Patent Application
20110210757
The present invention is directed towards a method and apparatus for the measurement of properties of porous media. In particular it is directed towards the relationship between pore structure and flow and diffusion in the porous medium.
Porous materials often contain pores that are interconnected thereby forming a network that allows fluid flow within the system. Understanding the pore size distribution and structure of this network is a very important step in understanding the characteristics of the flow through the porous medium. Similarly, the diffusion of species through the pores in the medium in the absence of flow is also affected (though in a different way) by the pore structure. There is therefore a need to understand in as much detail as possible not only the pore size within a material but also the pore structure.
Classically, the approach has been to make measurements of the permeability of the medium to a particular fluid (gas or liquid) and to measure the rate of species diffusion. Measurements have usually been made for the case where the medium is saturated with the gas or liquid—i.e. where there are no fluid interfaces in the medium. In such a case, the medium can be described as being saturated with the given fluid phase. However, these overall measurements do not give information about the link between pore structure and the fluid transport processes.
For many decades, attempts to forge this link have been made on the basis of data from porosimetry studies. Such data have been obtained by one of two main techniques:
(1) Mercury porosimetry in which the medium is first evacuated and is then surrounded by mercury. Pressurisation of the mercury leads to it penetrating the pores. The relationship between the diameter d of the smallest pore penetrated at a given applied pressure P is set out in Equation (1) below, (“A General Analysis for Mercury Porosimetry”, Powder Technology, vol. 33, p. 201, Smithwick, 1982).
where γ is the surface tension and θ is the contact angle of mercury.
The data obtained is usually presented in terms of pore volume penetrated as a function of the pore diameter calculated from the above Equation 1.
(2) Measurements of the uptake of a wetting liquid. Here, the capillary pressure is measured and the pore size distribution estimated from the measurements as set out, for example, in U.S. Pat. No. 4,211,106.
However, there is a basic difficulty in using just porosimetry data to interpret permeability and diffusivity measurements of a porous material. In practice, the pores have a complex path through the medium with some of the penetrated pores having “dead ends” (and therefore play no part in fluid transport) and others containing throats which restrict the flow. There is therefore a need for a new method and apparatus to determine properties of a porous material with greater accuracy.
According to the present invention there is provided a method to determine properties of a porous material, in which: a porous sample is prepared and sealed: first the sample is evacuated to near vacuum; then a non-wetting and electrically conducting fluid is passed through the sample at a known pressure and the volume of said fluid taken up by the sample is measured; then a small differential pressure is applied across the sample and the flow rate of said fluid through the sample is measured; and an alternating current which can be varied is passed through the sample and the resistance across the sample is measured.
Optionally, the method is repeated at different applied pressures. Preferably the pressure is increased in small increments and measurements are taken at each new applied pressure until there is no further fluid penetration (i.e. an increase in pressure does not force any fluid to be taken up by the sample as the porous material is saturated). Preferably the non-wetting and electrically conducting fluid is mercury.
The invention also extends to apparatus for determining properties of a porous material, the apparatus comprising: a sample cell for holding a sample of the porous material; two measuring tubes open at the bottom end to opposite ends of the sample and also connected to sumps for a non-wetting and electrically conducting fluid, and open at the top end to a vacuum pump; means for applying a fixed pressure to the system; means for applying a differential pressure across the sample; means for applying a current across the sample and means for measuring the resistance across the sample.
A novel method has been developed and an apparatus designed and tested to measure the characteristics of a porous material by penetrating the medium with, for example, mercury at a series of pressures. The amount of mercury taken up by the medium at a given applied pressure gives a measurement of the pore size distribution following the widely-used principle of mercury porosimetry. Having reached an equilibrium mercury penetration at a given pressure, a differential pressure (small compared with the total applied pressure) is applied across the medium and the rate of mercury flow through the medium measured. This allows the determination of the permeability of the medium for those pores penetrated at the given applied pressure.
The permeability rises with pressure (i.e. with decreasing size of penetrated pores) and a plot of permeability against pressure (or pore volume penetrated) gives unique information about the mechanism of flow through the medium. At a high enough pressure, the mercury permeability becomes equal to the permeability for a wetting fluid (gas or liquid). The flow is likely to be dominated by flow in the larger interconnected pores so the total permeability may be reached with only a small fraction of the pore volume penetrated.
Also at any applied pressure, the electrical conductivity of the medium is determined. For an electrically non-conducting medium, the measured electrical conductivity is governed by the electrical conductance through the mercury and is analogous to the process of diffusion through the penetrated pores. For low pressures, the conductivity will be zero (assuming the medium itself is an electrical insulator) but, with increasing pressure, the conductivity through the penetrated pores increases and eventually reaches a constant value at high pressures. The variation of the electrical conductivity with pressure gives an indication (analogous to that for permeability in the mercury permeability tests) of the contributions of the various pores to diffusive transport through the medium.
As the applied pressure is increased, both the mercury permeability and the conductivity increase, eventually reaching the values for the saturated medium. The way in which permeability and conductivity vary with pressure, coupled with the information on the pore volume penetrated, gives new insights into the pore structure and behaviour in the medium.
It will be noted that the three sets of measurements (porosimetry, permeability and conductivity) may be combined to give unique insights into the pore structure and its relationship to flow and diffusion in the medium.
The present invention will be demonstrated with reference to the following figures, in which:
In the nitrogen system, which is directly attached to the top of the measuring tubes 3a, 3b, there is a nitrogen inlet 5 connected to a nitrogen supply (not shown) and a nitrogen exhaust 6. Each of the supply and exhaust is controlled by means of a valve Vn.
To eliminate any fluid leakage around the edges of the sample and to ensure the sample is of the correct dimensions, the sample cores must be sealed with a non-permeable coating. This is achieved by first coating the whole surface of the sample with a layer of epoxy resin, for instance. When this has set, then the sample is set coaxially in an epoxy resin cylinder as shown in
Before any mercury is allowed into the system, both the apparatus and the sample must be evacuated. A pressure of 10 Pa is sufficient, this can be monitored on a Pirani gauge V (see
At the initial condition, the sample and its associated system are under vacuum. Opening valves V14 and 15 allows mercury from the sumps 2 to rise to be in contact with the sample faces and to further rise up the PTFE measuring tubes 3a and 3b to identical heights in each tube. At this stage, the pressure of the mercury in contact with the sample faces corresponds to the static head of the mercury in tubes 3a and 3b and must be kept lower than the pressure required for mercury to penetrate into the largest pores of the sample. To prevent mercury from flowing back to the sump when pressurising the system, valves V14 and V15 are closed. Valves V3 and V4 are then closed and nitrogen gas is admitted to the nitrogen system through nitrogen inlet 5 when valves V5, V7, V8 and V9 are opened. The pressure in the nitrogen system is monitored on pressure gauges P1 and P2. A gas exhaust valve V6 is installed to allow the nitrogen pressure to be reduced if required.
Porosimetry involves measuring the mercury volume uptake by the sample as a function of the pressure applied at the interface between the bulk mercury and the sample faces. As was stated above, the initial value of this pressure corresponds to the static head applied by the mercury in tubes 3a and 3b, which must be low enough to avoid penetration of the largest pores in the sample.
The system must be evacuated and set up as described above and the heights of mercury in each of the measuring tubes (which should be identical) are recorded. To begin the penetration into the largest pores, nitrogen gas is applied to the system at a known pressure. The nitrogen pressure is controlled using the inlet valve V5, and if necessary can be lowered using the exhaust valve V6. Turning the three-way valves V10 and V11 to open the nitrogen side will admit nitrogen gas to the mercury system and apply pressure to the mercury in the measuring tubes. The gas will force mercury at the faces of the sample to begin to penetrate the larger pores. As indicated by Equation 1, the pores of equivalent size or larger than the diameter that corresponds to the applied pressure, will be penetrated with mercury. Once the system has equilibrated, the new heights in the measuring tubes are recorded.
The difference in the initial and final heights of mercury in the measuring tubes enables the deduction of the volume of mercury that has penetrated the pores at the applied pressure and hence, the total volume of the pores open to channels of the given diameter.
Increasing the nitrogen applied pressure by small increments, repeating the process of pore penetration and taking measurements of the mercury volume that has filled the sample pores will give a pore size distribution.
After each incremental pore volume measurement is made, permeability and conductivity measurements should be carried out before a pressure increase is introduced, see discussion below.
This procedure may be repeated until there is no further mercury penetration.
Once the pore volume has been determined at a known pressure using porosimetry measurements as described above, the mercury permeability can then be measured.
A differential pressure must be applied across the sample to induce mercury flow. This can be achieved by closing valve V8 and opening valve V6 briefly to reduce the pressure slightly in measuring tube 3a, and then closing valve V6. The pressure in measuring tube 3a can be adjusted by opening valve V8 as required, and similarly valve V9 can be used to adjust the pressure in measuring tube 3b as required. The pressure differential must, however, be small compared to the total pressure of the system.
To begin flow measurements, the heights in measuring tubes 3a and 3b at the initial condition, and hence the difference in heights, Δho at time t=0 should be recorded.
The pressure in the nitrogen system should remain unchanged from the porosimetry measurements, and should not be notably affected by the small adjustments for the formation of the differential. The mercury will be forced through the sample in the direction of the pressure differential, to the low pressure side. Alternatively, the mercury heights can be reduced in measuring tubes 3a or 3b by opening then closing V14 or V15, respectively.
To measure the flow rate of mercury through the sample, the time taken for mercury to reach the final heights in the measuring tubes at time, t=t should be measured. The new mercury heights in tubes 3a and 3b and hence ΔHt should be recorded before V10 and V11 are turned to the off position.
This procedure can be repeated.
It is then possible to calculate the mercury permeability using equation (2) below when the difference in heights of mercury in the measuring tubes at t=0 and t=t and t are known. The rate at which mercury flows through the sample is characteristic of the permeability of the pore space containing mercury and is dependent on the applied pressure since it directly affects the pore space filled.
Referring to
Applying Darcy's Law:
where k is the permeability of the sample, Δh is the difference in height between tubes 3a and 3b, t is the time taken to reach the final heights, A1 is the cross sectional area of the manometer tubes A2 is the cross sectional area of the sample, μ is the viscosity of mercury, ρ is the density of mercury, L is the length of the sample, g is the acceleration due to gravity.
The volume of mercury in the sample can then be increased as the porosimetry technique is continued and both pore size and permeability can be measured at higher nitrogen pressures. Repetition of this technique over a range of nitrogen pressures can lead to the determination of a pore size and permeability distribution.
For gaseous diffusion of species A in species B in a porous medium, we may define a diffusivity ratio J which is the ratio of the diffusion coefficient measured for the porous medium (D) to the free gas diffusion coefficient DAB. If the medium is itself non-conducting and if the electrical conductivity of the medium saturated by mercury is σA, then the ratio σA/σT (where σT is the conductivity of the mercury itself) would be expected to be identical to the diffusivity ratio J, since the processes of conduction and diffusion are analogous. The value (σ) measured of the electrical conductivity in the apparatus for a medium not saturated with mercury will be a function of the applied pressure, the value eventually reaching σA at high pressure. Thus, the variation of σ with pressure can give information about the contributions of pores of various sizes to the diffusivity. This is analogous to the similar information given by the mercury permeability measurements and the combination of the two measurements provides valuable information about the pore structure.
The method of the present invention requires that once the porous sample has been coated, prepared and sealed in the sample cell, the apparatus and sample must be evacuated to a pressure in the range of 10 Pa. Mercury is then allowed to flood the mercury system of the apparatus, coming into contact with the sample faces and filling the measuring tubes. A first porosimetry measurement should be completed followed by the application of a small pressure gradient and then mercury flow measurements to determine the permeability that corresponds to the pore diameter deduced from the porosimetry. Measurements of the electrical resistance when an electrical alternating current is applied across the mercury in the sample should then be made. When completed, the nitrogen pressure should be increased by a small increment and the porosimetry, permeability and conductivity measurements repeated. The procedure should be carried out until the accessible pore volume has become fully saturated with mercury.
A first example of the method of the present invention showing how the pore size distribution, permeability and tortuosity of a sandstone sample are measured and calculated is given below. A list of the physical constants is given in table 1 below.
Equation (1) is used to calculate the pore diameter d that corresponds to the applied pressure. The cumulative pore volume is measured experimentally by noting at each variation of applied pressure, the change in height and hence volume of mercury in the sample.
The cumulative pore volume can be used to deduce that the sample of sandstone used in this example has a total pore volume of 3.93×10−7 m3 which, since the sample volume is known, represents a porosity of 12%.
At each incremental pressure applied the rate of flow of mercury through the sample was measured. Experimentally measured parameters were the height of mercury in the measuring tubes at time t=0, t=t and the time t taken for the change to occur. Equation (2) was used to determine the permeability k. The variation of the permeability with the applied pressure is illustrated graphically in
At low pressures there was no flow through the sample since the mercury had not penetrated any interconnecting pores. As the pressure was increased to around 250000 Pa, (1.53 μm pore diameter equivalent) the onset of mercury flow through the sample is evident. The permeability due to flow through pores of 1.25 μm diameter and above is approximately 0.3 mDarcy's (3×10−16 M2). As the pore diameter of saturated pores becomes smaller, the permeability of the porous system increases to 1.2 mDarcy's (1.2×10−15 m2). When the applied pressure reaches 320000 Pa (1.23 μm pore diameter) there is no longer a notable increase in the permeability. This indicates that pores smaller than 1.23 μm do not contribute to flow.
The electric resistance R across the sample was measured and the conductivity σ calculated from the equation:
σ=L/(AR)
where A is the cross sectional area of the sample and L is the sample length. At high pressures, σ approaches σA.
Using the apparatus and method of the present invention it is possible to measure and calculate properties of a porous material and in particular to understand the pore structure of the material and the relationship to flow and diffusion properties through the medium.
These examples further exemplify the present invention by reference to sandstone samples 1-4. The four samples have different porosities as set out in table 2 below.
The pore volume of Sample 2 was 5.25×10−7 m3. The permeability results shown in
Table 3 summarises the results obtained for the sandstone samples in this investigation.
The effective pore length of Sample 2 at low pore volumes is approximately 0.5 m and drops to 0.1242 m when the total pore volume is filled. The effective pore length of Sample 3 is 0.296 m. Electrical conductivity measurements indicate that the total effective pore length of Sample 4 is 3.56 m which is extremely high compared to the measurements taken of Samples 1 to 3.50% of the total volume of the sample is filled with pores of diameters less than 1.53 μm and the pores that are larger have a very low permeability (0.3 mD), the very large effective pore length would explain why the overall flow rate at low mercury volumes is low. As the volume of mercury in the sample is increased, more and more small pores are filled and they do not necessarily connect the large pores.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0716120.1 | Aug 2007 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2008/002754 | 8/14/2008 | WO | 00 | 2/17/2010 |
