ICE 2022


Several graphical and numerical methods characterize rocks deformation. These include elastic moduli cross-plots, Mohr circle, failure envelopes, and brittleness index. The natural fracturing probability curve is a common way to describe rock deformation based on elastic behavior. Poisson’s ratio and Young’s modulus are dynamic elastic variables that also quantify rock deformation based on density, shear, and compressional sonic wireline logs. These moduli are measured in the lab by subjecting rock-sample plugs to variable stress and measuring corresponding strain and deformation. Poisson’s ratio and Young’s modulus at the failure point of a rock plug define the point at which the rock stops deforming elastically. The measured failure point values from multiple plugs and from a given formation, are plotted on a Poisson ratio-Young modulus cross-plot. The best-fit trend line for those points represents the transition from ductile to brittle behavior. This trend line is known as the natural fracturing probability curve for a given lithological unit. Quantifying how far the plug readings are from this curve characterizes the sealing integrity of the corresponding plug. Data points from wells plotted below this curve are interpreted to be ductile. Meanwhile, data points above this curve are interpreted to be brittle and prone to breach. This two-dimensional analysis can be viewed in one-dimension to simplify the interpretation of the rock behavior, especially when looking at wireline logs. An equation was developed to use the Young modulus and Poisson’s ratio in to calculate the distance from the natural fracturing probability curve. The distance from the natural fracturing probability curve is called the Fracture Probability Factor (FPF). If the FPF values of a lithological zone are positive, the zone is characterized as a ductile healing region. However, if the values are negative, it is described as a brittle, failing region. This methodology can be practical in defining vertical compartments in a given well from wireline data. The FPF log against pressure data provides a basis to determine whether FPF positive zones can explain pressure differences between reservoirs. The significance of the FPF is to provide a variable to correlate with brittleness index measurements. These can be compared to elastic measurements and mineral content. Moreover, FPF can be used for seal effectiveness analysis or characterizing fracking intervals.