Mineralogy and reactive fluid chemistry evolution of hydraulically fractured Caney shale of Southern Oklahoma

This study investigates geochemical rock-fluid interactions as a potential cause of rapid loss of permeability and productivity in hydraulically fractured shale reservoirs. It also interrogates the effects of these reactions in transforming depleted shale reservoirs into impermeable carbon storage units. The study employs batch reactor experiments where rock-powder samples are reacted with field fracturing fluid under reservoir temperature (95^oC).

Results show significant changes in mineralogy and fluid chemistry following rock-fluid reactions up to 30 days. Initial mineralogy of the rock samples includes quartz, feldspar, carbonate, pyrite, and clay minerals. Post-reaction rock mineralogy reveals the breakdown of pyrite, carbonates and feldspars, and an increase of illite content. Results from reacted fluid analyses corroborate the mineralogical changes observed after different reaction periods. Mineralogical changes in rock powders and changes in fluids chemistry at different sampling intervals (0, 7 and 30-days) reveal complex trends of dissolution and precipitation of various components. In general, the reactions proceed as follows: Dissolved oxygen and oxidants in fracturing fluids cause the breakdown and oxidation of pyrite which introduces transient and localized acidity into fluids. The transient acidity catalyzes the breakdown of feldspars and carbonates leading to the release of primarily Na, Al, Si, Fe, and inorganic C into solution. These dissolved elements subsequently react to precipitate secondary minerals which may be detrimental to reservoir permeability in the long-term. Results from experimental modelling confirmed the above-mentioned dissolution, precipitation reactions.

Findings from this research serve an essential basis to help in finetuning fracturing fluid compositions to mitigate adverse reactions that cause rapid decline in permeability and productivity in hydraulically fractured shale reservoirs. The findings also have applications in geological carbon storage in depleted shale reservoirs in context of mineralogical alterations capable of transforming these reservoirs into impermeable carbon storage units and seals.