KG2B, a collaborative benchmarking exercise for estimating the permeability of the Grimsel granodiorite - Part 1: Measurements, pressure dependence and pore-fluid effects

Measuring the permeability of tight rocks remains a challenging task. In addition to the traditional sources of errors that affect more permeable formations (e.g. sample selection, non-representative specimens, disturbance introduced during sample acquisition and preparation), tight rocks can be particularly prone to solid–fluid interactions and thus more sensitive to the methods, procedures and techniques used to measure permeability. To address this problem, it is desirable to collect, for a single material, measurements obtained by different methods and pore-fluids. For that purpose a collaborative benchmarking exercise involving 24 laboratories was organized for measuring the permeability of a single low permeability material, the Grimsel granodiorite, at a common effective confining pressure (5 MPa). The objectives of the benchmark were: (i) to compare the results for a given method, (ii) to compare the results between different methods, (iii) to analyze the accuracy of each method, (iv) to study the influence of experimental conditions (especially the nature of pore fluid), (v) to discuss the relevance of indirect methods and models and finally (vi) to suggest good practice for low permeability measurements. In total 39 measurements were collected that allowed us to discuss the influence of (i) pore-fluid, (ii) measurement method, (iii) sample size and (iv) pressure sensitivity. Discarding some outliers from the bulk data set (4 out of 39) an average permeability of 1.11 × 10−18 m² with a standard deviation of 0.57 × 10−18 m² was obtained. The most striking result was the large difference in permeability for gas measurements compared to liquid measurements. Regardless of the method used, gas permeability was higher than liquid permeability by a factor approximately 2 (kgas = 1.28 × 10−18 m² compared to kliquid = 0.65 × 10−18 m²). Possible explanations are that (i) liquid permeability was underestimated due to fluid-rock interactions (ii) gas permeability was overestimated due to insufficient correction for gas slippage and/or (iii) gases and liquids do not probe exactly the same porous networks. The analysis of Knudsen numbers shows that the gas permeability measurements were performed in conditions for which the Klinkenberg correction is sufficient. Smaller samples had a larger scatter of permeability values, suggesting that their volume were below the Representative Elementary Volume. The pressure dependence of permeability was studied by some of the participating teams in the range 1–30 MPa and could be fitted to an exponential law k = ko.exp(–γPeff) with γ = 0.093 MPa−1. Good practice rules for measuring permeability in tight materials are also provided

The Geomechanics of CO2 Storage in Deep Sedimentary Formations

This paper provides a review of the geomechanics and modeling of geomechanics associated with geologic carbon storage (GCS), focusing on storage in deep sedimentary formations, in particular saline aquifers. The paper first introduces the concept of storage in deep sedimentary formations, the geomechanical processes and issues related with such an operation, and the relevant geomechanical modeling tools. This is followed by a more detailed review of geomechanical aspects, including reservoir stress-strain and microseismicity, well integrity, caprock sealing performance, and the potential for fault reactivation and notable (felt) seismic events. Geomechanical observations at current GCS field deployments, mainly at the In Salah CO2 storage project in Algeria, are also integrated into the review. The In Salah project, with its injection into a relatively thin, low-permeability sandstone is an excellent analogue to the saline aquifers that might be used for large scale GCS in parts of Northwest Europe, the U.S. Midwest, and China. Some of the lessons learned at In Salah related to geomechanics are discussed, including how monitoring of geomechanical responses is used for detecting subsurface geomechanical changes and tracking fluid movements, and how such monitoring and geomechanical analyses have led to preventative changes in the injection parameters. Recently, the importance of geomechanics has become more widely recognized among GCS stakeholders, especially with respect to the potential for triggering notable (felt) seismic events and how such events could impact the long-term integrity of a CO{sub 2} repository (as well as how it could impact the public perception of GCS). As described in the paper, to date, no notable seismic event has been reported from any of the current CO{sub 2} storage projects, although some unfelt microseismic activities have been detected by geophones. However, potential future commercial GCS operations from large power plants will require injection at a much larger scale. For such largescale injections, a staged, learn-as-you-go approach is recommended, involving a gradual increase of injection rates combined with continuous monitoring of geomechanical changes, as well as siting beneath a multiple layered overburden for multiple flow barrier protection, should an unexpected deep fault reactivation occur

Role of Poisson's ratios on the consolidation response of soils

This paper presents an analytical study of the classical consolidation problem related to deep soil deposit whose permeable or impermeable surface subjected to a perfectly flexible circular load. The deep soil deposit is modeled as a poroelastic medium of semi-infinite extent, saturated with compressible pore fluid. The analytical and numerical solutions for the surface settlements and excess pore pressure illustrate the significance and manner of the drained and undrained Poisson's ratios ν and νu on the consolidation response of saturated soils.link_to_subscribed_fulltex

Consolidation behaviour of embedded rigid foundations

The paper presents a summary of the results related to the consolidation behaviour of rigid foundations which are deeply embedded in poroelastic soil media. The results are derived from the complete mathematical formulation and numerical solution of the coupled integral equations governing such deeply embedded foundations. The numerical results presented in the paper illustrate the manner in which consolidation settlements of the embedded foundations are influenced by the drainage characteristics at the poroelastic soil-foundation interface and νu and ν which refer respectively to the undrained and drained values of the Poisson's ratio.link_to_subscribed_fulltex

On the mechanics of a rigid disc inclusion embedded in a fluid saturated poroelastic medium

The paper utilizes the classical integral transform techniques to develop the systems of Fredholm integral equations of the second kind, in the Laplace transform domain, governing the generalized displacement or loading of a rigid disc inclusion embedded in either permeable or impermeable bonded contact with a fluid saturated poroelastic infinite space. The generalized displacements correspond to an axial displacement, a rotation about the axial axis, a rotation about a diametral axis and an in-plane translation. The coupled integral equations in the Laplace transform domain are solved in a numerical fashion to generate results of technological interest. The numerical procedures focus on quadrature schemes for the solution of the integral equations and a procedure used for the inversion of Laplace transforms. The closed-form solutions of the integral equations as either t → 0+ or t → +∞ and as v → vu are also obtained. The numerical results are presented for the time-dependent displacements and rotations of the inclusion subjected to Heaviside-step function type loads and moments, and for the time-dependent relaxation of the force and moment resultants in an inclusion subjected to Heaviside-step function type displacements and rotations. In particular, the influence of the compressibility of the pore fluid on the time dependent responses of the inclusion is documented. © 1995.link_to_subscribed_fulltex

Eccentric settlements of a rigid foundation on a consolidating layer

The paper examines the eccentric settlement of a rigid foundation located at the surface of a poroelastic layer saturated with a compressible pore fluid. The base of the rigid foundation is circular and flat. The poroelastic layer rests in bonded contact with an impermeable, rigid subbase and the surface of the layer is considered to be either permeable or impermeable. The paper develops the integral equations governing the eccentric settlement of the rigid foundation associated with the consolidation process of the saturated poroelastic layer. The numerical results presented in the paper illustrate the time-dependent behavior of the rigid foundation by taking into account the effects of the relative layer thickness, eccentricity, drainage conditions, and the compressibility of the pore fluid.link_to_subscribed_fulltex

On the asymmetric indentation of a consolidating poroelastic half space

This paper examines the asymmetric indentation of a poroelastic half space by a rigid smooth circular indentor that is subjected to a moment resultant about a diametral axis. Three types of drainage conditions at the surface of the poroelastic half space are considered. The paper develops the governing coupled integral equations and reduces them to systems of Fredholm integral equations of the second kind in the Laplace transform domain. Efficient computational algorithms are proposed to evaluate the time-dependent behavior of the rigid circular foundation. The influence of the drainage boundary conditions and the compressibility of the pore fluid on the consolidation-induced rotation of the rigid circular foundation is discussed. © 1994.link_to_subscribed_fulltex