Experimental and Theoretical Modeling of Expansion in Pyritic Shale: Fid-Bau Portal

Experimental and Theoretical Modeling of Expansion in Pyritic Shale

Hoover, Shad E

Expansive pyritic shales are found in black carbonaceous shales throughout the United States as well as in other countries, including Ireland, England, Norway, Canada, and Sweden. Expansion occurs when the pyrite, which occurs either as finely disseminated syngenetic framboids, macroscopic crystals, or diagenetic replacement fossils, oxidizes to form sulfuric acid. Various hydrous sulfates could precipitate in the complex geochemical environment; however, gypsum typically precipitates as the sulfuric acid reacts with the calcareous (calcium carbonate) component of the shale. This paper explores kinetic and passive attempts at measuring the expansion of the shale and introduces a hybrid experimental testing procedure that uses hydrogen peroxide to initiate the expansion process. The normalized expansion (h/H) for the non-intact shale and intact shale core were 0.0008 and 0.0033, respectively, after 84 days. Expansion rates of 3.5 mm/year/m and 1.43 mm/year/m were calculated for the non-intact shale and intact shale core samples, respectively. A theoretical expansion model is developed that uses stoichiometric calculations to determine gypsum volume and discontinuity infilling theory to determine maximum total expansion. Input variables include shale type (intact bedrock, poorly-graded fragments, well-graded fragments), % pyritic shale (%S2), height of the expansion zone, and surcharge pressure. The theoretical model is used to predict maximum height of expansion and time to maximum expansion for the experiments studied and developed.