Evidence for the Heterogeneity of the Pore Structure of Rocks from Comparing the Results of Various Techniques for Measuring Hydraulic Properties

Laura L Schepp,Joerg Renner

doi:10.1007/s11242-020-01508-8

Abstract

We applied three oscillatory methods, the previously presented axial pore-pressure and pore-flow methods, and the laboratory application of the radial oscillatory pore-flow method, and performed steady-state flow-through experiments (Darcy tests), for comparison, in experiments on samples of Westerly granite and Wilkeson sandstone. The granite and the sandstone exhibit pore spaces dominated by micro-fractures and by the granular-medium character with a connected porosity of about 1 and 10 %, respectively. Permeability determined by the axial pore-pressure method shows the closest agreement with the results of the Darcy tests. Apparent porosity and drained modulus derived from specific storage capacity deviate from measured connected porosity and reference values, respectively. The observed deviations of the hydraulic properties between methods suggest that they bear information about the structure of the pore space. Only for the sandstone, anisotropy in hydraulic properties appears to contribute to differences between the results of the various methods. We argue that oscillatory testing provides three indicators for heterogeneity, period dependence, the relation between apparent and connected porosity, and the relation between amplitude ratio and apparent penetration depth, calculated from the simple scaling law for homogeneous materials. These indicators consistently classify the samples of Wilkeson sandstone as hydraulically homogeneous and those of Westerly granite as heterogeneous.

Highlights

A slightly compressible fluid injected into a saturated rock is partly transported through the connected pore space following the prevailing pore-pressure gradient and partly stored owing to its compressibility and that of the pore space
Developed in physical chemistry (Turner 1958, 1959) and applied to rock samples (Stewart et al 1961; Kranz et al 1990; Fischer 1992), the axial oscillatory pore-pressure method is insensitive to changes in boundary conditions for the samples and permits determination of permeability k and specific storage capacity s in a single experiment when the period is appropriately chosen
In addition to permeability k and specific storage capacity s, we report hydraulic diffusivity D = k∕(s ) and apparent porosity ap, defined as the ratio between specific storage capacity and fluid compressibility c f ap 1 + cpp cf where we follow the notation of Zimmerman et al (1986) for the various compressibilities

Summary

Introduction

A slightly compressible fluid injected into a saturated rock is partly transported through the connected pore space following the prevailing pore-pressure gradient and partly stored owing to its compressibility and that of the pore space. Experiments relied on the determination of permeability investigating steady-state flow at constant pressure gradients (e.g., Bernabé 1987; Darcy 1856). Since these methods are less suitable for low-permeability rocks, it has been practice for many decades to determine hydraulic properties of rocks from pressure transients recorded during field and laboratory tests, such as pulse tests (Bauer et al 1995; Brace et al 1968; Bredehoeft and Papadopulos 1980; Hsieh et al 1981; Neuzil 1982; Selvadurai and Carnaffan 1997; Zeynaly-Andabily and Rahman 1995), slug tests (e.g., Cooper et al 1967; Butler 1997), or experiments with a constant flow rate (Lin 1977; Trimmer et al 1980; Song et al 2004). In the current laboratory experiments, we employ a radial oscillatory pore-flow method analogous to periodicpumping tests in wells (Cardiff et al 2013; Rabinovich et al 2015; Rasmussen et al 2003; Renner and Messar 2006)

Objectives

Methods

Results

Discussion

Conclusion