Logging prediction and evaluation of fracture toughness for the shales in the Longmaxi Formation, Southern Sichuan Basin

Abstract

The mode-I fracture toughness is of great significance for evaluating the fracturing ability of shale reservoirs. In this study, the mode-I fracture toughness of the shale in the Lower Silurian Longmaxi Formation, Southern Sichuan Basin was determined with the Cracked Chevron Notched Brazilian Disc (CCNBD). Based on the experimental data, the relationships among the mode-I fracture toughness, the density, the acoustic time and the clay mineral content were analyzed. The shale samples for fracture toughness test from cores were commonly limited, the investigation of fracture toughness using well logs was necessary. Therefore, a prediction model was proposed by correlating the fracture toughness with well logs responses. The results indicate that the fracture toughness of shale samples is from 0.4744 MPa·m 1/2 to 1.0607 MPa·m 1/2 with an average of 0.7817 MPa·m 1/2 , indicating that the anisotropy of fracture toughness of the Longmaxi Formation shale. The clay mineral content and the density have a positive effect on the fracture toughness, whereas the acoustic time plays a negative role on the fracture toughness. The clay mineral content has an important effect on the relationships among fracture toughness, acoustic time and density. The prediction model can provide continuous data of mode-I fracture toughness along the wellbore for field hydraulic fracturing operation, and it has certain guiding significance in the exploration and development of oil and gas reservoirs