This paper presents a multi-scale lattice Boltzmann/finite element scheme that quantitatively links particulate mechanics to hydraulic properties of a grain assembly obtained from a simple shear discrete element simulation. A spatial homogenization is performed to recover the macroscopic stress from the micro-mechanical force chances. The pore geometries of the shear band and host matrix are then quantitatively evaluated through morphology analysis and flow simulations. Hydraulic properties estimated from multiscale flow simulations are compared with those inferred from volume averaging and geometric averaging schemes. Results from the discrete element simulations imply that grain sliding and rotation occur predominately within the dilatant shear band. These granular motions lead to dilation of pore space inside the shear band and increases in local permeability. While considerable anisotropy in the contact fabric is observed within the shear band, anisotropy of the permeability is, at most, modest in the assemblies composed of spherical grains.
Drilling with drag bits (PDC bits) simultaneously involves fragmentation of rock by the cutters and frictional contact on the cutter wear flats. While there is reasonable understanding of the forces arising from the cutting process, knowledge of the factors affecting the contact forces on the wear flats is still fragmentary. This paper focuses on determining the parameters that influence the mean normal stress σ on the cutter wear flats, and on mapping the dependence of σ on these parameters, by analyzing the idealized problem of a slightly inclined rigid slider moving on the surface of a Mohr-Coulomb elastoplastic half-plane.
Lavrov, A. (SINTEF Petroleum Research) | Torsæter, M. (SINTEF Petroleum Research) | Albawi, A. (Norwegian University of Science and Technology) | Todorovic, J. (SINTEF Petroleum Research) | Opedal, N. (SINTEF Petroleum Research) | Cerasi, P. (SINTEF Petroleum Research)
Integrity of the near-well area is crucial for preventing leakage between geological horizons and towards the surface during CO2 storage, hydrocarbon production and well stimulation. The paper consists of two parts. In the first part, a finite-element model of earlier laboratory tests on thermal cycling of a casing/cement/rock assemblage is set up. It is demonstrated that radial tensile stresses contributing to annular cement debonding are likely to develop during cooling of such an assemblage. The results of the modeling are in agreement with the results of the earlier laboratory experiments, with regard to the temperature histories, CT data, and location of acoustic emission sources. In the second part of the paper, a computational procedure is developed for upscaling of data about rock damage obtained from CT, to a finite-element model of flow in porous media around a well. The damaged zone is shown to dominate the flow along the axis of a compound specimen (a hollow cylinder of sandstone filled with cement). Implications for leakage along an interface between cement and rock in-situ are discussed.
A better understanding of the energy budget has important implications for enhancing the efficiency of hydraulic fracturing treatments. In particular, what percentage of the input treatment energy is released as radiated energy? What characterizes the deformation of the failure?
We use a bonded-particle modeling approach to investigate both the radiated energy release and the amount of brittle failure. To test our model, we simulate triaxial compression tests on calibrated sandstone samples. Our results show that much of the failure is marked by a tensile component, despite the development of one or two large shear planes crosscutting the samples. Additionally, only 2.5% of the input energy is radiated as seismic waves. We propose an updated empirical energy-magnitude relation: log ER = 1.9MW +8.5, where ER is the radiated energy and MW is the event moment magnitude. This relation is an alternative to the commonly used Kanamori relationship and more applicable for the small-magnitude acoustic emissions in triaxial tests and likely microseismic events in hydraulic fracturing experiments, which are both marked by strong tensile deformation. Close examination of the source mechanisms of the induced acoustic emissions reinforce the complex nature of the micromechanics behind rock fracturing in general, due to strong deviations of the local stress field from the applied external field.
The paper presents further developments of the boundary element technique for solving three-dimensional problems of piecewise homogeneous elastic media containing multiple cracks of arbitrary non-planar shapes (previous results were reported in [1, 2]). In the developed technique, the elastic fields are represented by integral identities. Triangular elements are used to discretize the boundaries and polynomial (linear and quadratic) approximations of the unknown variables are adopted. In-plane components of the fields and geometrical parameters are arranged in various complex-valued combinations to simplify the integration. No singular integrals are involved since the limit, as the field point approaches the boundary, is taken after the integration. Analytical integration over each element is reduced to that over the contour of the element via application of Cauchy- Pompeiu representation . The collocation method is used to set up the system of linear algebraic equations to find the boundary unknowns. Geoengineering applications of the method are discussed.
Lindfors, U. (Itasca Consultants) | Sjöberg, J. (Itasca Consultants) | Vedin, P. (Swedish Transport Administration) | Swindell, R. (Swedish Transport Administration) | Rosengren, L. (Rosengren Bergkonsult AB) | Holmberg, M. (Tunnel Engineering AB) | Stille, B. (Sweco Infrastructure AB0)
The Swedish Transport Administration is currently updating a handbook on the design of rock excavations for road and rail infrastructure. The aim of the handbook is to provide guidelines and practical engineering tools for the planning and design of rock mass excavations. This paper describes the content of the handbook regarding (i) description of the rules and regulations, and how these affect the design work of rock mass excavations, (ii) guidelines to describe the type of design activities that should be implemented during the planning, , constructions and operational (management) stages, with respect to the design of the load-bearing structure, (iii) description of how the input data to the design can be developed and presented during various steps in the process, and (iv) description of suitable design methodology and appropriate design methods for reinforcement for rock tunnels and rock slopes.
While the supercritical Co2 injection technology has been successfully employed for utilization operations such as enhanced oil recovery, there are some social and technical challenges with its large-scale implementation for storage purposes. Induced seismicity due to Co2 injection is one of primary concerns associated with the technology and it needs to be properly addressed. In this study, a set of 2D coupled Thermo-Hydro-Mechanical (THM) modeling was performed to investigate the effects of the reservoir porosity and thickness on the magnitude of induced seismic events. The model included a limited-dimension pre-existing fault which cannot be easily detected by geophysical surveys. The numerical modeling was used to simulate changes in the stress distribution in the reservoir, fault and surrounding rock due to Co2 injection. The fault slip obtained from the model was then correlated to the magnitude of earthquake for each case. Three reservoir thicknesses and three reservoir porosities were examined in parametric studies. The results showed that thin reservoirs have higher probability of failure and will result in larger-magnitude induced seismic events. Reservoirs with higher porosity were shown to have longer rupture time and larger events.
Gypsum (a mix of Hydrocal B-11 and Diatomaceous Earth) is used by the MIT rock mechanics group as a model rock material. Unconfined compression tests on pre-cracked specimens with high speed camera observation showed macro-cracking processes of gypsum similar to other materials; however in contrast to the other materials, no microcrack process zone could be observed. On the other hand, environmental scanning electron microscope (ESEM) observation of gypsum showed limited micro-cracking preceding the crack initiation. This indicates that a microcrack process zone in gypsum might exist but it is not visible in high speed camera photography because of the scale. It was therefore decided to use acoustic emission (AE) to determine if such microcracks occur. To this end, prismatic specimens containing pre-existing flaws were tested under uniaxial compression, and the cracking process was monitored with both AE and high-speed camera imaging. AE results revealed that although ESEM images show the occurrence of some micro-cracking before macro-cracking, so far it could not be detected by AE.
A rock bolt represents the most important element in ensuring stability of rock mass excavations. This paper presents weaknesses and strengths of methods which deal with evaluation of rock bolts. Most widely used is pull-out test, conducted for capacity determination in destructive manner. To overcome ‘destructivity’, a method of acoustic emission shows that the force close to the failure point can be determined, so that the rock bolt will remain usable for strengthening purposes. Another NDT system, called GRANIT, is developed in order to determine load level inside rock bolt by analyzing its natural frequencies. When it comes to evaluation of grouting quality, efforts were made in order to find reliable solution, but existing methods still have weaknesses which limit their applicability. First developed and today still widely used is Boltometer which is based on emission of energy in rock bolt and on analyzing returned energy. However, dissipation of energy, due to karstic phenomena may not yield good results. Some efforts in improvement of weaknesses of Boltometer have been conducted and are presented in paper. A basic concept is given for novel method for NDT evaluation of grouting quality, which is in development at Faculty of Civil Engineering in Zagreb.
The geological storage of Co2 and the extraction of geothermal energy from deep saline aquifers are promising methods, of mitigating global warming and obtaining a valuable source of renewable energy. In such processes the porosity and permeability of the reservoir rock are the two governing factors. Porosity can be easily predicted and is conventionally used to identify permeability properties. However, such identification is inaccurate, because the presence of irregularities such as bedding and authigenic quartz significantly reduces the permeability of even highly porous rock. This paper investigates the effect of bedding and the presence of authigenic quartz on brine permeability in sandstone using two types of sandstones with similar porosities, Warwick white (WWS) and Warwick golden (WGS), from the Queensland Basin, Australia. Permeability tests were conducted on 38mm diameter and 150mm long sandstone samples for a range of injection pressures (4 – 7MPa) under 10 MPa confining pressure at 30°C temperature, using a highly precise core flooding apparatus. The experimental data provide important data for the geological storage of Co2 and the production of geothermal energy from deep saline aquifers.