Abstract:
A method and an apparatus raise samples of liquid such as water from a low level to a higher level, for example for obtaining a sample of groundwater from a borehole. The apparatus comprises a pair of tubes ( 14  and  16 ) extending alongside each other, which are introduced into the borehole, linked by a connector ( 20 ) at their lower end, a valve ( 22 ) communicating between the inside and outside of one of the tubes ( 16 ) near its lower end, and a pig ( 46 ) insertable into the tube ( 16 ). By adjusting the pressure in each tube at their upper end, the pig ( 46 ) can be moved from one end to the other of the apparatus ( 10 ) and used to transfer samples of water.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a method and an apparatus for raising a sample of liquid such as water from a low level to a higher level, for example for obtaining a sample of groundwater from a borehole. 
   A variety of methods are known for obtaining samples of liquids from boreholes. For example in the oil industry coiled tubing units are commonly used to transmit sample fluids from a particular zone in the borehole to the surface. Alternative techniques use a tool suspended on a wireline. Both these approaches are mentioned in U.S. Pat. No. 5,289,875 (Stokley et al). Such tools may incorporate packers to restrict the section of the borehole from which the fluid is obtained. However, such tools are complex, and it would be desirable to provide a simpler way of obtaining samples, which would be applicable over a wide range of different depths. The samples may be withdrawn for analysis, or alternatively the process may be repeated many times to empty a section of the borehole, that is to say using the sampler as a pump. 
   BRIEF SUMMARY OF THE INVENTION 
   According to the present invention there is provided an apparatus for raising a sample of liquid from a lower level to a higher level, the apparatus comprising a pair of tubes extending alongside each other, for extending from the higher level to the lower level, linked by a connector at their lower end, a valve communicating between the inside and outside of one of the tubes near its lower end, a pig insertable into one of the tubes at the upper end, and means at the upper end to adjust the pressure in each tube. 
   The pig may be of spherical or generally cylindrical shape, so it seals to the tube it is in. It may for example be of dense plastic foam with end faces of polytetrafluoroethylene (PTFE); such pigs are known per se, and are used to clean out tubes or pipes. An alternative pig comprises a polyurethane or steel rod linking flexible polyurethane discs. 
   The present invention also provides a method for raising a sample of liquid from a lower level to a higher level, using an apparatus comprising a pair of tubes extending alongside each other, linked by a connector at one end, a valve communicating between the inside and outside of one of the tubes near the one end, a pig insertable into one of the tubes at the other end, and means at the said other end to adjust the pressure in each tube, the method comprising the steps of arranging the tubes so that the connector is at the lower level, inserting a pig into one of the tubes, adjusting the pressures so the pig moves to the end of the tubes adjacent to the connector, causing liquid to enter the tube through the valve, and then adjusting the pressures so the pig pushes the liquid that has entered the tube to the other end of the tube. 
   Thus in use, merely by adjusting the pressures at the upper ends of the tubes, the pig can be moved from one end of the tube to the other. The valve may be a simple check valve or non-return valve, so with the pig at the lower end of the tubes, application of reduced pressure opens the valve so that liquid enters the tube from the surroundings. Applying a pressure difference between the upper ends of the tubes can then push the pig along with the liquid that has entered the tube to the upper end of that tube. The tubes may be of considerable length, for example 900 m (3000 feet), but the pressure needed to raise the sample of water does not need to be large; it is effectively independent of the height difference between the top and bottom of the tubes. 
   In one embodiment the connector is a U-shaped turn block, so that the pig may be sent down one tube and returned up the other tube. Operation of this embodiment has the disadvantage that it is then necessary to transfer the pig from one tube to the other at the upper end (or use a new pig) if operation is to be repeated. In a preferred embodiment the pig remains in one tube throughout the operations, moving up and down that tube according to the changes in pressure. In this case the two tubes may be of different diameters. By repeating the pressure changes in a cyclic fashion liquid is effectively pumped from the lower level to the higher level, and so the apparatus may be used to empty a container. 
   The adjustments in pressure may be achieved using a supply of high-pressure gas, such as compressed air, combined with a jet pump for obtaining a reduced pressure. Alternatively, the adjustments in pressure may be achieved using pumps and compressors. The tubes are preferably flexible, and may therefore be stored coiled onto a reel. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which: 
       FIG. 1  shows the apparatus arranged to remove samples of water from a borehole; 
       FIGS. 2   a –   2   d  show diagrammatically successive steps in the operation of the apparatus of  FIG. 1 ,  FIG. 2   a  showing the step in which the pig is being driven down to the bottom;  FIG. 2   b  showing the step in which a sample of water is brought into the tube;  FIG. 2   c  showing the step in which the pig and water are brought up; and  FIG. 2   d  showing the step in which the sample of water is removed; and 
       FIG. 3  shows a longitudinal sectional view of the pig of the apparatus of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , the apparatus  10  is shown for extracting samples of water  11  from a borehole  12 . The apparatus  10  comprises two flexible tubes  14  and  16  which are supported at ground level coiled onto a reel  18 , and which are introduced into the borehole  12  so as to extend down to below the surface of the water  11 . By way of example, the borehole  12  may be many hundreds of meters deep, and the water level may be hundreds of meters below the surface. At their lower end, the tubes  14  and  16  communicate through a narrow-bore steel connector  20 . One of the tubes,  16 , is of wider internal diameter, and near the lower end of the tube  16  is a non-return valve  22  which would enable water  11  to enter the tube  16 , but prevents fluids from leaving the tube  16 . Pipes  24  and  26  emerge from the centre of the drum  18  at opposite ends, these pipes  24  and  26  communicating respectively with the flexible tubes  14  and  16 , and are connected through valves to a device to control the pressure in each (not shown in  FIG. 1 ). 
   Referring now to  FIG. 2   a , in which the apparatus  10  is shown diagrammatically, the pipe  24  communicates via a disentrainment pot  28  with a three-way valve  30 , while the pipe  26  communicates with a three-way valve  32 . The valve  30  enables the pipe  24  to communicate either with a pressure control line  34  or with an adjustable outlet restrictor  36 ; while the valve  32  enables the pipe  16  to communicate either with the pressure control line  34  or with a sample station  38 . The pressure control line  34  communicates with a jet pump  40  whose inlet is connected to a source of compressed air  42  and whose outlet is controlled by a valve  44 . Hence if the valve  44  is open, compressed air flows through the jet pump  40 , so the pressure in the control line  34  is reduced, while if the valve  44  is closed the compressed air flows into the pressure control line  34 . The tube  16  contains a cylindrical pig  46  (shown to a larger scale in  FIG. 3 ) of dense polyurethane plastic foam with PTFE end plates  47  which seal to the wall of the tube  16 . 
   In the step shown in  FIG. 2   a,  the valve  30  communicates with the outlet restrictor  36 , the valve  32  communicates with the pressure control line  34 , while the outlet valve  44  is closed. Consequently compressed air from the source  42  passes into the top end of the tube  16 , and pushes the pig  46  down to the bottom end of the tube  16 , where it comes to rest against the steel connector  20 . Air displaced from the tube  16  passes up the tube  14  to emerge through the restrictor  36 . 
   Referring now to  FIG. 2   b,  in this step the valves  30  and  32  both communicate with the pressure control line  34 , while the outlet valve  44  is open. The air flowing through the jet pump  40  considerably lowers the pressure in the pressure control line  34  and hence that in both the tubes  14  and  16 . Consequently the pressure at the bottom of the tube  16  is lower than that of the water  11  at that depth, so water  11  from the borehole  12  enters the tube  16  through the non-return valve  22 . The non-return valve  22  may be kept open in this way for sufficient time that the water level within the tube  16  becomes the same as or higher than that in the borehole  12 ; or alternatively the non-return valve  22  may be kept open for only a short period of time, so that only a small amount of water enters the tube  16 . The water that enters the tube  16  is above the pig  46 . 
   Referring now to  FIG. 2   c,  in this step the valve  30  communicates with the pressure control line  34 , the valve  32  communicates with the sample station  38 , and the outlet valve  44  is closed. Consequently the compressed air flows down the tube  14  and pushes the pig  46  and the water that has entered the tube  16  up the tube  16 . When the pig  46  reaches the top of the tube  16  the water therefore flows into the sample station  38 . 
   Referring now to  FIG. 2   d,  the water sample is shown in the sample station  38  and the pig  46  is shown at the top of the tube  16 . In this step both the valves  30  and  32  communicate with the pressure control line  34 , while the outlet valve  44  is closed. The pressure is therefore high in both the tubes  14  and  16 , and no gas flow takes place. By opening a valve  39  at the bottom of the sample station  38  the sample can be removed. 
   By repeating these steps, repeated samples of water are removed from the borehole  12 . The sequence of operating the valves is as follows, starting at the step shown in  FIG. 2   a.  When sufficient time has elapsed for the pig  46  to have reached the bottom of the tube  16 , the valve  30  is changed to communicate with the pressure control line  34  (so the pressures are equal in both tubes  14  and  16 ) and then the outlet valve  44  is opened (as shown in  FIG. 2   b ). When sufficient time has elapsed for water to enter the tube  16 , the outlet valve  44  is closed and the valve  32  changed to communicate with the sample station  38  (as shown in  FIG. 2   c;  the sequence of these valve changes is not critical). And then, when the pig  46  reaches the top of the tube  16  and the water has entered the sample station  38 , the valve  32  is changed to communicate with the pressure control line  34 . Finally, valve  30  is changed to communicate with the restrictor  36 , so that the pig  46  is again pushed down the tube  16  (as shown in  FIG. 2   a ). 
   It will be appreciated that the apparatus  10  may be modified in various ways while remaining within the scope of the present invention. For example the tubes  14  and  16  might instead be of equal diameter. In this case they may be linked by a U-tube of the same internal diameter, so that the pig  46  could go down one tube  14  and up the other, although this has the disadvantage that obtaining repeated samples of water would require the pig  46  to be transferred between the tubes at the top end. The tubes  14  and  16  may be completely removed from the reel  18  before operation, if they are of an appropriate length. Each of the three-way valves  30  and  32  may be replaced by a T-junction and a pair of two-way valves. 
   In some situations it is desirable to avoid lowering the pressure in the vicinity of the liquid, in order to avoid release of volatile organic compounds. In an alternative operating method, which avoids the need to lower the pressure, water is admitted into the tube  16  from the borehole  12  by the lowering the tubes  14  and  16  further into the borehole  12  and so further below the surface of the water  11  until the valve  22  is subjected to sufficient water pressure that it opens. Indeed, if the depth of submergence in the liquid  11  is greater than the desired length of liquid sample to be introduced into the pipe  16 , there is no need to generate a reduced pressure (so the jet pump  40 ,  44  can be omitted), and it may well be appropriate to have an elevated pressure in the tube  16  throughout the operation cycle. In another alternative the non-return valve  22  is replaced by an actuated valve, and this may be actuated by pneumatic, electrical, or mechanical means; such an actuated valve also avoids the need to lower the pressure. As described above, the source of the pressure is the cylinder of compressed air  42 , but it will be understood that other compressed gases such as nitrogen may be used instead; and indeed the raised and decreased pressures may alternatively be generated by devices such as compressors or vacuum pumps. 
   It will also be appreciated that the pig may differ from that described above, and for example may consist of flexible plastic disks linked by a rod. Furthermore the tubes  14  and  16  may be rigid pipes rather than flexible tubes, although flexible tubes are much more convenient to install where samples are to be obtained from a depth of more than a few meters. 
   The apparatus  10  may be used in a range of different contexts. In one example it may be used in measurements to assess if radioactive material is migrating in groundwater from a radioactive waste disposal site, by monitoring for the presence of any radioactive materials in the water in boreholes or wells around the site. Where it is necessary to first empty the borehole, this can be achieved by repeating the steps described above repeatedly until sufficient water has been removed.