An Experimental Investigation on the Rock Mechanical Behavior of Synthetic Layered Systems and Load-Cycling of its Individual Constituents

An Experimental Investigation on the Rock Mechanical Behavior of Synthetic Layered Systems and Load-Cycling of its Individual Constituents

Janmahomed, F.R.

Barnhoorn, A. (mentor)

Civil Engineering and Geosciences

Geoscience & Engineering

Petroleum Engineering

2016-08-26

Many authors already made an attempt to understand the effect of load-cycling on material strength and the evolution of elastic parameters. However, until now there was no study on the effect of load-cycling on the evolution of elastic parameters over the complete stress strain curve, i.e. the linear elastic regime, the fracturing regime, and the fractured regime. In addition, none of the authors focussed on the effect of load-cycling on fracture network improvement. Although previous studies already showed that elastic moduli of layered systems may be determined from properties and volume fractions of its individual constituents, there is no study done on the relation between rock mechanical properties, i.e. strains and yield or failure stresses, of (synthetic) layered systems and its constituents. Furthermore, in the literature no description is found on fracture characteristics of a (synthetic) layered system. Hence, an experimental investigation is conducted on the rock mechanical behavior of synthetic layered systems subjected to increased-loading and the effect of load-cycling on its individual constituents’ rock strength, elastic parameter evolution, and fracture network improvement. The increased-loading and load-cycling rock mechanical experiments are unconfined compression tests performed at room temperature. When comparing two of the same rock materials with a maximum deviation of 1% in porosity, load-cycling leads to failure at much lower stress levels when compared to increased-loading. Within the linear elastic regime, load-cycling returns a stabilized Young’s modulus which is always larger than its envelope value, while the Poisson’s ratio of the last load cycle coincides with its envelope value. Load-cycling generates an improved fracture network when compared to increased-loading. Characteristics of the improved fracture network are the increased fracture densities and the more uniform distribution of the fractures over the volume of the material. For vertically stacked synthetic layered systems, the elastic moduli and strains can be well-predicted by the Reuss Average which uses the average rock mechanical properties and volume fractions of the individual constituents. In contrast, for synthetic layered systems the stress level at failure point is independent of the volume fractions of its constituents and is observed to be in the vicinity of its weakest constituent. Despite this, fractures are still observed in the strongest constituents. The fracture propagation through the strongest constituent is ascribed to be due to amplified stress concentrations at the tip of the propagating fracture. However, from 2-layered systems it is observed that fracture propagation through the strongest constituent depends on the thickness of the weakest constituent. At the constituents’ boundary in synthetic layered systems, there is no offset in fracture path when going from one constituent to another nor is there a sudden change in aperture. However, there is a change in fracture inclination in a way such that the fracture inclinations of the individual constituents are respected. In addition, fractures in synthetics include cataclastics over the high porous intervals (±10-25%), while the same fracture is clean over the low porous intervals (±0,5-5%).

Rock Mechanics
Load-Cycling
Increased-Loading
Fracture Improvement & Characteristics

http://resolver.tudelft.nl/uuid:ebed9a0a-0fce-4005-8c0b-17455b2fa10d

Student theses

master thesis

(c) 2016 Janmahomed, F.R.