Mineral and Fluid Transformation of Hydraulically Fractured Shale: Case Study of Caney Shale in Southern Oklahoma

Abstract

This study explores the geochemical causes of permeability loss in hydraulically fractured reservoirs. The experiments involved the reaction of powdered-rock samples with produced brines in batch reactor system at temperature of 95^oC and atmospheric pressure for 7-days and 30-days respectively. Results show changes in mineralogy and chemistry of rock and fluid samples respectively, therefore confirming chemical reactions between the two during the experimental period. The shift in mineralogy of the rock included decreases of pyrite, feldspar, and carbonate content whiles illite content showed an initial increase before decreasing. Results from analyses of post-reaction fluids generally corroborate the results obtained for mineralogical analyses. In essence, the results reveal a complex trend of reactions between rock and fluid samples which is summarized as follows. Breakdown and oxidation of pyrite by oxygenated fluid causes transient and localized acidity which triggers the dissolution of feldspar, carbonates, and other minerals susceptible to dissolution under acidic conditions. The dissolution of minerals releases high concentrations of ions which subsequently precipitate secondary minerals. On the field scale, the formation of secondary minerals in the pores and flow paths of hydrocarbons significantly reduces the permeability of the reservoir, which culminates in rapid productivity decline. This study provides an understanding of the geochemical rock-fluid reactions that impact long term permeability of shale reservoirs. Findings from the study also reveal the potential of depleted hydraulically fractured shale reservoirs as carbon storage units.