The recent crash in the oil market has allowed the industry to reduce the pace of evaluation and completion decisions in unconventional reservoirs, and turn to a more science-based decision-making process for project execution. The traditional stimulation design based on the geometric spacing of induced fractures is now gradually changing to geological spacing (i.e., a design based on an understanding of the reservoir geology) to enhance hydraulic fracture stimulation effectiveness for drastically reduced cost. A methodical rock texture characterization of core samples and cuttings can provide powerful information that can be used reliably and cost-effectively to optimize fracture stimulation designs by placing frac stages based on rock characteristics. This paper presents a new method to quantify rock texture based on automated petrographic analysis that uses advanced microscopy image analysis from scanning electron microscopy (SEM) and optical microscopy. A procedure called "quantitative evaluation of minerals using a scanning electron microscope" (QEMSCAN) and optical microscopy analyses were used to image rock samples prepared from cores and cuttings. Rock texture parameters were extracted automatically using new digital data processing techniques. The information from automated petrographic analysis was used to determine the spatial distribution of all components including mineral composition, framework grains, matrix, cement, grain size and shape, pore size and shape, modes of contact between grains and the nature of porosity. The results showed that while mineral composition of rock is important, texture characterization is far more significant to understand rock behavior than has been reported in the industry. Our results demonstrate the importance of quantitative microscopy and how it can provide an understanding of the key relationship between rock texture and rock behavior.
A new method was produced to characterize rock texture quantitatively from advanced image analysis with the aid of an automated petrographic system.
We report the effect of micro-scale contact heterogeneities on fabric, stress, and elastic anisotropy in granular media using granular dynamics simulation. Uniaxial compaction (with zero lateral strain) of initially isotropic frictional grain packs is the source of anisotropy in our computational experiments. We provide evidence of fabric anisotropy using second order fabric tensor. Further, the horizontal-to-axial stress ratio (
The objective of this study was to discuss the influence of pore fluid on elastic properties of greensand. Gassmann''s equations generally work at low frequency and do not take into consideration the fluid related dispersion. In some cases Biot''s theory is used to describe the fluid related dispersion. However, Biot''s theory does not fully explain the frequency dispersion of sedimentary rocks. Greensands are composed of a mixture of quartz and micro-porous glauconite grains. In greensand, it is possible that the contrast between flow in macro-pores and micro-pores within glauconites gives rise to a local stiffening pressure gradient in the fluid. Then fluid flow in greensand could then be described as a kind of squirt flow. Greensand data from the North Nini filed was included in this study. Gassmann''s, Biot''s and squirt models were used to discuss the influence of pore fluid on elastic moduli. Biot''s critical frequency and NMR (nuclear magnetic resonance) T
Estimation of rock mass strength has become more critical in recent years due to the increase in the number of mining and civil projects at greater depths (> 2 km). Empirical approaches for the estimation of rock mass strength (e.g., Hoek-Brown/GSI) are primarily based on experiences at shallow depths and low confinement problems (e.g., tunnel wall failure), and therefore may not be representative for the strength in highly confined rock masses (e.g., for the core of pillars or abutments at depth). In this study an attempt is made to investigate the strength of rock masses, using the discrete element code PFC2D. For this purpose, the recently developed PFC2D Grain-Based Model (GBM) was used to match the laboratory response of intact and granulated Wombeyan marble. The term “granulated” refers to a heat treated sample where the grains have been completely separated at their boundaries due to anisotropy and contrast of their thermo-elastic properties. This material is considered to represent an analogue for a randomly jointed rock mass. It is shown that the PFC2D-GBM calibrated to unconfined and confined intact marble strengths and then to the unconfined granulated marble strength, underestimates the strength of the confined granulated marble. This problem was resolved by increasing the grain boundary friction angle in the granulated model to account for micro-scale roughness of the grain boundaries as observed in the microscopic image of the granulated marble. The calibration methodology taken to obtain micro-properties for both intact and granulated marble as well as implications for the determination of rock mass strength at various confinement levels are discussed.
With the increase in the number of mining and civil projects at great depths (>2 km), the validity of common empirical approaches calibrated at shallow depths needs to be evaluated. Moreover, at such depths due to high ratios of excavation-induced stress to rock mass strength, a number of challenges arise and therefore a better estimation of rock mass strength is critical for the safe and efficient development of underground infra-structures such as tunnels and pillars. Conventional approaches for the determination of rock mass strength (e.g., Hoek-Brown failure criterion  with the GSI classification system [2, 3]) have been primarily established based on experiences from tunnels at relatively shallow to moderate depths, and observations near excavations where confinement is relatively low (in the 5 to 10 MPa range). Therefore, the application of these techniques for the estimation of confined rock mass strength, relevant for the design of wide pillars, could be flawed and may lead to costly design errors.
In this study, the discrete element code, PFC2D  was used to investigate the strength of rock masses by calibrating the models to laboratory test results of undamaged and damaged samples. The damaged samples are considered to represent an analogue for a randomly jointed rock mass.
Different methods in PFC2D for modeling brittle failing rocks are first briefly reviewed. The PFC2D grain-based model (GBM) is then used to match the laboratory response of the intact and granulated Wombeyan marble reported by Gerogiannopoulos .
Systematic experiments on ice under increasing levels of confinement have established two distinct modes of brittle-like shear faulting. Similar modes of shear faulting appear to operate in rocks and minerals as well, and possibly in ceramics. One kind, termed a Coulombic (C) or frictional fault, is oriented at ~30° to the direction of shortening and is comprised of a narrow band of microcracks that nucleate prior to terminal failure and then link up to create the fault. The orientation of the fault is governed by the coefficient of internal friction, as predicted by the Mohr-Coulomb relationship. C-faulting is characterized by pressure hardening, grain size dependent strength, and the creation of fault gouge. The other, termed a plastic (P) or nonfrictional fault, is oriented at ~ 45° (i.e. sub-parallel to planes of maximum shear stress) and is comprised of a band of recrystallized grains. P-faults form once the degree of triaxial confinement is sufficient to suppress frictional sliding. P-faults are characterized by pressure and grain size independent strength. We show, for the first time, direct measurements of localized heat production during faulting. We observe that the level of heating is higher for P-faulting than C-faulting. For C-faults, the observed rise in temperature can be explained by the generation of frictional heat though sliding across the faces of the faults. In contrast, measured temperature rises in P-faults are too great to be explained by the generation of heat from frictional sliding. Instead, we find that the degree of heating in P-faults is consistent with heat generation associated with localized plastic flow.
Using insight into the micromechanics of failure learned from using ice as a model material, Renshaw and Schulson  demonstrated that the transition from brittle to ductile failure (BD transition) in a variety of crystalline materials, including crystalline rocks, occurs when the applied strain rate is sufficiently low to allow creep deformation to relax stress concentrations at flaw tips or when the confinement is sufficient to prevent frictional sliding along flaws.
To investigate the mode of failure at high strain rates under high confinement, we present here a systematic comparison of the structural and mechanical characteristics of polycrystalline ice rapidly deformed to terminal failure under triaxial compression with increasing degrees of confinement. Emphasis is placed on the physical processes underlying terminal failure. The results unambiguously confirm two distinct modes of compressive shear faulting under triaxial loading. Based on our earlier success in using ice as a model material for rock, we believe that the lessons learned from systematic experiments on ice can provide insight into similar processes likely occurring within crystalline rocks.
2.1. The Ice
Cubes 100 mm on side were machined from laboratory grown freshwater granular and columnar S2 ice following standard laboratory procedures described in Golding et al., 2010. The resulting material was free from cracks and completely transparent. Porosity was maintained less than 0.5 % for columnar ice and less than 1.0 % for granular ice.
In the context of CCS, we aim at simulating the migration of the CO2 for performance assessment purpose and risk management. In the case of the Krechba reservoir aquifer (In Salah - Algeria), we focus at modeling the reservoir pressure field and the induced ground surface displacement associated with CO2 reinjection. While coupling 3D fluid-flow and geomechanical modelings, we benefit from InSAR satellite surface displacement data in addition to conventional ones. Preliminary results show that adjustment over time of the reservoir mechanical properties is required to match observed displacement data and suggest considering rock properties evolution of media within an explicit coupling modeling approach.
CO2 storage in geological formations such as deep saline aquifers or depleted oil and gas reservoirs constitutes the Carbon Capture and Storage (CCS) option to fight against global warming. In this context, BP, STATOIL and SONATRACH started the In Salah Joint Venture to manage on a series of fields the CO2 produced by re-injecting it in the northern part of the aquifer of the Krechba gas field. The injection started in August 2004 and to date up to 3 million tons of produced CO2 have been re-injected, while 17 million tons being scheduled at least. In the frame of the CO2ReMoVe European project, IFP Energies nouvelles and his partners work at developing tools and methodologies to simulate the CO2 migration within the reservoir and the resulting site behavior of a series of pilot sites (including Krechba). The final objective of this project is to be able to model the short term behavior of the site and to predict its long term at the scale of the storage complex in order to assess the storage integrity and its long term performance. When considering modeling different kinds of data are available: some as input data, others as the targeted ones by the simulation; in this work, our objective is to reproduce the ground deformation data that have been measured by satellite.
2. THE IN SALAH PROJECT
The In Salah project concerns a series of gas fields located in central South Algeria (Fig. 1) that contain almost 1 to 10 % of CO2. To export the natural gas, it is necessary for operators to reduce the CO2 concentration to the sales gas export concentration threshold (0.3%). Hence, it was decided, in the In Salah project, to re-inject the captured CO2 into the Krechba reservoir aquifer: this will avoid the emission of approximately 17 millions tons of CO2 and in the same time this will help to study the CCS concept at an industrial scale. To have the most reliable fluid flow modeling representation of our problem, we start with an history matching approach in order to determine the best permeability distribution inside the reservoir that insures matching between the modeling results and the measured data. Preliminary works  show that a dual medium representation of the reservoir  is necessary to correctly mimic the field data, typical of fractured reservoir.
Palosz, Bogdan F. (Institute of High Pressure Physics UNIPRESS, Polish Academy of Sciences) | Stelmakh, Svitlana (Institute of High Pressure Physics UNIPRESS, Polish Academy of Sciences) | Grzanka, Ewa (Institute of High Pressure Physics UNIPRESS, Polish Academy of Sciences) | Gierlotka, Stanislaw (Institute of High Pressure Physics UNIPRESS, Polish Academy of Sciences) | Palosz, Witold F. (Brimrose Corporation)
Kane, Alexandre (Department of Engineering Design and Materials, Norwegian University of Science and Technology (NTNU)) | Østby, Erling (Department of Engineering Design and Materials, Norwegian University of Science and Technology (NTNU))
Pedersen, J.H. (Norwegian University of Science and Technology (NTNU)) | Hjelen, J. (Norwegian University of Science and Technology (NTNU)) | Solberg, J.K. (Norwegian University of Science and Technology (NTNU)) | Karlsen, M. (Norwegian University of Science and Technology (NTNU), Statoil ASA) | Akselsen, O.M. (Norwegian University of Science and Technology (NTNU)) | Astad, S.P. (Statoil ASA) | Østby, E. (SINTEF)
Otani, Jun (Nippon Steel Corporation) | Works, Oita (Nippon Steel Corporation) | Funatsu, Yuuji (Research & Rule Development, Indian Register of Shipping) | Inoue, Takehiro (Plate Steel Research Labs, Nippon Steel Corporation) | Shirahata, Hirouki (Plate Steel Research Labs, Nippon Steel Corporation)