SAMPLING PROTOCOL OPTIMIZATION USING FLUID ELECTRICAL CONDUCTIVITY LOGGING

Transcription

1 SAMPLING PROTOCOL OPTIMIZATION USING FLUID ELECTRICAL CONDUCTIVITY LOGGING C. Lasher 1 and J.M. Nel 2 1 SRK Consulting (Pty) Ltd, The Administration Building, 183 Main Road, Rondebosch, South Africa; clasher@srk.co.za 2 GCS, Leopard Court Building 1st Floor, South Wing Cnr of Jerome Rd & Kariba Street, Lynnwood Glen, Pretoria, South Africa; jacon@gcs-sa.biz Abstract Characterisation of fracture positions is important when dealing with groundwater monitoring, protection and management. Fractures are often good conduits for water and contaminants, leading to high flow velocities and the fast spread of contaminants in these conduits. Best practice guidelines related to groundwater sampling suggest that specific depth sampling with specialised bailers or low flow purging are the preferred methodologies to characterise a pollution source. These methods require knowledge about the fracture positions and, more importantly, flow zones in the boreholes. Down-the-hole geophysical and flow logging are expensive, complicated and time consuming. Not all fracture zones identified with geophysical logging seem to contribute to flow through the borehole. An efficient and cost effective methodology is required for the characterisation of position and flow in individual fractures. This research reviewed the use of Fluid Electrical Conductivity (FEC) logging to assist with the development of a monitoring protocol. FEC logging proved to be beneficial as it provided individual fracture positions, fracture yields and vertical groundwater flow directions. FEC logging proved to be fast, cost effective and practical in deep boreholes. The technique allows the development of a site specific sampling protocol. The information so obtained assists with the identification of the appropriate sampling depths during monitoring. 1. INTRODUCTION During groundwater monitoring, protection and management it is imperative that the groundwater being sampled is representative of the flow path one intends to characterise. Groundwater sampling is commonly practiced at, but not limited: to mining sites, waste disposal sites and petroleum industry sites. Best practice guidelines related to groundwater sampling suggest that specific depth sampling with specialised bailers and low flow purging are the preferred methodologies to characterise a pollution source. These methods require knowledge about the fracture positions and more importantly flow zones in the boreholes. Sampling at the wrong depth could lead to erroneous data and misinterpretation of contamination plumes or aquifer characteristics. Fractures are often good conduits for water and contaminants, leading to high flow velocities and the fast spread of contaminants in these conduits. Literature from 1990 to recent years (Tsang et al. (1990); Pedler et al. (1990); Doughty and Tsang (2000), (2002) and (2004); Doughty et al. (2008) and (2012); Kurikami (2008) and Lasher (2011)) indicate that groundwater in boreholes is not always stagnant under ambient (non-pumping) conditions. Depending on the hydraulic and/or piezometric head in individual fractures intersected by a specific borehole, groundwater will flow in and out of the borehole, as well as vertically within the borehole column. It has also been shown (Lasher, 2011) that while groundwater flows in different parts of the borehole column, zones of very low or no flow may also exist. It is therefore essential to strategically select the location at which one intends to sample. Common methods used to identify fractured and/or flow zones within the borehole are down-the-hole geophysical, packer tests and flow logging. These methods are expensive, complicated and time

2 consuming (Doughty et al., 2008). Not all fracture zones identified with geophysical logging seem to contribute to flow through the borehole (Lasher, 2011). Flow logging is further associated with some disadvantages such as over-or under- estimations of flow rates. This research reviewed the use of Fluid Electrical Conductivity (FEC) logging to determine transmissive zones within the borehole, identifying vertical and horizontal flow as well as quantify flow and as a result assist with the development of a monitoring protocol. Doughty et al. (2012) describes FEC logging as a hydrogeophysical method, in that it uses geophysical measurements to obtain hydrological information to understanding and reduce uncertainty about aquifer characteristics. The early development and applications of FEC logging using expensive high technology equipment was done by Tsang et al. (1990); Doughty and Tsang (2000), (2002) and (2004); and more recently by Doughty et al. (2008) and (2012) and Kurikami (2008). Lasher (2011) adapted and field tested the FEC methodology to an affordable low technology procedure suitable for developing countries. The key disadvantage about FEC logging is that it must be performed in open or screened boreholes. This paper reviews the application of FEC logging and concludes with a discussion of how it can be used to optimize sampling protocols using the results obtained. 2. METHODOLOGY This section briefly describes how FEC logging is applied in an open borehole. For further information regarding detailed methodology and analysis refer to Lasher (2011). This method can be applied under ambient pressure/piezometric and artificial pumping conditions. The latter is called Ambient Fluid Electrical Conductivity (AFEC) and Flowing Fluid Electrical Conductivity (FFEC) logging, respectively. The borehole is first logged with a down the hole Specific Conductance probe (If an EC probe is used the temperature correction must be done as a post processing step) so that ambient or background conditions are known. Then the Specific Conductance of the groundwater in the borehole is slightly increased. This can be done by either replacing the borehole water with water of a constant salinity which is noticeably different from that of the formation or by inserting table salt into the borehole column using an injection sock, hereby increasing the salinity. The method used is dependent on the ambient Specific Conductance of the groundwater as well as the ambient/natural flow within the borehole column. The injection sock works best for boreholes with high through flow rates, but provides visually less pleasing data. The interpretation is not influenced by either of the methods. Once the salinity in the column of water is altered the conductance of the groundwater is profiled repeatedly at known time intervals. In our experiments a multi-parameter probe with integrated depth sensor was used. The specific multi-parameter probe works best using a downward logging protocol where the water flows over the sensor while it is being lowered. The depth and Specific Conductance sensor are both found on this probe, eliminating any data mismatch, which is often the problem with geophysical logging. Once the test is completed the borehole is pumped clean to remove salt from the aquifer. Flowing FEC (FFEC) logging method is done in a similar way, however the borehole is pumped at a constant rate while the profiling is done at known intervals. Repeating the FFEC tests at a different discharge rate allows one to determine the individual fracture transmissivity (Doughty and Tsang, 2004), as well as the relative contributions of individual fractures. 3. DATA INTERPRETATION The Specific Conductance profiles will usually show typical dilution characteristics depending on the combination of fracture positions, pressure differences between the fractures and flow rate in the fractures. These dilution characteristics indicate approximate fracture locations which can be interpreted as aquifer feed points into the borehole. The qualitative interpretation of the fracture locations can be used as the initial guesses for the numerical model BORE II.

3 BORE II is a 2D finite element model used to simulate the fracture flow rates into and out of the borehole, using the salt dilution data and calibration data. In the BORE II software water inflow characteristics such as location, flow rate and concentration are adjusted by trial and error until the simulated and measured salt dilution data matches. If FFEC data is available the fracture yields and locations are more uniquely obtained using two different discharge rates. 4. EXAMPLE OF FIELD APPLICATION USING AFEC AND FFEC LOGGING AFEC and FFEC logging was used on an open borehole drilled into the highly fractured Table Mountain Group (TMG) aquifer on the farm Gevonden situated near Rawsonville. The groundwater in the area has a low ambient specific conductance and high natural flow through conditions. After logging the ambient groundwater quality the salinity of the borehole column was increased using the salt sock method. Ambient Fluid Electrical Conductivity (AFEC) Logging Results FEC logging was first conducted under ambient flow conditions with specific conductance profiles performed up to 85 minutes. Results obtained identified various distinct groundwater flow directions due to variation in fracture hydraulic head at different depths. The qualitative visual inspection of the data suggests inflows at: 74, 95 and 110 m and outflows at: 49, 130 and 140 m (Figure 1). Some early time peaks were seen at 63 and 69 m interpreted as minor inflow points. Figure 1. Ambient FEC Profiles Showing Main Contributing Fracture Depths and Flow Directions. Based on the dilution characteristics observed it is assumed that fresh water enters the borehole at 74 m and moves upward towards an outflow around 49 m. Fresh water also enters the borehole at 95 and 110 m and moves up and down the borehole, respectively. The zone between 94 and 109 m is a low flow zone. The water that enters the borehole at 110 m moves downward out flows out at between 130 and / or

4 140 m. Thereafter, no distinct peaks are seen and it is assumed that very little or zero flow occurs between 141 and 200 m. The best position to sample in this borehole would be between m. Flowing Fluid Electrical Conductivity logging Results FFEC logging was conducted at two pumping rates to determine fracture contributions and to understand how fracture flow rates vary with different pumping rates. As pumping conditions were induced, drawdown was observed and hence a change in hydraulic gradient towards the borehole. The latter conditions allow fractures to contribute to flow. FFEC logging at 0.3 L/s were conducted for 96 minutes while FFEC logging at 0.5 L/s was conducted for 209 minutes. Results obtained from these comparative datasets proved to be beneficial for fractured rock characterisation as it identified fractures contributing to flow, flow directions as well as relative flow rates. The specific conductance during the FFEC test (Figure2) decreased instantaneously in certain places as fresh water enters the borehole causing dilutions of salts; these are interpreted as inflow zones. In certain zones along the borehole the water did not dilute back to its background conditions. These zones are interpreted as very low flow zones. The low flow zones between m correlate with AFEC logging which confirms that no or very little flow occurs here. Figure 2. FFEC Logging at 0.3 and 0.5 L/s Showing Feed Points. During both FFEC logging tests major fractures contribute to flow and are observed at the same positions identified during AFEC logging. A few additional minor fractures also contribute to flow. All these fractures add fresh water to the borehole and flows upward due to pumping conditions. These feed points were observed during early time data, before interfering signature peaks occur.

5 Numerical Modeling of Ambient and Pumping/Induced Flow FEC Data. The BORE II software was then used to simulate fracture positions and flow rates by trial and error based on the qualitative assessment of fracture positions used for initial guesses. Figure 3. Calibration Graph for AFEC Logging at Borehole Obtained During Simulation of Fracture Location and Yields. The results obtained indicate that the initial guesses which include both fracture positions and flow directions are correct. The BORE II simulations also show that a few minor fractures also contribute to flow (Table 1).

6 Table 1. Fracture Yields Obtained During BORE II Simulations Under Ambient Conditions. Fracture depth (m) Yield (L/s) In flow Out flow The ambient total inflow into the borehole based on the BORE II simulations are L/s and the total outflows are L/s. AFEC logging assisted with flow under ambient hydraulic head conditions such as flow direction and flow contributions within the borehole. The inflow feed points for both FFEC logging tests were inserted as initial guesses at 74, 94, 110, 135, 142, 147, 180 and 183 m in the numerical model. Fracture depths, yields and concentrations were adjusted by trial and error until the simulated data matched the measured data.

7 Figure 4. Calibration Graph for FFEC Logging (0.3 L /s) Obtained During Simulation of Fracture Location and Yields.

8 Figure 5. Calibration Graph for FFEC Logging (0.5 L/s) at RAW_BH3 Obtained During Simulation of Fracture Location and Yields.

9 The sum of all fracture yields was calibrated against pumping discharge rates. The results obtained from the BORE II calibrated data indicate that additional fractures contributed to the borehole during FFEC testing, compared to the AFEC testing. It was interesting to note that some fractures remain outflows under the low stress pumping rates used for the FFEC tests (Table 2). Table 2. BORE II Simulations of Fracture Yields of RAW_BH3 Obtained During FFEC Logging at 0.3 L/s and 0.5 L/s. (Numbers in bold indicate additional fractures caused by change in discharge rate). Yield Fracture Yield Out Fracture Out In flow (0.5 In flow depth(m) (0.3 L/s) flow depth(m) flow L/s) BORE II calibrations indicate a fracture at 181 m was activated under pumping conditions, becoming an inflow and outflow zone. FFEC logging at different rates activate different fractures. When the borehole was pumped at 0.3 L /s a fracture at 188 m was activated. When pumping 0.5 L /s the fracture at 188 m no longer responds; however, a fracture at 107 m does. Modelling the data also shows that some inflow zones are caused by more than one transmissive fracture. Individual fracture transmissivity was not calculated as different fractures correspond at different rates e.g. 107 m and 188 m. It is therefore strongly advised that FFEC logging is repeated at a different rate as well as the existing rates for a longer period. However, it is assumed that fracture 36 and 44 are the most transmissive as it has a greater change in yield. Modelling of the data also show that an outflow fracture at roughly 120 m cannot be seen on FEC profiles and longer testing durations might assist with this. Based on this information the best position to sample would be between m.

10 5. CONCLUSIONS AFEC and FFEC logging characterizes individual fracture properties. The dilution characteristics indicate that, even under ambient conditions, inter borehole flow occurs between different flow zones intersected by the borehole, due to different pressure heads at different depths. Under pumping conditions more fractures contribute to flow indicating that not all transmissive fractures flow under ambient conditions. When fractures have equal head, flow will not occur. Modelling the data provided more precise fracture positions and fracture yields. They also showed that certain transmissive zones contain more than one contributing fracture. Modelling indicated that FFEC logging was too short and therefore certain outflow fractures could not be identified during visual inspection of FEC profiles. Overall, the FEC logging method identified transmissive zones, flow directions and quantified fracture contributions at low costs, provided you have the appropriate equipment. FEC logging also shows that different fractures contribute to flow under different pumping conditions. In spite of the complex nature of the fracture rock media FEC logging was successful, inexpensive and concise when characterising individual fracture properties. Understanding the aquifer characteristics before constructing a monitoring protocol is vital. It is therefore strongly advised that FEC logging is performed prior to monitoring. The information obtained from FEC logging can be used to select the appropriate position of sampling as well as the method used to sample. FEC results also provide essential information about the aquifer that one could use for modelling to predict scenarios should contamination take place. FEC logging also provides flow directions so that one could track contamination plumes if they should occur. FEC logging has proved to be a successful and cost effective, not to mention a time efficient method. FEC logging if a cost effective method which provides extensive information such as groundwater flow directions and flow rates in complex fractured rock aquifers. FEC logging proved to be beneficial as it provided individual fracture positions, fracture yields and vertical groundwater flow directions. FEC logging proved to be fast, cost effective and practical in deep boreholes. 6. REFERENCES Doughty, C., Tsang, C-F., Hatanaka, K., and Yabuuch, S. (2008) Application of direct, mass integral, and multirate methods to analysis of flowing fluid electric conductivity logs from Horonobe, Japan. Water Resources Research, Volume 44, W08403, doi: /2007WR Doughty, C., and Tsang, C-F. (2004) Signatures in flowing fluid electric conductivity logs. Journal of Hydrology. doi: /j.jhydrol Doughty, C., and Tsang, C-F. (2002) Inflow and outflow signatures in flowing wellbore electricalconductivity logs. Rep. LBNL Berkeley, CA: Lawrence Berkeley National Laboratory (available on-line at Doughty, C., and Tsang, C-F. (2000) BORE II A code to compute dynamic wellbore electrical conductivity logs with multiple inflow/outflow points including the effects of horizontal flow across the well. Rep. LBNL Berkeley, CA: Lawrence Berkeley National Laboratory (available on-line at Kurikami, H., Takeuchi, R., and Yabuuchi, S, (2008) Scale effects and heterogeneity of hydraulic conductivity of sedimentary rocks at Horonobe URL site. Physics and Chemistry of the Earth 33(2008) S37-S44. Lasher, C. (2011) Application of fluid electrical conductivity logging for fractured rock aquifer characterisation at the University of the Western Cape s Franschhoek and Rawsonville research sites. Unpublished PhD Thesis, University of the Western Cape, Cape Town. Pedler, WH., Barvenik, MJ., Tsang, C-F., and Hale, FV. (1990) Determination of bedrock hydraulic conductivity and hydrochemistry using a wellbore fluid logging method. Proceedings of the Outdoor Action Conference 1990, NWWA, Pages Tsang, C-F., Hufschmeid, P., and Hale, FV. (1990) Determination of fracture inflow parameters with a borehole fluid conductivity logging method. Water Resour. Res. 26 (4), Pages