Environmental Application of Flexible Liner Underground Technology (FLUTe)
FLUTe methods include a wide variety of flexible liner designs to perform functions. FLUTe provide a continuous seal of a borehole or pipe, and prevent migration of formation fluids through the open hole. No sealing grouts or bentonite seals are needed. FLUTe also could carry many useful devices such as tubing, instruments, absorbers, reactive covers, etc.. into place in the borehole while maintaining a continuous seal of the borehole. Borehole transmissivity can be mapped out while displacing the borehole water. The liner also support the borehole wall against collapse. Flexible liner is able to propagate through tortuous passages of varying diameters inaccessible to rigid piping or push rods, towing geophysical sondes of many kinds along the interior of the borehole while supporting the borehole against collapse onto the sonde. FLUTe liner can be removed by inversion without the liner touching any other portion of the borehole wall. There are liners fabricated in many different diameters and materials for many applications.
The most common application of FLUTe technologies would include the following:
The water flows from the formation into the spacer, through the port, into the tube which lies on the inside surface of the liner. The water flows from the port via the tube, to the bottom of the hole, and then upward through a Teflon ball check valve into the “U” shaped tube. The water rises in both legs of the U tube. In the larger (1/2″id) tube, the water level can be tagged from the surface. A gas pressure is applied to the large tube to drive the sample water through the second check valve to the surface. After purging the tubing, the sample water does not contact the drive gas. The large tube and pumping hardware are not everted into the borehole, but simply lowered as a tubing bundle following the liner to the bottom. Note that all the water in the borehole is isolated inside the liner.
The spacer, port, tubing and pump system shown are duplicated for each port. The liner is pressed against the borehole wall by the excess head in the liner above the local water table.
Pressure transducers are often attached to the sampling tubing just below the first check valve to measure the head in the formation at the port location. The transducer is upstream of the valves in the pumping system. The pressure transducer can be calibrated to the head measured in the sampling system with a tag line.
The NAPL FLUTe is a covering for a normal blank flexible liner. The liner is “blank” in that there are no attachments to the liner. The blank liner with a NAPL FLUTe cover is normally everted into a borehole to detect the presence of NAPL. There are many applications of the blank liner as described at blank liners. The NAPL FLUTe covering of a blank liner is everted into the borehole on the outside of an ordinary blank liner. The NAPL cover is pressed against the borehole wall by the interior liner pressure. The covering is hydrophobic and quickly wicks any NAPL contacted in the fractures or pore space into the cover. The cover is dye striped on the exterior of the cover. Contact with NAPLs such as TCE or PCE dissolves the dye stripes and carriers the dye to the interior surface of the cover. The cover material is white and the displacement of the dye to the interior surface produces a strong highly visible stain on the interior surface. That stain is the indication that the cover has contacted a NAPL. The size and location of the stain are indicative of the amount of NAPL present and the nature of the source.
FLUTe Activated Carbon Technique
The FACT (FLUTe Activated Carbon Technique) is a method developed by FLUTe for mapping the distribution of contamination in the pore space and fractures of a borehole wall. The technique incorporates a 0.125 x 1.5 inch strip of activated carbon felt into the typical hydrophobic cover of the NAPL FLUTe system normally used for mapping the subsurface presence of a wide variety of NAPLs. The NAPL FLUTe cover is typically installed into a borehole on the outside of an everting FLUTe blank liner. The installation of a NAPL FLUTe cover with the added activated carbon strip allows one to draw, by diffusion, the dissolved contaminants from the formation into the activated carbon. Recovery of the liner by inversion prevents the carbon from contact with any other portion of the borehole wall. At the surface, the carbon is then sectioned for chemical analysis. With the combination of the NAPL cover and the FACT, one can map both the NAPL and the dissolved phase of many other contaminants.
As the everting blank liner is installed, the water in the borehole is forced from the hole into the formation by whatever flow paths are available (e.g., fractures, permeable beds, solution channels, …). The liner descent rate is controlled by the rate at which water can flow from the hole via those paths. The everting liner is somewhat like the perfectly fitting piston sliding down the hole, except the liner doesn’t slide in the hole, it grows in length at the bottom end of the dilated liner at the “eversion point” as we call it. As the liner everts, it covers the flow paths sequentially. Each time that the liner covers a flow path, the transmissivity of the hole beneath the liner is decreased and the total flow rate out of the hole is reduced. This reduction in flow rate causes a reduction in the descent rate of the liner. Figure 1 is a drawing of the simple everting liner with two additional features. The roller at the wellhead measures the liner velocity and the pressure gauge measures the excess head in the liner which is driving the liner down the hole.
When the liner begins its descent in the hole, all of the flow paths are open and the descent rate is highest. As the liner sequentially covers those flow paths, the liner descent rate decreases to produce a monotonically decreasing velocity with depth in the hole. The velocity profile looks like below.
At each step change in velocity, one can determine the location of the flow path in the hole, and the magnitude of the velocity change is the measure of the flow that was occurring in that flow path before it was covered by the everting liner. From the velocity profile, one can calculate a transmissivity profile for the hole like that shown in picture.
In most transmissivity measurements, a transducer if first lowered to the bottom of the hole to directly measure the borehole driving head at the time each flow path is sealed. This direct measurement allows an even more precise calculation of the transmissivity profile. The measurements performed are of more than just the velocity and excess head. FLUTe Transmissivity Profiler™ measures all of the significant parameters which can influence the velocity of the liner descent. Those are incorporated into a software package that calculates the conductivity profile. In this manner, all significant flow paths can be mapped in the borehole in the time it takes to install the flexible liner. That time varies from half an hour to three to four hours depending upon the transmissivity distribution in the borehole.