Formatted Title

LNAPL Recovery from Low Permeability Strata by Means of Sand Layer Injection Using i-SAV^© Technology and Non-Ionic Surfactant Application

Background/Objectives

In a pilot test sand layers with a total of 8.5 tons of sand were emplaced in the contaminated subsoil by means of hydraulic stimulation (i-SAV^© technology) at one injection bore. Nine sand layers in a depth section between 16.6 m and 19.6 m were successively constructed at vertical depth intervals between 30 and 50 cm. The subsurface site strata in which the sand layers were emplaced consists low-permeable partially fissured fine sediments such as silts, clays and sands as wells as carbonates, and residual sulphates (gypsum), which were deposited in a shallow deltaic and lagoon environment during mid-Keuper times..

The viscous suspension of drinking water, carrier medium and sand as well as other additives were prepared in a mixing unit and immediately afterwards injected. The injection pressure exceeds the cohesion forces of the silty/clayey rock allowing the suspension to penetrate the rock as a dedicated layer.

Approach/Activities

The addition of non-ionic surfactants sourced from Ivey Inc. at concentrations below the critical micelle concentration improve the extraction of the gasoline phase (LNAPL) by mobilization while reducing the interfacial tension (IFT) of the water-oil system and changing the wettability (i.e. the wetting angle) of the strongly weathered sedimentary rock. As a consequence, the reduction of the IFT leads to a reduction in entry pressure and capillary number. The capillary number defines the ratio between the frictional forces (viscosity forces) and the capillary forces (interfacial tension). A reduction of the IFT has a particular effect on the continuous mobile LNAPL as it reduces the capillary forces, thereby increasing the capillary number. Therefore, the sand-gel was coated with a surfactant-water mixture immediately after injection to make the gasoline phase available to the sand layers.

The pilot test was aimed at assessing the effect of sand layers in combination with a surfactant addition for extraction of the mobile available gasoline phase from the low-permeable rock.

Results/Lessons Learned

The removal of the gasoline phase in the area of the pilot test was carried out after the placement of the sand layers by vacuum enhanced skimming (VES) and vacuum enhanced recovery by dual phase extraction (DPE). From the extraction well, directly at the location of the injection borehole, approximately 1,230 L of gasoline phase could be extracted over a period of 3 months. The output was far above the predicted extractable volume in the pilot test area. For comparison: Only 80 L LNAPL could be extracted via an extraction well located at 5 m distance from the injection borehole, but not entirely within but marginally intersecting the sand layers.

The coverage of the sand layers and their extent were recorded by means of tiltmeter measurements. In summary, a maximum radius of influence (ROI) of 6 m was recorded for most of the nine fracs.

In order to answer the question of whether a detectable increase in permeability of the sedimentary rock for the extraction of the mobile gasoline phase was made possible by the sand injections, existing data from CPT soundings and results from the groundwater potential measurements before and after as well as during the sand injections were evaluated. In addition, a small, detailed groundwater model (Visual Modflow) was created and LNAPL withdrawal evaluations were carried out with the program LNAST.

The sand layers injected have a permeability K of 2E-4 m/s to 1E-3 m/s and are significantly more permeable compared to the formation (K=1E-6 m/s to <1E-7 m/s). Calculations indicated that the average rock permeability in the injection interval was increased by 90 times and the porosity by 50% by the introduction of nine layers of sand with a cumulative thickness of 10 cm. The permeability of the aquifer with respect to the gasoline phase should therefore also increase by more than two orders of magnitude.

The additional effective porosities generated by the sand layers in the order of 2 m³ act like a macropore enabling the passive penetration of mobile LNAPL into this macropore/sand layers, if the penetration resistance is exceeded and the capillary number exceeds a certain threshold. This means that up to 2,000 litres of LNAPL can theoretically be stored in the sand layers. Compared to an exclusive increase in permeability, the combination of increased storage capacity and increased permeability enables the mobilization and thus extraction of larger quantities of gasoline phase. The injection of sand layers increased the average rock permeability by a factor of 6 compared to the initial situation.

The addition of surfactants at a concentration of about 0.7 wt% without the introduction of sand layers reduces the permeability of the aquifer to LNAPL by about 40% due to the assimilation of water into the LNAPL phase. Though adding surfactants to the high permeable sand layers, the entry pressure into the coarsely porous and permeable sand layers is reduced. In correspondence with a large pore throat radii of the emplaced sand a higher capillary number was achieved allowing an increased mobilization of the LNAPL phase in and out of the sand layers resulting in a higher LNAPL extraction.

We conclude that a significant LNAPL recovery can be achieved from low permeability strata by means of sand layer emplacement using i-SAV© injection technology and non-ionic surfactant application.