METHOD FOR SLENDER TUBE, MULTI-LEVEL, SUBSURFACE BOREHOLE SAMPLING SYSTEM

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a less expensive and more versatile multi-level ground water characterization system using the advantages of flexible borehole liners and other techniques to perform water level and ground water sampling in subsurface boreholes.

2. Background Art

A “borehole” is a hole, e.g., a drilled shaft, into the Earth's subsurface. The hydraulic conductivity profiling techniques described in my U.S. Pat. Nos. 6,910,374 and 7,281,422 have been used in over 300 boreholes since 2007. These patents, whose complete teachings are hereby incorporated by reference, describe a hydraulic transmissivity profiling technique which carefully measures the eversion of a flexible borehole liner into an open stable borehole. Other installations of flexible liners into boreholes by the eversion of the liners are used in the techniques disclosed in a variety of other patents by this inventor as well. Such liners are usually installed into the open boreholes using a water level pressure inside the liner which is significantly higher than the water table in the formation penetrated by the borehole. The use of the continuous flexible liner has a sealing advantage and other advantages used in other inventions, including those of my U.S. Pat. Nos. 7,896,578 and 5,176,207, the entirety of which also is incorporated herein by reference.

However, the current multi-level sampling systems in use have several significant limitations. Particularly, for example, the number of sampling intervals in a single borehole is limited by the diameter of the tubing used, as more intervals require more sampling tubes to be disposed on/in the flexible liner. Reducing the sample tubing diameter to allow more sampling ports in the liner, however, prevents the use of conventional water level meters in the tubes to measure the water table in the formation. Current systems do not allow within the pumping system the continuous monitoring of the water level in the formation, due to the check valves used in the sample pumping procedure. Location of the recording pressure transducers below the currently used pumping system prevents easy repair or replacement when the transducers fail. This limitation is a subject of my U.S. Pat. No. 8,424,377, entitled “Monitoring the Water Tables in Multi-level Ground Water Sampling Systems,” whose entire disclosure is incorporated herein by reference. But the best use of that technique nevertheless requires additional multiple tubing to be added to the multi-level system, and it requires the ability to measure the water table elevation.

Within the foregoing as background, the presently disclosed system and method were developed.

SUMMARY OF THE INVENTION

The present disclosure improves upon known sampling systems utilizing everted flexible borehole liners by re-routing the sampling tubing and altering the valve geometry and function. Further, a measurement procedure is disclosed to allow a full suite of hydrologic data to be obtained with substantial reductions in the cost. An advantageous feature of the present system and method is the ability to deduce the ground water table level, in the subsurface formation, while deploying an arrangement of sampling tubing too small for normally used water level measurement devices. Additional features of the disclosed methodology enhance the complementary use of other procedures taught in the other patents mentioned herein above.

There is disclosed a method and apparatus to allow the use of much smaller diameter tubing in a multi-level water sampling system, without loss of the ability to obtain the full suite of hydrologic information available from more expensive systems currently in use. Compared with known designs, the presently disclosed method and apparatus reduces the weight and expense of the sampling system, and increases the spatial resolution of measurements, yet without reducing the types of data usually collected. An added benefit is that the present system allows a significant cost reduction in the complementary use of the other methods of subsurface borehole monitoring and testing previously developed disclosed in the various previously granted patents. This system employs a flexible liner to seal the entire borehole (as a means of promoting a better seal) and yet allowing sufficient space in the liner interior for disposition of apparatus therein.

BRIEF DESCRIPTION OF THE DRAWINGS

The attached drawings, which form part of this disclosure, are as follows:

FIG. 1 is an elevation sectional view of a flexible liner system according to the prior art in use for multi-level water sample collection in a single borehole, with hydraulic head measurements at each sample elevation;

FIG. 1A is diagrammatic presentation, in side view, of alternative modes and systems for deducing the water level elevations in slender tubes using measured pressure changes, according to the present disclosure;

FIG. 2 is a side sectional view of a system according to the present disclosure, disposed in a borehole for less expensive and higher spatial resolution measurements;

FIG. 3 is a side sectional view showing a system according to FIG. 2, but with a pressure source attached;

FIG. 4 is a side sectional view of a system according to the present disclosure, illustrating an example of a normally open check valve system, located in the flexible liner at the port, to allow the function desired;

FIG. 5 is an enlarged diagrammatic side view of a portion of the presently disclosed system and apparatus, as used for performing water sampling and head measurements; and

FIG. 6 is a schematic side view illustrating the geometry of the presently disclosed system as used for the mathematical development of a means for deducing the water level in the geologic formation around the borehole, employing a system similar to that of FIG. 3.

DESCRIPTION OF THE INVENTION,
INCLUDING THE BEST MODE FOR PRACTICING THE INVENTION

Currently employed multi-level subsurface ground water sampling systems using a flexible liner in a borehole have limitations of cost, weight, and the number of ports that can be installed in a typical borehole of three to eight inches in diameter. A mode for reducing the weight and cost of the system is to reduce the tubing diameter used in the water pumping system, e.g., from 0.625 inch to 0.25 inch outside diameter (OD). However, it is not possible in current practice to measure with such small tubing (0.25 inch OD) the depth to the ground water table. The present system and method advantageously permits the use of a slender sample tube. Herein, “slender tube” refers to a tube having an inside diameter (ID) of approximately 0.25 inch or less. The presently disclosed apparatus and method also use a mathematical model and a procedure to measure the depth of the water table below the surface. Furthermore, the same apparatus and method may advantageously be used to monitor the ground water table over time (i.e. to detect changes in ambient water table depth). Herein, the term “formation water” is sometimes used to refer to ground water occurring in the geologic formation surrounding a subsurface borehole. With this system there are advantages of shipping weight and size, and substantial reductions in shipping cost for the disclosed apparatus and system, in part due to the ability to employ lighter and less expensive slender tubing.

Reference is made first to FIG. 1, which by way of introduction shows a known system geometry used in current practice to collect water samples, and to measure the water table, at multiple elevations in a single borehole below the surface of the ground. The system is usually emplaced in a borehole by eversion of the flexible liner, as described in patents referenced hereinabove. The arrangement includes an everted flexible borehole liner 11 which whose interior is filled with water to a fill level 12 a distance above the ground water level 13 in the subsurface geologic formation. A permeable spacer 14 is attached to the outer surface of the everted liner 11 (adjacent the borehole wall), defining an interval from which a water sample is drawn from the formation. Ground water, i.e., formation water, to be sampled enters through the spacer 14, passes into the pumping system via a liner port 111 and through an intermediate tube 115, and then fills a large tube 15 to a first tube water level 16 corresponding to (substantially equal in depth from surface) the water level 13 in the formation. The first tube water level 16 can be determined and measured by lowing from the surface an ordinary water level meter to the water level 16 in the large tube 15. A water sample may be pumped to the surface by applying a gas pressure to the large tube top 17 of the large tube, which in turn closes a first check valve 18 at or near the bottom of the large tube 15, with the result that water in the large tube is forced through a second check valve 113 in a small tube 19. The sample water then is forced to the surface (above the ground), where it is collected in a sample container for analysis. In a multi-level sampling system in a single borehole, the single-port configuration according to FIG. 1 may be effectively duplicated for each of a plurality of different sampling ports (i.e., at different height elevations (depths) in the borehole) in the installed liner. The multiple tubing subsystems (i.e., the plurality of large tubes and small tubes (in communication with their respective check valves) are gathered into a bundle of tubes supported (e.g., by a tether or cable) from the wellhead 110 located at the top of the well casing.

There are modes for deducing a water level (table) in a slender tube without the need to lower physically a conventional water level meter to the water level in the tube, as is commonly done in an open well hole. It is known, for example, to deploy a bubbler monitor, which is a slender tube lowered down the well, with the bottom end of the tube disposed below the water level in the well (or below some other water surface to be determined). With a bubble monitor, a constant air flow rate (discharge) is applied to the top of such tube. A pressure transducer at the top of the tube measures the pressure required to expel air from the bottom of the tube. If the elevation of the bottom of the tube is known, the height difference between the water level and the submerged bottom end of the tube can be determined. This basic method has been used for decades or longer. In some circumstances of a very slender well or tube, however, the air flow out of the bottom of the tube may cause an air lift pumping effect which can confuse the measurement by expelling water out of the well (or otherwise develop flow viscous losses which are confusing to the measurement). This relatively cumbersome and sometime inaccurate method is sometimes called the bubbler method.

The presently disclosed method and system harnesses other, innovative approaches to making such determinations and measurements, and combines them into flexible liner underground technologies using everted liners in boreholes. Referring now to System A in the drawing of FIG. 1A, one method according to the present disclosure uses a vacuum applied to the top of a slender sample tube, which vacuum lifts the water level (in the sample tube) a distance ΔL above the level of the formation water table WT to a level above the surface; this permits a measurement of both the vacuum required and the height H of the water level above the surface. The subsurface water table is then calculated from the vacuum applied, in equivalent length of water column, ΔP, minus the height length H. But this technique works only if the bottom end of the sample tube used is supplied with water that can be drawn into the open end, as from a well, in a lake, or connected to an aquifer which can readily supply the volume of water drawn up into the sample tube. This method may be used, for example, in recently developed ACT (air coupled transducer) tubes such as described in U.S. Pat. No. 8,424,377. A limitation on the method is that the vacuum required to lift the formation water level to above the surface must be less than about three-fourths of an atmosphere (that is, less than about 25 feet).

Reference is made to System B of FIG. 1A, illustrating a second alternative method and system according to the present disclosure. This embodiment includes a port which allows formation ground water to flow into a manometer-style sampling system, but with a one-way check valve at a port in the everted flexible liner, instead of at the bottom of the tubing bundle as seen in FIG. 1. A slender sample tube is disposed as a single open loop in the borehole, with both tube ends above the ground surface GS. In this case, formation fluid may enter the sample tube via the check valve CV, but no back flow is permitted through the check valve CV into the formation. The formation water enters the slender sample tube from the port via the check valve CV, and rises in both “sides” of the sample slender tube to a first level corresponding to the level of the ambient water table WT. Thus when a gas pressure, ΔP, is applied at the sample tube second end bn, the water level in the second tube side (below tube second end bn) descends below the first water level (water table WT) a distance AL to second water level b; concurrently the water level in the first tube side (below tube first end s) rises an equal distance, ΔL, to second water level a. A transducer (seen in FIG. 1A as an unlabeled component on top of the tube first end s) preferably sealably closes tube first end s. The transducer measures the pressure in the slender tube between the tube first end s and the ground water in the slender tube. The transducer thus can detect and read the pressure increase, in the air volume within the first tube side above the water table and between second level a and the tube first end s, due to the water level rise in the tube from water table WT to second level a. In this configuration, the applied pressure is measured, and the pressure increase in the first tube side is measured at tube first end s. It is strongly preferred that the air volume in the first tube side be at ambient pressure before the pressure is applied to the sample tube second end bn.

The pressure relationship to the level changes is ΔPbn=2 ΔL+ΔPs, where ΔPbn is the pressure increase applied at tube second end bn and ΔPs is the pressure change (here, a pressure increase) within the first tube side measured with the transducer at the tube first end s. The depth to water table determination for this situation thus is:

WT depth=Vo/At=(Po ΔPbn/ΔPs+ΔPbn−Ps)/2

where Vo is the total volume of the gas initially between the tube first end and the first water table WT in the tube first side (before pressure is applied at the tube second end), At is the cross sectional area (using inside diameter) of the slender tube, Po is the initial gas pressure (assumed everywhere to be atmospheric pressure), and Ps is the final pressure measured in the sample tube first side (after pressure is applied at the tube second end).

The geometry of the alternative embodiment of System C in FIG. 1A is similar to that of System B, but with some notable differences. The check valve, NOCV, is normally open and closes only with a strong back flow out the liner port. By injecting a known number of moles of air, Δn, near the top of the first tube side, at tube first end s, the water level in the first tube side is depressed below the ambient water table WT by a distance ΔL. The injection of a known number of moles of air is readily accomplished with a syringe attached to the tube first side below the transducer at tube first end s. In this embodiment, the relationship between the level change and the pressure change measured is ΔPs=ΔL. The water table determination for this situation is:

WT depth=Po Vs/(ΔPs At)−Ps,

where Vs is the volume of the syringe and At is the cross sectional area (measured using the inside diameter) of the slender sampling tube. Ps is the final pressure in the first side of the slender tube (e.g., between the first end s of the tube and the water in the tube) after the gas injection.

A fourth alternative embodiment is disclosed as System D of FIG. 1A. It is similar to the embodiment of System C, but features the distinction that the check valve is not normally open, and therefore is closed against any flow into the surrounding geologic formation. In this case, the relationship of the pressure change at the tube first end s is: ΔPs=2ΔL. This is because any water level displacement of distance ΔL to second level a causes a corresponding water rise in the second tube side to second level b, with a net elevation change of (2·ΔL). The depth to water table for this situation is:

$\frac{(Po) (Vs)}{(Δ Ps) At} - \frac{Ps}{2}$

where Vs is the syringe volume and At is the sample tube cross section area.

It is noted that the configuration of System C of FIG. 1A allows the ACT system to continuously monitor the water level changes, if the changes are not abrupt causing the check valve NOCV to close. For the other configurations of the present system, the pressure, vacuum or air injection can be done with a single instrument box equipped with the proper valves, transducer, and syringe. An advantage is that any water table can be measured through a slender tube (i.e., less than about 0.25 inch ID) for any depth, without fear of entrapment of the level meter cable. In formation water tables greater than 200 feet, it can also be an advantage in a typical flexible liner underground water pumping system, such as seen in FIG. 1, where tag lines have be torn apart by the wet film adhesion during removal of the tube.

The method embodiments of Systems B, C, and D above can be executed in reverse. That is, the pressure application or gas injection can be a vacuum application or a gas extraction.

It is observed that the methods of the embodiments of Systems B, C, and D of FIG. 1A involve a step of changing a condition of a gas within the slender tube between the tube first end s and the ground water in the slender tube. This step can be accomplished various ways. It preferably may include applying a gas pressure to the tube second end bn, thereby causing an increase the gas pressure within the slender tube between the tube first end s and the ground water in the slender tube. Alternatively, the step changing a condition of a gas within the slender tube between the tube first end s and the ground water in the slender tube may be the step of injecting a known volume (i.e. a known number of moles of gas (e.g., air) at a known temperature) of gas, Δn, near the top of the first tube side, at or near tube first end s.

Also, in the embodiments of Systems B, C, and D of FIG. 1A, WT=Vo/At. Because Vo is assumed to be the total volume of the gas above the water table in the first tube side below its first end s, these water table determinations preferably should be corrected by subtracting the volume, Vabs, in the system above the ground surface GS divided by At, or WT−Vabs/At, for calculating the actual water table depth below the ground surface. Po is the initial pressure and it is assumed that that is the atmospheric pressure everywhere including in the gas volumes in the tubes.

The air injection into the system can be relatively precise. The entire calculation of the water table depth requires that the total volume of sample tubing above the ground surface be known, and the diameter of the tube below the surface be substantially constant. The basic System C of FIG. 1A is the more reliable system, in that it does not require an instant closure of the check valve at the port. For the “normally closed” valve configuration, the instant closure is less a problem.

Attention now is invited to FIG. 2, which shows in further detail a preferred geometry of the one embodiment of the apparatus and system according to the present disclosure. In this embodiment, the general tubing system of FIG. 1 is replaced by a U-shaped open loop of slender tube that has a first tube first portion 23 extending from the ground's surface to a check valve 27 at a port in the liner, and a tube second portion 29 extending from the check valve 27, to the bottom of the liner (normally, but not necessarily, at the bottom of the borehole), and then returning up to the surface at a tube second end 26. The first side of the slender tube's “U” shape is its segment descending from the first end 23 to the bottom of the everted liner; the second tube side ascends from the bottom of the “U” at the bottom of the liner up to the tube second end 26. The second portion 29 of the tube, extending down from the second end 26 to the bottom of the liner, is supported on a strong cord called a tether 24. A tube segment, including at least the tube first portion 23, running from the bottom of the liner to above the surface, is contained in a sleeve 22 of flexible fabric welded to the interior surface of the everted liner 25.

At a sampling location, a port conducts formation water from an external permeable spacer 28 on or annularly surrounding the liner 25, through the liner via the port in the liner, then through the check valve 27 and on into the slender sample tube. The water in the geologic formation flows to an equilibrium level in both portions 23, 29 of the slender sample tube. The slender tube is too small in diameter to permit access of a water level meter normally lowered into the tube to the water level in the tube. In the geometry of the present system, the water can be drawn to the surface through the tube-in-the-sleeve, first tube portion 23, extending from the port (at check valve 27) to the surface (as seen in System A of FIG. 1A). FIG. 2 shows only a single tube and port configuration; in preferred practice, a plurality of such tube and port combinations are connected to be in fluid communication with a corresponding plurality of respective ports disposed at different elevations in the borehole, thereby to draw water samples from many different elevations.

FIG. 3 depicts further a configuration for an embodiment generally according to that of FIG. 2, including a tube system from tube first end 33 to tube second end 32, with the addition of a gas pressure source 31 in communication with the tube second end 32 of the slender sample tube 36, 39 via a pressure transmission tube 34. The gas pressure source 31 and transmission tube 34 supply the gas for driving the water in the tube portion 39 out of the tube at the tube first end 33. By controlled application of a gas pressure to the second end 32 of the slender tube, the water in the U-shaped loop of tubing (which is at least partially disposed in an interior sleeve on the inside of the liner) is displaced toward (and potentially out) the tube first end 33. However, for such displacement or discharge to occur, there must be a check valve 37 at the port in the everted liner, adjacent the permeable spacer 38. A possible configuration and arrangement for such a check valve is shown in FIG. 5.

Applying gas pressure at the tube second end 32 of the second tube side closes the port check valve 37, which allows the water fill within the tube-in-the-sleeve to be forced to the surface through the tube first portion of the second tube side. It is useful that the careful measurement of the volume of water expelled can be used to determine the volume of the tube filled. Knowing the dimensions of the U shaped tube, on can then determine the level to which h the tube was filled and therefore the water level in the tube which is also the water level in the formation. When the gas pressure at source 31 is reduced to the ambient atmospheric pressure, the check valve 37 opens, and the U-shaped tubing system is refilled by flow from the surrounding formation and through the port. The exterior spacer 38 defines the interval of the borehole from which the formation water is drawn.

A significant difference between the embodiment of FIG. 2 and the geometry seen in FIG. 1 is that the check valve 37 in the system is located at the liner port in FIG. 2, instead of at the bottom of the large tube 15 seen in FIG. 1. In the practice of the embodiment of FIG. 3, a water sample is easily collected from the tube-in-the-sleeve at its first end 33.

While the foregoing descriptions allow a relatively easy pumping method for obtaining a water sample from the sampling interval at the spacer 38, a user of the system is still unable to obtain a measurement of the depth to water table in the formation itself, excepting the method using the water volume expelled. Hereafter is described a means and method for determining the water level in the formation.

Attention is invited to FIG. 4, showing the disposition of a pressure transducer 41 attached to the first end 42 of the first tube side (the first tube side situated in the liner sleeve), after the U-shaped open tube loop 49 has refilled with water. A valve 43 at the first end 42 of the sleeved tube first portion allows water to flow from the tube first portion during purging and sampling. The transducer 41 seals the top of the sleeved first tube side and traps a volume of air, in the sleeved tube first portion between the water level in the tube first portion and the transducer 41. With the valve 43 closed, the controlled application of a small pressure from the pressure source 46 to the second end 45 of the slender tube depresses downward the water level in the second tube side, and raises the water level in the first tube side above the water table elevation 44.

The magnitude of the applied pressure is measured by a gauge 410 in the pressure transmission tube. The resulting water flow in the U-shaped tube 49 closes the check valve at the port 48, and causes the water level 44 in the volume Vo in the first tube side beneath the transducer 41 to rise by the same amount as the downward displacement in the second tube side below its second end 45. Were the transducer 41 not sealing the top of the volume Vo, in the first tube side, the water displacement would be half the pressure applied (as measured at gauge 410) in units of water head. However, the water level rise in the volume Vo in the first tube side is less than half the applied pressure as measured at the gauge 410, due to the pressure rise in V_Ocaused by the compression of the gas in due to reduction of the volume Vo. The pressure rise in the first side can be used as described in FIG. 1A system B to deduce the original volume, Vo, and therefore the original water level at the bottom of Vo.

FIG. 5 depicts in an enlarged view a suitable check valve (e.g. valve 37 in FIG. 3), for installation at the liner port, in accordance with the system as described in FIG. 1A, System C. FIG. 5 shows one preferred embodiment for the liner port check valve; there are other, commercially available “normally open” type check valves that are suited for use according to the systems and methods disclosed hereinabove. The liner port 52 is a sealed aperture defined through the liner 51 at a location to correspond, after liner installation, to the preselected sampling elevation in the borehole. The valve body 53 is located below the port 52, so that a heavy check ball 55, with density greater than 1.0 gm/cc, rests by gravity on the bottom of the hollow valve body's interior space. In such configuration, formation water at the permeable spacer outside the liner flows through the port 52 and through the valve body 53, past the ball support 56 at the bottom of the inside of the valve body 53, and into the valve tube 54. The flow of formation water continues from the valve tube 54 to the slender sample tube 57. At least a portion of the length of the slender sample tube 57 preferably is snugly enclosed in an open-ended sleeve 59 defined or disposed on the inside surface of the everted liner 51. The tube 57 seen in FIG. 5 normally is the tube first portion, of the first tube side, that rises to the surface from the check valve (e.g., valve 37 in FIG. 3, as would be adjacent to the port 48 of FIG. 4).

When a pressure is applied (e.g., from source 46 in FIG. 4) to the second portion of the tube 57 below the valve body 53, the resulting fluid flow toward the port 52 moves the ball 55 (against the force of gravity) upward in the valve body to seal the valve seat 58 at the top of the valve body. After that valve closure, the water in the tube 57 below the valve 53 rises upward in the tube 57, above the port 52 toward the surface. When the pressure in the tube 57 is dropped to ambient pressure, the ball 55 falls within the valve body interior, and the tube 57 refills with formation water entering from the port 52. A low flow rate from the tube 57 may not be sufficient to lift the ball 55 to cause closure of the valve. If this same valve is relocated to a position above the port 52, and inverted with the ball resting by gravity on the valve seat 58, it functions as a “normally closed” check valve, which would allow water to flow from the formation into the tube-in-the-sleeve 57, but remain closed to any flow back into the formation. The common “duck bill valve” is an example of another normally closed valve that may be beneficially used at a liner port 52.

FIG. 6 illustrates in further detail a configuration and function of an embodiment of the presently disclosed system, near the top of a borehole, and depicts the volume Vo, labeled 61, beneath the transducer 62. Controllably applying a gas pressure from the source P to the second end 64 of the U-configured slender sample tube 69 causes the initial or first water level 65 in the second tube side slender to be depressed to a second level 66. The water level change in the second side of the slender tube 69 from the first level 65 to the second level 66 (as indicated by the down directional arrow in FIG. 6) is substantially the same as the water level change in the volume Vo 61 in the first side of the tube, i.e., from a volume first level 67 to the volume second level 68 in the tube as suggested by the upward directional arrow in FIG. 6. The gas pressure increase in Vo (61) due to the rise in the water level from volume initial first level 67 to the new, second, equilibrium level 68 is recorded by the transducer 62.

The resulting increase in gas pressure in the volume 61 is well defined by the ideal gas law. Therefore, the pressure in the volume 61 ises from P_Oto P due to the rise of the water level, and the volume between the water level and the transducer seal decreases by an amount Δwhich equals Vo−V), where V is the second, compressed, volume of gas. Writing the initial condition and the final condition for the perfect gas law:

Po Vo=n R To

where n is the number of moles of gas in the volume Vo, and R is the universal gas constant. PV=n R To for the smaller volume after the water level rise. Then (Po)·(Vo)=PV, where V can also be written as Vo−ΔV. Then PoVo=(Po+ΔPt)(Vo−ΔV), where ΔPa is defined as the pressure increase applied to the second end of the slender tube, Po is the initial pressure measured in the volume 61 of the first side of the tube, ΔPt is defined as the pressure change (in units of length of a water column) measured by the transducer on the first end of tube-in-the-sleeve, and At is defined as the diametric cross sectional area of the tube-in-the-sleeve, then:

ΔPa=2 ΔL+66 Pt,

where ΔL is the water level change in each side of the tube 69. This level change is that shown in FIG. 6 from first level 65 down to second level 66, and from initial volume level 67 up to volume equilibrium level 68.

By solving for Vo and using the form for ΔPa, one can show that:

Vo/At=(Po/ΔPt+1)(ΔPa−ΔPt)/2 ,

where At is the cross sectional area of the tube 61.

Vo is the original volume of gas trapped above the water table in the sleeved segment of the first tube side. Therefore, Vo/At is the axial “length” of the gas volume or the depth below the transducer 62 to the water table. It is therefore possible to determine the depth to the water table in the formation, which is the same as the depth to the water level in the first side of the tube-in-the-sleeve, from the measurement of the pressure change in the gas caused by the application of a known pressure change to the top of the small tube from the source P. Since the measurement can be done in a short time, the effects of temperature changes are not significant.

This sampling and head measurement geometry can be duplicated for many different port elevations in the same borehole. The foregoing slender tube geometry and methodology allows the total number of useful sampling elevations to be greatly increased in a single given borehole of typical diameter (3-8 inches), as the use of a plurality of slender tubes (which cannot receive a conventional meter within their interior) does not unacceptably encumber the everting liner system.

Methods according to the forgoing descriptions of the system are evident to one skilled in the art, but shall be summarized. There is provided a method for determining groundwater condition (such as but not limited to depth to ground water table) in a borehole beneath the earth's surface, including at least some of the steps of: defining a sleeve on a flexible liner; disposing in the sleeve at least a portion of a slender tube thereby to hold upon the liner at least the portion of the slender tube, the slender tube having a tube first end and a tube second end; defining a port in the liner; placing the slender tube in fluid communication with the port; everting the flexible liner into a borehole below the surface; situating the tube first end and the tube second end above a ground water table in the borehole; allowing ground water to flow from the port through the check valve into the slender tube; permitting the ground water to rise in the slender tube to a first level corresponding to the ground water table; closing the tube first end; changing a condition of a gas within the slender tube between the tube first end and the ground water in the slender tube to affect a change of a ground water level in the slender tube from the first water level to a second water level; and determining, from the condition of the gas or from the change of the ground water level in the slender tube, the depth of the ground water table.

The step of placing the slender tube in fluid communication with the port preferably includes the steps locating a check valve adjacent to and in fluid communication with the port, and placing the slender tube in fluid communication with the check valve. The step of situating the tube first end and the tube second end above a ground water table in the borehole preferably further includes the step of situating the tube first end and the tube second end above the surface.

A pressure transducer preferably is provided near the tube first end, and the step of closing the tube first end preferably entails sealably disposing the pressure transducer on the tube first end to seal airtight closed the tube first end.

The step of changing a condition of a gas within the slender tube between the tube first end and the ground water in the slender tube can be accomplished various ways. It preferably may include applying a gas pressure to the tube second end, thereby causing an increase the gas pressure within the slender tube between the tube first end and the ground water in the slender tube. In such an embodiment, the step of determining the depth of the ground water table preferably includes measuring the gas pressure applied to the tube second end, and measuring with the transducer the increase in the gas pressure within the slender tube between the tube first end and the ground water in the slender tube. The step of determining the depth of the ground water table further thus may include calculating the depth of the ground water table using the formula:

WT=(Po ΔPbn/ΔPs+ΔPbn−Ps)/2,

wherein WT is the depth of the ground water table, Po is an initial gas pressure, ΔPbn is the pressure increase applied at the tube second end, Ps is the final pressure measured within the slender tube between the tube first end and the ground water in the slender tube tube (after the pressure change ΔPbn), and ΔPs is the gas pressure increase measured with the transducer.

In an alternative embedment, the check valve is a normally open check valve, and the step of changing a condition of a gas within the slender tube between the tube first end and the ground water in the slender tube includes injecting with a syringe a known number of moles of gas into the slender tube between the tube first end and the ground water in the slender tube. The step of determining the depth of the ground water table then includes the step of measuring with the transducer a change in the gas pressure within the slender tube between the tube first end and the ground water in the slender tube. In this embodiment, the step of determining the depth of the ground water table further includes calculating mathematically the depth of the ground water table using the formula:

WT=Po Vs/(ΔPs At)−Ps,

wherein WT is the depth of the ground water table, Po is an initial gas pressure, Vs is the volume of the syringe, ΔPs is the gas pressure increase measured with the transducer, At is the cross sectional area of the slender tube, and Ps is the final pressure measured within the slender tube between the tube first end and the ground water in the slender tube.

In yet another alternative embodiment, the check valve is a normally closed check valve, and the step of changing a condition of a gas within the slender tube between the tube first end and the ground water in the slender tube includes injecting with a syringe a known number of moles of gas into the slender tube between the tube first end and the ground water in the slender tube. In this alternative methodology, the step of determining the depth of the ground water table preferably includes measuring with the transducer a change in the gas pressure within the slender tube between the tube first end and the ground water in the slender tube. Accordingly, the method step of determining the depth of the ground water table further features calculating mathematically the depth of the ground water table using the formula:

WT=Po Vs/(ΔPs At)−Ps/2,

An additional advantage of the disclosed system and method is that the method of U.S. Pat. No. 8,424,377 can be used to monitor the history of water level changes at each port, without the otherwise costly addition of the tube-in-the-sleeve for each port used in the design. Both the transducer used and the tube-in-the-sleeve are important, innovative, components of the present system, although the transducer need not be dedicated to the use of this system.

The overall advantages of this system and method support the innovative character of the system. This application is not an attempt to patent the spacer design, or the liner concept, which have been in prior use. Further, the installation procedure for the present system is generally in accordance with those disclosed in this applicant's previously issued patents. However, the unique features including a normally open valve at the port of sufficiently small dimensions that it can be emplaced in a borehole in the normal everting manner, plus the ability to deduce the water table from the pressurization procedure, offer significant improvements over known systems. The water table measurement is a central feature of the procedure of U.S. Pat. No. 8,424,377, and must remain possible for the use of that method and system.

Other acceptable methods for measurement of the water level in a slender tube system of this design are useful as described hereafter. A variation of the water table measurement in the slender tube can be performed for shallow water tables less than ˜25 ft below the surface. In such a case, one can apply an increasing vacuum to the first end of the tube-in-the-sleeve until the water level in the tube is viewed in the tube above the ground surface. The magnitude of the vacuum measured by the same pressure transducer, with the water level in the tube measured from the ground surface, is sufficient to determine the water table in the tube before the vacuum is applied. This is a somewhat more direct measurement, but not possible for deeper water tables for which a typically applied vacuum cannot lift the water to above the surface.

A variation of the water level measurement is to inject a small measured amount (moles) of gas into the volume Vo 61 shown in FIG. 6. This injection adds to the number of moles, n, in the ideal gas equation above to cause a rise in the pressure in the volume Vo. Such a measured injection, Δn, is possible with a small syringe of known volume. The check valve at the liner port (e.g., FIG. 5, valve body 53) can be a normally closed valve for this procedure. The pressure change caused by the injection is recorded and used to calculate the volume Vo which, in the same manner as described, defines the distance to the water table at the bottom of the volume Vo. In this case, ΔPt=2 ΔL, where ΔL is a downward displacement. This relationship is used to solve for Vo/At. This method is new and very useful to the utility of the presently disclosed system and method.

A third variation of the water level measurement is the same as the above-described second variation, except that the valve at the port (e.g., FIG. 5., check valve 53) is a normally open check valve which allows the water displaced by the injection of a small volume of gas to flow into the formation. However, the gas injection should be done slowly in order to not close the check valve. In that situation, the correct relationship is ΔPt=ΔL in the calculation of the volume change. This method is new and very useful to the utility of the present system and method. For convenience, the pressure transducer, digital pressure display and syringe can all be included in an instrument case with a quick connect fitting to the tube-in-the-sleeve to provide a portable means of measuring the water table depth at numerous ports without a dedicated transducer on the tube-in-the-sleeve. A recording transducer is also convenient.

This design has significant advantages over the multi-level measurement system currently in use by the inventor. The use of slender tubing has the advantages of reduced cost and the ability to use many more measurement intervals in a single hole. The ability to measure the water table depth in the slender system using the several methods devised makes this design only slightly less convenient than when larger tubing with a water level meter is used in the current design. The ease of connection to a surface transducer is a major advantage when a normally open check valve is used at the port which allows the continuous monitoring of the water level as described in U.S. Pat. No. 8,424,377. Additional advantages are the weight reduction of the system and the ability to ship the more flexible slender tubing system on smaller diameter reels to foreign countries at reduced cost. A further advantage of this system is that it can be constructed in a manner to greatly reduce the labor of construction.

Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. Unless specifically indicated otherwise herein, the steps of a method need not necessarily be executed in the order recited within a claim. The entire disclosures of all patents cited above are hereby incorporated by reference.

METHOD FOR SLENDER TUBE, MULTI-LEVEL, SUBSURFACE BOREHOLE SAMPLING SYSTEM

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