FLOWING FLUID ELECTRICAL CONDUCTIVITY PROFILING AN INVENTIVE AND COST EFFECTIVE ALTERNATIVE TO CONVENTIONAL DOWNHOLE HYDRAULIC PROFILING TECHNIQUES

Transcription

1 FLOWING FLUID ELECTRICAL CONDUCTIVITY PROFILING AN INVENTIVE AND COST EFFECTIVE ALTERNATIVE TO CONVENTIONAL DOWNHOLE HYDRAULIC PROFILING TECHNIQUES Abstract S.D. Mohr 1 and L. Smith 2 1 Environmental Resources Management Southern Africa, Cape Town, Western Cape, South Africa samuel.mohr@erm.com 2 Environmental Resources Management Southern Africa, Johannesburg, Gauteng, South Africa liam.smith@erm.com Flowing fluid electrical conductivity (FFEC) profiling provides a simple and inexpensive way to characterise a borehole with regards to the vertical location of transmissive zones, the hydraulic properties of the various transmissive zones and the intra-well flow conditions which may be present in the well under ambient conditions. The method essentially involves analysing the time evolution of fluid electrical conductivities in a borehole under pumped and ambient conditions using a down-hole conductivity/temperature data logger. The premise of the method is that the borehole column of water has its electrical conductivity altered by adding saline water into the borehole. This results in a contrast in electrical conductivity (EC) between the water in the borehole and the water in the adjacent formation. At depths where transmissive zones are present, decreases in EC values in the FFEC profile will be observed where formation water with a lower EC (relative to the borehole water column) enters into the well whilst pumping at low abstraction rates (between 500ml and 1 liter per minute). By altering the EC of the well bore water and maintaining a constant pumping rate, the sequence of FFEC profiles depicts the dynamic flow and transport response which is dependent upon the hydraulic properties of the formation. In this paper the authors present several examples where FFEC profiling has been used to identify transmissive zones in boreholes where no information existed with regards to the vertical distribution of transmissive zones. Furthermore, the authors present case studies where FFEC profiling has been employed as an alternative technology to more conventional hydraulic profiling techniques. This includes a comparative technology case study where down-hole impeller flow meter technology was employed in addition to FFEC profiling and a multi-rate FFEC profile test which was used to determine discrete fracture transmissivity values in a borehole where packer testing equipment could not be installed. Within the context of groundwater contamination investigations, the method holds several attractions as it generates minimal waste water to be managed and disposed of, is inexpensive and can be completed within a relatively short time period.

2 1. INTRODUCTION Within the context of hydrogeological investigations within fractured bedrock environments, knowledge of the location, spatial density and hydraulic properties of transmissive features is critical in understanding the governing flow and transport processes within the fractured aquifer environment. This information becomes even more critical in the context of groundwater contamination investigations where contaminant mass flux is often concentrated into discrete flow zones which are critical to identify, but are often difficult to detect. The advancement of boreholes or coreholes into the subsurface is the hydrogeologist s primary gateway into the subsurface to allow assessment of these features and a host of down-hole investigative techniques have been developed over the years to facilitate the understanding of the subsurface environments and compilation of site specific conceptual models (National Research Council,1996). Core logging and downhole geophysical tooling are able to identify the locations of fractures with appreciable accuracy, but are unlikely to provide meaningful information on the fluid flow properties of the fractures themselves. Straddle packer testing is able to deliver accurate information on the hydraulic properties of the considered intervals, but are expensive and time consuming to perform. Packer tests also have reduced applicability when used in rotary percussion drilled boreholes as varying borehole diameters often prevent the packers from achieving an acceptable seal between the testing intervals. Flow logging techniques provide a viable alternative to the abovementioned options in many cases. Several forms of flow logging techniques exist such as spinner flow meters, electromagnetic or heat pulse flow meters, tracer dilution tests and flowing fluid electrical conductivity (FFEC) profiling. Borehole flow meter logs can provide information on the location of transmissive zone, but can be strongly influenced by the wellbore radius and need to be calibrated with a concurrent caliper log since the measurement of flow rate is inversely proportional to the wellbore radius. Heat pulse flow meters are able to detect lower flow rates than spinner flow meters, but are considerably more expensive and spinner flow meters are generally the more common flow meter tool being used. Spinner flow meters also require a moderate flow rate within the wellbore to function optimally and may not yield meaningful information under low flow conditions or in low permeability environments. Tsang et al (1990) were the first to introduce the FFEC profiling method and the technique has been used in several deep borehole to depths in excess of 1,500m (Kelley et al., 1991, Doughty et al, 2008) and is also gaining momentum as a useful hydraulic characterization tool in shallow (i.e. <100m) boreholes studies as well. Doughty & Tsang (2005) conducted extensive research in the area of FFEC profiling as an alternative to packer testing or spinner flow meters in order to investigate fracture locations and to provide estimations of individual fracture transmissivities. The method entails altering the EC in a borehole (either by circulating de-ionised water or water with increased salinity), pumping the borehole at low abstraction rates and measuring the time evolution of dilution across borehole interval as formation water enters the borehole in response to the head differential induced by the abstraction. By altering the EC of the wellbore water and maintaining a constant pumping rate, the sequence of FFEC profiles depicts the dynamic flow and transport response which is dependent upon the hydraulic properties of the formation. The method was found to be more accurate than spinner flow meters and less costly than inflatable packer testing. The method allows for the identification of points of inflow (fractures) and further work conducted by Doughty & Tsang resulted in the construction of the simulation program BORE II which is able to model fracture inflow and outflow points as well as being able to calculate individual fracture transmissivities.

3 3. METHODOLOGY As mentioned in Section 1, the method entails altering the EC of water in a borehole, pumping the borehole at low abstraction rates (<1l / minute) and measuring the time evolution of dilution across borehole interval as formation water enters the borehole at fluid bearing fractures in response to the head differential induced by the abstraction. The alteration of the wellbore EC can be achieved by circulating deionized water throughout the entire wellbore or by increasing the salinity of the wellbore by the addition of salt. In the case of the studies mentioned in this paper, the wellbore EC was raised by the introduction of salt to the wellbore water. This was completed by placing approximately 500g of table salt in a porous material pouch and lowering the pouch to the bottom of the borehole and then raising it back to the surface. It must be noted that it is important to not raise the salinity of the water by such an amount that significant density contrasts between the differing waters occurs which may result in density driven flow. In general EC values were raised to a maximum of 4 ms/cm. Following the addition of the salt, a small submersible pump with a variable flow control would be lowered to approximately 1m below the static water level and would commence abstraction at rates of between 500ml and 1l per minute. Flow rates were adjusted accordingly so as to not create excessive drawdown in the boreholes. Following the commencement of pumping, a Schlumberger CTD Diver logging tool capable of measuring fluid electrical conductivity, fluid temperature and probe depth at one second intervals was lowered down the borehole at a constant rate taking measurements of the abovementioned parameters at the specified time intervals. The logging tool would be lowered down the borehole (and subsequently raised back up) once every fifteen to twenty minutes for a period of three to four hours. Following the completion of the test the boreholes would be purged to ensure complete removal of the introduced salt and that ambient EC conditions were once again present in the boreholes. Figure 1 displays a typical field assembly required for conducting the profiling as well as key dilution and hydraulic response processes which occur during the FEC profiling.

4 Figure 1. A typical FFEC profiling assembly. Formation wáter will be primarily sourced from higher yielding zones resulting in increased dilution at these depths. 3. FIELD EXAMPLES Site 1 Petroleum hydrocarbon groundwater contamination investigation Site 1 is located within a fractured bedrock Karoo aquifer. Site investigations have confirmed that a release of petroleum hydrocarbon product has affected the shallow fracture system aquifer down to a depth of 8-12m below ground level. Several private residences within close proximity to the site have boreholes which are used for irrigation and domestic purposes, and were identified as potential receptors to dissolved phase contaminants which may be migrating off of the site under investigation. Regional hydrogeological information suggests that significant transmissive zones are located at depths below 25m below ground level (bgl) and are often the primary water bearing features which are intersected by boreholes within the region. FFEC profiling was conducted in order to determine whether the nearby residential boreholes are sourcing water from the shallow affected fracture system, or a deeper more transmissive system. The borehole was profiled for a period of three (3) hours whilst being pumped at a rate of 500ml/min. Figure 2 displays a typical FFEC profile of the residential boreholes surrounding the site. The profile suggests that downward flow was occurring in the borehole during the test, with formation water entering into the well at a shallow fracture zone at 8m bgl and migrating to a major transmissive zone at 30m bgl. Confirmation of residential boreholes intersecting both the shallow potentially affected fractures and the deeper unaffected transmissive zone below 30m bgl assisted in completing the source-receptor-pathway exposure assessment at the site.

5 Figure 2. Green arrows at 8m bgl and 30 m bgl indicate the presence of flow zones within the FFEC profile. Site 2 Chlorinated solvent groundwater contamination investigation Site 2 has a 40-year history of industrial and manufacturing use, most notably the historical storage and handling of chlorinated solvents such as trichloroethylene. Considering the site history, activities on the site are considered to have a high probability of having resulted in soil and groundwater contamination. The site is underlain by gabbronorite and norite mafic igneous rocks of the Main Zone of the Rustenburg Layered Suite. Upon commencement of the investigation, three (3) onsite boreholes were found to be present on the site. FFEC profiling of the three existing boreholes was conducted in order to obtain preliminary information regarding the location and depth of significant flow zones within the aquifer in order to develop a preliminary site conceptual model which would inform further phases of work. Downhole geophysics including caliper tool, gamma tool, EAL resistivity tool, acoustic televiewer tool, optical televiewer and spinner flow meter surveys were performed on the three boreholes in addition to FFEC profiling. Figure 3 displays the FFEC profile of one of the boreholes in conjunction with the associated caliper tool log and spinner flow meter log. Downhole spinner flow metering was conducted on the borehole whilst pumping the borehole at a rate of 0.75l/s. The FFEC data suggests two transmissive zones in the borehole as indicated by the observed dilution at the depths of 20m and 27m bgl. The flow zone at 20m is considered to be the dominant transmissive zone in the borehole. This is consistent with the caliper tool log and with the regional conceptual understanding that transmissive zones are generally present at the base of the weathered zone of the gabbronorites at approximately 20m bgl. Of interest is to note that the flow meter results were not able to detect the flow zones at 20m and 27m. This is considered to be partly due to the relatively low abstraction rate maintained during the flow meter test. It must be noted that within the context of contamination investigations the management of large volumes of potentially contaminated groundwater needs to be

6 considered, as disposal costs of contaminated groundwater can be considerable and may place limits on the volume of groundwater which can be abstracted and disposed of. Limitations on volumes to be abstracted will in turn affect the maximum possible pumping rate and duration of the tests. In this particular investigation approximately 2,000l of potentially contaminated water was generated during the spinner flow meter test, whilst only 110l of water was generated by the FFEC profiling technique. Figure 3. Green arrows indicate a major flow zone at 20m bgl and a smaller flow zone at 27m bgl. The accompanying caliper tool log is consistent with this however the spinner flow meter log does not detect the flow zones. Site 3 Remediation of private borehole impacted by petroleum hydrocarbon compounds Site 3 is located in a similar geological setting to that of Site 2 with the subsurface geology being dominated by the norites of the Main Zone of the Rustenburg Layered Suite. Historic petroleum hydrocarbon releases have affected several nearby private boreholes, with elevated concentrations of dissolved phase petroleum hydrocarbons being detected in the boreholes under consideration. Site investigations concluded that source zone remediation (i.e. reduction of contaminant mass on the site) would not be effective in mitigating the risks posed by the dissolved phase concentrations in the private boreholes and subsequent remedial intervention actions focused on mitigating the associated risks at the boreholes themselves. A key step in the investigation was to determine the location and transmissivity of flow zones within the boreholes. This would be followed up with depth discrete sampling in order to determine vertical contaminant mass distribution within the boreholes if multiple flow zones were identified. If it could be confirmed that only the upper section of the borehole was affected by dissolved phase hydrocarbon compounds and a deeper unaffected transmissive zone was present, then a potential remedial option would be to reinstall the boreholes and seal off the upper affected portion while only drawing water from the deeper unaffected zone. It was originally intended to use straddle packer testing

7 to obtain this information, but site specific limitations including access restrictions rendered the packer testing option unfeasible. FFEC profiling was thus used to obtain this information. The borehole under consideration was profiled for a period of four (4) hours whilst being pumped at a rate of 800ml/min followed by another profiling period of two (2) hours at 1,200ml/min. Figure 4 displays the FFEC profile data for the 800ml/min profiling step of the abovementioned borehole. The purpose of the multi-rate tests was to be able to model individual fracture transmissivities of identified flow zones within the borehole. The FFEC data suggests two transmissive zones in the borehole as indicated by the observed dilution at the depths of 22m and 28m below ground level. Figure 4. Green arrows indicate the presence of flow zones at 22m bgl and 28m bgl. Following the identification of the flow zones, the numerical model BORE II (Hale and Tsang, 1988; Doughty et al., 2000) was used to model the individual flow contributions for the identified flow zones at the different abstraction rates. Initial data such as borehole depth, feed point depth (i.e. flow zone depth), feed point EC (assumed from the baseline profile) and total abstraction rate are inputted into the model. The model is then calibrated to the real data (i.e. the FEC profiles shown above) by altering the feed point flow rates until the modeled data fits the real data. Upon achieving an acceptable fit, the modeled feed point flow rates are considered to be representative of the individual fracture fluid flow contributions and sum to the total abstraction rate as defined by the equation: Q Tot = q i Where Q Tot is the total abstraction rate and q i is the individual fracture flow rate. Using the multi rate flow data, individual fracture transmissivities could be calculated using the equation:

8 T i T Tot = q i Q Tot Where T i is the individual fracture transmissivity (variable to be solved), T Tot is the bulk transmissivity of the borehole (determined by a short duration pump test), Q Tot is the change in total flow rate between the two profile tests and q i is the change in individual fracture flow rate between the two profile tests (modeled from BORE II). Table 1 displays the key fluid flow contribution data obtained during the multi rate tests including the modeled individual flow contributions from the identified fractures zones, whilst Table 2 displays the calculated transmissivity values for the individual fracture zones. Table 1. Key fluid flow contribution data obtained during the multi rate tests Fracture Depth Profile Test Q 1 Tot (l/min) q 2 i (l/min) T 3 Tot (m 2 /day) 22m m Total abstraction rate measured during profile tests 2 Individual fracture flow contribution - modeled from BORE II 3 Obtained from short duration pump test of the borehole Table 2. Calculated transmissivity values for individual fracture zones Fracture Depth Q Tot (l/min) q i (l/min) T i T Tot T i (m 2 /day) 22m m A transmissivity range of 9-11m 2 /day was calculated for the fracture zone at 22m bgl whilst a transmissivity of 1-2m 2 /day was calculated for the fracture zone at 28m bgl. This is consistent with the regional hydrogeology where the most transmissive zones are often found on the interface between the weathered zone and the more competent underlying bedrock (at approximately 20m bgl), with deeper fractures generally being poorer yielding due to poor fracture development as a result of less weathering and increasing compressional forces as depth increases. The results of these tests assisted the project team in assessing the feasibility of remedial actions related to borehole replacement and selective borehole screening. 4. CONCLUSIONS The authors have presented three cases studies where FFEC profiling has been used to good effect in gaining a greater understanding of the distribution, density and transmissive properties of fractures present in boreholes which were profiled. The data from the FFEC profiling was generally complementary of other investigative techniques employed at the sites and in certain cases yielded more accurate information than what could have been achieved by other means. As with all investigations, the FFEC method is considered to be one of a selection of hydrogeological investigative tools which may be used to further characterize a site and develop the conceptual model and is best used in conjunction with other applicable techniques dependent on the objectives of the study. The technique does however hold

9 several attractions since it is relatively inexpensive to conduct, is simple in its execution and can be performed in a relatively short time period. Within the context of groundwater contamination investigations the technique holds further attractions since it generates relatively low volumes of waste water which would be required to be removed, treated and disposed of in an environmentally responsible manner. 5. REFERENCES Doughty, C., 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-46833, Lawrence Berkeley National Laboratory, Berkeley, CA Doughty, C., and C.-F. Tsang (2005), Signatures in flowing fluid electric conductivity logs, Journal of Hydrology., 310, Doughty, C., Tsang, C-F., Hatanaka, K., Yabuuchi, S., and Kurikami, H., (2008) Application of directfitting, mass integral, and multirate methods to analysis of flowing fluid electric conductivity logs from Horonobe, Japan Water Resources Research., 44, W08403, doi: /2007wr Hale, F.V., Tsang, C.-F., (1988). A code to compute borehole conductivity profiles from multiple feed points, Rep. LBL , Lawrence Berkeley Laboratory, Berkeley, CA. Kelley, V.A., Lavanchy, J.M., Lo w, S., (1991). Transmissivities and heads derived from detailed analysis of Siblingen 1989 fluid logging data, Nagra Tech. Rep. NTB 90-09, pp. 184, Nagra, Wettington, Switzerland. National Research Council (1996), Rock Fractures and Fluid Flow: Contemporary Understanding and Applications, Natl. Acad. Press, Washington, D. C. Tsang, C.-F., Hufschmeid, P., Hale, F.V., (1990). Determination of fracture inflow parameters with a borehole fluid conductivity logging method. Water Resources Research. 26 (4),