Investigating the Volume and Structure of Porosity in Fractured and Unfractured Rock From the Newberry Volcano, Oregon, USA: Evaluation of Two- and Three-Dimensional Methods

摘要

Porosity is a fundamental characteristic of rock critical to its mechanical and hydrologic behavior, yet a study of the open and accumulated healed porosity measurements of nine core samples from Newberry Volcano shows that different measurement methods produce significantly different estimates of pore volume and structure. We compare traditional 2D point count, petrographic image analysis, and 3D micro X-Ray Computed Tomography (micro CT). This comparison reveals that detailed petrographic mapping provides the most accurate characterization of fracture porosity, and its history of development, owing to its high spatial resolution and accuracy of phase identification as well as insights afforded from mineralogic and textural relationships. However, this analysis lacks the three-dimensional characterization necessary to determine pore shape and interconnectedness, especially in highly anisotropic and heterogeneous fracture porosity. Thus micro CT, although it consistently underestimates porosity, can usefully augment the petrographic analysis. High resolution mapping of petrographic thin sections also provides a means to characterize the roughness of fracture surfaces associated with repeated slip that generates porosity recorded by the development of healed porosity. Analysis of 19 slip events on a small, early stage fracture experiencing ≤ mm-scale slip, indicates that this roughness is preserved across multiple slip events and is consistently associated with dilation. Characteristic length scales intrinsic to rock such as the primary grain and pore size distributions of the ＞ 0.2 mm size fraction primarily influence the roughness of fractures until the mechanism of fracture growth transitions to linkage among macroscopic fractures. This correlation among primary rock characteristics such as grain size, fracture roughness, repeated fracture slip, and dilation suggests that these key attributes for successful stimulation to generate an Enhanced Geothermal System might be readily assessed.