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Non-destructive Evaluation of the Stable Behavior of a Quasi-brittle Sandstoner
Luong, M.P. (LMS (Solid Mechanics Laboratory) CNRS UMR 7649, Civil Engineering, Departement of Mechanics, Ecole Polytechnique) | Tabrizi, M. Emami (LMS (Solid Mechanics Laboratory) CNRS UMR 7649, Civil Engineering, Departement of Mechanics, Ecole Polytechnique) | Halphen, B. (LMS (Solid Mechanics Laboratory) CNRS UMR 7649, Civil Engineering, Departement of Mechanics, Ecole Polytechnique) | Eytard, J.C. (LMS (Solid Mechanics Laboratory) CNRS UMR 7649, Civil Engineering, Departement of Mechanics, Ecole Polytechnique)
ABSTRACT The paper introduces a non-destructive testing technique in use to detect the occurrence of geomaterial instability subsequently causing damage and a specific data reduction procedure to assess damage accumulation. It is assumed that the evolution of non-linear energy increase corresponds to the evolution of the damage extent. The damaging process was detected by analyzing the signal evolution of ultrasonic pulses propagating through a sandstone specimen subjected to increasing static compression loads up to failure. An input-output non-parametric technique and a non-linear analyzer for data reduction procedure were chosen to portray the non-linear behavior. Based on a multi-dimensional Fourier transform, the non-linear analyzer permits to separate linear and non-linear parts. It can be used to monitor non-destructively and continuously the overall alteration or damage process of sandstone so that damage mechanisms could be quantitatively estimated by a dimensionless parameter, the so-called non-linearity ratio. 1 INTRODUCTION Interest in the non-destructive evaluation and testing for civil engineering projects, particularly in inspection and monitoring of geotechnical structures has increased very much in recent years, for example, in the use of elastic wave, seismic and electrical methods in the evaluation of concrete structure, foundations, etc.; because these methods are rapid, inexpensive, economic and portable in comparison with the other methods. On the basis and the fact that the effect of scale and the climatic and biological events are very important in the behavior and strength of rocks, application of non-destructive evaluation for inspection and monitoring of geotechnical structures seems to be in underdevelopment. This paper introduces a non-destructive technique based on ultrasound propagation characteristics to detect the material instability occurred in sandstone; thus providing a very useful early warning for the security of underground structure or rock slopes at high risk for environment. A non-destructive evaluation of material stability in some sandstone specimens under uniaxial compressive loading was successfully performed in laboratory. It uses a special testing technique to detect the signal evolution of ultrasound pulses propagating through the specimen subjected to increasing axial compression loading.An input-output non-parametric technique based on ultrasonic pulse propagation (Luong et al. 2005) and a non-linear analyzer for data reduction procedure (Liu & Vinh 1991) were chosen to portray the non-linear behavior of a sandstone subjected to increasing static compressive loads. Based on multidimensional Fourier transform, the non-linear analyzer permits to separate linear and non-linear parts. It can be used to monitor non-destructively and continuously the overall alteration or damage process of a sandstone so that damage mechanisms could be quantitatively estimated by a dimensionless parameter the so-called non-linearity ratio. 2 EXISTING TECHNIQUES Common experimental methods have been traditionally used to obtain information concerning deformation, strains, structural integrity and failure mechanisms in rock infrastructure, using strain gauges, photoelasticity, moire, ultrasound and radiography, as well as acoustic emission and thermographic methods (Luong 1992). Unlike most metals that are mass homogeneously produced, properties of rock are not unique especially in rock mass. In most rocks, both acoustic velocity and attenuation vary greatly.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Geological Subdiscipline > Geomechanics (0.70)
ABSTRACT Upper Paleozoic to Mesozoic-age greywackes are widespread throughout New Zealand. This paper describes the characteristics of the greywacke rocks based on field mapping, laboratory testing and rock mass classification from sites around the country. The rocks comprise hard sandstones, interbedded sandstones and mudstones, and mudstones. Where unweathered, intact rock materials have unconfined compressive strengths generally above 100MPa and moderate to high modulus ratios. The rock masses, which are typically closely-jointed and commonly tectonically disturbed, have an unusual combination of very high intact strength and joints with low persistence. The effect of these properties on rock mass deformability and strength is illustrated by estimation of dam foundation deformability from tiltmeter measurements and assessment of critical foundation failure mechanisms from estimates of defect and global rock mass strengths. 1 INTRODUCTION In New Zealand, the term ‘greywacke’ is applied to the very well indurated to slightly metamorphosed, interbedded mudstones and muddy sandstones belonging to the Torlesse Supergroup. These Upper Paleozoic to Mesozoic-age basement rocks are widespread throughout New Zealand (Fig. 1) where they are the bedrock of many of the country's engineering projects and also an important source of roading and concrete aggregate. Greywacke rocks are commonly closely-jointed as a result of their complex geological history. This paper is part of a long-term New Zealand research project into the engineering properties of unweathered greywacke. Studies at three sites (Aviemore Dam, Belmont Quarry, Taotaoroa Quarry – Fig. 1) have involved engineering geological mapping, laboratory testing of intact rock and assessment of rock mass properties (Read et al. 1999, 2003, Richards & Read 2006, Read & Richards 2007). Postgraduate studies involving mapping, laboratory testing and analysis of earlier in situ testing (e.g. Cook 2001) also form part of the project. This paper summarises geotechnical characteristics of greywacke rock masses and discusses influences of defect characteristics on their strength and deformation properties. 2 GREYWACKE PROPERTIES 2.1 Rock material Greywacke sequences (Begg & Mazengarb 1996) consist of interbeds of:Sandstone – coarse to medium grained, and medium to dark grey. Individual grains are poorly sorted angular quartz and feldspar, plus fragments of metamorphic and igneous rocks. The intergranular filling is clay minerals formed during induration or slight metamorphism. Mudstone – layers of clay, silt or mud, generally dark grey to black, sometimes red from iron minerals. Proportions of mudstone to sandstone vary between localities. For example, at Waitaki near Aviemore, mudstone is the dominant lithology, while elsewhere (e.g. Karapiro near Taotaoroa) sandstone dominates. More massive beds of both lithologies may be up to tens of metres thick, although more cyclic deposition can result in interbedding with discrete beds < 0.5m thick. Figure 2 summarises intact compressive strength and deformation parameters for greywackes from a number of sites. Sandstones, which have moderate modulus ratios, have strengths > 100MPa with stronger rocks being coarser grained. Mudstones, with moderate to high modulus ratios, are generally weaker than the sandstones and strong to very strong.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock (1.00)
New Model For the Volumetric Strain of Rocks Under High Differential Stress And Large Shear Displacement
Takahashi, M. (Research Center for Deep Geological Environments, AIST) | Takemura, T. (Institute of Geology and Geoinformation, AIST) | Kato, M. (Faculty and Graduate school of Engineering, Hokkaido University) | Kwasniewski, M. (Silesian University of Technology) | Li, X. (Institute of Rock and Soil Mechanics, Chinese Academy of Science)
ABSTRACT To investigate precisely the internal structural changes that occur in highly stressed Shirahama sandstone with increasing axial strain and confining pressure, we measured the total porosity and bulk density using mercury injection porosimetry and specific surface area using the gas adsorption method. Under low confining pressures, the specimen is characterized by the existence of a main fault and deformation is dominated by dilatancy. Under higher confining pressures, the specimen undergoes ductile deformation; volumetric strain indicates progressive compaction throughout the experiment. The measurements of total porosity and specific surface area support the finding of increasing porosity at higher confining pressures. Macroscopically, the specimens show persistent compaction, yet microscopically the total porosity increases with increasing confining pressure. This phenomenon is attributed to the formation of stress-induced microcracks caused by grain crushing and the development of small open spaces around rock fragments that arose due to high degrees of compaction and large shear displacement. 1 INTRODUCTION Permeability and specific storage are important parameters in evaluating the transport property and storage capacities of soils, rocks, and geomaterials. These properties are analyzed using permeability measurements undertaken in laboratory tests. Test results indicate that permeability decreases with increasing effective confining pressure, but the specific storage does not showa clear correlation with changes in effective confining pressure. Zhu and Wong (1997) measured permeability as a function of various stress states across the brittle-ductile transition in five types of sandstone. They compiled triaxial compression data for permeability versus effective pressure, differential stress, porosity, and axial strain, and discussed the evolution of permeability in various porous sandstones in terms of mechanical deformation and failure mode. Aspects of the microstructural changes that occur under conditions of high stress and large shear displacement have yet to be studied sufficiently to yield definite conclusions regarding their mode of occurrence. Additional, data are also required in terms of the changes in volumetric strain and bulk density that accompany different deformation patterns. In the present study, we focuse on the relationship between changes in the internal structure, especially volumetric strain and changes in bulk density change at a microscopic scale, and mechanical deformation using data obtained from mercury intrusion porosimetry and the gas adsorption method. 2 EXPERIMENTAL TECHNIQUES In this experiment, we used the following two methods to evaluate volumetric strain and the total volume of the specimen. 2.1 Pore volume apparatus When the pore pressure was constant during deformation of the specimen, the volume change of the pore water that flowed out or was extracted from the specimen was measured using a micro- metering valve in which the inner piston could be moved forward or backward to maintain constant pore pressure. The change in pore volume change was calculated based on the diameter of the metering valve piston and the degree of rotation of the valve handle. The sensitivity of this system was about 2.7×10−4 cm3 corresponding to a 6.36 micro-strain in terms of volumetric strain.
- Geology > Geological Subdiscipline > Geomechanics (0.90)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.67)
ABSTRACT The objective of the investigation was to carry out pull out tests on a scale model of rock anchors in intact rock samples and to determine the shape of the different failure surfaces formed. Specifically, failurewas forced to take place through the rock, not through any of the interfaces or through the anchor itself, to allow studying the behaviour of the rock at failure. For this purpose, specific equipment has been designed and constructed in the Geotechnical Laboratory of the CEDEX (Centre for Studies and Experimentation on Civil Engineering) in Madrid, Spain. The testing equipment has been used with different materials (granite, limestone and sandstone blocks), carrying out several tests for each one of these materials. Different aspects of the geometry of the test results have been analyzed. This experimental data can be employed to contrast and validate an analytical calculation method to obtain the tensile capacity of rock anchors based on nonassociated plasticity and the variational method, and to get a deeper insight in the behaviour of rock at failure. 1 INTRODUCTION At construction sites where many anchors are needed, it is usual to make specific in situ pull out tests at the site to improve the anchor design. These tests are seldom carried out to reach the rock failure, because of too low anchor resistance or problems with the testing equipment, particularly when the quality of the rock mass is high. Usually, the anchor itself fails much before the rock, and even if failure takes place through the rock, the failure surface is not measured. During the works presented in this text, model rock anchors have been tested, and the failure of the anchors have been thoroughly studied (García-Wolfrum, 2005). 2 TESTING EQUIPMENT A specific testing equipment has been designed and constructed in the Geotechnical Laboratory of CEDEX (Centre for Studies and Experimentation on Civil Engineering) to carry out the pull-out tests. This testing equipment is composed by a hydraulic jack, a computerized control system and a connection system with the different blocks (see Figure 1 and 2). With testing device rock blocks with a base of 1m×1m can be subjected to an increasing force of up to 100 kN. The vertical displacement of the jack and the pull-up force are measured with a precision of 0,25mm and 0,5KN, respectively. The whole testing process is controlled by a personal computer. 3 TESTED MATERIALS Tests were carried out using as a base material some of the rock kinds that are most usual in Spain: sandstone, limestone and granite. 3.1 Rock Three sandstone blocks, two limestone blocks and two granite blocks with a size of 1×1×0,4m respectively were used for the tests. The basic properties of these rocks have been studied in laboratory through a wide range of tests.The most important results are shown in Table 1.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.71)
- Geology > Rock Type > Igneous Rock > Granite (0.71)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Limestone (0.71)
ABSTRACT In this paper an extension of the known Hoek-Brown failure criterion to three dimensions is presented. First of all, a recompilation of compression, extension and true triaxial tests made with rocks, present in the technical literature, is made. The main conclusion drawn from those tests is the importance of the intermediate principal stress in the failure strength of rocks. Taking this idea into account, the Hoek-Brown failure criterion is modified. The laboratory test results are modelled with this new criterion with great success, which proves the goodness of the modification done. 1 INTRODUCTION In many rock engineering problems it is necessary to have a failure criterion able to reproduce correctly the real stress condition at failure under three-dimensional configuration. Particularly, the spread in the use of Finite Element Method programs that let calculate three-dimensional models requires compulsoryly the definition of three-dimensional failure criterion to be able to make adequate calculations. By other hand, among other criteria present in the technical literature, probably the most used model in Rock Mechanics is the one developed by Hoek – Brown established in 1980. where σ 1 is the major principal stress at failure, σ 3 is the minor principal stress at failure, σ c is the uniaxial compression strength of the rock matrix and mb and s are constants that depend on the characteristics of the rock and its degree of fracturing. The values of m0 are given for different rocks in Hoek and Marinos (2000). GSI is the Geological Strength Index (Hoek et al, 1992) of the mass rock and D is a factor which depends upon the degree of disturbance to which the rock mass has been subjected by blast damage and stress relaxation. If the study is based on laboratory specimens, GSI can be considered equal to 100 and D equal to 0, which means, from a practical point of view, a minimal disturbance. Having into account these previous ideas, in this paper an extension of the Hoek – Brown failure criterion is made to reproduce the failure in three-dimensional stress states. 2 RECOPILATION OF LABORATORY TEST RESULTS 2.1 Compression and extension triaxial tests The first results that proved the influence of the intermediate stress (σ 2) in the failure strength were those obtained in the so-called triaxial compression tests in which the intermediate stress is fixed equal either to the minimum principal stress (as in the compression test under confining pressure) or to the maximum principal stress (as in the extension test under confining pressure). Examples of these kinds of tests can be found in Mogi (1967) who made 26 extension and 21 compression triaxial tests, in which Westerly granite, Dunham dolomite and Solenhofen limestone were used. The results obtained in that research can be seen in Figure 1. The fact that the extension curve lies above the compression curve indicates the marked influence of the intermediate principal stress on the failure strength. This idea is one of the main conclusions that can be drawn from those particular results.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock (0.56)
ABSTRACT Samples of a fine-grained sandstone were tested under conventional (CTC) and true triaxial compression (TTC) conditions in an attempt to reveal the effect of confining pressure, intermediate principal stress and minimum principal stress on the mechanical behavior of rocks. Under CTC conditions, an increase in confining pressure resulted in a strong increase in pre-dilatant compaction, in the threshold of dilatancy, in pre-peak ductility and in the ultimate strength. Under TTC conditions, both the intermediate principal stress and the minimum principal stress cause some increase in the ultimate strength and the threshold of absolute dilatancy. However, the effect of these stresses on the deformational properties is different: an increase in the intermediate principal stress arrests the process of microcracking and causes the rock to behave in an increasingly brittle manner, an increase in the minimum principal stress causes an increase in ductility. Mogi's empirical failure criterion has been found the most appropriate to fit all the triaxial strength data. 1 INTRODUCTION This paper is a follow-up to a paper that was presented last November at the 4th Asian Rock Mechanics Symposium in Singapore (Kwa'sniewski&Takahashi 2006). In that paper the effect of, independently, confining pressure (p), intermediate principal stress (σ 2) and minimum principal stress (σ 3) on the ultimate strength and post-failure behavior of a fine-grained sandstone was discussed. In the present paper, results of conventional triaxial compression and true triaxial compression tests will be presented in more detail with special emphasis on the effect of p, σ 2 and σ 3 on the volumetric deformation mode of the rock in both the pre-and post-failure range (Fig. 1). Koide andTakahashi of the Geological Survey of Japan, having built a slightly modified version of Mogi's apparatus, tested samples of three different sandstones, a shale and a marble (Takahashi & Koide 1989). In the 1990s Haimson and Chang of the University ofWisconsin in Madison built another, highly sophisticated version of aMogitype apparatus and studied the behavior of a KTB amphibolite and a Westerly granite under high σ 2 and high σ 3 conditions (Haimson&Chang 2000, Chang&Haimson 2000). However, in all these [those] studies it was only the onset of dilatancy that was investigated in detail. Moreover, no attempts were made to study the effect of not only σ 2 but also the effect of σ 3 on the volumetric deformational mode of the rocks tested. As is shown in Figure 1, the so-called onset of dilatancy (C') or the threshold of relative dilatancy (OD) is the stress level at which the volumetric strain – differential stress characteristic starts to deviate from the straight reference line typical for linear elastic materials that undergo compaction when under compression. At the threshold of absolute dilatancy (TD) the compactant volumetric strain attains a maximum value, which means that the volume of the loaded rock body becomes minimum and the instantaneous Poisson's ratio assumes a value equal to 0.5.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Europe > United Kingdom > England > London Basin (0.91)
- Europe > Poland (0.91)
ABSTRACT The unconfined compressive strength (UCS) of a rock is a basic parameter for many characterization systems, strength criteria and calculation methods. It is well-known fact that it depends on the water content of the samples, and decrease when the water content increases. The paper discusses the possible causes of this reduction. From published data by Vasarhelyi and co-workers and others authors some empirical tentative guidelines for this reduction are proposed, which can be used in rock engineering problems where changes in water content occur regularly (dam and bridges foundations, harbors...). 1 INTRODUCTION The unconfined compressive strength (UCS) is probably the most used of the rock index properties for their characterization. So all the standards have detailed regulations on the test and many authors have published on the effect of the sample size on the results of the test. The standards detail also the form and dimensions of the sample, the conditions of parallelism of the faces, even the speed of load application. But almost none of the standards say anything about the humidity of the samples. This is a surprising lack because the samples can be absolutely dry, air dry, semi saturated or saturated.And the water content, or the saturation state, has a clear influence on the results of the test. As a rule the strength diminishes when the water content increase, with a minimum in saturated samples. So some experienced engineers advise to test the rock in the same humidity conditions in which the rock mass is going to stay. This is especially important in dam foundations (which are going to be saturated) or in rockfills. Some rules of thumb have been proposed to cope with this problem (Romana, 2003) when working with geomechanics classifications. There are a scarcity of published data on the unconfined compressive strength (UCS) of saturated samples with the exception of the work by Vasarely and co-workers. For instant Vasarhely and Ledniczky (1999) say that "it is known that saturated materials have lower strengths…than air-dry ones". The aim of this paper is to point at the problem, to recollect the scarce published data, and to offer a first tentative quantitative approximation of the reduction in unconfined compressive strength of saturated rocks. 2 DECREASE IN UNCONFINED COMPRESSIVE STRENGTH OF SATURATED ROCKS Figure 1 (Pells, 1993) shows a Deere-Miller diagram (failure strength vs. deformation modulus at 50% of failure strength) containing data from compression tests in dry and saturated Hawkesbury sandstone. Saturation implies a reduction almost Figure 1. Strength data on Hawkesbury Sandstone (Pells, 1993). proportional in both parameters, but the relationship between them would remain approximately constant. Unfortunately no numerical result can be deduced due to the lack of numerical definition of the data. Hsu and Nelson (1993), in a preliminary research for the not built Super Collider, correlated the unconfined compressive strength of many types of shale (from Canada and USA) with thewater content.Their results (Fig 2), showa marked negative correlation between water content and compressive strength.
- Asia > Middle East (0.29)
- Europe > Hungary (0.29)
- North America > United States (0.25)
- North America > Canada (0.24)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.53)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.47)
- North America > Canada (0.93)
- Europe > United Kingdom > North Sea > Central North Sea > Central Graben > Block 22/7 > Nelson Field > Forties Formation (0.93)
- Europe > United Kingdom > North Sea > Central North Sea > Central Graben > Block 22/6a > Nelson Field > Forties Formation (0.93)
- (4 more...)
A Constitutive Model For Elastic Visco-plastic Behavior of Weak Sandstones
Weng, Meng-Chia (Department of Civil and Environmental Engineering, National University of Kaohsiung) | Tsai, Li-Sheng (Department of Geotechnical Engineering, China Engineering Consultants) | Jeng, Fu-Shu (Department of Civil Engineering, National Taiwan University) | Lin, Ming-Lang (Department of Civil Engineering, National Taiwan University)
ABSTRACT Squeezing means large time-dependent convergence during tunnel excavation. In the pastwhile tunneling through the weak rock strata in western Taiwan, severe squeezing was encountered and significant remediation costs were required to repair the damage. This phenomenon is closely related to rock mechanical properties, but the mechanism is still not clear. Therefore, it is necessary to explore the mechanical properties of these weak rocks and construct an adequate constitutive model to predict their behaviors. In this paper, we first introduce the deformation characteristics of weak rocks, and then a constitutive model comprising nonlinear elastic and visco-plastic deformation based on the characteristics is proposed. This model has such features as:non-linear elastic deformation; apparent shear dilation, large plastic deformation occurs prior to the failure state, the creep deformation under shear loading. After comparing the actual data with the predictions, it is shown that the proposed model describes well with the elastic, plastic and creep behaviors of weak rock under hydrostatic and triaxial loadings. 1 INTRODUCTION In western part of Taiwan, weak sedimentary rocks (including sandstone, shale and mudstone), which have undergone through juvenile rock forming process are often encountered during tunnel constructions, and these materials exhibit relatively low shear strength and stiffness. As a result, the typical strength of these rocks ranges from 5 to 80 MPa. When compared to hard rock, it was found that the deformation behavior of these rocks is characterized by large amount of nonlinear deformation, shear dilation, creep and plastic deformation prior to the failure state. In civil and rock engineering, it is very important to predict the complex behaviors of theseweak porous rocks for the understanding of various geotechnical problems, such as underground excavation, slope stability and foundation. However, from the simulations based on several widely used constitutive equations, the results reflect poor coincidence between experiments and modeling. Therefore, it is essential to construct a constitutive model to properly simulate the elastic, plastic and creep deformations for weak rocks. In this paper, we first present the deformation characteristics of weak rocks, including nonlinear elastic behavior, apparent plastic and creep deformation. Then according to the real behaviors, a model composed of nonlinear elasticity and visco-plasticity is formulated. Finally verification and comparison of the predicted model with the experimental data is presented. 2 DEFORMATION CHARACTERISTICS OF WEAK ROCKS In order to investigate the deformation characteristics of weak rocks, one kind of weak rock, Mushan sandstone in which squeezing has occurred during tunnel construction (Jeng et al., 1996) is adopted as the specimen. Triaxial tests along pure shear stress path were performed (Bernabe et al., 1994; Jeng et al., 2002). The creep study was performed in triaxial tests following a step-wise loading procedure. The step-wise loading procedure permitted the evaluation of creep behavior at reasonable times, in terms of laboratory requirements.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.86)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock (0.54)
Studies of Mechanisms Associated With Sand Production Using X Ray CT Scan In Real Time
Santos, J.B. (Dept. of Geology Federal University of Rio de Janeiro) | Barroso, E.V. (Dept. of Geology Federal University of Rio de Janeiro) | Vargas, E.A. (Dept. of Civil Engineering Catholic University Rio de Janeiro) | Castro, J.T. (Dept. of Mechanical Engineering Catholic University Rio de Janeiro) | Gonçalves, C. (CENPES Research Division of Petrobras) | Campos, E. (CENPES Research Division of Petrobras)
ABSTRACT: Simultaneous production of solid particles originating from the surrounding rock mass frequently occurs during production stages of an oil well. This process is called sand production. Stress concentrations around the borehole may cause loss of cohesion of the rock, thus creating a region of loose granular material susceptible to drag by seepage forces. This paper deals with X ray CT scan studies in real time, carried out with a special cell, of the early stages of sand production, primarily the ones related to the damage processes around the borehole. 1 INTRODUCTION Sand production is the denomination given to the simultaneous outflow of oil and solid particles during production stages of a well in an oil producing field, bringing with it a number of operational problems and considerable money loss. This problem may occur both in poorly consolidated rocks (mainly sandstones) and consolidated rocks. In the latter case, sand production is associated with stress concentrations around the borehole. Sand production processes in boreholes are generally preceded by the formation of damaged zones around it (Dusseault et al., 1992).The present paper reports on the study of the formation of breakouts/damage around boreholes in soft rocks. The experimental program is carried out by simulating on a rock sample the stresses arising during excavation stages. The sample is typically a poorly consolidated sandstone or an artificially assembled material simulating sandstones EXPERIMENTAL PROGRAM 2.1 Characterization of the sandstone and artificial sandstone samples Samples of Rio Bonito sandstone were used in the context of the present work. 2.2 Experimental setup A especially designed cell was built in order to carry out the experimental program described in the previous section (Figure 2). The self contained pressure cell is capable of applying both axial and radial pressures to the sample. 2.3 Test methodology Four samples, two of the Rio Bonito sandstone and two of the synthetic sandstone, were tested in the context of the research. The present paper will focus only in samples V8–2(synthetic) and 698-RB(Rio Bonito), one of each material, whose basic characteristics are described in Table 1. A methodology was developed for assembling the samples inside the cell (Santos 2004) and for the positioning of the cell inside the scanner (Figure 3). 2.4 X Ray CT Scan Analyses and Discussion After the cell containing the sample is positioned inside the X-Ray CT Scanner, a radiographic test is made. This test, denominated pilot test, consists of a longitudinal examination of the sample. Next, the CT Scanner transversal images were obtained, initially with no pressure applied to the sample followed by pressure changes in planned loading stages until failure of the sample. In the case of sample V8–2, the sequence of confining pressures used was: 5, 10, 15, 20, 25 and 40MPa whereas for the 698-RB sample the corresponding sequence was 10, 20 and 30MPa. In each of these stages, CT Scanner images were acquired as presented in Figure 4.
- Health & Medicine > Diagnostic Medicine > Imaging (1.00)
- Energy > Oil & Gas > Upstream (1.00)
ABSTRACT: Drilling experiments under pre-existing true-triaxia l stress in an oil-producing limestone and several sandstones revealed three distinct modes of failure leading to borehole breakouts. The failure micromechanism in Austin Chalk limestone begins at the borehole wall, where two conjugate shear cracks initiate, straddling the σhspringline, and advance toward each other along shear stress paths. AV-shaped breakout is formed when the two conjugate fractures intersect. SEM images of failed boreholes in arkosic sandstones show that borehole instability begins behind the borehole wall along the σhspringline, where a cluster of extensile microcracks subparallel to the borehole wall develop, and then follow a shear stress trajectory toward the borehole wall. The microcracks dislodge thin layers of sandstone, which are flushed off by drilling fluid, creating a V-shaped breakout. In quartz-rich sandstones, grains are bonded by sutured contacts, and breakouts are slot-shaped, aligned with σh springline. Failure begins with the localized grain debonding along the σhspringline. Loosened grains compact, forming a compaction band. Debonded grains at borehole wall are flushed off by the drilling fluid, creating a slot-shaped breakout, similar in width to the compacted band, and can therefore be described as an emptied compaction band. 1 INTRODUCTION Rock failure around boreholes leading to borehole cross section elongations called breakouts is a common phenomenon in reservoir rocks such as sandstones and limestones. Numerous theories have been employed to explain the mechanism responsible for borehole breakouts. Pressure-dependent elasticity, microstatistics, elastoplasticity, bifurcation process, and fracture mechanics are just some of the approaches taken to explain borehole instability, and most are discussed in a comprehensive review by Germanovich & Dyskin (2000). Extensive laboratory drilling experiments in typical oilproducing sandstones and a limestone, varying in porosity between 15 and 30%, led to a better understanding of the correlation between in situ stress and breakout dimensions, and on the process of breakout formation (Haimson & Song, 1993; Haimson & Lee, 2004; Sheets, 2004). The present paper reports on observations of grain-scale mechanisms that bring about borehole breakouts, based solely on optical and scanning electron microscopic studies of specimens tested in the laboratory during a multi-year research program at the University ofWisconsin. 2 EXPERIMENTAL SETUP AND PROCEDURE Drilling experiments at the University of Wisconsin were designed to simulate more realistically actual field conditions in which rock is already under in situ stress prior to drilling. Hence, rather than applying external loads to specimens containing pre-existing boreholes, central vertical holes are drilled in rock blocks already subjected to three unequal principal stresses. Drilling experiments are conducted on rectangular prismatic rock samples (typically 127×127×178mm3). An electric drill rig is mounted on top of a loading frame that enables the application of the vertical stress to the sample. The loading frame is unique in that it enables vertical drilling into a rectangular prismatic specimen that is subjected simultaneously to three unequal mutually perpendicular loads, simulating the most general in situ state of stress (Figure 1).
- North America > United States > Wisconsin (0.50)
- North America > United States > Texas (0.35)
- North America > United States > Louisiana (0.35)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Limestone (0.96)
- North America > United States > Texas > West Gulf Coast Tertiary Basin > Austin Chalk Formation (0.97)
- North America > United States > Texas > Sabinas - Rio Grande Basin > Austin Chalk Formation (0.97)
- North America > United States > Texas > East Gulf Coast Tertiary Basin > Austin Chalk Formation (0.97)
- (7 more...)