THE MECHANISMS BY WHICH THE MARCELLUS GAS SHALE SEQUESTERS RESIDUAL TREATMENT WATER (RTW)

The international rush to produce gas within shale by horizontal drilling and massive slickwater hydraulic fracturing (i.e., fracking) has prompted concerns that fracking may harm global health through groundwater contamination. Concerns involving water quality arise because as much as 2 x 10⁴ m³ of water-based fluid with various treatment chemicals is injected into gas shale to open fractures near each horizontal well. Once fractures within the gas shale are opened, the treatment water reacts with constituents of the shale to gather up salt, some metals, and a few radionuclides. During the lifetime of a gas-shale well less than half of the treatment water is recovered as flowback or later production brine. While recovered treatment water is easily managed in closed-loop systems at the surface, residual treatment water (RTW) slips beyond the control of engineers. Presently, the fate of this RTW, more than 10⁴ m³ per horizontal well, is uncertain. The question is whether the RTW might someday appear in groundwater by natural upward migration. The answer is that it is unlikely or never on time scales that matter for several reasons, the most important three are: 1.) the lack of an upward transport mechanism; 2.) the presence of a long-term seal; and 3.) the ability of a shale to imbibe RTW. First, while not a mechanism for sequestration, per se, flowback and subsequent production immediately reduce pore pressure in the vicinity of the borehole, thus setting up an inward pressure gradient. For RTW to migrate upward along natural fractures, for example, an outward pressure gradient is necessary. Second, if imbibed pervasively in the stimulated reservoir volume, the RTW could be absorbed by a porosity of < 1%. Spontaneous imbibition is a form of wicking made possible by a high capillary pressure within the water-wet matrix of very fine grained rocks like gas shale. Third, the Marcellus is a good seal. The strata surrounding the Marcellus contains water, and so a capillary seal was formed at its margin. Such a capillary seal forms automatically in the presence of a porous media with variations in porosity and two fluids. The seal is virtually indestructible, and for this reason traps fluids in basins virtually forever. If RTW is caught in the tip regions of fractures, even less imbibition is necessary for total sequestration.