test fluid injector means 16 
provides the source of the fluids which are to be introduced into the test 
chamber 2 for pre-conditioning the test chamber or which are to be the 
test material. In the preferred embodiment the test fluid injector means 
16 includes a stirring autoclave with a quick opening top and a heating 
jacket as are known in the art. The test fluid injector means 16 injects 
the fluid into the test chamber under pressure. The test fluid injections 
can be either heated or unheated, stirred or unstired as test conditions 
require. 
The system disclosed in FIG. 1 also includes an exit fluid receiver means 
24 having an inlet 26 connected to the outlet 6 of the test chamber 2 by 
means of a conduit 28 having a valve 30 and a valve 32 disposed therein. 
The exit fluid receiver means 24 also includes an outlet 34 and a drain 
36. The exit fluid receiver means 24 is any suitable pressure vessel 
capable of receiving a pressurized fluid from the test chamber 2. 
Connected to the outlet 34 of the exit fluid receiver means 24 is a 
specific volume transfer chamber 38 which is also within the system shown 
in FIG. 1. The connection from the outlet 34 is made by means of a conduit 
40 joining with an inlet 42 of the transfer chamber 38. Disposed in the 
conduit 40 is a needle valve 44. The transfer chamber 38 permits measured 
volumes of fluids to be injected into or withdrawn from the test chamber 2 
while maintaining a constant or substantially constant pressure. This is 
achieved by appropriately controlling the flow through the conduit 40 and 
the valve 44 between the exit fluid receiver 24 and the transfer chamber 
38 because the volume of a high pressure gas transferred from a high 
pressure chamber to a lower pressure chamber is approximately equal to the 
volume of the low pressure chamber multiplied by the high pressure and 
divided by the low pressure. In the system shown in FIG. 1, the high 
pressure chamber is the exit fluid receiver 24 and the lower pressure 
chamber is the specific volume transfer chamber 38. The transfer chamber 
38 is any suitable apparatus as known in the art. A volume calibrated hand 
pump, such as a Ruska pump, or other suitable device can be substituted 
for the chamber 38. 
The system also includes a high pressure nitrogen source 46 connected 
within the system as illustrated in FIG. 1. The purpose of the high 
pressure nitrogen source 46 is to supply nitrogen to the points needed to 
facilitate drainage of the test chamber under a high system pressure. 
Appropriate manifolding means is used in the nitrogen source 46. 
Thus, the system as illustrated in FIG. 1 provides means for flowing fluid 
into the test chamber 2 under pressure at a first time, means for flowing 
another fluid into the test chamber 2 under pressure at a second time, and 
means for maintaining pressure in the test chamber 2 between the first 
time and the second time substantially constant. Additional fluids can 
also be introduced into the test chamber 2 at additional times as 
required. The fluids include brine, drilling mud, cement slurries, 
flushes, washes, spacers, or other suitable fluids. 
With reference primarily to FIG. 2 the preferred embodiments of the test 
chamber 2, the drive means 12 and the collecting means 14 will be 
described. The test chamber 2 includes a compact housing preferably 
comprising a pressure vessel as shown in FIG. 2 to include a substantially 
cylindrical container 48 having a lid 50 threadedly and pressure-sealingly 
connected thereto. The container 48 has an opening defined in a side wall 
thereof to provide the outlet 6 and has another opening defined in a 
bottom wall thereof to provide the outlet 8. The lid 50 includes openings 
defined therein to provide the inlet 4 and the vent 10. The valve 22 is 
shown threadedly connected to the inlet 4, and a vent valve means 52 is 
shown threadedly connected to the vent opening 10. 
Suitably attached to the exterior surface of the container 48 is heating 
means for heating a fluid within the vessel. As schematically illustrated 
in FIG. 2 the heating means includes a plurality of strip heater elements 
54 of a type known in the art. The heating means also includes a 
thermocouple 56 for controlling the strip heater elements 54. 
The pressure vessel has a cavity 58 defined therein by the interior 
surfaces of the container 48 and the lid 50. The cavity 58 can receive a 
fluid through the inlet 4. In the preferred embodiment the pressure vessel 
is one which can withstand approximately 5,000 to 10,000 pounds per square 
inch of pressure on a fluid contained in the cavity 58. A fluid in the 
cavity 58 can also be heated in the preferred embodiment to a temperature 
of up to approximately 500.degree. F. by means of the strip heater 
elements 54. 
Disposed in the cavity 58 of the housing is a filter means for providing a 
path through which a fluid in the cavity 58 can pass from an exterior 
surface of the filter means to the interior of the filter means. In the 
preferred embodiment the filter means includes a medium of porous or 
permeable material specifically shown as comprising a substantially 
cylindrical core 60 having an outer surface 62 which defines the exterior 
surface of the filter means when no filter cake is deposited thereon. A 
filter cake can be deposited on the core 60 by the means of the system 
shown in FIG. 1 acting on the surfaces 62 through the introduction of 
driling mud into the cavity 58. The core 60 further has an interior 
opening 64 defined therein. In the preferred embodiment the opening 64 is 
axially disposed within the substantially cylindrical core. The core 60 is 
any suitable material (such as sandstone, metal or plastic, for example) 
which can be used to simulate the mineral formation of the downhole 
environment in which the test fluid is to be used. 
Fluid which passes through the porous material of the core 60 into the 
opening 64 is vented to the exterior of the container 48 through a mandrel 
tube 66. The mandrel tube 66 has a wall defining a hollow interior region 
in the tube, and the tube is constructed so that fluid passing through the 
porous core 60 into the opening 64 enters the hollow interior region of 
the tube 66. In the preferred embodiment this construction includes a 
plurality of holes, illustrated in FIG. 2 as holes 67, defined in and 
through the wall of the mandrel tube 66. The tube 66 extends out of the 
opening 64 defined in the core 60 and passes through the second outlet 8 
so that fluid in the hollow interior region of the tube 66 can pass to the 
exterior of the vessel The portion of the tube 66 extending into the 
opening 64 of the core 60 lies adjacent the interior surface of the porous 
means defining the opening 64. 
In the preferred embodiment the opening 64 passes through the entire length 
of the core 60 so that the mandrel tube 66 passes through the core 60 and 
attaches to a core cap 68 capping one end of the core 60. The other end of 
the mandrel tube 66 passes out of the container 48 through the outlet 8 
and is there engaged by a nut 70 which pulls the mandrel tube 66 and the 
core cap 68 connected thereto down to thereby lock the core 60 in place 
within the cavity 58 of the vessel. The nut 70 works against a yoke 72 
which is disposed between the nut 70 and the bottom wall of the container 
48. 
As shown in FIG. 2 the first end of the core 60 is sealed by a sealing 
gasket 74 disposed between the first end surface of the core 60 and the 
core cap 68, and the second end of the core 60 is sealed by a sealing 
gasket 76 disposed between the second end surface of the core 60 and the 
bottom interior surface of the container 48. The passage of the mandrel 
tube 66 through the outlet 8 in the bottom wall of the container 48 is 
sealed with a packoff assembly 78 disposed in the yoke 72. 
The present invention also includes movement means for moving the fluid 
within the cavity 58 at a controllable fluid shear rate relative to the 
filter means along an interface between the fluid and the filter means. 
This interface includes the exterior or outer surface 62 of the core 60 or 
the exterior surface of a filter cake if one is applied to the outer 
surface 62 by suitable means of the system. This fluid movement means 
specifically is used for moving the fluid in the cavity adjacent the core 
60 or the filter cake when a portion of the fluid is passing through the 
core 60 into the hollow interior region of the mandrel tube 66. 
The preferred embodiment of the movement means includes a cage paddle 
having a plurality of stirring elements 80 spaced from the outer surface 
62 of the core 60. In the preferred embodiment the stirring elements 80 
are disposed substantially parallel to the axially extending opening 64 
and the mandrel tubing 66 disposed therein. As shown in FIG. 2, each 
stirring element 80 is elongated and extends along the length of the core 
60. 
The stirring elements 80 are retained in spaced relation to each other and 
to the exterior surface of the core and the filter cake, if one is applied 
to the core, by suitable means, such as an upper retaining ring 82 and a 
lower retaining ring 84. The lower retaining ring 84 defines a central 
opening through which the core 60 extends in substantially coaxial and 
concentric relationship with the cage paddle. The cage paddle is rotatably 
mounted in the cavity 58 of the pressure vessel so that it can be rotated 
relative to the core 60. The rotatable cage paddle enables stirring of the 
test fluid to continue during an actual fluid loss test. The elongated 
stirring elements 80 eliminate uneven fluid shear rates at the filter 
medium surface. 
So that the speed of rotation of the cage paddle, and thus the rate of the 
fluid shear, can be varied over a desirable range, the movement means also 
includes the drive means 12. In the preferred embodiment the drive means 
12 includes a variable speed motor 86 illustrated in FIG. 1. The preferred 
embodiment of the drive means also includes magnetic means for 
magnetically coupling to the cage paddle a variable rotative force 
provided by the variable speed motor so that the cage paddle moves the 
fluid in the cavity 58 adjacent the surface 62 of the core 60 (or adjacent 
the surface of a filter cake applied to the core) to impart a controllable 
shear at the interface between the fluid and the surface 62. 
Alternatively, the variable speed motor can be directly mechanically 
coupled to the cage paddle by means of a shaft packed off with a suitable 
high pressure packing. The preferred magnetic form is illustrated in FIG. 
2 by the structure identified by the reference numeral 88. In the 
embodiment shown in FIGS. 1 and 2, the motor 86 is connected to the 
structure 88 by means of a suitable drive belt 90. When the cage paddle is 
driven, it rotates around the fixed core 60 resulting in an even 
distribution of fluid shear rate across the surface 62 of the core 60 or 
the surface of a filter cake applied to the surface 62. 
To measure the quantity of fluid lost during a fluid loss test, the 
apparatus further includes the collecting means 14 for collecting the 
fluid passing through the core 60 to the opening 64 and the interior of 
the tubing 66. The collecting means may include any suitable structure, 
but is preferably shown in FIG. 2 to include valves 92 and 94 and a 
pressurized receiver means 96. This structure is connected to the mandrel 
tube 66 by means of a suitable connector element 98. 
Broadly, the method of the present invention by which a fluid loss test is 
conducted on a fluid used in a well defined by a permeable structure 
includes the steps of placing the fluid in fluid communication with the 
structure-simulating permeable filter core 60 so that a portion of the 
fluid can permeate the filter core and of moving the fluid relative to the 
filter core 60 so that a shear rate is achieved at an interface between 
the fluid and the filter core as the fluid permeates through the filter 
means. The method also includes collecting the quantity of the fluid 
permeated through the filter means. The method also includes initially 
mounting the filter, such as the cylindrical permeable core 60, in the 
pressure vessel and pressurizing the fluid in fluid communication with the 
core. 
In the embodiment shown in FIG. 2 the test fluid is flowed substantially 
axially or longitudinally past the exterior surface 62 of the core 60 as 
the cavity 58 is filled with the fluid. A portion of the fluid permeates 
the core 60 and then flows substantially axially or longitudinally out of 
the opening 64 through the hollow interior of the mandrel tube 66. 
Simultaneous with the substantial axial flow and permeation by the fluid, 
a circumferential component of movement can be imparted to the fluid by 
rotating the cage paddle substantially co-axially with respect to the core 
while the core is maintained fixed. 
To determine the quantity of fluid lost over a predetermined time, the step 
of collecting the quantity of fluid permeated through the filter means is 
conducted for a predetermined period of time as known in the art. 
Collecting the fluid is performed by appropriately operating the elements 
92, 94 and 96 as known in the art. 
The operation further comprises, prior to the step of flowing the test 
fluid past the core, the step of treating the core 60 to closely simulate 
the permeable structure of the well. This step includes saturating the 
core with simulated formation water, such as brine, and applying a filter 
cake of drilling mud to the exterior surface 62 of the core 60. An 
illustration of a filter cake 100 applied to the exterior surface 62 of 
the core 60 is illustrated in FIG. 3. 
The method furter includes, after the step of applying a filter cake but 
before the step of flowing the test fluid past the core, the step of 
flowing a wash fluid past the filter cake. A flushing fluid can also be 
flowed past the filter cake. 
The method still further comprises, after the step of flowing a flush 
and/or wash fluid, the step of removing the core from the vessel for 
determining the amount of filter cake remaining applied to the core 60. 
The method also includes the step of heating the fluid which is in fluid 
communication with the filter core 60. The method still further includes 
the step of maintaining pressure in the vessel during the steps of 
treating the core, flowing the test fluid past the core, simultaneously 
imparting a circumferential component of movement to the fluid, and 
collecting the quantity of the fluid passing into the opening of the core. 
By way of a more specific example of the operation of the present 
invention, a filter means treatment substance such as simulated formation 
water (e.g., brine) is initially placed in the cavity 58 and the core 60 
is saturated with the simulated formation water by using either a 
flow-through or vacuum procedure which are understood by those familiar 
with the art of high temperature, high pressure fluid loss testing. Once 
the core has been saturated, the formation water is drained through the 
outlet 6 and the valve 30. 
Next, a known quantity of drilling mud or the like is injected into the 
cavity 58 through the valve 22 and the inlet 4. The pressure vessel is 
then pressurized to the specified test pressure, such as a pressure up to 
10,000 pounds per square inch. 
With the vessel pressurized, the cage paddle is rotated by the motor 86 at 
the specified speed for the particular test in progress. 
Heat is next applied to the vessel by means of heater elements 54 as 
controlled by the thermocouple 56. When the test temperature is reached, 
valve 92 is opened to start a fluid loss test period. The fluid lost 
during this period is measured in accordance with procedures understood by 
those in the art. 
As required by the specific test being performed, the drilling mud can be 
agitated or allowed to remain static during the test period when the fluid 
is permeating the core. After the drilling mud test and/or the 
pre-conditioning period, the drilling mud is drained while maintaining 
total system pressure. Although the drilling mud is drained, some of the 
drilling mud remains adhered to the core 60 and forms a filter cake. The 
approximate thickness of the filter cake is known from the quantity of 
drilling mud originally injected and the amount lost during the test and 
drained. 
A measured volume of wash or spacer fluid is then injected into the test 
chamber, agitated as required and then removed. 
Next, the test slurry is injected into the cavity 58 through the inlet 4 
and the valve 22 and the amount of fluid lost through the core 60 is 
measured using procedures similar to those described above and understood 
by those in the art. 
To evaluate a wash fluid for mud and filter cake removal, the following 
tests are performed. Initially, the foregoing steps are terminated while 
the cement slurry is still fluid. The fluid slurry is drained from the 
chamber and the core 60 is removed from the cavity 58. The filter cake 
remaining on the core is cut through to the surface 62 of the core 60 and 
partially removed so that the thickness of both the mud and cement filter 
cakes can be determined. The effectiveness of the wash which was used 
during the wash sequence is then compared to the results of another test 
during which either no wash or a plain water wash was used. 
To determine the reverse flow permeability, the fluid permeability of the 
core is measured from the inside to the outside of the core both before 
and after the fluid loss test is performed. The result is compared to 
another test using a plain water wash. To determine the permeability of 
the core, a known volume of fluid is injected through the valve 92, the 
mandrel tube 66, the core 60, and into the cavity 58. The pressure 
differential between the valve 92 and the cavity 58 is measured, and the 
time period is measured. Knowing the volume injected, the pressure 
differential, the time period and the geometry of the core 60, the 
permeability is calculated as known in the art. 
From the foregoing it is apparent that the various tests can be conducted 
under continuous agitation throughout the entire fluid loss test period. 
Additionally, the shear rate over the entire core surface is substantially 
the same. The shear rate can be varied from zero to high turbulence and 
can be duplicated on successive tests. 
Using the system illustrated in FIG. 1, test fluids can be changed without 
opening the test chamber 2 or relieving the pressure therein. Thus, it is 
possible to simulate a period of mud circulation followed by a period of 
mud flushing before measuring the fluid loss of a cement slurry. 
Furthermore, no outside mechanical pumps are required to circulate the 
test fluid past the fluid medium as required by previous systems capable 
of changing test fluids under pressure. 
Still further, the test chamber 2 is easy to disassemble and clean and is 
constructed to permit the filter medium to be easily removed for 
inspection or replacement. 
Thus, the present invention is well adapted to carry out the objects and 
attain the ends and advantages mentioned above as well as those inherent 
therein. While a preferred embodiment of the invention has been described 
for the purpose of this disclosure, numerous changes in the construction 
and arrangement of parts can be made by those skilled in the art, which 
changes are encompassed within the spirit of this invention as defined by 
the appended claims.