Hazim Hussein Abass

Understanding Stress Dependant Permeability of Matrix, Natural Fractures, and Hydraulic Fractures in Carbonate Formations

Most carbonate reservoirs behave as dual porositypermeability systems in which the rock matrix and both natural and created hydraulic fractures contribute to the hydrocarbon transport in a very complex manner. Understanding the behavior of the permeability of the matrix frame, natural fractures, and created hydraulic fractures, as a function of reservoir depletion, is vital to designing optimum stimulation treatments and to maximize the carbonate formation’s exploitation. Core samples were selected from a carbonate reservoir and a testing procedure was applied to determine the stress dependant permeability as a function of various combinations of effective stresses. A tensile natural fracture was simulated by splitting a whole core by failing it under tension using a Brazilian test procedure. The stress dependant permeability was evaluated under varied effective stresses simulating a reservoir depletion scenario. A shear fractured core was selected from a given carbonate formation and a stress dependant permeability was established. The tensile fractured core was then propped with a low concentration of small mesh proppants and the permeability of the simulated propped fracture was determined. Using a new reservoir simulator the testing results and selective functions were used to predict the production performance of a carbonate reservoir under the effect of the stress dependant permeability. The experimental results indicate that the tensile fractures are much less conductive than shear fractures and the shear fractures are less conductive than propped fractures. The concept of effective stress within the rock matrix is totally different than that of natural fractures; therefore, the effective stress function for both matrix and natural fractures should be separately evaluated to obtain representative functions for any simulation study. The tensile fractures lose conductivity at very early stages of reservoir depletion. Recommendations to manage these tensile fractures for optimum hydrocarbon recovery are suggested.

An Analytical Elasto-Plastic Analysis for Stability of Axisymmetric Wellbore

In this paper, a general shear strain hardening and softening Drucker-Prager model based on the existing triaxial compression test results has been introduced to model the rock behaviour which captures well the mechanical characteristic of typical soft rock formations. This elastoplastic model is then adopted to develop a rigorous analytical solution for the drained wellbore drilling problem subjected to in-plane isotropic stress field, and to simulate the borehole collapse failure. It is found that the wellbore boundary value problem can be reduced to solving a system of first order ordinary differential equations in the plastic zone, with the radial, tangential, and vertical stresses as well as the volumetric strain and plastic shear strain being the five basic unknowns. The illustration numerical example shows the distributions of stress components and volumetric strain around the borehole, in addition to the evolving plastic deviatoic strain and stress path for a rock point at the borehole surface due to the wellbore drilling. The critical mud pressures necessary to prevent borehole collapse, predicted by the elastoplastic analysis based on different wellbore instability criteria, are compared with the value corresponding to the elastic theory, which are found to be considerably lower.Copyright