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Discontinuities such as fault planes, joints and bedding planes in a rock mass may be filled with different types of fine-grained material that are either transported or accumulated as gouge due to weathering or joint shearing . Filling materials are of the most important geotechnical parameters of discontinuities that have great effect on the shear strength of the joints. This research tries to find a logical relation between uniaxial compressive strength (UCS) of the filling materials with shear strength of the filled discontinuities. For this purpose, joints are made artificially in laboratory scale and connected to each other with different combination of gypsum & clay mortar as the filling materials in dry condition.
According to the conducted tests and analysis of the relevant diagrams, it is concluded that joints with higher UCS of filling materials have higher shear strength so that they have greater value in cohesion (C) and smaller value in friction angel (Φ).
Generally rock masses present in nature are characterized by discontinuities such as joints, fractures and other planes of weakness. Discontinuities that are infilled with fine-grained material which is either transported or appears as a result of weathering or joint shearing, will adversely affect the behaviour of the rock mass . These fine infill materials may drastically reduce the shear strength of the rock joints compared to an unfilled or clean joint, because they may prevent the walls of the rock joint from coming into contact during shear. Hence, the investigation of shear behavior of the joints is of prime importance .
The shear strength of a filled joint is often assumed to be that of the infill material alone, if the infill thickness is higher than a certain critical value. for smaller values of infill thickness, the rock-to-rock contact influence becomes increasingly prominent.
In this study, a series of laboratory test carried out on artificial and idealized models of rock joints in order to determine the relation between the uniaxial compressive strength of the filling material with cohesion and frictional properties of joints using Mohr-Coulomb criteria. The tests carried out in dry condition and joints have no roughness (smooth joints).
ABSTRACT: Wellbore strengthening is an extensively-used method to reduce lost circulation in the petroleum drilling industry, with adding Lost Circulation material to the drilling mud and bridging the fractures on the wellbore to increase maximum stable pressure. In this study, the finite element and Kirsch analytical methods used to model the hoop stress distribution and its effective factors, in one of South Pars gas field's formations, based on Persian Gulf. Findings showed that the compressive stress, in the single fracture model, is raised up to the area of 30° in the fracture initiation state and it will be more in the bridging location across the fracture faces. Furthermore, the hoop stress at the tip of the fracture tends to be tensile; moreover, the compressive stress with higher wellbore pressure on the wellbore, before the area of 60° and after bridging the fracture, is greater than the compressive stress with lower wellbore pressure on the wellbore wall and it will be reversed after the area of 60°. In the multi-fracture model, by moving away from the first fracture, the compressive stress decreases around the 90°, due to the existence of second fracture and the compression stress is raised by increasing the horizontal stress contrast.
Wellbore strengthening is a practical method for reducing lost circulation while drilling formations with narrow drilling mud weight windows. It increases the wellbore's maximum sustainable pressure by bridging drilling induced or natural fractures with lost circulation material (Feng and Gray, 2016). To keep downhole pressure within the mud-weight window, drilling fluids and lost circulation material (LCM) are considered to make wellbore-hydrodynamic pressure low enough to evade downhole lost circulation but high sufficient to avoid borehole instability or kicking(Feng et al., 2015). These drilling fluids and additives cause in the formation hoop stress enhancement, called stress cage, which is a near wellbore area of high stress induced by propping open and sealing narrow fractures at the wellbore/formation boundary (Alberty and McLean, 2004). All lost circulation materials are not same and their type plays a role in terms of both plugging and toughness to better endure displacement pressures. It also, has been confirmed that, mostly, combinations of LCMs act more efficiently compared with the practice of only one type in wellbore strengthening (Savari et al., 2014). Some companies are produced a designer mud which effectively increases fracture resistance while drilling, which can be valuable in both shale and sandstone.it acts by forming a stress cage, using particle bridging and some type of fluid loss mud (Aston et al., 2004). In recent years many deep fundamental studies has been done, related to the lost circulation and wellbore strengthening (Feng and Gray, 2017; Feng et al., 2016). To better understanding of basics of the process of Wellbore strengthening, the effects of several parameters are still not fully understood, and a complete parametric study for each type of formations is necessary to improving field operations. There are plenty of numerical models and analytical solutions which have been developed in recent years for that reason. (AlBahrani and Noynaert, 2016; Wang et al., 2007; Mehrabian et al., 2015; Zhong et al., 2017;Salehi and Nygaard, 2014; Kiran and Salehi,2016; Salehi and Nygaard, 2011; Shahri et al., 2015; Zhang et al., 2016; Zhang et al., 2017;Wang et al., 2018; Chellappah et al., 2018; Feng et al., 2018; Wang, 2018); besides, some research has been done for the usage of wellbore strengthening methods for depleted reservoirs.(Shahri et al., 2014). Furthermore, a set of analytical equations, considered their advantages and disadvantages, are developed for parametric analysis of typical wellbore strengthening approaches. (Morita and Fuh, 2011). A finite-element method is the most important numerical technique, used today to model the wellbore strengthening problems, has been developed to research the effects of major parameters on the distribution of near wellbore hoop stress and fracture width (Feng and Gray, 2016; Arlanoglu et al., 2004; Towler, 2007). In this research, the term hoop stress is generally used to mean the circumferential stress at the wellbore wall. The hoop or tangential stress around a wellbore wall is the main factor in borehole stability and integrity analysis. This research investigates different and effective parameters of wellbore strengthening, related to the formations of south pars gas field in Persian gulf and numerical and analytical methods are used for this purpose; besides, new numerical model with multi fractures has been created to better understanding of wellbore strengthening mechanism and related effective parameters, to investigate of hoop stress around the wellbore and the width of the fractures.
In this paper, the forces acting on tunnel lining due to dry condition and groundwater pressures were studied. Firstly, the effect of the forces acting on tunnel lining under dry condition was studied numerically for the Sabzkouh tunnel as an actual case study. This tunnel is a deep tunnel that is bored through Zagros Mountains in Iran by a hard rock Tunnel Boring Machine (TBM). Secondly, the effect of hydrostatic pressures on tunnel lining was evaluated. The lining of a bored tunnel usually consists of precast concrete segments that are reinforced by steel bars. These segments must be capable to withstand all loads caused by earth (e.g. rock and water pressures), construction conditions (e.g. thrust forces) and utilization (e.g. traffic loads) without unallowable deformations. A Finite difference code was used to analyze Sabzkouh tunnel lining. The final results show that the values of bending moments, axial forces and shear forces in the precast concrete lining can be reduced under fully-drained conditions, although, the drainage is more effective in weak rocks rather than strong rocks.
In recent years, mechanized tunneling has developed increasingly, and the benefits of full-face tunnel boring machines have been recognized. Design methods for segmental tunnel linings used in mechanized tunnel constructions typically employ numerical bedded beam models and/or classical analytical solutions for the determination of structural forces (i.e. moments and shear and axial forces) and simple load spreading assumptions for the design of the reinforcement in joint areas (Gall et al., 2018). Basically, the forces acting on the tunnel lining depend on construction procedures and in many cases, these forces enhances during construction rather than after construction. The measurement of the induced bending moments and normal forces are difficult, but the numerical analyses give more reliable results than analytical and closed form solutions. The behavior of lining segments is affected by the complex construction features, for example the sequential excavation process and backfill grouting. Therefore, developing a framework to accurately predict the lining forces and deformations is essential for the purpose of structural safety and optimum design (Zhao et al., 2017).
ABSTRACT: Asmari and Sarvak limestones are two main oil producer formations in Iran and the Middle East. The production and optimal utilization of these reservoirs will have a significant impact on the economy of the petroleum industry. Geomechanical modelling of oil reservoirs are widely used in optimum drilling, production and reservoir compaction. Hence, the static Young’s modulus (Es) is one of the most essential parameters for any reservoir geomechanical modelling. However, information on the values of Es along the well depth is often discontinuous and limited to the core locations. Therefore, dynamic Young’s modulus (Ed) determined from open-hole log data such as density and compressional and shear wave velocities could result in continuous estimation of elastic properties of the formations versus depth. Nevertheless, static parameters are more reliable than the dynamic parameters and they are widely accepted by geomechanics around the world. The relationship between the static and dynamic elastic modulus in rock materials has been frequently addressed in scientific literature. Overall, when it comes to the study of materials with a wide range of elastic moduli, the functions that best represent this relationship are non-linear and do not depend on a single parameter. Therefore, finding a valid correlation between static and dynamic parameters could result in a continuous and more reliable knowledge on elastic parameters. In this study, published data of the tests which were carried out on 45 Asmari and Sarvak limestone core specimens are used. Then, as an artificial intelligence method, artificial neural networks were developed to correlate Es and Ed data. After comparing the results of the suggested method with correlations which were established between dynamic and static measurements, a good agreement was observed. The accuracy of the obtained results have shown that artificial neural networks are appropriate tools to predict the values of Es based on Ed data of limestone formations.
Houshmand, N. (Amirkabir University of Technology (Tehran Polytechnic)) | Shahriar, K. (Amirkabir University of Technology (Tehran Polytechnic)) | Zarei, H. (Amirkabir University of Technology (Tehran Polytechnic)) | Kamali, A. (Amirkabir University of Technology (Tehran Polytechnic))
Stability analysis is a crucial task in the caverns excavation that depends on the site characteristics including geological and geomechanical parameters. In this study, geo-mechanical properties of rock masses, field stress and joints parameters of Rudbar Lorestan powerhouse cavern in Iran were investigated and displacements, as well as stresses, were evaluated using 3D numerical simulation and with considering Hoek-Brown criterion. This infrastructure has the main cavern with dimensions of 26.8 × 50 × 130.5 meters in width, height and length respectively. The stability of the cavern has been studied by investigating the side wall and crown deformation with and without a support system. Results show that installation of support system—including reinforced shotcrete lining of 35 cm, 10 meters grouted dowel and tendons—can reduce wall horizontal displacement and crown vertical displacement up to 65% and 75%, respectively. Therefore, the comprehensive method incor-porating numerical analyses, as well as empirical study and filed observation has been proven to be very promising in deformation and even stability prediction of surrounding rock mass in underground caverns to delineate excavation damage zones.
Underground infrastructures stability of rock masses is of significant elements in geotechnical engineering, in both design and construction stages (Baria et al, 2008). The main objective of this study is to evaluate the effect of support system on the stability of staged excavation of the powerhouse cavern in dolomitic limestone. Determining the strength parameters of rock masses and analysis of rock stability are some of the fundamental subjects in rock mechanics (Singh, 2005). As an empirical failure criterion for rock, Hoek-Brown failure criterion has been widely used in rock tunnel engineering, rock slope engineering, and other fields (Wang et al, 2017). However, empirical methods do not provide the stress distributions and deformations around the cavern (Gurocak et al. 2007). At present, a variety of numerical methods are available for stability analysis of rock masses (Abdollahipour, 2012). In overall, displacement, stress, and strain are easy to acquire using numerical calculations. Displacement is a vital parameter in engineering design and construction. The effective parameters including GSI, disturbance factor, discontinuities and rock mass characteristics, overburden depth, and coefficient of lateral pressure are considered as the input parameters to predict the displacements and stresses which may result in instability. It is suggested that for stability analysis of caverns 3D simulation utilize, as the structure of these excavations is complex.
Saberhosseini, Seyed Erfan (Islamic Azad University) | Mohammadrezaei, Hossein (Islamic Azad University) | Saeidi, Omid (Iranian Offshore Oil Company) | Zadeh, Nadia Shafie (Natural Resources Canada) | Senobar, Ali (Iranian Offshore Oil Company)
Summary Pre-analysis of the geometry of a hydraulically induced fracture, including fracture width, length, and height, plays a crucial role in a successful hydraulic-fracturing (HF) operation. Besides the geometry of the fracture, the injection rate should be optimal for obtaining desired results such as maintaining sufficient aperture for proppant placement, avoiding screenouts or proppant bridging, and also preventing caprock-integrity failure as a result of an extensively uncontrolled fracture in reservoirs. A sophisticated numerical model derived from the cohesive-elements method has been developed and validated using field data to obtain an insight on the optimal fracture geometry and injection rate that can lead to a safe and efficient operation. The HF operation has been conducted in an oil field in the Persian Gulf with the aim of enhanced oil recovery (EOR) from a limestone reservoir with low matrix permeability in a horizontal wellbore. The concept of the cohesive-elements method with pore pressure as an additional degree of freedom has been applied to a 3D fully coupled HF model to estimate fracture geometry, specifically fracture height as a function of the optimal injection rate in a reservoir porous medium. It was observed that by increasing injection rate, all the fracture-geometry parameters steeply increased, but the fracture height must be controlled to be in the reservoir domain and not surpass the caprock and sublayer. For the reservoir under study with the maximum height of 100 m, length of 250 m, width of 100 m, permeability of 2 md, and porosity of 10%, the optimal fracture height is 73.4 m; the average fracture width and half-length are 12.8 mm and 55.4 m, respectively. Therefore, the optimal injection rate derived from the fracture height and geometry is in this case 4.5 bbl/min. The computed fracture pressure (49.55 MPa = 7,283.85 psi) has been compared with the field fracture pressure (51.02 MPa = 7,500 psi), and the error obtained for these two values is 2.88%, which showed a very good agreement.
Abstract Objectives/Scope: In this study, Managed Pressure Drilling (MPD) is investigated in a naturally fractured Iranian oil field as a tool to mitigate under balanced drilling hazards due to its high Sulfur content; and fluid losses and other problems that are inherent to conventional over balanced drilling. Methods, Procedures, Process: MPD was identified, planed, and applied in an already drilled well, as an alternative technology, to calculate the increase in its rate of penetration (ROP). The calculations were made by Schulumberger Drilling Office package and quality checked by using Signa ERDS software. The software simulates drilling fluid dynamics to investigate application of drillind hydraulics to better chose the optimum drilling technique. Well drilling fluid properties, bottom-hole assembly, casing/completion design and rock formation properties are implemented as an input to the software to start calculations. Results, Observations, Conclusions: The subject of this study was drilled in south west of Iran. Drilling experiences in this area identified the rock formation as a brittle, highly fractured, which negatively impacts drilling time and cost. Also, in certain instances, elevated mud weights are required to deal with high-pressure high sulfur content gas and/or water flows. Calculation shows that in shallow depths (0-470.8 ft, dd) due to low pore pressure, drilling mud is in balance and within pressure window. At deeper depths (470.8-7211.3 ft, dd) drilling is MPD and increase in mud pressure is required. At (7211.3-9530.8 ft, dd) depths, sharp changes in reservoir pore-pressure cause different increases in mud pressure (from 90 to 600 psi). At deeper depths (9530.8-13154.5 ft, dd) an increase in mud pressure may cause formation fracture, so it is suggested to continue overbalance drilling or separate the well bore by casing. If the formation is separated in previous section, drilling from 9530.8 to 13154.5 ft, dd, pressure management can be continued with an increase in applied back pressure in order to stop toxic gas hazards. At deeper depths (more than 13154.5 ft) according to high depth and dramatic pressure increase, it is suggested to continue MPD to prevent formation damage or induced fracturing and also prevent H2S influxes into annulus. Novel/Additive Information: Tts the first time to apply MPD technique in a drilling well design in this filed as a solution to challenging drilling conditions like narrow pressure window, Toxic gas hazards, sever mud loss problems, differential stocking and blow out risks. Such results may result in a significant improvement in drilling economics.
ABSTRACT: Geomechanical modeling of a reservoir has a very important role in all parts of a field lifecycle. In this paper, we demonstrate a new method for modeling the distribution of elastic properties in the whole reservoir using the concept of geomechanical units (GMUs). In this study, a GMU is a cluster of Young’s, Bulk and shear modulus, Poisson’s Ratio and unconfined uniaxial strength. To establish these GMUs we used eight wells and the Post-stack seismic data in the field of interest. Dynamic elastic parameters were computed from logging data of mentioned wells. To convert these dynamic parameters to static values, empirical equations were determined in a neighboring field of Salman, in the interval of Kangan and Dalan formations. In the next step, Multi-resolution graph-based clustering was applied to these static elastic parameters to construct five distinct GMUs. For three-dimensional modeling of GMUs, the 3D acoustic impedance model of the field was made by genetic inversion and used as a secondary parameter of Co-kriging. The amounts of elastic parameters of each GMU at the location of well number six in the final 3D model are found to be in good agreement with the known values of this well.
Geomechanics is a petroleum engineering sub-discipline developed to address the mechanical behavior of the reservoir and bounding rocks during exploration and production activities (Zoback, 2010; Aadnoy and Looyeh, 2011). In this regard, Three-dimensional modeling of geomechanical parameters plays a significant role in whole life of a reservoir. These models are used for seismic modeling, interpretation, hydraulic fracture design, assessing borehole stability and stress calculations in geological studies. Therefore, any improvement in one of these momentous applications could lead to better and more sufficient field development plans, at the same time save the considerable amount of money and operation time.
This study employs an efficient approach to construct a 3D reservoir geomechanical model based on the concept of Geomechanical Units (GMUs). A GMU is a single unit for design and modeling purposes. A GMU can be selected from logs, cores, or judgment (Dusseault, 2011). The advantages of GMU use in engineering studies have been discussed by a number of authors including Uwiera et al. (2011) and Nygaard (2010). In this work, a GMU is a set of rock mechanical properties, such as: Young’s modulus, bulk modulus, shear modulus, Poisson’s ratio, and uniaxial compressive strength. These elastic parameters are clustered by using different clustering methods to establish the best GMU. The purpose of using this method is to determine the distribution of elastic parameters in whole parts of the field of study. The Kangan and Dalan formations are the reservoir layers in the field of study. These formations are consisted of carbonate and dolomite (Figure 1).
One of the main methods in hard rock quarry mining is the controlled fracture method that is carried out by the introduction of a slowly advancing crack by Non-Explosive Expansion Material (NEEM). The method of rock breakage is without noise and vibrations and its operation, compared to blasting method, is more controllable, very safe and easy and without extra undesirable cracks in the rock block (Hinze and Nelson, 1996; Gambatese 2003; Huynh, et al. 2009; Laefer, et al. 2010).
In this method, some circular holes are drilled closely with equal length, diameter and spacing (center- to-center distance) in a rock block. Subsequently, the holes are filled by the NEEM, which it can be expanded and generates an incremental static loading into the holes after about two to four hours. If the spacing of the holes to be adequate, it will create a crack between two neighboring holes and the rock will fracture along the high-stress concentration path between the holes. In this paper will be try to introduce a new numerical modelling approach for showing location and length of the first cracks around the holes and predicting of the crack growing path with the geometry.
2 Experimental data and a case study
A granite quarry mine was selected as a case study entitled “Ahrar Mine” which it has located near to Natanz in Iran (Fig. 1). Some laboratory tests have been done on the granite specimens for measuring of some rock mechanics’ properties such as uniaxial compressive strength, Brazilian tensile strength, Young’s modulus, Poisson’s ratio, specific gravity and fracture toughness by ISRM standard methods (Fowell, 1995), (Tab. 1). Process of rock fracture in due time by the NEEM has been shown in Figs. 1 and 2. The drill holes were 38 mm with 1 to 3 meters depth and the average of the hole’s spacing was 12 cm (10-14 cm) in the mine.
Kamali, A. (Amirkabir University of Technology) | Shahriar, K. (Amirkabir University of Technology) | Sharifzadeh, M. (Curtin University) | Gholami, M. A. (Mahab Ghodss Consulting Engineering) | Mossaei, N. (Tehran University)
ABSTRACT: The geometrical parameters of 639 discontinuities that surveyed in powerhouse cavern of Rodbar Lorestan pumped storage power plant project have been investigated by scanline and areal sampling methods. As regards the processing and correction of bias types, one bedding and three joint sets are existed in the site. The tectonic activities and direction of principal stresses have caused for each of trace length and spacing characteristics, the probability distribution function of joint sets differ to each other as regards their genetic types. The calculated mean trace length by scanline and areal method are very close together for one joint set and for another one the difference is 28%. The actual intensity differences between circular and rectangle sampling windows for joint sets J1 and J2 are 7% and 38%, respectively. Meanwhile, the calculation of volumetric intensity by various methods shows the estimation of this characteristic is very difficult in the field.
Discontinuities in rock mass have profound effect on deformation, strength, stress-strain relation and failure of rock mass (Ye et al. 2012). So, besides the description of mechanical properties of discontinuities and intact rock the description geometrical properties of discontinuities is very important to investigate of rock mass behavior and to determinate interaction between rock mass and structures. The paper represents geometrical properties of discontinuities in powerhouse cavern of Rodbar Lorestan pumped storage power plant in Iran. They are investigated with using Scanline and areal sampling methods (circular and rectangular). The location of cavern in tensile zone of anticline and the area is tectonized due to the existence of high Zagros zone are one the main characteristics of the study area.
Recognizing of homogenous statistical zone in study area is the first step in modeling of joint geometry in rock mass (Kulatilake et al. 1990). As regards the discontinuities surveyed locations are in different areas, the various statistic tests have been done to control the data homogeneity and to investigate of uniformity of rock mass zones. As regards the 639 surveyed discontinuities and their clustering two joint sets, bedding and another joint set as strike joint set exist in the site. According to uncertainty on surveyed data, the correction of type of errors on dip, dip direction, spacing, frequency and trace length of discontinuities have been done. On the other side, as regards the intrinsic statistical properties of discontinuities geometry, the statistical analysis of the corrected data has been done. Meanwhile, the Fisher constant, intensity (areal and volumetric) and other characteristics are calculated for each joint set.