A Geomechanical Approach To Evaluate Brittleness Using Well Logs: Mississippian Limestone, Northern Oklahoma
Fracture density, or brittleness, of a rock is an important factor in understanding the characteristics of a reservoir and the reservoir's predisposition to fracturing. This becomes particularly critical when exploring and producing in tight or fracture dominated reservoirs such as that of the Mississippian limestone in the study area on the Northern Shelf of the Anadarko basin, Oklahoma. Previous work has defined brittleness on the basis of mineralogy or elastic rock parameters. This study explores a new definition of brittleness based on geomechanical principles that incorporates linear elastic rock properties as well as fracture toughness. The equation being used for this purpose was derived from a relationship published by Sih (1985). According to Sih, "the surface and volume energy density of each material element are related by the rate of change of volume with surface." This would suggest that (Fd)(Ua) = Uv, where Fd is the fracture density (fracture surface/volume); Ua is the energy needed to create fracture area A (related to surface energy density); and Uv is the strain energy density.Data gathered from acoustic, density, and micro-resistivity image logs taken from two vertical wells drilled through the Mississippian formation in Northern Oklahoma were used to create and attempt to validate a brittleness log based on the new equation. The equation assumes linear elasticity, and mode I fracturing. Based on these assumptions, the equation relates fracture density, or brittleness, to KIC, (the critical fracture toughness for mode I fractures), Young's Modulus, shear modulus, Poisson's Ratio, and the strain state. The resulting brittleness log created from this equation is compared to brittleness logs derived from other methods of determining brittleness from well logs and checked against the fracture frequency observed from image log data. This more comprehensive method of determining brittleness could aid in developing a more effective approach to reservoir characterization and in improved targeting of brittle intervals within a formation.