Abstract:
A sampling system and a method for sampling fluid from a target zone within a well bore without commingling fluid from other zones in the well. The sampling system includes a hydraulic flow control system and a differential flow logging system. The hydraulic flow control system has a plurality of multi-level, vertically disposed pumps with fluid extraction rates set to generate hydraulic zones above and below a center one of the pumps. The hydraulic zones isolate fluid flow in a target zone surrounding the center pump, and the center pump collects a sample from the isolated target zone without mixing fluid from other zones of the well. The differential logging system has a plurality of flow devices. Each of the plurality of flow devices is disposed near one of the hydraulic zones to monitor vertical flow in the well and confirm isolation of the target zone.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is related to and claims the benefit of priority to Provisional Application U.S. Ser. No. 61/662,712, titled “Discrete Groundwater Sampling System for Open Borehole Bedrock Wells in Fractured Rock,” by Philip T. Harte, filed Jun. 21, 2012 in the U.S. Patent and Trademark Office, the contents of which are incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, and licensed by or for the U.S. Government for governmental purposes without payment of any royalties thereon. 
    
    
     BACKGROUND 
     The present invention relates in general to fluid sampling of zones in wells intersected by fractures and in long-screen wells in unconsolidated aquifers. 
     The GroundWater Protection Council (1999) indicates that thousands of waste disposal sites are present in the northeastern United States where fractured-rock aquifers predominate. Innovative sampling systems that can effectively and efficiently sample fluids in heterogeneous aquifers are particularly relevant in helping to identify contaminant distribution and pathways. 
     Fluid sampling in open boreholes tapping fractured-rock aquifers is particularly challenging because of vertical mixing of fluid within the borehole from flow from multiple fractures. The use of downhole pumps to pump water or other fluid from long, open-hole bedrock wells or long-screen sections of wells is problematic because the well bore acts as a conduit and induces vertical flow and mixing of fluid from multiple fractures at different zones (depths) throughout the entire well opening based on the transmissivity of each fracture (Shapiro, 2002). The source of the pumped fluid in an open borehole is a mixture of the well bore fluid and the flow-weighted contribution of fluid from multiple fractures. The unknown source origin of the resultant fluid sample is less than ideal for the identification of natural and anthropogenic contaminant pathways. 
     Packers that straddle the pumps have been used to isolate flow from discrete fractures or fracture sets to reduce capture of well bore fluid (Shapiro, 2002). However, the use of packers can be time consuming and logistically difficult. An easier and more efficient sampling system is needed. 
     SUMMARY 
     A sampling system with a multilevel pumping system is described herein that uses hydraulic containment to collect specific discrete zone fluid samples in wells having long open-hole (non-cased) bedrock wells or long-screen sections of wells without mixing and drawing of fluid from different zones (depths). The sampling system couples hydraulic control of flow in open-hole or long-screen wells with differential logging of vertical flow to surgically remove well fluid samples and quantitatively assess zonal transmissivity. The hydraulic control of flow uses pumps to isolate samples from discrete fractures within specific zones without mixing the fluid samples with fluid throughout the well. The absence of mixing is confirmed through differential flowmeter logging or dual tracer tracking. The sampling system aids scientists and water managers tasked with the challenge of identifying contaminant pathways and sources in complex fractured-rock aquifers or heterogeneous unconsolidated aquifers. 
     In accordance with an embodiment of the invention, a sampling system for use in a well has a sample pump disposed in a target zone of the well to collect a fluid sample from the target zone. An upper pump is disposed above the sample pump and a lower pump is disposed below the sample pump. A first flow device is disposed between the upper pump and the sample pump, and a second flow device is disposed between the sample pump and the lower pump. The upper pump, the lower pump, and the sample pump have extraction flow rates set to generate a first hydraulic zone between the first flow device and the sample pump, and a second hydraulic zone between the sample pump and the second flow device. The first and second hydraulic zones isolate the fluid in the target zone from fluid in other zones of the well, and the first flow device and the second flow device monitor fluid flow within the well to confirm isolation of the target zone. 
     In accordance with another embodiment of the invention, a sampling system for use in a well has a hydraulic flow control system and a differential logging system. The hydraulic flow control system has a plurality of multi-level, vertically disposed pumps with fluid extraction rates set to generate hydraulic zones above and below one of the pumps used as a sample pump. The hydraulic zones isolate fluid flow in a target zone surrounding the sample pump, and the sample pump collects a sample from the isolated target zone without mixing fluid from other zones of the well. The differential logging system has a plurality of flow devices. Each of the plurality of flow devices is disposed near the respective hydraulic zones to monitor vertical flow in the well and confirm isolation of the target zone. 
     In accordance with another embodiment of the invention, a method is provided for sampling fluid from a target zone within a well bore without commingling fluid from other zones in the well. In the method, locations and flow rates of fractures in a geologic formation surrounding the well and the total extraction flow rate of the well are determined by performing conventional flowmeter logging. A fracture to sample is selected and a fracture target zone is determined based upon the log results. A hydraulic flow containment system having a plurality of pumps is provided that isolates fracture fluid in the target zone from the well bore fluid. A differential flow logging system having a plurality of flow devices is disposed above and below the target zone to confirm isolation of the target zone. A fluid sample is collected from the isolated target zone using one of the pumps located in the target zone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings. The drawings are not necessarily drawn to scale. In the drawings: 
         FIG. 1  is a schematic cross-sectional view of a differential flow logging system within an open hole bedrock well intersected by fractures, according to an embodiment of the invention; 
         FIG. 2A  is a schematic cross-sectional view of a multilevel pumping system within the well of  FIG. 1  that operates to create stagnation hydraulic zones within the well, according to an embodiment of the invention; 
         FIG. 2B  is a schematic cross-sectional view of a multilevel pumping system within the well of  FIG. 1  that operates to create divergent hydraulic zones within the well, according to an embodiment of the invention; 
         FIG. 3A  is a schematic cross-sectional view of a sampling system combining the differential flow logging system of  FIG. 1  and the multilevel pumping system of  FIG. 2A ; and 
         FIG. 3B  is a schematic cross-sectional view of a sampling system combining the differential flow logging system of  FIG. 1  and the multilevel pumping system of  FIG. 2B . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A sampling system is described for the collection of specific (discrete) zone (depth-dependent) fluid samples in long open-hole (non-cased) bedrock wells or long-screen sections of wells, without mixing and drawing of fluid from different zones (depths). The sampling system couples differential logging of vertical flow with hydraulic control of flow to surgically remove the well fluid samples. 
     A differential coupled flow logging system  100  (hereinafter “flow logging system”) is illustrated in  FIG. 1 . The flow logging system  100  allows for synchronous recording of vertical-flow differences in an open borehole  102 . In the exemplary embodiment shown in  FIG. 1 , the flow logging system  100  uses synchronous measurements of vertical flow from two vertically positioned flow devices  104  and  106  in the well bore  102  to facilitate estimation of vertical flow, indicated by arrows  108 , from discrete fractures  110 A to  110 C in bedrock  112  within an aquifer. Fracture flow, such as fracture flow  114  from the fracture  110 B, enters the borehole  102  and mixes with fluid from the well bore  102 . 
     The flow devices  104  and  106  are attached to surface instrumentation  116  that interprets vertical velocity up and down the borehole  102 . The flow devices  104  and  106  can be, for example, flowmeters that synchronously measure vertical flow, or dual tracer devices that track direction of flow. Synchronous measurement of vertical flow using flowmeters reduces the error associated with the repositioning of a single flowmeter during logging caused by transient hydraulics and small fluctuations in ambient and pumped flow. Differential flowmeter logging can be accomplished using, for example, heat pulse flowmeters (Hess, 1982, incorporated herein by reference), and borehole dilution logging (Pitrak et al., 2007, incorporated herein by reference). Estimates of discrete fracture flow can be facilitated using the analytical solution FLASH, developed by the U.S. Geological Survey (Day-Lewis et al., 2011, incorporated herein by reference). Alternatively, flow logging can be done using emplaced tracer devices rather than flowmeters to determine vertical flow directions. 
       FIG. 2A  illustrates hydraulic control of flow (i.e., hydraulic containment), which is achieved by a multilevel pumping system  200  that allows for vertically positioned extraction from multiple locations in the borehole  102  by isolating discrete fracture zones for sampling. In the embodiment shown in  FIG. 2A , three pumps  202 ,  204 , and  206  are used to isolate a discrete target sample zone  208  adjacent to a selected fracture. For purposes of illustration, the fracture  110 B has been selected. The middle pump  206  is a sample pump. The upper and lower pumps  202  and  204  are “waste” pumps in that they are used to extract well fluid only for hydraulic purposes rather than for sampling. The three pumps  202 ,  204 , and  206  are vertically positioned in the well bore  102  and are variable-rate controlled pumps. The pumps  202 ,  204 , and  206  allow for the surgical extraction of fracture fluid by isolating the fracture fluid from the well bore fluid. The target sample zone  208  is isolated based on volumetric mass balance between the fracture flow  114 , the rates of extraction of the sample pump  206  and the waste pumps  202  and  204 , and the borehole  102 . A multiple port, multiple line deployment/extraction system (not shown) is used to deploy and extract the pumps, and to vary the vertical position of the three extraction pumps  202 ,  204 ,  206  based on the desired size of the target sample zone  208 . The sample and waste fluid are pumped to the surface independently of each other. The sample is pumped through a sample line  214 , and the waste fluid is pumped from the waste pumps  202  and  204  through waste lines  216  and  218 , respectively. 
     In  FIG. 2A , rates of extraction from the pumps  202 ,  204 , and  206  are adjusted to create stagnation or minimal velocity flow zones (i.e., hydraulic zones) between adjacent vertically positioned pumps. These hydraulic zones contain the vertical extent of the fluid source of the target sample zone  208 . In the embodiment shown in  FIG. 2A , the waste pump  202  extracts well fluid  210   a  and  210   b , the sample pump  206  extracts well fluid  211   a  and  211   b , as well as fluid  114  from the target fracture  110 B, and the waste pump  204  extracts well fluid  212   a  and  212   b . The rates of extraction of the pumps  202 ,  204 , and  206  are adjusted to create stagnation zones  220  and  222  between the sample pump  206  and the waste pumps  202  and  204 , respectively. The principles of hydraulic boundaries and zones including no-flow zones are described in image well theory (Ferris et al., 1962, incorporated herein by reference). 
     Alternatively, isolation of the target zone is also achievable by inducing divergent flow away from the target zone  208  using a sampling system  250 , as shown in  FIG. 2B , which prevents mixing of borehole fluid with fluid in the target zone  208 . In the embodiment of  FIG. 2B , the upper and lower waste pumps  202  and  204  are set at flowrates greater than the rate of the sample pump  206  causing a predominance of vertical flow  210   c  and  211   c  to move away from the target zone  208 , excluding the fluid captured by the sample pump  206 . 
     The duration of extraction of the waste pumps  202  and  204  and the sample pump  206  is dependent on the total pump rate from the three pumps, the flow rate  114  from the fracture  110 B, the summation of flow rates from all other fractures excluding the target fracture  110 B in the borehole  102 , and the amount of fluid in the target zone  208 . A minimum of about 2.3 times the volume of fluid in the target zone  208  is evacuated prior to sampling. These same principles with regard to flow in the target zone  208  and flow outside of the target zone  208  can be used to deploy the pumping systems  200  and  250  in long-screen wells set in unconsolidated aquifers. 
     The flow logging tool  100  and the multilevel pumping system  200  are combined to yield the sampling system  300  that generates the stagnation zones  220  and  222 , as illustrated in  FIG. 3A . Also, the flow logging tool  100  and the multilevel pumping system  250  are combined to yield the sampling system  350  that generates the diverging flows, as illustrated in  FIG. 3B . The sampling systems  300  and  350  allow for hydraulic containment and isolation of flow from discrete fractures using the multilevel pumping systems  200  and  250 , while isolation is confirmed with the differential flow logging system  100 . 
     Prior to deployment of the sampling systems  300  and  350 , conventional flowmeter logging (ambient and stressed) is performed to identify locations of fractures in the geologic formation  112  surrounding the well  102 , and to determine their flow rates and the total extraction rate of the well  102 . Additional differential logging can be done to better quantify flow from the fractures. The sampling systems  300  and  350  are configured based on the fracture flow distribution, which is used to design the proper pump rates of the waste and sample pumps  202 ,  204 , and  206  to create either the stagnation zones  220  and  222  ( FIG. 2A ) or the divergent (i.e., outward) flow ( FIG. 2B ). The fracture to be sampled is selected. For purposes of illustration, the fracture  110 B has been selected in the embodiment shown in  FIGS. 3A and 3B . A preliminary extraction zone  302  is determined, which is the distance between the target fracture  110 B and the next adjacent upper and lower fractures  110 A and  110 C. The precise target sample zone  208  is determined after the positioning of the lower and upper waste pumps  204  and  202  and the formation of the stagnation zones  220  and  222  between the waste pumps  202  and  204  and the sample pump  206  of  FIG. 3A , or the formation of the divergent flow conditions as shown in  FIG. 3B . The target sample zone  208  is defined as the borehole capture zone of the sample pump  206 . Under divergent conditions, the target zone  208  is restricted to the location of the sample pump  206 . 
     The pumps and flow devices are then positioned in the well  102 . The lower waste pump  204  is positioned in the well  102  below the target fracture  110 B and between the target fracture  110 B and the next adjacent lower fracture  110 C. The exact placement distance of the lower waste pump  204  below the target fracture  110 B is determined based on well hydraulics including fracture inflow rates and rates of extraction of the waste and sample pumps  202 ,  204 , and  206 . The lower flow device  106  is positioned between the lower waste pump  204  and the target fracture  110 B. 
     The sample pump  206  is positioned adjacent to the target fracture  110 B. The upper flow device  104  is positioned between the sample pump  206  and the planned position of the upper waste pump  202 . The upper waste pump  202  is positioned above the sample pump  206  and the upper flow device  104  between the sample fracture  110 B and the next adjacent upper fracture  110 A. The exact placement distance of the upper waste pump  202  above the sample pump  206  is determined based on well hydraulics including fracture inflow rates and rates of extraction of the waste and sample pumps  202 ,  204 , and  206 . For long-screen wells, the sample pump  206  is positioned at a desired depth within a selected screen interval (target zone), and the waste pumps are set based on the volumetric flow of the well screen, anticipated extraction rate of the waste pumps  202  and  204 , and the length of the well screen. 
     The upper waste pump  202  is set a minimum distance of about 3 feet above the sample pump  206  to a maximum distance of about 1 foot below the upper fracture  110 A. The lower waste pump  204  is set a minimum distance of about 3 feet below the sample pump  206  to a maximum distance of about 1 foot above the lower fracture  110 C. The Theis equation (Theis, 1935, incorporated herein by reference) can be modified to further constrain the locations of the waste pumps  202  and  204  within the minimum and maximum specified distances by calculating a distance  304  between the waste pumps based upon the expected volume pumped in a desired sample time subtracted by the estimated Darcian radial flow over the target zone  208  during that same sample time, divided by 4.6 times the volume of fluid along the distance  304  in the borehole  102 . 
     The extraction flow rate of the sample pump  206  is set at a rate equal to or less than the flow rate of the flow  114  from the target fracture  110 B, as determined from the conventional and differential flowmeter logging. The extraction flow rates from the upper and lower waste pumps  202  and  204  are initially set at the total extraction flow rate of the well  102  minus the extraction flow rate of the sample pump  206 . The rate of extraction from each waste pump  202 ,  204  is based on the position of the pump relative to the well opening, the flow rate of the target fracture  110 B, and the summed inflow rates of the non-target fractures  110 A and  110 C, as determined by the conventional flowmeter logging. The flow rate of the upper waste pump  202  is the sum of the upper borehole flow, and the flow rate of the lower waste pump  204  is the sum of the lower borehole flow. 
     When initiating the pump sequence, the waste pumps  202  and  204  are turned on first. The extraction flow rate of the lower waste pump  204  is set initially at the summation of flow  212   b  from the lower borehole. The extraction flow rate of the upper waste pump  202  is set initially at the summation of flow  210   a  from the upper borehole. After stabilization of the extraction rates from the waste pumps  202  and  204 , the sample pump  206  is turned on at the extraction rate commensurate with the flow rate from the target zone  208 . The stagnation zones  220  and  222  ( FIG. 3A ) are created between the waste pumps  202  and  204  and the sample pump  206  by minimizing vertical flow between the middle sample pump  206  and the upper and lower waste pumps  202  and  204 . After stabilization of the stagnation zones  220  and  222 , isolation of the target sample zone  208  is confirmed using the differential flow logging system  100 . Isolation is confirmed under stagnation conditions when there is no flow between the target sample zone  208  and the zones above and below the target sample zone  208 . 
     Alternatively, confirmation that the borehole fluid is not mixing with the sample fluid can be aided by performing injection tracer tests to ensure that the sample fluid is clear from the injected tracer. The exact composition of the injected tracer, such as whether it contains a dye, is based on the compounds of interest, the field geochemistry, and whether a dual tracer device was used to confirm vertical flow. The injection tests are performed to insure that the target sample zone  208  is minimally disturbed either by balancing injection with additional extraction or by applying a non-fluid tracer (e.g., heat). Also, confirmation of the stagnation zones  220 ,  222  can be determined using a borehole dilution technique, which involves creating tracer-free zones in the target zone  208  after an initial uniform tracer (under pre-pump conditions) is introduced into the entire length of the borehole  102 . 
     For example, the tracers can be frozen in porous bottles and deployed between the sample pump  206  and the waste pumps  202  and  204 . Once in the well bore  102 , the tracers thaw and slowly seep out of the bottles. The direction and rate of vertical flow are confirmed by tracking the presence of the tracers in the fluid extracted from the three pumps in the surface instrumentation  116 . The presence of the tracers in the waste pumps  202  and  204  confirms flow away from the target zone  208  and thus no mixing of the borehole water at the target zone  208 . The target zone  208  is recharged only by the flow from the targeted fracture  1106 . 
     Preferably, the pump rates are adjusted to create stagnation zones  220  and  222  between the waste pumps  202  and  204  and the sample pump  206 . However, divergent flow away from the sample fracture  110 B ( FIG. 3B ) also facilitates the collection of discrete samples that do not contain mixed borehole water. Isolation of the target sample zone  208  is confirmed using the flow logging system  100 . Isolation is confirmed under divergent conditions ( FIG. 3B ) when there is vertical flow away from the target zone  208  (i.e., there is no downward flow above the target zone  208  or upward flow below the target zone  208 ). If tracers are used, the absence of tracers in the sample pump  206  and the presence of tracers in the waste pumps  202  and  204  prove that the target zone  208  is isolated from borehole mixing. 
     Sample collection of the extracted fluid from the sample pump  206  begins after evacuating a minimum volume of fluid equivalent to about 2.3 times the volume of the borehole  102  between the waste pumps  202  and  204 . Samples are collected from the target sample zone  208  using the sample pump  206  while extracting well fluid above the target zone  208  using the upper waste pump  202  and below the target zone  208  using the lower waste pump  204 . The chemistry of the fluid extracted from the sample pump  206  and the waste pumps  202  and  204  is monitored to aid in identifying capture of fluid from the different zones in the well  102  and in the confirmation of tracer movement if dual tracer devices are deployed. 
     For purposes of hydraulic isolation and sampling from the target sample zone  208 , either no flow above and below, or vertical flow outward or away (diverging flow) from, the target sample zone  208  produces a representative sample collected by the sample pump  206  from the fracture flow  114 , while well fluid  210   a  above the upper stagnation zone  220  in  FIG. 3A  or above the target zone  208  in  FIG. 3B  is extracted by the upper waste pump  202  and well fluid  212   b  below the lower stagnation zone  222  in  FIG. 3A  or below the target zone  208  in  FIG. 3B  is extracted by the lower waste pump  204 . Thus, the zone above the target zone  208  is a no-flow or up-flow zone, and the zone below the target zone  208  is a no-flow or down-flow zone. 
     While the sampling system described herein has been described with respect to sampling in discrete heterogeneous fracture networks, it has applications as a zone-specific sampling system using hydraulic containment in a variety of geologic formations. The sampling system works in wells with long open holes (non-cased) or long-screen sections, as well as in short open-hole or short-screen sections. In screened wells, the use of external filter packs outside the screen precludes the use of packers to isolate fluid because of short-circuiting whereas hydraulic containment can still be effective in isolating target zones. Also, discrete sampling with the sampling system can be used in the identification of emerging contaminants because of its ability to reduce mixing and dilution of groundwater. Further, the sampling system can be used as a screening tool to assist in the deployment of well screens in open boreholes. This helps ensure proper well screen placement based on vertical chemistry differences. 
     The sampling system described herein provides several advantages over conventional zone-specific sampling systems. For example, it uses hydraulic control to surgically extract water from discrete sample zones in wells, minimizing vertical mixing within the open hole bedrock well or long screened section of well to obtain non-mixed (homogenous) samples that are representative of the sample zone. Also, the sampling system is not cumbersome to use. It does not require the use of heavy-duty equipment (e.g., cable tools and rigs) to run or physical containment devices such as packers and, thus, can be deployed quickly and efficiently, allowing for quicker data collection of discrete chemistry. In addition, in boreholes that have undergone hydraulic fracturing, such as boreholes set in the Marcellus Shale Formation, the borehole surfaces are rough, which prevents the use of conventional zone specific sampling that relies on physical containment (e.g., use of packers) through sealing of the target zone. Hydraulic containment, utilized by the sampling system described herein, is unaffected by rough, irregular surface walls of the boreholes and can be used as an effective sampling system to isolate flow and collect discrete zone specific samples. 
     Collection of groundwater samples in open bedrock boreholes is problematic because of the nature of flow in fractures. The sampling system described herein allows samples to be collected in an efficient manner and therefore offers significant cost benefits to the groundwater and petroleum communities, as well as the ability to determine solute transport in fractured-rock aquifers and the effect of fracture connectivity on borehole flow and sampling, such as whether fracture connections outside the borehole can be identified. 
     Thus, it will be appreciated by those skilled in the art that modifications and variations of the present invention are possible without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents. 
     GENERAL BIBLIOGRAPHY ON THE SUBJECT 
     The following bibliography provides citations to the references cited in the above text. The references are provided merely to clarify the description of the present invention and citation of a reference either in the bibliography below or in the specification above is not an admission that any such reference is “prior art” to the invention described herein.
     Day-Lewis, F. D., Johnson, C. D., Paillet, F. L., and Halford, K. J. 2011. A computer program for flow-log analysis of single holes (FLASH). Ground Water, doi: 10.1111/j.1745-6584.2011.00798.   Ferris, J. G., D. B. Knowles, R. H. Brown and R. W. Stallman, 1962. Theory of aquifer tests, U.S. Geological Survey Water-Supply Paper 1536 E, 174p.   Hess, A. E., 1982. A heat-pulse flowmeter for measuring low velocities in boreholes: U.S. Geological Survey Open-File Report 82-699, 44 p.   Ground Water Protection Council, 1999. Ground Water Report to Congress: Summaries of State Ground Water Conditions, 109p.   Pitrak, M, Mares, S., and Kobr, M. 2007. A Simple Borehole Dilution Technique in Measuring Horizontal Ground Water Flow. Ground Water, V45, no. 1, 89-92 p.   Shapiro, A. M. 2002. Cautions and suggestions for geochemical sampling in fractured rock. Groundwater Monitoring and Remediation, V.22(3), p. 151-164.   Theis, C. V., 1935. The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using groundwater storage. Trans. Amer. Geophys. Union, V16, p. 519-524.