In-Situ Remediation of Brine Impacted Soils and Groundwater Using Hydraulic Fracturing, Desalinization and Recharge Wells Craig Robertson, M.Sc., P.Geol. 1, Ion Ratiu, M.Sc. 2, and Gordon H. Bures, M.Eng., P.Eng. 3 ABSTRACT The remediation of brine impacted fine grained soils present significant technical problems. Traditional recovery methods, including tile drainage and recovery wells are limited due to the low hydraulic conductivities of the soil matrix. Current designs either require very tight spacing, which can be technically and economically restrictive, require enhanced recovery methods such as mole drains or leave areas between the drains or wells that are not effectively leached. Other technical issues include leaching of salts from the vadose zone. This not only includes the zone above the historical water table but also the area above the drain inverts. This can be especially troublesome in semi-arid areas. The low hydraulic conductivities and resulting low flows require lone timeframes to achieve closure. Disposal of effluent is generally via deep well injection. Depending on transportation distances and disposal fees, this can be expensive. In an attempt to provide effective pathways for brine removal, flush salts from the vadose zone and reduce disposal cost; the combined technologies of hydraulic fracturing, desalinization and recharge wells has been employed to remediate brine impacted soils and groundwater. The remediation techniques used at the site include fracing the impacted soil zones at multiple depths and locations using hydraulic fracturing. A frac media is injected into the fractures to increase the permeability. Spacing of the fracing locations is selected to allow for overlap between fracture zones. Drainage is directed toward a gravel sump where the leachate is collected and treated using a flash distillation desalinization unit. This resulted in two streams, one for reapplication to the site and the second, concentrated brine, for disposal. Prior to reapplication the treated water is first blended to ensure chemistry compatibility with the receiving soils. The treated water is then injected at the upgradient end of the formation to enhance salt recovery. INTRODUCTION Remediation of brine impacted soils is a major issue in the western Great Plains area of Canada. As a result of oil and gas operations, highly concentrated brines have been released into the environment. Generally resulting from spills or pipeline breaks the highly concentrated fluids infiltrate into the soil where they leach down and enter the groundwater. For relatively course grained materials the traditional concept of leaching the soils with natural precipitation and recovery by either pumping wells or tile drainage systems has proven to be fairly successful. However, with fine grained soils, generally glacial tills and lacustrine deposits, the removal of 1 Wiebe Environmental Services Inc., Calgary, Alberta 2 GeoGrid Environmental Inc., Calgary, Alberta 3 Frac Rite Environmental Ltd., Calgary, Alberta
the impacted fluids becomes difficult and time consuming. Historically remediation of oil and gas facilities has centered on dealing with hydrocarbon issues while leaving the brine impacted soils and groundwater in place. Current regulations require that a site be remediated to the former land use capability. For many sites treatment and accumulated costs over decades has not been feasible. Therefore, alternative remediation options are being explored. The concept of improving the permeability of the soil along with leaching salts with the application of irrigation water was considered for a former battery owned by Canadian Natural Resources Limited (Canadian Natural) in the Calmar area of central Alberta. BACKGROUND Development of the oil and gas reserves in the Calmar and Leduc fields started in the early 1940 s and the Calmar Battery likely began operations in the 1940 s. Development and environmental information about the site and surrounding area is limited since Alberta Energy and Utility Board (EUB) facility files and spill records only date back to 1975. Files obtained from the Surface Reclamation Council of Alberta do indicate salt water spills occurring in 1962 and again in 1966 due to operations at the Okalta Oil Ltd. battery and associated salt water pipelines. A surface reclamation certificate was issued on 1972 to Oakwood Petroleum Ltd. by the Department of the Environment under Section 15 of the Surface Reclamation Act. Figure 1: Aerial Photograph 1962
An aerial photograph search indicates a facility was present in 1949. Aerial photos for subsequent years indicate the site was decommissioned by 1969. Figure 1 presents the layout of the facility as it appeared in 1962. Through the acquisition of Sceptre Resources in 1996, Canadian Natural has retained responsibility for environmental issues pertaining to the former Calmar Battery. The current land owner had reported to Alberta Environment (AENV) a lack of vegetation and crop growth in the general vicinity of the former battery site. AENV contacted Canadian Natural in mid 1998 specifying the land owner s concerns. In September, 1998 Canadian Natural contracted Wiebe Environmental Services Inc. (WES Inc.) to conduct an environmental inspection of the site. The inspection found stressed vegetation in certain areas and soil probing with a hand-auger indicated hydrocarbon contaminated soil at a depth of approximately 30 to 40 cm below ground surface. A preliminary Environmental Site Assessment (ESA) was conducted in 1999 to determine the likely cause and extent of impacted soil and groundwater. A detailed ESA was completed in 2004. Characterization of the subsurface conditions of the Calmar Battery included conducting an electro magnetic (EM) survey, advancing conductivity probes, drilling of 137 boreholes and advancing 100 test-holes. The investigations were conducted to a maximum depth of 7.5 m below grade. In order to identify potential impacts on groundwater quality, and assess the Figure 2: Site Diagram Showing Phase II Site Assessment Program
characteristics of the shallow groundwater table, 15 boreholes were completed as groundwater monitoring wells. The areas under investigation are presented on Figure 2. The conclusions of the investigation were as follows: A volume of approximately 8,000 m 3 of heavy end hydrocarbons, trace elements (metals), and heavily brine impacted materials was identified in the former tankfarm (Area 1), and the buried flare pit (Area 2) areas. Evidence of residual brine constituents was identified within the soil profiles of the northwest (Area 4), and the southwest portions of the lease (Area 6). A volume of approximately 62,000 m 3 of brine impacted materials with potential for in-situ remediation through non-disruptive methods was anticipated for Areas 1, 2, 4, and 6. Figure 3: August 5, 1962 Aerial Photograph
Figure 4: EM31 Conductivity Data Figure 5: Direct Push Conductivity Cross Sections
A volume of approximately 1,000 m 3 of medium to light end hydrocarbon impacted materials was identified in Area 10. The groundwater flow direction in the unconsolidated clay deposits is predominantly northwest. Groundwater flow direction in the mudstone bedrock appears to be to the southwest. The average linear groundwater flow velocity is estimated to range between 3 and 6 mm/year. Brine impacts on groundwater quality were identified in the former tankfarm and the buried flare pit area at monitoring wells P03-1, P03-3, P03-5, and P03-13. Regions of elevated EM31 conductivity values, suggesting the presence of inorganic impacts were identified within the former tankfarm and the buried flare pit areas (Figure 3). Areas of adjacent low and high EM31 values were identified in the west and southwest and were attributed to the presence of buried metal objects (i.e. pipelines) (Figure 4). Vertical conductivity and EM31 data correlate well in delineating the lateral extent of the inorganic impacts. Potential impacts appear to extend to 6.0 m below grade (Figure 5). REMEDIATION PLAN Detailed environmental site assessments conducted at the Calmar Battery characterized the environmental status of the former lease and the adjacent areas. The assessment programs identified the presence of a relatively extensive brine impacted plume within the former tankfarm and the buried flare pit area. A phased remediation and reclamation plan was recommended for the Calmar Battery. The initial stages of the remediation program targeted the removal of the heavy end hydrocarbons, trace elements (metals), and the most heavily brine impacted materials from the former tankfarm and the buried flare pit areas. Residual brine impacts on soil and groundwater quality remaining at the limits of the excavations were addressed through in-situ remediation. Monitoring and remediation activities will continue until satisfactory remediation criteria are achieved. DECOMMISSIONING The decommissioning program included removal and landfill disposal of the hydrocarbons, trace elements (metals), and the most heavily brine impacted materials. The excavated materials were stockpiled on-site for landfill disposal. Confirmatory samples from the limits (base and sidewalls) of the excavations were collected to verify the removal of the impacted materials and to determine a base line for the remediation program. To remediate the residual brine impacted materials remaining in-situ at the limits of the excavations, a leachate recovery sump was installed in the central portions of the former flare pit. To facilitate the recovery of impacted groundwater a volume of approximately 650 m 3 (1059.04 tonnes) of? 1½ diameter washed rock was placed in the central portion of the flare pit excavation. For water recovery, a 610 mm (24 ) diameter galvanized steel culvert was installed.
Figure 6: Leachate Recovery Sump Installation Detail Figure 7: Leachate Recovery Sump
The leachate recovery system was backfilled with clean imported subsoil. On-site originating topsoil was replaced over the disturbed area and recontoured. Refer to Figure 6 and Figure 7. Lease cleanup operations included removal and disposal/recycling of the abandoned buried lines. All steel piping was removed from the lease and transported to an appropriate recycling facility. At the landowner request, the cobbles, larger aggregates, and the industrial debris scattered over the lease were periodically removed and landfill disposed during the 2005 summer season. Weed control completed the lease cleanup activities. HYDRAULIC SOIL FRACTURING Hydraulic fracturing is a process whereby a fluid is pumped into a well at a rate and pressure high enough to overcome the in-situ confining stress and the material strength of a formation (i.e. soil or rock) resulting in the formation of a fracture. This process has been used for decades in the petroleum industry to enhance production rates from low-yielding oil and gas wells. When a hydraulically fractured well is produced, the induced fractures serve as pathways to promote fluid flow at greater rates than would otherwise be possible. The fractures also increase the effective drainage area of wells, such that fewer wells are required for extraction from a petroleum reservoir. Hydraulic soil fracturing technology was adapted for environmental applications in contaminated soils and groundwater in the early 1990s. In practice, a slurry mixture containing a proppant (sand) and a viscous fluid (guar gum and potable water) is pumped under pressure into subsurface soils to create a fracture. Fracturing is facilitated by the use of specialized down hole fracturing equipment designed for various soil types, drilling configurations, and operating pressures. After the slurry has been placed in the subsoil, sand proppant holds the fracture open while an enzyme or chemical additive breaks down the viscous fluid. The fluid subsequently drains from the fracture pathways, leaving only permeable sand inside the induced fractures. Multiple fractures emplaced by soil fracturing intersect the system of natural, vertical microfractures that often exist in clays, thereby creating a network of drainage pathways. The fracture network increases the bulk permeability of subsoil, thereby increasing contaminant recovery or treatment rates. Hydraulic soil fracturing has been successfully applied to accelerate site cleanups with conventional remediation technologies and enhance geotechnical dewatering rates for slope stability and foundation applications. Knowing the configuration and geometry of individual fractures initiated and propagated in subsoil is extremely useful for assessing the subsequent performance of remediation technologies at fracture-enhanced sites. This is especially true in cases where the soil fracturing process includes delivering chemical or biological amendments for expediting in-situ remediation. Fractures placed in the subsurface can be mapped remotely using surface mounted tiltmeters. Tiltmeters are highly sensitive instruments (to microradian resolution) used in the hydraulic fracturing industry to measure the minute ground surface deformations created during the
fracturing process. The direction and magnitude of ground surface deformation or tilt measured by tiltmeters is used to determine the shape, thickness, extent and orientation of fractures in the subsurface. This information is useful in confirming the extent and coverage of fractures placed in the subsurface, as few fractures ever propagate in a perfectly radial or horizontal manner. Typically, an array of 12 to 25 surface-mounted tiltmeters is set up in a grid or concentric configuration around each fracture borehole location. The tiltmeter spacing and configuration is determined by the number of fractures placed per borehole and the depth of fracture placement. During fracturing, ground surface tilt is measured at each tiltmeter station and the information is stored in on-site dataloggers. Tiltmeter data is downloaded onto laptop computers in the field and subsequently analyzed and interpreted using principals of tiltmeter geophysics. The output is generated into three dimensional graphical representations of individual fractures using advanced computer modelling software. These images can be user-manipulated via computer output files to view individual fractures, or the entire fracture network, from any perspective in space (Figure 8). Figure 8: Conceptual Model of Hydraulic Fracturing The project involved the creation of a hydraulically connected subsurface drainage network in clay soils within the 1.5 ha area salt plume. This involved fracturing salt-impacted soils at vertical increments of 0.5 m to 1.0 m to the bedrock surface located at approximately 4.5 metres below the ground surface. A total of 221 horizontal fractures each containing almost one tonne of sand were emplaced between 2 to 4.5 metres depth at 64 borehole locations in clays. The network of horizontal sand drains emplaced in this manner was hydraulically connected to a gravel filled sump in the former flare pit area. Fracture mapping was conducted on 47 fractures at 13 fracture borehole locations to confirm the extent of fracture placement and connectivity.
Fracturing break pressures (i.e. pressure at which soil parts or breaks and fracture propagation begins) typically ranged from 1,000 kpa to 1,500 kpa for the majority of fractures. Break pressure is a function of the depth of fracture initiation, soil cohesion, slurry viscosity, and sand loading. The results appear to be consistent with shallow overburden pressures and cohesive strength values typical of clay soils. The overall placement efficiency of sand fractures (i.e. ratio of sand emplaced to sand pumped) was 96 %. Some sand slurry emerged through the ground surface in the latter stages of pumping in shallow fractures initiated at 2.0 metres depth. This pattern appeared to be consistent until the starting fracture depth was increased to 2.5 metres, whereupon all sand slurry stayed emplaced in the subsoil. The results of fracture mapping to determine fracture orientation and geometry are presented based on analysis of tiltmeter geophysical data. Tilt vectors measured at each tiltmeter station were used to model each fracture as a discrete planar feature. Pressure-time plots and slurry flow rate data were likewise considered in the interpretation of the tiltmeter data. The tiltmeter data was processed to produce a three-dimensional depiction of fractures placed at each fracture borehole location. The results of fracture mapping on 47 fractures indicated a preferred trend in fracture orientation along a southwest-northeast axis. Approximately 80 % of fractures exhibited this orientation while 20 % of fractures followed a southeast-northwest trend. Fractures generally tended to propagate preferentially northwards (63 % of fractures) than southwards. Tiltmeter data analysis indicated that two-thirds of fractures mapped were near-horizontal or slightly ascending to the ground surface (Figure 8). Approximately one-quarter of the fractures mapped were classified as moderately ascending, while less than 10 % of fractures were classified as strongly ascending to the ground surface. The steepest ascending fracture was 52 which occurred at 4.4 m depth in FB16-4. INJECTION WELLS Injection wells were selected as the means of inducing recharge and return flow for the flushing system. Two factors were decisive in the selection. The upper 1.5 metres of soil was not impacted and the fracture pattern was installed in a series of progressively deeper horizontal layers. In order to by pass the upper soil zone and to maximize the exposure of the infiltrating water to the fractured medium vertical injection wells were installed around the perimeter of the contaminated area. The injection well locations are shown on Figure 9. With the pumping of the culvert installed in the leachate recovery sump near the centre of the impacted area and injection of treated water around the perimeter the hydraulic gradient would result in radial flow from the injection wells to the leachate sump. This would result in the flushing of salts from the area around the fractures. The water treatment system was located on the edge of the field near the access road adjacent to a secondary highway. This allowed access to electrical and natural gas utilities and allowed for
Figure 9: Injection Well Location Map Figure 10: Injection Well Installation Profile
trucking of waste water. The location also reduced restrictions on the farming operations. A sump pump was placed in the recovery culvert with electrical and discharge lines buried below grade. Once the water was treated it was returned to the injection wells by gravity feed through buries return lines attached to each well through a manifold. To allow maximum use of the arable land, the recovery culvert was completed below grade (Figure 10). WATER TREATMENT The selected water treatment method consisted of a flash distillation unit. The process works on the principle that liquids boil at a lower temperature when placed in a vacuum. The collected saline leachate is placed in a concentrate container (Figure 11). The leachate is then pumped through a heat exchanger where the temperature is raised to approximately 60 o C. The water stream in then sprayed into a vacuum chamber where distillation takes place. The evaporated water is passed though a condenser and the recovered distilled water is stored in a clean water tank. Water that does not volatilize in the chamber is returned to the concentrate tank and pumped back through the system. The process continues until the concentrate reached a desired level of salinity when it is discharged into a waste water disposal tank and a fresh batch of leachate is introduced to the system. Flash distillation has been used extensively for the recovery of metals in the plating industry. The construction materials are very resistant to corrosive liquids. The major advantage of flash distillation over most other available systems is that it can handle brine concentrations significantly above reverse osmosis capabilities and uses less energy than traditional distillation units. Identified concerns to date include controlling discharge concentrations and scaling. Figure 11: Flash Distillation Process
The configuration of the treatment pad consists of a corrugated metal berm constructed to contain three storage tanks. The tank configuration consisted of a leachate tank, a treated water tank and a waste water tank. The flash distillation unit is mounted adjacent to the tanks and is connected to electrical and natural gas utilities, the leachate inflow and pump power lines and the injection well outflow lines. CONCLUSIONS To date the infrastructure and equipment have been installed at the site. The distillation unit has being test operated to maximize operating efficiencies. Full scale operation is expected to commence in the spring of 2007 with remediation expected to take several years.