SKA was retained by the Federal Reserve Bank of Dallas, Houston Branch to design and install a remediation system to recover a gasoline product plume from beneath the planned location of a new post 11 September 2001 Federal Reserve Bank building, which began construction in May 2003 in Houston, Texas. The recovery system included two dual phase extraction horizontal wells which replaced a vertical well recovery system previously installed at the location of the planned building.
The horizontal well dual phase extraction system was designed using a numerical hydrogeologic flow model which was constructed using a 3-D geologic model constructed for the site and hydrogeologic test data.
The design addresseed the unique challenges of the site which included:
- time constraints for building construction;
- limited access due to the planned building structure;
- subsurface soil heterogeneities;
- hydraulic gradient; and
- the need to integrate vapor and water extraction function from a horizontal well. The horizontal wells are unique in their configuration because they serve to both dewater beneath the building and extract hydrocarbons in the vapor phase.
Because of the unique client requirements to stage all remediation equipment offsite on adjacent City of Houston property, each blind horizontal well terminated by intersecting a 20-inch-diameter gravel backfilled sump 40 feet below grade. A curvilinear well geometry design was used to enhance plume capture within the logistical constraints of the site and future building structure. To take advantage of the hydraulic gradient beneath the building the well screens were installed inclined in the direction opposing the dip of the water table at the site. This created a clean break between the saturated screen for dewatering and exposed screen for soil vapor extraction. In addition, an approximately one to three percent grade of the horizontal screens facilitated gravity drainage of the liquids. The irregular well path placed the screen within clean producing sand units in an alluvial sedimentary system that contains interbedded sand and clay.
The use of horizontal wells enabled construction of the Federal building to proceed on schedule and unconstrained by environmental issues. The design and implementation of this program draws on the specialized capabilities and unique experience of the project team. The system began operation in October 2003 and the site received final regulatory closure in January 2006.