Size Effect in Strength Assessment by Indentation Testing on Rock Fragments

Size DependencyIn this study, uniaxial compressive strength (UCS) of limestone rocks was estimated by indentation testing on small rock fragments. Here, the effects of rock fragment dimensions (particularly area and thickness) on indentation indices were studied. The investigation shows size dependency of the conventional indentation parameters. The results indicate the fragment area normal to loading direction has little effect on the indentation indices, while the sample thickness has major influence on the results. To reduce size dependency, the results of indentation test were normalized by a thickness function. The proposed empirical equations were verified against independent data pertaining to other limestone rocks not used in developing the correlations, which showed agreement between the estimated and measured UCS. Statistical analysis was used to determine the minimum number of required indentation tests in relation to the project importance. This study shows that the uniaxial compressive strength of rocks can be estimated reasonably well from indentation testing of small rock fragments

A laboratory and full-scale study on the fragmentation behavior of rocks

Rock fines produced in rock blasting and crushing processes is a major problem for aggregate producers. Laboratory and full-scale studies were carried out to explore the possible relationship between fines production and water content of rock material. The Brazilian tensile test was selected for the laboratory study while a cone crusher machine was employed for the full-scale trials. Results of the laboratory work showed that the percentage of fines fraction produced was a function of the type of rock tested and the tensile strength of individual specimens. Different rocks produced various amounts of fines, but for the rocks investigated, a correlation between its strength and its fines generation could be attained. The influence of water content on production of fines was also examined and showed that increased water content reduces both tensile strength and fines generation in the laboratory. The impact of water content on aggregate production was also studied in full-scale but the results were not as clear as those of the laboratory tests. (c) 2006 Elsevier B.V. All rights reserved

Subsurface fracture analysis and determination of in-situ stress direction using FMI logs: An example from the Santonian carbonates (Ilam Formation) in the Abadan Plain, Iran

The relationship between the present-day stress field and natural fractures can have significant implications for subsurface fluid flow. In particular, fractures that are aligned in orientations favourable for reactivation by either shear or tensile failure in the in-situ stress field often exhibit higher hydraulic conductivities. The Ilam Formation of southwestern Iran is an important hydrocarbon reservoir containing numerous natural fractures. However, little is known about the state of stress in this region, or any of Iran's petroleum provinces. We conducted analysis of the present-day maximum horizontal stress orientation and the density, orientation and hydraulic conductivity of natural fractures in the Ilam carbonates using high resolution Formation Micro Imager resistivity logs in two wells. A total of 51 breakouts with an overall length of 215 m were observed in the two wells, indicating a maximum horizontal stress orientation of 68°N (± 7.6°) in well A and 58°N (± 6.3°) in well B. Furthermore, the wellbore-derived stress orientations determined herein are consistent with those inferred from nearby earthquake focal mechanism solutions, indicating that stresses in the sedimentary cover are linked to the resistance forces generated by Arabia–Eurasia collision. Furthermore, the correlation between stress orientations estimated from earthquake focal mechanism solutions and breakouts indicates that focal mechanism solution data, which is often considered to be unreliable for stress field analysis near transform margins, may provide reliable information on the stress orientation near continental collision zones. The image log data also reveals three sets of open, and presumably hydraulically conductive, fractures with strikes of (i) 160–170°N, (ii) 110–140°N and (iii) 070–080°N. Fracture set (iii) is consistent with being formed and open in the present-day stress field. However, fracture sets (i) and (ii) strike at a high angle to the present-day maximum horizontal stress, and are interpreted herein to be the result of either pre- or syn-folding related forces. The observation that different sets of open fractures in the field can be either sensitive or insensitive to the present-day stress is critical for improving hydrocarbon recovery.Mojtaba Rajabi, Shahram Sherkati, Bahman Bohloli and Mark Tinga