Impact of subsurface geochemical rock-fluid reactions on the permeability of hydraulically fractured Caney Shale of Southern Oklahoma

Production rate in hydraulically fractured reservoirs is expected to decline with time linearly in log-log space, with a slope of -1/2. However, most hydraulically fractured reservoirs witness a faster decline within the first 2 to 5 years of production, indicating a rapid loss of permeability. Several studies have tried to diagnose the predominant mechanism driving this loss, but the findings remain divergent at best. While some researchers aver subsurface geo-mechanical stress dynamics, others promote subsurface geochemical rock-fluid reactions as the predominant cause of this decline. It however remains imperative to understand the causes of the rapid permeability loss in order to develop the right solutions to mitigate the challenge. This study evaluates permeability decline from the perspective of geochemical rock-fluid reactions considering these reactions are pervasive and impact micro and nano-scale fractures and pores within the reservoir. In this research, six experimental designs categorized under three thematic areas are employed to study geochemical rock-fluid reactions in hydraulically fractured Caney Shale. The first and second thematic areas employ static batch experiments where fracturing fluids are reacted with powdered rock and polished cored rock respectively. This helps to understand crucial post-fracturing reactions that adversely affect permeability. In the third category, dynamic core flooding experiments are adopted using artificially fractured rocks to assess the impact of geochemical reactions on petrophysical and mechanical properties of shales. Results from this study reveal complex post-fracturing geochemical reactions in shale reservoirs. This is simplified as follows: Injection of highly oxygenated fracturing fluids into the subsurface triggers the oxidative dissolution of pyrites and other sulfides formed in reduced environments. This causes transient localized acidification leading to the dissolution of proximal minerals and increasing the ionic concentration of the fluid. These ions precipitate new minerals to occlude fractures and pore throats in the reservoir. This process ultimately causes the rapid permeability decline observed in hydraulically fractured shale reservoirs. This study provides insight on adverse post-fracturing geochemical rock-fluid reactions. It also proffers substantial evidence to confirm the predominance of geochemical reactions in permeability loss in hydraulically fractured shale reservoirs. The study also has applications in geological carbon storage.