Miljøprojekt, 1113; Teknologiudviklingsprogrammet for jord- og grundvandsforurening – Hydraulisk frakturering udført ved vandret boreteknik

Hydraulisk frakturering udført ved vandret boreteknik

Summary and conclusions

At the test site, a heavy contamination with chlorobenzene, dichlorobenzene, dichloronitrobenzene, anilines, BTEX and ammonium/ammonia has been detected. Furthermore, chloride levels are high.

The contamination has been located from the soil surface to a depth of approx. 5 meter below ground level (blg), whereas the horizontal distribution of the contamination is estimated to make up an areal of approx.500 m².

The site geology comprises fill in the upper 1-2 meter bgl and beneath this moraine clay to a depth of approx. 23 meters blg, where the “Danien” chalk layer is found. At a depth of 14-18 meter bgl, lenses of moraine gravel are located in the moraine clay. This layer is water-bearing, separated from the primary groundwater reservoir (in the chalk aquifer) by a 5-7 meter thick moraine clay layer. The quartenary moraine clay sediments comprise a continuous secondary water reservoir. The primary reservoir is artesian with a hydraulic pressure level 1.5-2 meter blg.

Before the remediation action was initiated, the hydraulic pressure level in the top of the moraine clay (approx. 2 meters bgl) was approx. 0.5-0.75 meter higher than measured in the primary chalk reservoir. Repeated water table measurements of wells screened in the upper and lower part of the moraine clay formation demonstrated a downward vertical pressure gradient of approx. 0.03 m/m within the ’hot-spot’ area.

The objective of this project is to verify whether hydraulic fracturing is a cost-effective remediation technique in a low permeable moraine clay deposit.

During the detailed system design phase, it was decided to install 2 horizontal drainpipes 5 meters blg in fractured moraine clay in preference to 3-5 horizontal drainpipes installed in non fractured moraine clay deposit. Each drainpipe measures 35 meter.

Remedial pumping of water and soil gas from the drainpipes is carried out by a Dual Phase Extraction (DPE) technique. Both water and gas is thereby treated prior to discharge into the recipient and the atmosphere.

By choosing this remediation technique, it was expected that an upward gradient could by established between the primary and the secondary water reservoir within the ‘hot-spot’ area. Additionally, the fracturing technique was expected to be cost-effective, due to the reduced number of drainpipes needed.

In this report, the remedial and operational experiences achieved during the first year of remediation on the site are presented.

Due to a request from the site owner, a unfractured reference drain, originally installed to assess the hydraulic effect obtained by hydraulic fracturing of the 2 drains unfortunately had to be sealed off. Therefore, it has not been possible to state the time-related development of the hydraulic effect from drainpipes installed in both fractured and non-fractured moraine clay deposits as originally planned. Additionally, the delayed sealing of a production well screened in the primary chalk aquifer meant that the conditions for the implemented remediation technique were not optimal during the first approx. 12 months of operation.

The primary experiences achieved in the operation period from late November, 2000 till early April, 2002 are summarized below:

During the first 9 months of operation, an overall downward pressure gradient is observed between the upper and the lower part of the moraine clay deposit and most possibly also between the secondary and the primary aquifer. The latter statement has not been possible to document, as no monitoring wells screened in the primary aquifer are present at the test site.

After approx. 9 months, an overall upward gradient developes as intended in the remedial design. This tendency is observed from ultimo September, 2001 until April, 2002, and only two monitoring wells (MB4 and MB5) deviates from this pattern.

The fractured drain system affects a zone on either side of each drain of up till approx. 4.5 meter, in accordance with the design parameters and results predicted by the modelling /4/. In the northernmost section of each drainpipe the affected zone is reduced due to the fact that no fractures were implemented here.

A total average of approx.1.3 m³ water is drained per day from the 2 horizontal drainpipes and the hydraulic capacity of the drainpipes is very constant with time. An attempt to increase the hydraulic capacity by acidifying the drains did not succeed. The overall hydraulic tendency is positively supported by the results of 2 Dual Phase Extraction Test carried out on drainpipe A-A with intervals of 20 months. Unlike the test results obtained from drainpipe A-A, test results for drainpipe B-B are not directly comparative, due to the fact that heavy rainfall occurred during the 2nd test period using this drainpipe.

Prior to the implementation of the remediation technique, the hydraulic capacity of a fractured drain was estimated to be 3-5 times the capacity of a non-fractured drain on the basis of Groundwater modelling, see /ref. 4, chapter 3.4.2/. Unfortunately, this proportional relation has not been demonstrated in practice, as only 20-40% of the expected hydraulic capacity has been achieved as yet. This fact has caused a delayed hydraulic effect of the remediation technique. Furthermore, the delayed sealing of the production well screened in the primary aquifer has affected the technique in a negative way, too, since this well produced approx. 5-10 m³per day until its closure in December 2001.

With an average zone of affect for each drain of up to 4.5 meters, the total influential area of the 2 drainpipes corresponds to an approx. area of 600-800 m². Based on the actual rainfall data and the present types of consolidation on the site, approx. 5-15% of the drained water volume is considered to be represented by infiltrated precipitation, whereas the remaining portion is assumed to originate from the ground water. A high degree of uncertainty is associated with this estimate – and future measurements will try to verify this estimate, and possibly contribute to a more accurate quanfication concerning the origination of the inflow to the drains.

Except from the concentration level of anilines, ammonium/ammonia and chloride, no clear drop in concentration is detected for the primary contaminants. Moreover, an overall tendency to increasing concentrations of primary contaminants and chloride is found in monitoring well no. MB-2(lower screen section) and MB-5(lower screen section). These wells are both located 4.5 m away from the drainpipes and both screened approx. 4.5 meter blg – which is identical with the installation depth for the horizontal drainpipes.

With the exception of dissolved oxygen and pH, concentrations of the redox related parameters decreased in both drainpipe samples and samples representing the level in adjacent monitoring wells.

The development measured in the upper part of the moraine clay deposit is found to be controlled by the change in the pressure conditions of the aquifer as well as by the natural dilution of the polluted water body. In the lower part of the moraine clay deposit, represented by the lower screen section of monitoring well no. B17 (screened 14-16 m blg), the same overall pattern is documented with respect to both development in the contamination levels of primary contaminants and redox related parameters.The absolute change in concentrations are small and makes it difficult to draw any conclusions.

After approx. 14 months of operation approx. 25 kg of primary contaminants and approx. 800 kg chloride are estimated to be remediated from the moraine clay. The average removal rate has been approx. 0.08 kg per day for the primary contaminants. The mass flux rate has dropped from 0.1 kg per day till 0.03 kg per day; the former value being measured 10 days after initiating the remediation, and the latter value representing the mass flux after 310 days of operation.

Whether or not 3-5 non-fractured horizontal drainpipes would have been more effective is hard to estimate. The main reason is that no DPT could be done on a drainpipe installed in a non-fractured and immediately comparable geological formation.

Assuming that the operation of 3-5 non-fractured horizontal drainpipes would have increased the hydraulic capacity of the remediation system the total operation period might have been reduced.

Various technical problems have occurred on the treatment plant, e.g. clogging of the raw water pump and adsorption of iron, presence of air in the vacuum tank, and errors in the data transmissions between the treatment facility and the client’s monitoring system. Despite the operation problems, all primary contaminants have been treated to a level, which meets the discharge permit conditions, whereas heavy metals (zinc and cadmium) still causes problems.

At present, the treatment facility is therefore being optimised with regard to removal of zinc and cadmium. The operation costs per treated m³ of contaminated groundwater have been approx. 1,000 DKr. If alternatively, 3-5 non-fractured drainpipes had been installed, the operation costs per extra drainpipe is estimated to be approx. 15% higher than the present unit cost per fractured drainpipe.On the other hand installation of extra drainpipes might have caused an increased hydraulic capacity of the remediation concept and lowered the total operation costs. There is, however, no information to confirm these aspects, as the effect of non-fractured drainpipes has not been tested on the site.

Based on the experiences obtained so far, it seems that hydraulic fractured drainpipes do not meet the hydraulic capacities as specified in the design base. However, even with the relatively low hydraulic capacity of the remediation system, data obtained strongly indicates that the intended hydraulic effect on the pressure level in the water reservoirs has been accomplished within the ‘hot spot’ area. Further operation of the system will verify whether this tendency will be continue.