Abstract
Establishing bulk rock properties in friable material such as coal is difficult simply because retrieval of a sufficient sample is challenging particularly because fractured/cleated coal disintegrates in the coring process. This paper describes the use of synthetic rock with embedded simple discrete natural fracture (DFN) systems to establish key rock mechanical properties in synthetic rocks with varying DFN complexity and varying degrees of depletion. The ultimate goal of the work aims to inform late-life technology choices in the depleted CSG reservoirs.
To achieve this, we measured the deformation behaviour of the printed intact matrix and the printed interface (fracture) and expanded the rock mass equivalent continuum theory by Huang et al (1995) to related P32 (Area of Fractures / Volume or Rock) to Young’s modulus and Bulk modulus. 3D printed synthetic right cylinder rock specimens with zero, one and two fractures were printed in a vertical building and horizontal building direction to investigate the stress strain behaviour and any anisotropy inherent in the 3D printing process. Five vertically printed and horizontally printed intact (zero fractures) specimens were tested in axial compression (zero stress boundary condition) to measure the intact stress-strain behaviour and calculate the intact Young’s Modulus and three vertically printed specimens with zero, one and two specimens were tested in isotropic stress conditions to measure bulk modulus (inverse of bulk compressibility).
Five horizontally printed specimens were not reproducible and had an average Young’s Modulus of 4.95 GPa. The horizontally printed specimens with the same one and two fracture system, were not repeatable, and had measured fractured stiffness of 181.2 and 81.5 MPa/m for one fracture specimens and 114.8 and 142.9 MPa/m for two fracture specimens. The five vertically printed specimens were reproducible, with an average Young’s Modulus of 5.37 GPa. The one fracture system had a fracture stiffness of 86.6 and 97.8 MPa/m and the two fracture had a fracture stiffness of 58.3 and 63.8 MPa/m. The equivalent continuum theory suggests that the joint stiffness should be fracture intensity (P32) independent, therefore equal, which was not the case. The change in volumetric strain due to change in isotropic stress was also measured to calculate the bulk modulus of a specimen with zero, one and two persistent vertical fractures. Results from the testing showed that as the fracture area increased, the volumetric strain behaviour was increasingly nonlinear, until a stress magnitude where the bulk modulus became linear and equal for zero, one, and two fracture specimens (3.68, 3.68, 3.51 GPa).
Results show reduction in modulus as a function of P32i and P32 which, however, does not fit the developed theory but is promising to continue the work with the 3D printed specimens. Adjustments to the controls on the printing process may be made to reduce the specimen variability and improve repeatability. In all fractured cases, the synthetic rocks showed initial non-linear behaviour, which was expected, and important for future work and analysis. Therefore, the results of this program are sufficient to formulate a broader test matrix that will be conducted to establish fundamental rock physical parameters and in particular bulk compressibility of coals of varying permeability related to P32/P33 characteristics.
