Strain Stress Path Research Articles

Uniaxial Testing of Soil–Cement Composites to Obtain Correlations to Be Used in Numerical Modeling

This paper presents the results of laboratory testing of samples obtained from soil–cement composite columns produced on a real-world test site as part of a research and development project. The introduction presents the motivation of the research and the goals that guided the layout of a research program. The general geotechnical conditions, initially assumed methodology of soil–cement composite sampling, and finally, the methodology of strength tests were presented. In tests conducted with the measurement of the strain-stress path, the strength and stiffness of the material were determined in various modes of the stress–strain path, including unloading/reloading cycles. The test results were presented in the form of graphs presenting soil–cement composite stiffness in the function of material strength and subjected to a short critical discussion against the background of reference samples from composites prepared in laboratory conditions. This allowed for their qualitative and quantitative assessment and the formulation of conclusions and guidelines, concerning the execution of works and especially the potential practical outcomes (benefits for numerical modeling), juxtaposed in the summary. Some prospects and needs for future developments were also formulated on the basis of actual experience.

Measurement of Unloading Pore Volume Compressibility of Frio Sand Under Uniaxial Strain Stress Path and Implications on Reservoir Pressure Management

Measurement of Unloading Pore Volume Compressibility of Frio Sand Under Uniaxial Strain Stress Path and Implications on Reservoir Pressure Management

Grain- to Reservoir-Scale Modeling of Depletion-Induced Compaction and Implications on Production Rate

Summary The reduction of pore pressure caused by depletion can induce significant reservoir compaction and loss of permeability, especially in unconsolidated reservoirs. In this paper, we develop a numerical approach on the basis of computer-based simulations of unconsolidated rock samples subjected to mechanical tests that replicate the one-dimensional (1D) strain depletion path and allow for a prediction of permeability loss. The 1D strain stress path is a good approximation for long and thin conventional reservoirs with a compliant caprock. The numerical sample consists of crushable stiff and soft grains (proxies for sand and shale) simulated with the discrete element method (DEM) coupled with the bonded-particle model (BPM). Model parameters are calibrated through numerical single-grain-crushing tests which reproduce the experimentally measured sand strength. Grain crushing induced by the uniaxial strain stress path results in a pronounced reduction of porosity and permeability, which manifests more readily for samples with large grain size. The change of particle-size distribution indicates that high effective stresses cause grain crushing and production of a significant amount of fines that lower permeability. Simulation results indicate that the presence of soft grains and inclusions (e.g., shale fragments) facilitates grain crushing. Reservoir simulations—incorporating the change of porosity and permeability as a compaction table—show that the compaction can enhance cumulative production due to compaction drive but also reduces production rate by impairing the reservoir permeability. This multiscale numerical workflow bridges grain-scale compaction behavior and field-scale reservoir production.

Discrete element modeling of grain crushing and implications on reservoir compaction

Depletion-induced formation compaction is a concern for production in hydrocarbon reservoirs. In this study, numerical mechanical tests and digital samples are proposed as an alternative to uniaxial strain experiments for investigating the effects of compaction. We propose a grain crushing model with two-sized subgrains based on the discrete element method and bonded-particle model (DEM-BPM) and perform numerical single grain crushing tests to calibrate the model parameters against the experimental sand grain characteristic strength. Numerical uniaxial strain tests are used to predict porosity and permeability reduction caused by reservoir compaction. Grain crushing incurs a significant reduction of porosity under the uniaxial strain stress path, which becomes more evident for digital samples consisting of larger grains. The stress-dependent changes in porosity and permeability obtained from the DEM simulations can be used as inputs for a reservoir-scale correlation to predict the productivity loss due to reservoir compaction.

A constitutive model for porous rock including effects of bond strength degradation and partial saturation

We present a constitutive model to simulate the geomechanical response of porous rocks including the effect of bond strength degradation (or cementation), and the role of capillary pressure in partially saturated conditions. It is oftentimes difficult to obtain enough experimental data for the proper hydro-mechanical characterization of the rocks. This, for example, is what generally occurs with the geological reservoirs to be used for CO 2 storage, located about 600 m below the ground level. Therefore, the model proposed entails a compromise between the ability to reproduce realistic strain–stress paths and the number of parameters. A review and analysis of a wide range of tests show that it is not easy to put forth formulations that are sufficiently general with less than eleven parameters. With this number of parameters, the model proposed has enabled to obtain a satisfactory simulation of brittle/ductile behaviour, pre-peak dilatancy, the bond effect, the effects of chemical degradation on bond strength, and the unsaturated behaviour of porous rocks.

An Assessment of Compressive Size Effect of Plane Concrete Using Combination of Micro-Plane Damage Based Model and 3D Finite Elements Approach

In recent years, the material behavior dependence of laboratory concrete specimens built with the same concrete mixture under the same load conditions to their geometrical sizes is well established. This phenomenon which is observed not only in concrete but also in most quasi-brittle materials such as rock, ceramic or composite materials is now called as size effect. Many of the existing structural analyzing codes are not able to consider this important feature of concrete structures especially under compressive loadings. However we know that the main purpose of concrete application in structural members is to resist compression. The aim of this study is to show the ability of author's recently developed 3D finite elements code equipped with the proposed author's newly micro-planes damage based model for considering of compressive size effect of plane concrete. To do so, two different sizes of cubic concrete specimens are modeled with mentioned code under the uniaxial compressive test and their fracture mechanisms, pre-peak and post-peak strain-stress paths are investigated. Obtained results reveal the good coincidence with experimental evidences. In fact, the combination of proposed micro-planes damage based model and developed presented 3D finite elements technique creates a powerful numerical tool to capture and predict precisely strain localization and fracture mechanism in the specimens and consequently to assess properly the compressive size effect of plane concrete in analysis and design.