A Review of Brittleness Index Correlations for Unconventional Tight and Ultra-Tight Reservoirs

Abstract

Brittleness is a key parameter in the development of the unconventional shale and tight carbonate reservoirs as it plays a role in the design of the hydraulic fractures as well as the selection of the sweet-spot locations for perforation and fracture initiation. The brittleness index (BI) is utilized to indicate if the formation rocks are brittle, which are preferable to form a complex network of fractures, or ductile, which are occasionally desirable to seal the fractures from growing. However, there is a wide variety of BI methods in the literature that lead to inconclusive BI values. The Mineral-based brittleness index (MBI), which is a method based on the mineral composition of the formation, can be derived from mineral well logging data or laboratory core testing. Another approach in describing the brittleness is the Fracability Index (FI), which is a combination of Young’s modulus and Poisson’s ratio. Differentiation is also made between the dynamic FI, which is calculated from well logging data, and the static FI, which is derived from laboratory core testing such as uniaxial compressive strength, Brazilian tensile strength and triaxial testing. Hence, to understand the complexity of the brittleness, it is crucial to consider all dependencies such as the lithology, mineral composition, TOC, porosity, temperature and pressure amongst others. In this work, a comprehensive review and analysis of the existing equations and correlations for the calculation of the MBI and FI was conducted. These methods were applied on different low porosity and low permeability rocks. A thorough comparison has also been conducted between the MBI and FI correlations as well as between the dynamic FI and the static FI to ultimately clarify and improve the definition of brittleness as a function of lithology. High content of quartz and carbonates result in high values of MBI, and high Young’s modulus values yield high FI values. On the other hand, high clay content and high porosity lead to low MBI values.