Numerical simulation of deep-well injection in Geismar, Louisiana

Industrial waste water has been injected into a permeable part of the Upper Miocene injection reservoirs near Geismar, Louisiana since 1971. The injection zone is a series of alternating extremely permeable sands and impermeable clays, hydraulically isolated from the surficial aquifer system by a thick layer of clay, which acts as a confining layer. The study area encompasses four industrial facilities and 12 active injection wells. Numerical simulations were performed to evaluate injection-well design and potential for movement of injected waste water within the hydrogeologic system. Numerical modeling was conducted in both cylindrical coordinates and Cartesian coordinates. A convection cell developed around a single prototype injection well in cylindrical-coordinate system with a buoyant injection rising to form a lens within the injection zone. Around an ideal, fully penetrating well cased into the injection sand and open from a depth of 5,056 feet to 5,456 feet, simulations show the injectant moves upward to a depth of 4,866 feet, forms a lens about 400 feet thick, and spreads radially outward to a distance of 1,670 feet after 10 years. Comparison simulations of injection through wells having open depth intervals of 5,056 to 5,256 feet and 5,256 to 5,456 feet demonstrate that such changes in well design have little effect on the areal spread of the injectant lens or the rate of upward movement. Multiple injection well simulations in Cartesian coordinates indicate that about 5, 10, and 80 percent of the study area would be underlain by injected waste water after 10, 20, and 100 years of injection, respectively. Outward movement of waste water proceeds even after injection stops. The rate of outward movement under the post-operational period (after injection ceases) is much smaller than under the operational period (injection period).