HPT (Hydraulic Profiling Tool)
What is Geoprobe® Direct Image® HPT?
- HPT produces a detailed log of relative formation permeability
- Can be used to estimate hydraulic conductivity in the saturated zone
- Logs HPT injection pressure, flow rate and electrical conductivity
- Use pressure dissipation tests to measure hydrostatic pressure
- Determine piezometric profile and water table depth
- Can be used to map salt and brine contaminant plumes
- HPT logging is easy to learn and operate
- Interpretation of HPT logs is straight forward and intuitive
The Hydraulic Profiling Tool is a logging tool that measures the pressure required to inject a flow of water into the soil as the probe is advanced into the subsurface. This injection pressure log is an excellent indicator of formation permeability (Figure 1). In addition to measurement of injection pressure, the HPT can also be used to measure hydrostatic pressure under the zero flow condition. This allows the development of an absolute piezometric pressure profile for the log and prediction of the position of the water table. The piezometric profile can be used to calculate the corrected HPT pressure. This data along with the flow rate can then be used to calculate an estimate of hydraulic conductivity (K) in the saturated formation.
Figure 1: A typical HPT Log displaying graphs (left to right) electrical conductivity (EC), HPT injection pressure (top axis/blue fill) with the absolute piezometric pressure profile plotted (bottom axis/black line) and HPT flow rate. The bottom three triangles on the absolute piezometric pressure line represent hydrostatic pressure measurement points. The inflection point (red dot) in the absolute piezometric pressure line is the predicted water table.
Measurement of the injection pressure in the HPT system is made using a downhole pressure transducer (Figure 3). Use of a transducer in the downhole position allows measurement of the injection pressure at the HPT screen only and excludes frictional losses through the flow tube of the HPT trunkline. The downhole transducer position is also necessary for making hydrostatic pressure measurements at the probe.
Figure 2: This probe is robust and percussion drivable (hammerable) with Geoprobe® direct push machines. Standard HPT probes are equipped with an EC array for measurement of soil electrical conductivity as a complementary log to the HPT log.
Uses of HPT:
The primary log from the HPT system is the log of HPT injection pressure with depth. Again, refer to Figure 1 for an example of this log. Injection pressure correlates well with formation permeability and contributes to the chief use of the HPT. The HPT log provides the user a view of permeability with depth. Together with the EC log and targeted core samples the user can interpret lithology and hydrostratigraphy. With this log an experienced site investigator can determine permeable zones, potential contaminant flow paths, zones that will yield groundwater to samplers, or zones to set monitoring or production wells.
HPT logs are also used to develop cross sections of the subsurface (Figure 4). This is a powerful application of HPT, allowing the site investigator to trace contaminant migration pathways (aquifer materials) or confining zones across a site. It also allows the comparison of lithology between locations based on apparent permeability and electrical conductivity.
Figure 4: Cross section developed using HPT pressure (black traces on logs). The HPT logs are shown here with collocated MIP-XSD logs which indicate the presence of halogenated volatile contaminants at logs 05 and 07 near the middle of the section. The dashed green line across the logs represents the contact between a glacial till (high HPT pressure/low permeability) and the overlying sands and gravels. The contact here appears to outline a buried stream valley (migration pathway) where the volatile contaminants (blue filled red) detected with the MIP-XSD are migrating away from the source area.
Formation hydraulic conductivity can be estimated from HPT logs using an empirical relationship developed for the tool (Geoprobe® Tech Guide, 2010). These estimations can be made automatically using the DI Viewer software. A graph of hydraulic conductivity estimated from an HPT log is shown in Figure 5. Data from this estimate is readily transferable to groundwater flow models.
Figure 5: An HPT log showing (left to right) EC, HPT pressure (top axis/blue fill)) with absolute piezometric Pressure (bottom axis/black line) and estimated hydraulic conductivity (K). The K estimation can be made automatically in the DI Viewer software that is used to review HPT logs. Estimated hydraulic conductivity is only calculated for the saturated zone (below the water table).
The HPT is also useful for the detection of brines or other high electrical conductivity fluids in soil. These brines may originate from oilfield production or storage activities. Other high ionic fluids amenable to this technique include road salts and remediation fluids. These fluids are detected as an anomaly between the EC and HPT log. This occurs when the EC increases while the HPT indicates a zone of high permeability (low HPT pressure). An example of this is shown in Figure 6 along with a site cross section in Figure 7.
Figure 6: In fresh water conditions an increase in EC and an increase in HPT pressure typically indicate increased clay content. However, when ionic contaminants are present EC may rise significantly while the HPT pressure remains low, as seen here. This is an example of the detection of ionic remediation fluids (sodium persulfate) in groundwater using the HPT and EC logs. In this case, the EC begins to increase just below 14ft before HPT pressure starts to rise from baseline to maximum value in the 16 to 19 foot range and then returns to baseline by 20 feet. HPT pressure doesn’t begin to rise until 19 feet, indicating that this is a zone of high permeability. The shape of the EC graph in this interval is also characteristic of ionic contamination.
Figure 7: This site cross section of EC (top axis/brown fill) with HPT pressure (bottom axis/blue line) highlights the location of ionic remediation fluids. At this site, the background EC as seen in the far left and right logs and doesn’t exceed much more than 50mS/m and is a poor indicator of the changing lithology (not typical) because of the soil mineralogy. The HPT pressure graph displays the change from the high to lower permeability of the formation as the probe enters the finer grained soils. The 4 logs in the middle of this cross section have EC readings that increase between 140mS/m to 300mS/m just before the low permeability zone indicated by the HPT pressure graph and then return to baseline even though the HPT pressure is still high.
Tooling and Equipment:
The equipment to perform HPT logging is simple (Figure 8 and 9). In addition to the Field Instrument (FI6000) for data acquisition, HPT requires the use of the K6300 Controller. This instrument provides the pump and pressure and flow measurement required to perform HPT logging.
HPT probes are available in both 1.75 in. (44.5mm) diameter for use with 1.5 in. (38mm) and 1.75 in. (44.5mm) probe rods and 2.25 in. (57mm) diameter probes for use with 2.25 in. probe rods. Tools string diagrams for these probes may be found at geoprobe.com/hpt-tool-string-diagrams. HPT probes are robust, drivable under all Geoprobe® 54 series and 60 series hammers, and can be factory rebuilt when they wear out (provided remaining thread life is deemed sufficient).
Figure 8: The HPT log displays changes in soil permeability as the HPT probe is driven into the subsurface with the direct push rig. HPT and EC results are displayed in real time on the field laptop at the surface.
Figure 9: Instruments used for HPT Logging. The K6300 HPT Controller (bottom) provides metered high pressure flow for HPT injection. Data collection and EC functions are performed by the FI 6000 instruments stacked above the HPT controller. Logging data is acquired and stored using the laptop computer.
Geoprobe Systems® offers training programs for operators who are new to the HPT system. This training is designed to teach proper set-up and operation of the HPT system and also includes practice of log Quality Assurance (QA) and Quality Control (QC) procedures, field exercises, troubleshooting, system maintenance and log interpretation. Persons who have performed MIP, EC or OIP logging can readily upgrade and use an HPT system with some additional training. Interpretation of HPT logs should be performed by an experienced site investigator with knowledge of the site geology/hydrogeology. Targeted sampling should be used to confirm log interpretation.
HPT Combined With Other Logging Techniques:
The HPT is often combined with other Geoprobe® DI logging technologies. These include the following:
MH6534 MIP-HPT-EC Probe
MIP: the “MiHpt“ probe includes the MIP, HPT and EC sensors. The MIP’s information on position of dissolved phase to NAPL VOC’s is always more useful when formation permeability is known.
OH6570 OIP-HPT-EC Probe
OIP: the “OiHPT” probe includes the OIP, HPT and EC sensors. The OIP induces fluorescence on a minimum of residual NAPL fuels or light oils to determine vertical and horizontal extent of these contaminants. The most useful high resolution site characterization determination comes with the added information of formation permeability provided by HPT.
CPT: The HPT has been used as a surrogate for U(2) pore pressure measurements, especially in unsaturated zones. The HPT is also a dependable indicator of seepage zones from dams and levees.
HPT-GW Sampler: This unique tool allows the user to perform HPT logging then stop at appropriate depths to obtain groundwater samples.
Information on all of these logging technologies can be found on this web site. In each of these tool combinations the HPT serves as the “workhorse” for permeability definition. The Standard K6300 HPT controller is used in the instrument set for each of these techniques.