A Fast Method to Estimate the Correlation Between Confining Stresses and Absolute Permeability of Propped Fractures

Abstract

Depletion of oil and gas reservoirs during production from fractured wells, or fluid injection through frack-packs may cause significant changes in the effective stress that proppants experience inside the fractures. Previous studies have shown that closure stress may change absolute permeability, however, deriving this relationship requires time-consuming and costly experiments that need to be repeated with the change of proppant or field parameters. Here, we propose a methodology to estimate absolute permeability magnitudes of the propped fracture direction by just using limited knowledge about grains' geometry. In this study, Lattice Boltzmann Methods (LBM) is used to simulate fluid flow to resemble flow in propped fractures under reservoir conditions. Due to the large number of particle and complexity of the sitting of proppant particles next to each other, intensive computational power is required to carry out simulations. Therefore, a parallelized LBM code is utilized to accelerate computations. The three-dimensional granular geometry was constructed from two-dimensional micro-Computed Tomography images. The geometry was then used as an input for the finite element analysis to simulate grain deformation/sliding upon applying different stresses on the samples. The captured deformation of the grains' geometry was then exported to the LBM code for calculation of permeability. Upon increasing confining stress on the samples, the proppant particles began to deform, slide on each other and the pore network evolved. While the standard API cell measures only conductivity changes under different normal stresses, the presented method gives a greater advantage by allowing determination of the simultaneous effect of normal and shear stresses on the components of the permeability of propped fractures. It is believed that parts of complex fracture networks may experience some shear stresses, but these effects could not be measured in the lab easily. Preserving core samples with the same conditions for a long time is not feasible. Therefore, using LBM, we present a fast and reliable method to measure absolute permeability of proppants during flowback and production using limited data in the form of micro-CT images or 3D images of the grains forming proppant packs.