A Novel Model of Brittleness Index for Unconventional Reservoirs: Confining Pressure and Pore Pressure Effect

Abstract

Brittleness indices (BI) commonly used in the petroleum industry are based on elastic modulus or mineralogy that can be calculated from well logs. However, they both ignore the effect of confining pressure and pore pressure. Shale is usually distributed at various conditions with different confining pressure and pore pressure. Models without considering the influence of confining pressure and pore pressure will directly lead to inaccuracy in BI calculation, thus resulting in the failure of hydraulic fracturing. In this study, first a model considering the effect of confining pressure in BI calculation has been built by introducing “fracture toughness”. Considering the ratio k of the minimum horizontal effective stress to vertical effective stress and its relation with Poisson's ratio, a relationship between confining pressure and pore pressure has been obtained. Then, a BI model considering pore pressure has been built by replacing confining pressure in the first model with pore pressure. X-ray diffraction analyses, triaxial and Brazil disk split tests have been done to get mineral content and rock mechanics parameters of samples, respectively. Also, Kaiser acoustic emission probe has been used to monitor the generation and expansion of cracks in triaxial texts. Our study shows that BI is usually larger at low confining pressure than at high pressure. Also, BI gets improved with the consideration of pore pressure. The effective stresses in the reservoir decrease at elevated pore pressure, which is equivalent to reducing the influence of confining pressure and thus increases the brittleness of rocks. The results calculated by the new model, which considers the influence of Young's modulus, Poisson's ratio, tensile strength, confining pressure, pore pressure and fracture toughness in BI calculation, match well with experimental results. This new model can be used to quantitatively calculate BI of rocks at different confining pressures and pore pressures, which is essential in analyzing rock mechanics and selecting fracturing section.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016