Effect Of Overburden Pressure Research Articles

Numerical study of the effect of overburden pressure on the stability of mine dump slope

Numerical study of the effect of overburden pressure on the stability of mine dump slope

Study on the Mechanical Parameters and Permeability of Coal Reservoirs at Different Temperatures.

Deep coal reservoirs, as opposed to their shallower counterparts, exhibit characteristics of higher temperatures and pressures. These conditions affect the fracture structure and mechanical properties of coal, which in turn controls permeability. Substantial studies have been conducted to determine the effects of overburden pressure on permeability, but the correlation between the temperature and mechanical parameters/permeability of coal remains unclear. This study focused on low-rank bituminous coal from the southern edge of the Junggar Basin in Xinjiang. Using experiments conducted on seepage and mechanics at different depths (considering effective stress and temperature), the study investigated how temperature affects the mechanical parameters and permeability of coal column samples. A permeability prediction model was established incorporating temperature, mechanical parameters, and effective stress. The results show that from 20 to 80 °C, the elastic modulus of coal column samples decreases by 31.0%, and the Poisson ratio increases by 72.0%. Permeability decreases between 48.37 and 90.12% under different depths. The stress sensitivity coefficient under various temperature conditions decreased exponentially as the effective stress increased, and the temperature sensitivity coefficient under various effective stress conditions decreased with increasing temperature. The permeability was more sensitive to a temperature below 40 °C. In the permeability prediction model, the fracture compressibility coefficient is bifurcated into two coefficients, each controlled by temperature and effective stress. The permeability prediction error of the model was 12.7% under constant effective stress and 17.2% under varying effective stress and temperature conditions. The study could provide guidance for fracturing and coalbed methane production in deep coal reservoirs.

Numerical study on the effects of scour on monopile foundations for Offshore Wind Turbines: The case of Robin Rigg wind farm

Scour around monopiles supporting Offshore Wind Turbines (OWT) may cause a significant reduction in the foundation capacity and the natural frequencies of the soil–structure system. The decommission of two monopiles at the Robin Rigg wind farm after only six years of operation due to massive unexpected scour demonstrates the hazard that an active marine environment poses to OWT. Assessing and predicting the quantitative impact of scour on monopile foundations from a point of view of the utilisation of the foundation capacity is hence crucial to its long-term structural and operational safety. Using finite element analysis, the present study quantifies the influence of scour through moment M and lateral load H (M−H) interaction curves and Load Utilisation (LU) ratios. It analyses the first and second natural frequencies of OWT exposed to different scour morphologies. This methodology is applied to a large-diameter monopile embedded in sand and to the scour-induced failed foundation of the Robin Rigg wind farm. The results indicate that, with increasing scour depth, the M−H capacity and the natural frequencies are reduced due to the loss of lateral confinement. The response of the system is found to depend on the rigidity of the pile governed by the slenderness ratio, the capacity of the soil to redistribute loads with increasing scour, and the effects of overburden pressure on deeper unscoured soil layers. The LU methodology has proven to be an appropriate and efficient approach to predict the long-term performance of scoured monopiles.

Statistical Inference of Sound Speed and Attenuation Dispersion of a Fine-Grained Marine Sediment

Acoustic recordings of signals in the 1.5–4.0-kHz band were analyzed for information about the sound speed and attenuation frequency dispersion of a fine-grained sediment found in the New England Mudpatch. Analysis of piston cores established prior bounds for a geophysical parameterization of a seabed model that predicts Kramers–Kronig dispersion relations. Sediment layers are described by the Buckingham viscous grain shearing (VGS) model that accounts for the effects of overburden pressure of compressional and shear speeds and attenuations. A statistical inverse problem was solved by using multiple samples of received levels recorded on two vertical line arrays as a function time and hydrophone depth for six frequencies in the 1.5–4.0-kHz band. A statistical inference model that assumed both model parameters and data samples are random variables quantified information content from marginalization of a conditional posterior probability distribution for the geophysical parameters that characterize the mud layer. From the inferred geophysical parameter point estimates the sediment sound speed and attenuation frequency dispersion are predicted and compared to previously reported direct measurements. Also, the predicted sound-speed gradient in the mud sediment from the VGS model is compared to a previous inference that utilized explosive sources.

The effects of heterogeneity on pressure derived porosity changes in carbonate reservoirs, Mishrif formation in SE Iraq

The effects of overburden pressure on porosity and permeability are often neglected in formation evaluation, while it has important influences on reservoir storage and flow capacities. The present work focuses on the effects of overburden pressures on porosity changes in carbonate Mishrif Formation, Iraq. The porosities of 73 core plug samples have been measured under 4 overburden pressures. Winland Equation was reconstructed using the pore throat size of 165 samples which were measured by mercury injection. The Winland, FZI, Lorenz, and porosity data ranges rock typing methods have been used to manage the heterogeneity. Winland recognized as the most suitable method, because porosity, permeability and pore throat sizes are considerably affected by overburden pressures. Two factors of porosity and permeability are affected by overburden pressures in FZI method. In data grouping using ranges of porosity, the effective factors reduce to one. Lorenz method groups successive data and so is not effective for grouping the samples. Results showed that the best methods of managing heterogeneity for overburden tests are Winland, FZI, porosity data range, and Lorenz method, respectively. These new findings can help for better understanding of porosity–pressure relationships with respect to the reservoir heterogeneity.

Prediction the shear strength and shear modulus of sand-clay mixture using bender element

Employing the conventional laboratory geotechnical methods such as shear box test to measure shear strength and shear modulus require destroying the samples which is seen as time consuming and costly. Whilst the bender element technique (BE) maintains the sample condition, time, and cost efficiency. Several sand-clay mixtures were compacted and subjected to bender element test as well as sheared using shear box test to measure and correlate shear modulus (t), shear strength (G) and the maximum shear modulus (Gmax). The results showed the critical stage (transition fines-grained) at fine-grained (FG) equal to 50% where any further increment beyond this value led to decrement the soil mixture strength. Both t and G were normalized using moisture content, density, and applied normal stress. Five empirical equations from the normalized shear strength tN were applied on the previous field data to exam their reliable and limitations. The equations indicated the importance of including the effect of overburden pressure for the natural sample as well as the in-situ moisture content and field density to avoid uncertainty in the predicted value of the soil shear strength and modulus. At no depth limitation, all empirical equations (tN1, tN2, tN3, tN4, and tN5) exceed ±20% lines which indicated a large variation in results. At depth limitation (< 5 m), only one equation corresponding to N4 showed reasonable validity and reliability to predict the shear strength. Similar was on the prediction of the shear modulus. The 5 m depth limit was recommended to apply the equation consistently.

Outlet facility to pass coal flow to armored face conveyor

The authors review the international experience in thick coal seam mining. Advantages and disadvantages of multi-slice and full-seam longwall mining technologies are compared. The low-efficiency multi-slice longwall mining is little used and is being replaced by the full-seam longwall top coal caving technology with undercut and with top coal disintegration under the physical effect of overburden pressure. In the recent decade, the longwall mining systems used in top coal caving have been given additional functions connected with control of coal extraction above and behind the powered roof support. The authors have examined the patent documentation and invention activity in the field of the LTCC methods and equipment in the top coal mining countries. This paper describes the revealed trends in engineering facilities capable to enhance efficiency of longwall top coal caving in mining of thick coal seams. Different type feeders are described, and recommendations on their application are given.

Stress-strain behavior and liquefaction strength characteristics of Ottawa F65 sand

The stress-strain-strength response of soils is of significant interest to development and calibration of realistic constitutive models that can be used in numerical simulation of geotechnical engineering problems. An extensive characterization of Ottawa F65 sand along with various monotonic and cyclic tests conducted during the course of the Liquefaction Experiments and Analysis Projects (LEAP) are presented here. The specimens in these tests were prepared using a meticulous sample preparation technique to facilitate their consistency and repeatability. Monotonic drained and undrained triaxial tests shed light on the steady-state (critical state) of Ottawa sand, while stress-controlled and strain-controlled cyclic triaxial and direct simple shear tests provide key information on the cyclic stress-strain behavior and liquefaction strength of this soil. The triaxial tests identify the liquefaction strength of the soil at different densities, while the direct simple shear tests evaluate the effect of overburden pressure on the cyclic response. The results of these experiments are also compared to the experimental results available in the literature. The data obtained from the cyclic triaxial and direct simple shear tests were further analyzed by plotting the computed shear modulus degradation and damping curves. The results show how the soil stiffness degrades as cyclic shear stress is applied for soil at different densities and confining stresses. It can be seen that the rate of shear modulus degradation increases with the increase of confining stress and decreases with the increase of soil density. The damping curves consistently show an increasing in damping ratio until a shear strain of about 0.05%, followed by a plateau at about 20–25% damping ratio for shear strain between 0.05 and 0.5%, and ending with a decrease in damping until reaching a final damping ratio of about 10%.

Pore Pressure and Kσ Evaluation at High Overburden Pressure under Field Drainage Conditions. I: Centrifuge Experiments

AbstractThis article is the first of two companion papers studying the effect of overburden pressure on the liquefaction behavior of saturated Ottawa sand. A series of four centrifuge tests were co...

Effect of overburden pressure on determination of reservoir rock types using RQI/FZI, FZI* and Winland methods in carbonate rocks

Rock typing is an important tool in evaluation and performance prediction of reservoirs. Different techniques such as flow zone indicator (FZI), FZI* and Winland methods are used to categorize reservoir rocks into distinct rock types. Generally, these methods are applied to petrophysical data that are measured at a pressure other than reservoir pressure. Since the pressure changes the pore structure of rock, the effect of overburden pressure on rock typing should be considered. In this study, porosity and permeability of 113 core samples were measured at five different pressures. To investigate the effect of pressure on determination of rock types, FZI, FZI* and Winland methods were applied. Results indicated that although most of the samples remain in the same rock type when pressure changes, some of them show different trends. These are related to the mineralogy and changes in pore system of the samples due to pressure change. Additionally, the number of rock types increases with increasing pressure. Furthermore, the effect of overburden pressure on determination of rock types is more clearly observed in the Winland and FZI* methods. Also, results revealed that a more precise reservoir dynamic simulation can be obtained by considering the reservoir rock typing process at reservoir conditions.

Capillary pressure–saturation curves of thin hydrophilic fibrous layers: effects of overburden pressure, number of layers, and multiple imbibition–drainage cycles

Unsaturated fluid flow in thin porous media depends on hydraulic properties, such as the capillary pressure, P c, as a function of saturation, S. We measured this relationship for two different types of compressible thin hydrophilic fibrous layers under varying conditions. Among other factors, we changed the number of layers and the overburden pressure (i.e. the confined solid pressure applied on top of the sample) imposed on one layer or a stack of layers. Applying an overburden pressure drastically affected the [Formula: see text] curves. However, increasing the number of fibrous layers had little impact on the capillary pressure–saturation curves. We also investigated the effect of multiple imbibition–drainage cycles on the [Formula: see text] data. Measured data points were used to find general expressions for the [Formula: see text] relationships of compressible thin porous media. Existing quasi-empirical correlations used in vadose zone hydrology, notably expressions by van Genuchten (Van Genuchten MTh. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci Soc Am J 1980; 44: 892-898) and Durner (Durner W. Hydraulic conductivity estimation for soils with heterogeneous pore structure. Water Resour Res 1994; 32: 211–223) for single- and dual-porosity media, respectively, were employed to fit the measured data points.

An analytical model for fracture initiation in elasto-plastic soft formations based on stress path analysis

Most models developed for hydraulic fracturing initiation of borehole are based on the elasticity mechanics, and tensile fracturing is considered as the mechanism of fracture initiation. However, the lab and field data show that these models cannot properly predict the fracturing mechanism of soft formations. We proposed an elasto-plastic model to analyze the fracturing mode and fracture initiation pressure in the soft formations that considers the effect of overburden pressure and fluid flow. The analytical stress solutions under six different stress state are obtained, thus the effective stress path on the borehole wall is analyzed to determine the fracturing mode. The effective stress path analysis indicate that both tensile and shear fracturing are considered as the fracturing mode. The fracturing mode and fracture initiation pressure are influenced by the burial depth significantly. The tensile fracturing is more likely to occur in the shallow formation, while the shear fracturing usually occurs in the deep formation. In addition, the formation strength and mud cake also affect fracturing mode and fracture initiation pressure. The shear fracturing is more easily to be induced in the formation with low strength and tight mud cake. The preyielding around the borehole due to the low strength will decrease the fracture initiation pressure without affecting the fracturing mode of the borehole wall.

Numerical modelling of bottom-hole rock in underbalanced drilling using thermo-poroelastoplasticity model

Stress analysis of bottom-hole rock has to be considered with much care to further understand rock fragmentation mechanism and high penetration rate. This original study establishes a fully coupled simulation model and explores the effects of overburden pressure, horizontal in-situ stresses, drilling mud pressure, pore pressure and temperature on the stress distribution in bottom-hole rock. The research finds that in air drilling, as the well depth increases, the more easily the bottom-hole rock is to be broken. Moreover, the mud pressure has a great effect on the bottom-hole rock. The bigger the mud pressure is, the more difficult to break the bottom-hole rock is. Furthermore, the maximum principal stress of the bottom-hole increases as the mud pressure, well depth and temperature difference increase. The bottom-hole rock can be divided into three main regions according to the stress state, namely a) three directions tensile area, b) two directions compression areas and c) three directions compression area, which are classified as a) easy, b) normal and c) hard, respectively, for the corresponding fragmentation degree of difficulty. The main contribution of this paper is that it presents for the first time a thorough study of the effect of related factors, including stress distribution and temperature, on the bottom-hole rock fracture rather than the well wall, using a thermo-poroelastoplasticity model.

STRESS EFFECT ON CEMENTATION FACTOR OF SANDSTONE ROCKS TYPE EVIDENCE FOR A VARIABLE ARCHIE POROSITY EXPONENT “m”

Ancient sandstones include important reservoirs for hydrocarbons (oil and gas). This research provides an experimental investigation on the influence of variations in confining pressure on cementation factor of two wells located in Sirt basin. Measurements of formation resistivity factor versus cementation factor for sandstone samples which were collected from both wells at different confining pressures were carried out. The Effects of confining stress on the electrical properties of the rock samples were studied and analyzed. The results showed that the confining pressure variations depend on the pore structure of the studied rock. Furthermore, it can be concluded that the electrical resistivity of rocks is function of pore size distribution and how the fluids were distributed in the pore spaces. The effect of overburden pressure and the porosity variation on cementation factor is also investigated for different rock types. It has been concluded that the compaction due to overburden pressure generally causes a considerable increase in resistivity, especially in poorly cemented rocks. In low-porosity rocks, resistivity measurements determined under representative overburden pressures that are strongly recommended to improve well log interpretation.

Novel liquid-gas corrected permeability correlation for dolomite formation

Standard practice for determining permeability is to perform gas permeability measurement and apply correlations to receive corrected permeability as the value of absolute permeability. However, these correlation are dedicated for the specific reservoirs or unconsolidated porous media and cannot be applicable for other rock formations. The objective of this study is to establish correlation between liquid and gas permeability for the pure dolomite. A series of liquid and gas permeability measurements were performed on cylindrical rock samples from formation located in Poland. To determine Klinkenberg-corrected gas permeability, measurements of gas permeability at three (randomly selected) different inverse mean pressure were extrapolated to infinity. Then values of water and corrected-gas permeability were contrasted against each other. The results showed that Klinkenberg permeability overestimates water permeability and the observed difference is in range of 11–24%. This fact can be caused by water polar nature. Hydrogen bonds are formed with neighboring molecules and cause high surface tension and capillary forces. Later, the effect of overburden pressure on water permeability was studied and results showed reduction in permeability as a function of increasing stress. In the last part of the research, the permeability values obtained by using novel equation has been compared with values using other mathematical formulas designed for sandstone and limestone. We learned that our correlation provides results with higher accuracy than the previous equations.

Stress distribution and its influencing factors of bottom-hole rock in underbalanced drilling

The underbalanced drilling has been widely used due to its advantages of high drilling efficiency and low cost etc., especially for hard formation drilling. These advantages, however, are closely related to the stress state of the bottom-hole rock; therefore, it is significant to research the stress distribution of bottom-hole rock for the correct understanding of the mechanism of rock fragmentation and high penetration rate. The stress condition of bottom-hole rock is very complicated while under the co-action of overburden pressure, horizontal in-situ stresses, drilling mud pressure, pore pressure and temperature etc. In this paper, the fully coupled simulation model is established and the effects of overburden pressure, horizontal in-situ stresses, drilling mud pressure, pore pressure and temperature on stress distribution of bottom-hole rock are studied. The research shows that: in air drilling, as the well depth increases, the more easily the bottom-hole rock is broken; the mud pressure has a great effect on the bottom hole rock. The bigger the mud pressure is, the more difficult to break the bottom-hole rock; the max principle stress of the bottom-hole increased with the increasing of mud pressure, well depth and temperature difference. The bottom-hole rock can be divided into 3 regions respectively according to the stress state, 3 direction stretch zone, 2 direction compression area and 3 direction compression zone; the corresponding fragmentation degree of difficulty is easily, normally and hardly.

Experimental Study of the Effects of IFT and Hysteresis on Resistivity and Capillary Pressure of Carbonate Rocks

Characterizing the electrical resistance of the formation rock is of great importance in, for example, interpreting the well log data, calculating the porosity, and water saturation for the formation rocks. The relation between the resistivity factor and porosity and also between the resistivity index and water saturation (Archie equation) in the formation rock uses the parameters, such as the cementation factor and saturation exponent. However, the direct application of Archie equation to carbonate rocks that are very heterogeneous and mostly oil-wet needs further investigation. In this experimental work, the effects of overburden pressure, temperature, and interfacial tension on the capillary pressure and resistivity characteristics during the imbibition and drainage processes are studied. The results showed a decrease of the saturation exponent when the rock changes from the oil-wet to the water-wet, during the primary drainage (e.g., from 5.6 to 2.29), imbibition (e.g., from 3.98 to 1.93), and the secondary drainage (e.g., from 4 to 2.04). As a result, this study improves understanding of the reservoir rock behavior especially when the enhanced oil recovery processes are studied.

HydrogeoSieveXL: an Excel-based tool to estimate hydraulic conductivity from grain-size analysis

For over a century, hydrogeologists have estimated hydraulic conductivity (K) from grain-size distribution curves. The benefits of the practice are simplicity, cost, and a means of identifying spatial variations in K. Many techniques have been developed over the years, but all suffer from similar shortcomings: no accounting of heterogeneity within samples (i.e., aquifer structure is lost), loss of grain packing characteristics, and failure to account for the effects of overburden pressure on K. In addition, K estimates can vary by an order of magnitude between the various methods, and it is not generally possible to identify the best method for a given sample. The drawbacks are serious, but the advantages have seen the use of grain-size distribution curves for K estimation continue, often using a single selected method to estimate K in a given project. In most cases, this restriction results from convenience. It is proposed here that extending the analysis to include several methods would be beneficial since it would provide a better indication of the range of K that might apply. To overcome the convenience limitation, an Excel-based spreadsheet program, HydrogeoSieveXL, is introduced here. HydrogeoSieveXL is a freely available program that calculates K from grain-size distribution curves using 15 different methods. HydrogeoSieveXL was found to calculate K values essentially identical to those reported in the literature, using the published grain-size distribution curves.

Interfacial characterization of soil-embedded optical fiber for ground deformation measurement

Recently fiber-optic sensing technologies have been applied for performance monitoring of geotechnical structures such as slopes, foundations, and retaining walls. However, the validity of measured data from soil-embedded optical fibers is strongly influenced by the properties of the interface between the sensing fiber and the soil mass. This paper presents a study of the interfacial properties of an optical fiber embedded in soil with an emphasis on the effect of overburden pressure. Laboratory pullout tests were conducted to investigate the load-deformation characteristics of a 0.9 mm tight-buffered optical fiber embedded in soil. Based on a tri-linear interfacial shear stress-displacement relationship, an analytical model was derived to describe the progressive pullout behavior of an optical fiber from soil matrix. A comparison between the experimental and predicted results verified the effectiveness of the proposed pullout model. The test results are further interpreted and discussed. It is found that the interfacial bond between an optical fiber and soil is prominently enhanced under high overburden pressures. The apparent coefficients of friction of the optical fiber/soil interface decrease as the overburden pressure increases, due to the restrained soil dilation around the optical fiber. Furthermore, to facilitate the analysis of strain measurement, three working states of a soil-embedded sensing fiber were defined in terms of two characteristic displacements.

Finite element analyses of two-tier geosynthetic-reinforced soil walls: Comparison involving centrifuge tests and limit equilibrium results

This study presents the procedure and results of the finite element (FE) analyses of a series of centrifuge tests on geosynthetic-reinforced soil (GRS) two-tier wall models with various offset distances. The objectives of this study were to evaluate the applicability of FE for analyzing GRS two-tier walls with various offset distances and to investigate the performance and behavior of GRS two-tier walls in various stress states. The FE simulations were first verified according to the centrifuge test results by comparing the locations of failure surfaces. The FE results were then used to investigate the effective overburden pressure, mobilization and distribution of reinforcement tensile loads, and horizontal deformation at the wall faces. The interaction between two tiers was investigated based on the FE results, which were also used to examine the modeling assumption of reinforcement tensile loads in limit equilibrium (LE) analysis and to evaluate the design methods in current design guidelines. This study demonstrated favorable agreement between FE and the centrifuge model in locating the failure surface. The FE results indicated that as the offset distance increased, the reinforcement tensile load and wall deformation decreased in both the upper and lower tiers, suggesting the attenuation of interaction between the two tiers. The maximum tensile loads of all reinforcement layers at the wall failure predicted using FE analysis and LE method assuming uniform distribution of reinforced tensile loads were comparable. Compared with the FE results, the Federal Highway Administration (FHWA) design guidelines are conservative in determining the effect of overburden pressure, required tensile strength, location of maximum tension line (for designing the reinforcement length), and the critical offset distance. Furthermore, the FHWA design guidelines do not account for the influence of the lower tier on the upper tier that was observed in this study.