Chemical reactions between hydraulic fracture fluids and oil/gas shales can increase or decrease rock porosity depending on a number of factors. For example, acidic fracture fluids increase porosity through dissolution of minerals, particularly carbonates. In contrast, oxidative dissolution of pyrite caused by oxic fracture fluids releases Fe(II), which when oxidized precipitates as Fe(III)-(hydr)oxides that occlude pores and pore necks, resulting in a loss of porosity. Depending on the mineralogy and organic content of the host shale, the rate of Fe(III) oxidation/precipitation and the type of solid phase precipitated can vary markedly. This study used a combination of laboratory- and synchrotron-based methods to monitor these changes in four oil/gas shales with highly variable mineralogy and organic matter.
Hydraulic fracturing of unconventional shales has been an important addition to the energy portfolio of the United States. Though this practice has been highly successful, a significant amount of oil/gas resources remain in shales that have been hydraulically fractured [1]. While a large amount of the research on unconventional shale plays has focused on the geophysical aspects of the shale deposits, alteration of the rock due to chemical reaction has received comparably little attention. Geochemical reactions in the subsurface can either increase porosity/permeability through mineral dissolution or decrease porosity/permeability as secondary minerals precipitate in the system. The impact of these alterations on long term hydrocarbon production is a first-order question that remains unanswered. This project lays the foundation to address this problem.
We investigated the mineralogical, geochemical, and physical changes of four shales with varying mineralogy and organic contents [Marcellus shale (NY), Barnett shale (Central TX), Eagle Ford shale (South TX), and Green River shale (CO)] (Table 1) following reaction with a synthetic fracture fluid based on the fracture fluid composition used by the National Energy Technology Laboratory (NETL) in Marcellus Shale Gas Well E, located in Greene Co., PA (Table 2). This fracture fluid formulation is standard for the majority of the Marcellus shale gas plays.
