The Impact of Water on Brittleness and Fracturability: Evaluating Organic-Rich Argillite from the Naparima Hill Formation

The Late Cretaceous Naparima Hill Formation argillites are the primary source for most of the oil and gas production within the last century in Trinidad. Hydrocarbon production has declined in recent years, necessitating the exploration of new petroleum plays and the use of improved techniques to increase production. The Naparima Hill Formation is also considered as an unconventional reservoir due to its low permeability and high hydrocarbon content. Hydraulic fracturing, which depends on mechanical properties such as brittleness and fracturability, is necessary for the exploitation of this formation. In this study, we measured the unconfined compressive and tensile strengths of four organic-rich outcrop lithofacies (siliceous-calcareous argillite, calcareous argillite, carbonate rich siliceous argillite, and siliceous argillite), under dry and water-saturated conditions. The strength measurements were then used to determine the strength-based brittleness index and fracture toughness. A fracturability evaluation model that integrates brittleness index, fracture toughness, and minimum horizontal insitu stress was used to evaluate the fracturability. Our results showed that saturation reduced the uniaxial compressive and tensile strengths up to 51% and 59%, respectively. The siliceous calcareous argillite and siliceous argillite experienced the highest reductions in strength due to their relatively high porosities (21% and 31%, respectively) and lack of carbonate cement. The brittleness index and fracturability ranged from 60 to 75% and 0.17 to 0.43 MPa^-2.m^0.5, respectively, under dry conditions, and increased to 69 to 78% and 0.26 to 0.55 MPa^-2.m^0.5, respectively, when saturated. Our results imply that all the argillites are brittle, and only the siliceous calcareous argillite and siliceous argillite are easily fractured when water saturated for hydraulic fracturing operations. Thus, the siliceous calcareous argillite and siliceous argillite are more susceptible to tensile fracture initiation and propagation during hydraulic fracturing.