Measuring and testing – With fluid pressure – Porosity or permeability
Reexamination Certificate
1999-08-12
2001-01-30
Williams, Hezron (Department: 2856)
Measuring and testing
With fluid pressure
Porosity or permeability
Reexamination Certificate
active
06178808
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
BACKGROUND OF THE INVENTION
The invention relates generally to geological test instruments, and, in particular, to instruments for detecting how well fluids are conducted through geological materials such as clays, rocks, and soil.
The hydraulic conductivity of geological materials is important for determining their drainage capacity and the mobility of liquids such as oil or water in subsurface strata. The hydraulic conductivity of less permeable geological media may be less than 10
−4
cm/sec.
Measurement of such hydraulic conductivities presently entails placing the sample in a test cell so that liquid flow may occur only between an inlet and outlet opening in the cell. Tubing connects the openings to burettes containing the test fluid, typically water. The burettes have adjustable heights and graduation lines so that the column heights of the contained fluid may be easily measured.
In the “fallinghead risinghead” method of measuring hydraulic conductivity, the water level in one burette is placed above the water level in the other to establish a pressure differential across the sample and the rate of flow measured by comparing, at periodic intervals, the changing heights of the liquid columns in the burette.
In the “fallinghead” method, only one burette is used and the remaining opening in the test cell drains into a graduated cylinder or similar device.
In both methods, the burettes are typically open to the air. However, they may be closed and pressurized, for example with compressed air, to achieve a greater pressure difference. This pressurization addresses the problem that steady state hydraulic flow necessary to establish conductivity can take many hours or weeks to occur.
In a “constant volume” method originally described by Bjerrum, L. and Huder, J., in their publication
Measurement of Permeability of Compact Clays
, Proc. 4
th
Intl. Conf. on Soil Mech. and Foundation Eng., 1957, a closed loop is established between the inlet and outlet to and from the test sample so that the test sample always has a constant volume of test fluid. A falling column of mercury incorporated into this closed loop provides the pressure difference across the test sample. While this method has decreased measurement times (by decreasing the time to steady state hydraulic conductivity where inflow equals outflow), the test results can still take many hours or days to stabilize.
All of these methods have produced erratic results if insufficient time is allowed for stabilization to occur.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a test of hydraulic conductivity suitable for geological materials, but where test results are obtained in minutes rather than hours and where variations in the measurements are much reduced.
In the invention, a closed loop of test fluid is maintained with constant pressure differential. A “constant head mercury column” is used to provide constant differential pressure in a closed loop environment ensuring a constant fluid volume in the sample.
Specifically, the present invention provides a method for testing geological materials for hydraulic conductivity including the steps of enclosing the sample material in a cell having fluid impermeable walls and a first and second opposed port separated by the sample. The geological sample is saturated with a test fluid and placed under a constant pressure difference across the first and second ports while fluid flow of the test fluid through the ports is matched so that a constant volume of test fluid is within the sample.
It is thus a principal object of the invention to eliminate variations of pressure difference in a closed loop system such as may affect the stability of the measurement of geological materials of complex characteristics. Although the inventors do not wish to be bound to a particular theory, it is believed that the changing pressure difference inherent in prior art constant volume devices may unpredictably influence the measurement by changing, the hydraulic gradient in the sample, the pore water pressure and the stress on the sample. By providing a constant pressure difference, as well as a constant volume of test fluid in the sample, these and other second order effects on the measurement of hydraulic conductivity are eliminated.
The greater stability in the measurement further shortens the necessary measurement time.
The constant pressure difference across the first and second ports may be ensured by the use of a tube, a portion of which is filled with a material of greater specific gravity than the test fluid. A vertically oriented portion of this tube is completely filled with the material of greater specific gravity and connects at its upper end with horizontal tube at partially filled with the material of greater specific gravity. An interface exists separating the material of greater specific gravity from the test fluid. The free ends of the tube are attached one to each of the first and second ports.
It is thus another object of the invention to provide a means for producing a constant pressure difference across the sample that may be used in a closed loop and that is compatible with the goal of a constant volume of fluid flow. The vertically oriented flexible tube may be part of a constant volume connection between the first and second ports that ensures the same fluid flow into the first port as out of the second port. The horizontal tube, which adds no hydraulic head to the hydraulic head already applied by vertically oriented flexible tube, is used to measure the volume of fluid flow
The connections between the vertically oriented tube and the horizontal tube and the first and second orifices may provide interfaces between the material of greater specific gravity and the test fluid and these interfaces may have an identical cross- sectional area.
Thus, it is another object of the invention to eliminate the effect of pressure drop caused by meniscus capillary pressure of the various fluids.
One interface between the material of greater specific gravity and the test fluid may be an upwardly extending tube terminating in an orifice of a predetermined cross-section and surrounded by a well.
Thus, it is another object of the invention to provide a connection between the material of greater specific gravity and the test fluid that is both fixed in height and of a predetermined and constant cross-sectional area.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention.
REFERENCES:
patent: 2618151 (1952-11-01), Leas
patent: 6055850 (2000-05-01), Turner et al.
patent: 1268577 (1961-06-01), None
“Permometer P700000 Instruction Manual,” (Jul. 1995), 14 pgs., Trautwein Soil Testing Equipment, P.O. Box 31429, Houston, TX 77231.
“WCC—Clifton Laboratory Test Procedure for Permeability Test wtih Backpressure Using Constant Vol.—Falling-Head Apparatus,”(Apr. 1981), 7 pgs.
L. Bjerrum and J. Huder, “Measurement of the Permeability of Compacted Clays,”.
Proceedings of the Fourth International Conference on Soil Mechanics and Foundation Engineering, London, vol. 1, Div. 1a/2, pp. 6-8, Aug. 1957.
Benson Craig H.
Wang Xiaodong
Cygan Michael
Quarles & Brady
Williams Hezron
Wisconsin Alumni Research Foundation
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