Modeling elastic properties and assessing uncertainty of fracture parameters in the Middle Bakken Siltstone

Abstract

A statistical rock-physics technique, based on well data that provides estimates and associated uncertainty of fracture density in the Middle Bakken Siltstone, is presented. Geologic and hydrocarbon-charging history of the Middle Bakken indicate multiple sets of fractures that justify treating this unit as elastically isotropic. The generalized n-phase self-consistent model relates the elastic properties to composition, matrix porosity, and fracture porosity, where an assigned aspect ratio and volumetric fraction corresponds to each input. The modeling of bulk density as a function of total porosity supplies deterministic estimates of the composition. Analysis of in situ stress and pore-stiffness calculations provide a range of fracture aspect ratios, corresponding to open fractures. Stochastic simulation of fracture porosity initiates the statistical nature of the technique. This treatment of fracture porosity enables the rock-physics model to be treated statistically through multiple realizations. Modeling results explain the measured bulk and shear moduli, with the bulk modulus more accurately described, and the results also provide statistical estimates of fracture porosity. Calculations using these estimates of fracture porosity, along with fracture aspect ratios, result in statistical estimates of fracture density for each depth value in the Middle Bakken unit. Values of fracture density fall within imposed limits (< 0.10). The results and technique demonstrated here could be applied to field seismic data to identify locations of increased fracture density. These locations might indicate areas of increased permeability in the Middle Bakken Siltstone.