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ABSTRACT ABSTRACT: The group of Alvand granitic rocks is distributed widely around the city of Hamadan, western Iran, and a number of important civil engineering structures have been founded on and in this rock group. Thus, the weathering characteristics of Alvand granites are of considerable practical importance. The granites are generally medium to coarse-grained and porphyritic, light-grey to buff in colour. To assess the influence of weathering on the geomechanical characteristics of these rocks, a sampling and testing program was carried out. Field work involved assessment of the rock mass weathering grades, execution of simple field tests and collection of representative samples from different weathering grades. This paper presents the results of laboratory investigations, including weathering characteristics and physical, chemical and mechanical properties. Interrelationships are determined for all test results. From the field and laboratory studies, robust linear relationships are found between the weathering degree of the tested samples and their geomechanical properties. 1 INTRODUCTION The geomechanical properties of rocks vary considerably depending on the rock type, nature of discontinuities and weathering. The effects of weathering on the geomechanical properties of rocks have been studied by numerous investigators and various weathering indices and classifications have been proposed. In nature, the inevitable process of weathering produces significant changes in almost all the chemical and physical properties of rocks and it is universally recognized that this process will have affected many of the geomechanical properties of the rock mass (Gupta & Rao 2001). The main aim of the present study was to demonstrate the effect of weathering on the geomechanical properties of Alvand grantitic rocks. The group of Alvand granitic rocks is distributed widely around the city of Hamadan, western Iran, and a number of important civil engineering structures have been founded on and in this igneous rock group.
- Geology > Structural Geology > Tectonics > Compressional Tectonics (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
ABSTRACT ABSTRACT: This paper presents a comparative study used for the analysis of the stability of slopes of the Amiya area, near Kathgodam Nainital, Uttaranchal, based on assumptions that the rock mass follows the MohrโCoulomb failure criterion. An extensive field study has been carried out. Laboratory experiments have been conducted to calculate the various mechanical properties of the rock mass. These properties have been used as input parameters for the numerical simulation. The computed finite element deformations and the stress distribution, along the failure surface, have been compared with the field measurements and are found to be in good agreement. The study shows that the slope is perilously unstable. 1 INTRODUCTION The stability of large rock slopes which are vulnerable to rock mass (rotational) failures can be analyzed by traditional limit equilibrium methods such as Bishopยกยฏs method (Bishop 1955), Janbuยกยฏs simplified method (Janbu 1954) or the later improved sophisticated method (Janbu 1957, Spencer 1967) based on assumptions regarding the inclination and location of the interslice forces. For many cases, the limit equilibrium methods established have given relatively reliable results despite their limitations. However, they tend to give conservative values of factor of safety, since the full shear strength is assumed to be mobilized simultaneously along the failure surface. Duncan (1996) and Griffiths & Lane (1999) summarized the results of a survey on slope stability analysis using FEM based on the shear strength reduction technique and provided a number of valuable lessons concerning the advantage and disadvantage of FEM for use in practical slope engineering problems. DEM (Cundall & Strack 1979) and discontinuous deformation analysis (DDA) (Chihsen & Bernard 1996) can be used to simulate large block movements in complex geological media consisting of many blocks which can break on presumed fracture planes during the calculation process without any external intervention.
- Asia > India (0.29)
- North America > United States (0.29)
ABSTRACT ABSTRACT: The oil and gas industry is exploring reservoirs under increasingly difficult geological conditions. Accessing these reservoirs requires drilling through unconsolidated formations, faulted rocks, rubble zones and salt structures. The formations not only have highly overpressured pore pressure and abnormal insitu stresses, but also have very low fracture gradients. With extremely narrow drilling mud window, drilling engineers experience a high frequency of wellbore stability problems during well construction, causing substantial non-productive time in drilling operations. To keep wellbore from shear failure and tensile failure in this narrow mud weight window, wellbore strengthening is crucially important. This paper presents a new method to strengthen wellbore by reducing both rock shear and tensile failures. This novel method is based on the premise that building an impermeable boundary condition at wellbore wall can avoid drilling mud invasion into pore spaces of the formation. This reduces pore pressure increase near the wellbore, and also reduces effective tangential stress and increases effective radial stress. Poroelastic solution with permeable and impermeable boundary conditions is developed to analyze pore pressure and drilling fluid interaction and wellbore stresses changes induced by drilling. Numerical study of this poroelastic solution shows that for the same mud weight the wellbore is stable for the impermeable mud condition; however, it is unstable for the permeable case. 1 INTRODUCTION A considerable number of oil and gas reservoirs in the world are located in unconsolidated sands or in the naturally fractured formations. These complicated formations usually have not only highly overpressured pore pressure and abnormal in-situ stresses, but also very low fracture gradients. Consequently, this causes an extremely narrow safe drilling mud window (Figure 1), and drilling engineers experience a high frequency of wellbore stability problems during well construction, inducing substantial non-productive time in drilling operations.
- Europe > Norway > Norwegian Sea (0.34)
- North America > United States > Texas (0.29)
ABSTRACT ABSTRACT: A petroleum industry trend towards the use of water-based drilling muds is driving research into the chemohydromechanical interaction between the drilling fluid and shale formations, which cause the majority of drilling problems. Examination of solutions for simple geometries lends important insights into shale/mud interaction, provides benchmarks for numerical models, and can give a basis for the development of new laboratory tests. This paper presents a chemoporoelastic solution for a chemically active shale ball that is subjected to both hydraulic and chemical loading at its surface. It is shown that the size of a ball subjected to an increase in surrounding ion concentration eventually decreases, but depending on the chemomechanical and chemo-hydraulic coupling parameters it can follow a path of monotonic contraction, contraction to a minimum size followed by recovery, or initial swelling followed by contraction. These results demonstrate the importance of chemoporoelastic considerations for predicting the response of a shale to changes in drilling mud composition. 1 INTRODUCTION The petroleum industry has long relied on oil-based drilling fluids to stabilize the wellbore during drilling operations. However, the environmental constraints and associated costs which surround the disposal of oil-based waste products are driving a trend towards the use of water-based drilling fluids. One major technological concern with water-based drilling fluids arises from the interaction of the fluids with shales which, along with mudstones, siltstones, and claystones, comprise 75% of drilled sections in oil and gas wells and cause 90% of the drilling problems related to wellbore instability (Tan et al. 2002). One common means for improving wellbore stability is to add various salts to the drilling fluid. Success of this approach relies on alteration of the (coupled) pore pressure and stress fields in the immediate vicinity of the wellbore, which in turn relies on the shale possessing certain chemo-mechanical properties.
ABSTRACT ABSTRACT: Drilling mud weights are typically selected based on the requirement that hole quality is sufficient to successfully reach target depth. Unfortunately, acceptable hole enlargements while drilling may exceed safe limits for completion performance, e.g., to achieve effective zonal isolation or to avoid damage to expandable screens. Breakout analyses to select optimal mud weight to maintain wellbore stability typically define hole quality in terms of the width of failed zones and do not provide quantitative information about breakout depth. Although laboratory experiments have been run and theoretical models have been developed to predict enlargement depth as a function of breakout width, relationships derived from these studies have never been validated using field data. We present here a subset of analyses of detailed measurements of breakout widths and depths from image logs and caliper data within the reservoir sections of more than 20 wells. In sands, breakout depth increases with breakout width, in agreement with models and laboratory results. Although in some cases shales have more severe enlargements, breakouts with similar widths are in most cases deeper in sands than in shales. One explanation for this observation is that in these wells the sands have a lower residual strength, whereas in shales which are less brittle the nominally failed material provides sufficient support that further breakout growth is inhibited. There is some evidence that in extremely weak sands this relationship may be reversed (i.e., shales are more brittle). While models of stable breakout shape in materials with residual strength support this hypothesis, other factors may be important, including initial strength and internal friction. 1 INTRODUCTION Wellbore breakouts are zones of enlargement due to failure of the rock at the wellbore wall at orientations where the greatest stress concentrations around the well exceed the strength of the rock.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.85)
ABSTRACT ABSTRACT: The paper deals with the geotechnical study and optimum ultimate pit slope design of Ashok opencast project with special reference to the highwall stability of slopes created by surface miner. It was also aimed to know the influence of slope design parameters on the safety factor by sensitivity analysis. Geotechnical mapping was done on the exposed benches of the surface mine as per the norms of International Society of Rock Mechanics. The different geo-mechanical properties of lithological units were determined. The failure analysis was done by GALENA software based on limit equilibrium method and optimum slope design was recommended. 1 INTRODUCTION The geotechnical study was conducted for the optimum ultimate pit slope design of the highwall slopes at Ashok opencast project. The mine is producing 6.5 million tonnes per year. It is located in Jharkhand state of India and being mined by Coal India Ltd. The coal is of non-coking category and is suitable for use in powerhouses. It was also aimed to know the influence of slope design parameters on the safety factor by sensitivity analysis, which tells the importance of the parameter in the critical slope. A more justified and suitable remedial measure can be planned for any critical slope after sensitivity analysis. The geotechnical characteristics of joints in overburden and coal cleats were measured on the exposed benches of the surface mine as per the norms of International Society of Rock Mechanics (ISRM 1978). The different geo-mechanical properties of lithological units were determined at the Rock and Soil Mechanics Laboratories of CMRI. Initially the average orientations of the discontinuity sets determined from the geologic structural mapping were analyzed to assess kinematically possible failure modes involving structural discontinuities in the sandstone as well as coal slope faces.
- Geology > Geological Subdiscipline > Geomechanics (0.88)
- Geology > Rock Type > Sedimentary Rock > Organic-Rich Rock > Coal (0.70)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.70)
- Well Drilling > Wellbore Design > Wellbore integrity (0.56)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.56)
- Well Drilling > Wellbore Design > Rock properties (0.55)
Monitoring the Behavior of a Sill Pillar At Failure In a Narrow-vein Mine
Labrie, D. (CANMET Mining and Mineral Sciences Laboratories, Natural Resources Canada) | Boyle, R. (CANMET Mining and Mineral Sciences Laboratories, Natural Resources Canada) | Anderson, T. (CANMET Mining and Mineral Sciences Laboratories, Natural Resources Canada) | Conlon, B. (CANMET Mining and Mineral Sciences Laboratories, Natural Resources Canada) | Judge, K. (CANMET Mining and Mineral Sciences Laboratories, Natural Resources Canada)
ABSTRACT ABSTRACT: Trials were undertaken in a narrow-vein mine of Northern Quebec, Canada, to monitor stress changes in sill pillars due to mining. Trials were carried out in active mine areas using vibrating wire proving rings and short extensometers, installed in boreholes to monitor radial and axial displacements in three orthogonal directions. Trials took place at intermediate depths, 600 meters underground, in areas with extraction ratios exceeding 80%, and lasted several months. The present paper focuses on a successful trial, showing regular stress changes within the sill pillar all along its loading curve. Stress changes were relatively slow during the first half of the experiment. Changes increased significantly during the second half, resulting in the definition of a clear inflection point when the stope reached about 75% of its final height. Mining was stopped when a rock burst resulted in complete sill pillar failure, which prevented any return to the burst area. Monitoring and numerical modeling records were reviewed. These show reasonable agreement and validate the trial and the data gathered during the period of monitoring. This experiment provides insights into the unpredictability of rock burst events, and ways to prevent or alleviate their damage. 1 INTRODUCTION Mining and excavation of rock material inevitably produce a redistribution of stresses around mine openings, within pillars and abutments. Loading of pillars and abutments is limited by the strength of rock materials. Once this limit is reached, rock material will either deform plastically, in the case of soft rocks, or fail violently, in the case of hard brittle rocks. Intermediate rock materials show an intermediate behavior, with failure progressing through limited ductile deformation and relatively small bursts (Labrie et al. 2002). Trials were done in a narrow-vein mine of Northern Quebec, Canada, to monitor stress changes taking place within pillars during mining.
- North America > Canada > Quebec (1.00)
- North America > Canada > Ontario > National Capital Region > Ottawa (0.15)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Mineral (0.70)