On the Bio-Chimo-Mechanical Characterization of Eagleford Shale-Part I
Authors: Hayatdavoudi A. , Mokhtari M. , Nizamutdinov R. S. , Sawant K. G. , Parrish A. F. 
Published: March 2017

The Eagle Ford formation, often referred to Eagleford Shale, is a complex one. Some field personnel involved with drilling this formation argue that in their experience Eagle Ford formation is not "shale" at all; however, they do not offer a morphological, mineralogical, and mineralogical reason supporting their claim. To design, plan, engineer, and carry out an economically viable drilling, completion, and production, we need a deeper understanding of both general and local characteristic of the Eagle Ford formation, whether shale or not! Following this goal, with the help of industry, we received on loan, full cores of Eagle Ford formation, and began our tests and analysis. Using an interdisciplinary approach, it is the object of this paper to characterize the formation in depth.

Having learned from our several decades of shale studies, our analyses include but are not limited to: (1) determining solubility of Shale specimen in de-ionized water, (2) using ion selective electrodes, measure the potential, Eh, Temperature, and Hydraulic Potential over the submerged portion of specimen, (3) setting up a video system to record all measurements with time to see (a) which ion leaves the mass of specimen first at a given instance of time and (b) to see whether the timing of this event coincides with the release of first bubble of gas and appearance of fractures in the shale mass, (4) analyzing the slopes of Eh vs. Time (5) examining the possible beneficial and non-beneficial effects of bacterially produced minerals, i.e. combined carbonate-silicates, Marcasite/Pyrite (FeS2), on hydrocarbon development in source rock, and finally (6) determining how these interactive, bio-geo-chemo-systems could affect the mechanical properties of the Eagleford formation.

Our results show: (a) Na+ diffuses from specimen to water first, almost instantaneously, next is Ca+2, from ½ to several-hours, and Mg+2 up to 20-hours, (b) gas bubbles appear about 0-2-hours after Na+ release, (c) time for ionic permeability to reach equilibrium (steady-state) is within the range of 20-50 hours depending on the concentration of and type of release of, (d) pH remains acidic between 5.5 and 6.5, (d) diffusion of Na ion from specimen to water appear to initiate first at the pressurized fissility planes (planes of weakness), which is rapid at the beginning but slows down thereafter, possibly indicating activation of the small and smaller pores in the specimen, (e) the bacterium, shown by arrows in Fig 10, converts Fe II to FeIII then to Marcasite crystals, along with secretion of associated minerals, where these minerals could act as thermal insulator and allowing maturation of the hydrocarbons in-situ to continue, and (f) in this process it appears that the excess supply of H2S, the by-product hydrogen, along with growth of crystals, all lead to pressure build-up in the fissility planes, thus prying them open. The Shale plates, due to dissolution of cement bonding them, appear unable to bond back, which could be attributed to the presence of water and sulfuric acid, reacting with the rock material, rendering it thin, weak, and brittle. Understanding these results could determine a more favorable drilling strategy for constructing a stable wellbore, mitigating lost circulation, designing a better fracturing method, designing a more compatible completion and fracturing fluid, and implementing better corrosion mitigation while producing the well.