CASE STUDY: Diesel Remediation in Central Saudi ArabiaRecovering LNAPL with varied transmissivity from a perched aquifer.
At an active bulk fuel facility in central Saudi Arabia, historical spills and leaks had resulted in up to 2.8 metres of diesel floating on a perched water table. The site and geographic location required a flexible, low-infrastructure solution that could withstand the extreme climactic conditions without interfering with the facility’s operations.
Location: Central Saudi Arabia
Client: Saudi Arabian Environmental Consulting Firm, on behalf of a major Oil & Gas Company.
Duration: 3 years
Project Value: $350,000 CDN
Geology: Perched aquifer, Silty sand.
The oil & gas company hired a local environmental consulting firm to characterize the site. Characterization revealed two important factors:
- Perched aquifer: The LNAPL (Light Non-Aqueous Phase Liquid) was located on a perched aquifer. Ensuring that the underlying aquitard was not penetrated during remedial activities was imperative to containing the diesel.
- Varied NAPL transmissivity: NAPL transmissivity testing indicated transmissivity varied by 4–5 orders of magnitude. Despite thick layers of LNAPL (> 2 m), the LNAPL would not flow into the extraction wells quickly; so, an aggressive technology like dual-phase extraction would not work.
On behalf of their client, the consulting firm tendered an initial pilot project to three companies for bid. IRSL earned the pilot project based on their price and innovative approach.
Vacuum-Enhanced Pneumatic Skimming
To meet the challenges associated with this site, IRSL recommended extracting the LNAPL through vacuum-enhanced extraction wells instrumented with pneumatic skimmers. A full-time engineer remained on-site for the project’s 3-year duration to continuously adjust and optimize the system to compensate for the varying degrees of NAPL transmissivity.
Free product removal was completed through vacuum extraction wells instrumented with pneumatic skimmers equipped with hydrophobic filters. The filters allowed only the LNAPL to pass, leaving the water behind, while the vacuums increased the radius of the skimmers.
This system was flexible enough to fulfill a number of important criteria:
- Little infrastructure, and minimal power, leaving a very small ecological footprint.
- Minimal equipment to be transported to the site also meant minimal maintenance on-site.
- The vacuum system and skimmers could be easily moved, adjusted and optimized.
- The efficiency of the filtration negated the need for a treatment system.
Perched Water Table
A Perched Water Table is a temporary accumulation of groundwater that is above the water table in the unsaturated zone. The groundwater is usually trapped above an impermeable soil layer, such as clay, and actually forms a lens of saturated material in the unsaturated zone.
Transmissivity is a function of the mobility of the NAPL and aquifer. The higher the transmissivity, the better able to recover the NAPL. Lower transmissivities require slower, lower maintenance, and less operationally intense technologies because the NAPL doesn’t flow into the wells as quickly. This characteristic does not vary over time.
A number of conditions made this project exceptionally challenging:
- Large variations of 4–5 orders of magnitude in the NAPL’s transmissivity across the site required proactive, extensive, and continuous optimization with regard to cycle length and numbers, as well as skimmer position.
- Careful attention was required to avoid penetrating the aquitard and spreading contamination.
- The remote location necessitated minimal and low-maintenance equipment as the equipment, and its replacement parts, had to be transported from North America.
- The extreme climatic conditions required uncomplicated equipment designed to withstand very high temperatures, humidity, corrosion, and dust.
- The active bulk fuel facility’s rigorous health and safety regulations required all equipment to be explosion-proof and highly secured.
- Over 71,000 litres of diesel were removed over the course of the project.
- The extracted liquid was greater than 99.997% diesel, minimizing the need to treat extraneous water and translating into real cost savings for the client.
- The purity of the recovered diesel enabled the company to stream it into their
production, effectively paying for the project.
- The pilot project was deemed a complete success, initiating plans for several full-scale projects.