Mechanical Properties Of Shale Research Articles

Preliminary Application of X-ray Computed Tomograph on Characterisation of Polish Gas Shale Mechanical Properties

Preliminary Application of X-ray Computed Tomograph on Characterisation of Polish Gas Shale Mechanical Properties

Investigating the mechanical behavior of shale: A micro-scale approach

Hydrocarbon production from unconventional shale-gas reservoirs has increased dramatically in the last decades, and a good understanding of the mechanical behavior of the reservoir rock is of crucial important. Characterization of shale is challenging because this rock exhibits a multi-scale heterogeneity, and thus an improved experimental insight into the deformation and damage mechanisms at micro-scale is required. In this context, an innovative experimental approach, consisting of uniaxial compression tests in environmental scanning electron microscopy (ESEM) and full field strain measurement by digital image correlation (DIC) techniques, is proposed. Some preliminary results from two tests on samples with distinct water contents (i.e., one is dry and the other is wet) are presented. The results show that the microstructure plays a decisive role in the macroscopic properties of shale. Moreover, the mechanical properties of shale is strongly affected by water content.

The mechanical properties of shale based on micro-indentation test

Abstract In view of the difficulty to get mechanical characteristics of shale reservoirs, a quantitative evaluation method based on micro-indentation test technology was proposed to research the meso-mechanical properties of shale. Through micro-indentation test of shale outcrop samples from the Cambrian Longmaxi Formation in the Changning area, Sichuan Basin, the meso-mechanical properties of shale were analyzed, and the relationship between the macro-scale and meso-scale shale mechanical properties was evaluated. The analysis results of micro-indentation tests show that the mesoscopic elastic modulus and indentation hardness are heterogeneous in distribution. The comparison of macro and meso experiments shows that the statistical mean value of meso-elastic modulus is approximate to the value of the macro-elastic modulus. The relationship between composition and mechanical properties of shale was obtained based on the packing density model theory. The result shows that mesoscopic elastic modulus and indentation hardness increase nonlinearly with the increase of the packing density. Based on the hardness-packing density model, the reverse analysis of cohesion and friction angles shows that the mesoscopic value is slightly smaller than the macroscopic value. The micro-indentation test technology can evaluate shale meso-mechanical properties and predict the macro-mechanical properties effectively.

Experimental study of microstructure and rock properties of shale samples

The study of shale mechanical properties is very important for well completion and stimulation in shale gas reservoirs. Thus,the geomechanical properties of Paleozoic marine shale in Lower Yangtze Basin were studied.With the application of argon ion polisher,high-solution CT and scanning electron microscope,the shale samples were processed and their microstructures were observed. Besides, mineral compositions of this shale were analyzed by XRD technique. In order to get the compression characteristic of shale, the scratch and uniaxial experiments were performed. In addition, the loading and unloading situations were simulated under different effective stress conditions in order to reduce negative effects of natural fissures and rock relaxation. Finally,the multi-stage triaxial compression deformation experiment was performed on the triaxial rock testing system for calculating cohesive strength and inherent friction angle. The study of shale mechanical properties is veryimportant for well completion and stimulation in shale gas reservoirs. Thus, a systematic experiments was performed on Paleozoic marine shale samples collected from Lower Yangtze Basin, and the microscopic structure,mineral compositions and the geomechanical properties of marine shale samples were studied. Testing result shows that except for shale sample with natural fractures,both the values of Youngs modulus and Poissons ratio increase with the increase of confining pressure. But in the unloading condition, the Young s modulus increases with the decrease of confining pressure,whereas the Poisson s ratio is greater than that in its loading condition. The inherent friction angle is within approximately 40°and the shale sample with high brittleness index tends to crack as fracture network according to the multi-stage triaxial compression test. This research is greatly helpful for the study of shale mechanical properties,and provides technical parameters for well completion design and stimulation.

The Importance of Geochemical Parameters and Shale Composition on Rock Mechanical Properties of Gas Shale Reservoirs: a Case Study From the Kockatea Shale and Carynginia Formation From the Perth Basin, Western Australia

Evaluation of the gas shale mechanical properties is very important screening criteria for determining the potential intervals for hydraulic fracturing and as a result in gas shale sweet spot mapping. Young’s modulus and Poisson’s ratio are two controlling mechanical properties that dictate the brittleness of the gas shale layers. These parameters can be determined in the laboratory by testing the rock sample under different conditions (static method) or can be calculated using the well-logging data including sonic and density log data (dynamic method). This study investigates the importance of the shale composition and geochemical parameters on the Young’s modulus and Poisson’s ratio using log data. The data set of this study is coming from five different wells targeting the Kockatea Shale and Carynginia formation, two potential gas shale formations in the Perth Basin, Western Australia. The results show that converse to the common idea the effect of organic matter quantity and maturity on the rock mechanical properties of the gas shale reservoirs is not so much prominent, while the composition of the rock has an important effect on these properties. Considering the weight percentage of shale composition and organic matter quantity it could be concluded that effect of these parameters on rock mechanical properties is dependent on their weight contribution on the shale matrix. As well as effect of thermal maturity on the shale matrix and consequently on the rock mechanical properties of the shales is dependent on the organic matter content itself; therefore, obviously with a low organic matter content thermal maturity has no prominent effect on the brittleness as well.

An Experimental Investigation of Shale Mechanical Properties Through Drained and Undrained Test Mechanisms

Shale mechanical properties are evaluated from laboratory tests after a complex workflow that covers tasks from sampling to testing. Due to the heterogeneous nature of shale, it is common to obtain inconsistent test results when evaluating the mechanical properties. In practice, this variation creates errors in numerical modeling when test results differ significantly, even when samples are from a similar core specimen. This is because the fundamental models are based on the supplied test data and a gap is, therefore, always observed during calibration. Thus, the overall goal of this study was to provide additional insight regarding the organization of the non-linear model input parameters in borehole simulations and to assist other researchers involved in the rock physics-related research fields. To achieve this goal, the following parallel activities were carried out: (1) perform triaxial testing with different sample orientations, i.e., 0°, 45°, 60°, and 90°, including the Brazilian test and CT scans, to obtain a reasonably accurate description of the anisotropic properties of shale; (2) apply an accurate interpretative method to evaluate the elastic moduli of shale; (3) evaluate and quantify the mechanical properties of shale by accounting for the beddings plane, variable confinement pressures, drained and undrained test mechanisms, and cyclic versus monotonic test effects. The experimental results indicate that shale has a significant level of heterogeneity. Postfailure analysis confirmed that the failure plane coincides nicely with the weak bedding plane. The drained Poisson’s ratios were, on average, 40 % or lower than the undrained rates. The drained Young’s modulus was approximately 48 % that of the undrained value. These mechanical properties were significantly impacted by the bedding plane orientation. Based on the Brazilian test, the predicted tensile strength perpendicular to the bedding plane was 12 % lower than the value obtained using the standard isotropic correlation test. The cyclic tests provided approximately 6 % higher rock strength than those predicted by the monotonic tests.

Influence of Occurrence Environment on Mechanical Properties of Shale

One of the main content of the environmental geotechnical engineering is engineering disaster problem brought by environmental change, the most basic problem is the influence of occurrence environment on the mechanical properties of rock. The water-saturated and freeze-thaw cycling experiment were conducted for dried shale under the open system, then carried out the mechanical properties experiment on dry and water-saturated state and on certain times of freeze-thaw cycles, the characteristics of the water softening, freeze-thaw damage deterioration, deformation and failure for shale were studied systematically. It is shown that the moisture has an important impact on mechanical effect of shale. With the increasing of the freeze-thaw cycles, the elastic modulus and strength of the shale reduce significantly, then approach stably after experiencing a certain number of freeze-thaw cycles, while the ultimate strain is increasing continuously. The engineering problems brought by environmental change must be solved from moisture and temperature in geotechnical engineering in cold regions.

Wellbore Stability in Oil and Gas Drilling with Chemical‐Mechanical Coupling

Wellbore instability in oil and gas drilling is resulted from both mechanical and chemical factors. Hydration is produced in shale formation owing to the influence of the chemical property of drilling fluid. A new experimental method to measure diffusion coefficient of shale hydration is given, and the calculation method of experimental results is introduced. The diffusion coefficient of shale hydration is measured with the downhole temperature and pressure condition, then the penetration migrate law of drilling fluid filtrate around the wellbore is calculated. Furthermore, the changing rules of shale mechanical properties affected by hydration and water absorption are studied through experiments. The relationships between shale mechanical parameters and the water content are established. The wellbore stability model chemical-mechanical coupling is obtained based on the experimental results. Under the action of drilling fluid, hydration makes the shale formation softened and produced the swelling strain after drilling. This will lead to the collapse pressure increases after drilling. The study results provide a reference for studying hydration collapse period of shale.

Fracturing-Fluid Effects on Shale and Proppant Embedment

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 146658, ’The Effects of Fracturing Fluids on Shale Rock-Mechanical Properties and Proppant Embedment,’ by Ola Akrad, SPE, Jennifer Miskimins, SPE, and Manika Prasad, SPE, Colorado School of Mines, prepared for the 2011 SPE Annual Technical Conference and Exhibition, Denver, 30 October-2 November. The paper has not been peer reviewed. Development of shale reservoirs has grown significantly during the past few decades, driven by evolving technologies in horizontal drilling and hydraulic fracturing. The productivity of shale reservoirs depends highly on the design of the hydraulic-fracturing treatment. To design a successful treatment, a good understanding of the shale mechanical properties is necessary. The results of this study show that the greatest reduction of Young’s modulus occurs at high temperature and in samples with high carbonate content. Introduction In geology, shale traditionally has been defined as a sedimentary rock containing a high percentage (>50%) of clay and lower percentages of silica or carbonate minerals. However, many current shale prospects are not shales as defined in geology. Instead, they are “prospective-shale” reservoirs: fine-grained clastics characterized by ultralow permeability and usually composed of silica and carbonate minerals with a small amount of clay. Success in these shale systems depends largely on excellent hydraulic-fracturing designs that require a good understanding of the mechanical properties of the subject and confining formations. In hydraulic-fracturing design, Young’s modulus is a criterion used to determine the most-appropriate fracturing fluid and other design considerations. Young’s modulus provides an indication of the fracture conductivity that can be expected under the width and embedment considerations. Without adequate fracture conductivity, production from the hydraulic fracture will be minimal, or nonexistent. This study of Young’s modulus shows a significant decrease in the presence of certain fluids. Such decreases in Young’s modulus lead directly to decreases in fracture conductivity and, therefore, should be taken into account when designing the treatment to obtain a required fracture conductivity in a given reservoir. Methods and Testing Procedures It is known that fracturing fluids can affect the formation face of the created factures and cause damage. This damage can manifest itself as filter cake, fluid imbibition, and other damage mechanisms. A possible additional damage mechanism in prospective-shale systems is the potential weakening of the rock frame or modification of Young’s modulus, which could allow additional proppant embedment to take place. Such embedment, if occurring, could affect the fracture conductivity of a given treatment adversely.

Drilling With a Balanced-Activity Invert-Emulsion Fluid in Shale: Is It Sufficient for Maintaining or Enhancing Wellbore Stability?

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 145969, ’Drilling With a Balanced-Activity Invert-Emulsion Fluid in Shale: Is It Sufficient for Maintaining or Enhancing Wellbore Stability?,’ by Terry Hemphill, SPE, Halliburton, prepared for the 2011 SPE Annual Technical Conference and Exhibition, Denver, 30 October-2 November. The paper has not been peer reviewed. Most wellbore-stability problems encountered while drilling occur in shale formations. Invert-emulsion fluids (IEFs) are used to drill reactive shales. Testing of a deepwater shale was performed with a new apparatus, which allows shale samples to be tested under downhole conditions. Results show that even at an activity level higher than the known shale-activity level, water was able to enter the shale and weaken it. The “chemomechanical-balance” concept was used to explain measured results in terms of the coupling of shale-mechanical properties and IEF chemical activity and to explain the resulting consequences of activity-level selection for IEF. Introduction The interaction of invert-emulsion drilling fluids with exposed shales has received much study because well-bore-stability problems occurring in shale are in the 80% range of all cases cited in literature. While usually not found in the reservoir sections, when these shales become unstable, significant nonproductive time is spent in efforts to stabilize the wellbore, and sometimes entire drilling intervals are lost. To understand the interaction of IEFs with shale during the drilling process better, it is helpful to review the pertinent theories used to describe such action. Semipermeable-Membrane Theory. At the face of the shale that is exposed to aqueous fluid, a semipermeable membrane exists that regulates the flow of water in or out of the specimen. Fluids having high membrane efficiencies, such as IEFs, have the ability to pull water from the rock efficiently. But fluids with low membrane efficiencies, such as common water-based drilling fluids, do not pull water efficiently. There is still uncertainty about whether the membrane is provided by the IEF or by the shale itself. Osmotic-Pressure Theory. Fluids having lower chemical activity compared with the shale-pore fluid can generate significant amounts of osmotic pressure that can be used to remove water from the rock. Conversely, an IEF with higher chemical activity in the drilling-fluid water phase than in the shale-pore water can push water into a shale.

Experimental Study on the Influence of Temperature on Shale Mechanical Properties under Conventional Triaxial Compression

High temperature conventional triaxial compression test of shale are carried out by the MTS815 servo-controlled testing machine, based on the experimental results, the relationships between temperature and shale peak strength, elastic modulus, Poisson's ratio, cohesion, internal friction angle are investigated. Although the experimental results are discrete comparatively, the general law is obvious. When the confining pressure imposed on shale is constant and the temperature changes form 25°C to 120°C, with the increasing of the temperature, the triaxial compression strength, shear strength gradually increase, while average elastic modulus, Poisson's ratio has a slightly decrease. The thermal stress generated by the high temperature plays a role to accommodate the deformation and the function of preventing crack propagation, thus the bearing capacity of shale samples are strengthened. But the influence of temperature on shale mechanical properties mutates when the temperature is at 80°C. Shale peak strength dramatically decreased, average elastic modulus decreased slightly, and Poisson's ratio also increased slightly, which indicated that at 80°C, different thermal expansivity of mineral particles of shale may cause cross-grain boundary thermal expansion incongruous, creating additional thermal stress, thus the sample’s bearing capacity decreased.

An Experimental Investigation of the Impact of Diffusion Osmosis and Chemical Osmosis on the Stability of Shales

Wellbore instability in shales is the most challenging and costly issue in drilling operations. Wellbore instability in shales can be attributed to many factors, some of which have been well studied and documented. However, the physicochemical and mechanical properties alterations in shales that eventually lead to wellbore failure have been largely ignored. Water and ion movements in and out of shales play a major role in the alteration of the physicochemical and mechanical properties of shales, thus leading to wellbore instability problems and possible hole collapse. Water and ions can move in and out of shales by many mechanisms, including, but not limited to, diffusion osmosis, chemical osmosis, convective flow, and capillary suction. This work presents experimental data analyzing the impact of chemical osmosis and diffusion osmosis on water and ion movements when shale interacts with drilling fluids. The adopted experimental work minimized the effect of convective flow and capillary suction. Results show that water movement is not only controlled by chemical osmosis (water activity) as previously thought but is also influenced by diffusion osmosis. This insight provides information and guidelines to optimize drilling fluids to effectively control and mitigate wellbore instability when drilling through troublesome shale.

An Experimental Investigation on the Stability and Reservoir Hydrocarbon Capacity of Shales

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 121451, ’An Experimental Investigation on the Impact of Capillary Pressure, Diffusion Osmosis, and Chemical Osmosis on the Stability and Reservoir Hydrocarbon Capacity of Shales,’ by Talal M. AL-Bazali, U. of Kuwait; Jianguo Zhang, SPE, BP plc; and Martin E. Chenevert, SPE, and Mukul M. Sharma, SPE, U. of Texas at Austin, originally prepared for the 2009 SPE Offshore Europe Oil and Gas Conference and Exhibition, Aberdeen, 8-11 September. The paper has not been peer reviewed. Wellbore instability in shales is the most challenging and costly issue in drilling operations. Wellbore instability in shales can be attributed to many factors, some of which have been well studied and documented. However, the physicochemical- and mechanical-properties alterations in shales that eventually lead to wellbore failure largely have been ignored. Water and ions movement in and out of shales plays a major role in the alteration of the physicochemical and mechanical properties of shales, thus leading to wellbore-instability problems and possible hole collapse. Introduction The unfavorable interaction between shale and drilling fluids is considered to be the primary cause of many wellbore-instability problems. Such interactions are very complicated and include mechanical, chemical, physical, hydraulic, thermal, and electrical phenomena. The overall effect of these interactions mainly is related to the movement of water and ions into or out of shales. The physicochemical and mechanical properties of shale around the wellbore, such as permeability, strength, pore pressure, and elastic modulus, can be altered greatly by such movement. It is well-accepted that the adsorption of water results in shale-strength and elastic-modulus decrease, in swelling, and in pore pressure increase. These changes around the wellbore may cause wellbore instability problems during drilling and completion operations. The magnitude of water movement and the effect of absorbed water on shale properties are influenced by the presence of ions in the solution. Ionic diffusion also may result in the movement of ions into the shale formation, resulting in chemical alteration. Water and ions can move in and out of shales by many complicated mechanisms. These mechanisms include, but are not limited to, chemical osmosis, diffusion osmosis, capillary suction, and convective flow. In the full-length paper, the focus is on the diffusive flow of water and ions. Specifically, the effect of both chemical osmosis and diffusion osmosis on shale alteration was investigated. The experimental setup and procedure were designed to minimize the effect of convective and capillary flow. Shales play a major role in petroleum exploration and production because they commonly are considered to be both source rocks and seals. Their ability to exhibit good sealing characteristics arises from their small, water-wet pores. The small pore throats in shales are responsible for generating high capillary pressures, which exclude hydrocarbons.

Estimating Mechanical Properties of Shale From Empirical Correlations

Summary Borehole-stability prediction requires knowledge of the mechanical properties of the formations which is rarely available in shale sections. This paper presents empirical correlations to assist in predicting shale mechanical properties. The correlations are based on extensive laboratory testing of shale cores primarily from the North Sea. The acoustic P-wave velocity is a primary input parameter in several of the correlations; thus, various sources of the P-wave velocity such as sonic wireline, sonic measurement while drilling (MWD), and acoustic measurements on cuttings may be used to obtain somewhat continuous estimates of shale mechanical properties. Borehole-stability evaluations can be made at different stages in the drilling process (planning, while drilling, and post-analysis). Other applications of the correlations where shale mechanical properties are required are evaluation of overburden compaction during depletion and optimization of the drilling process (selection of bit type, bit parameters, etc.).

A wellbore simulator for mud-shale interaction studies

A wellbore simulator for investigating mud/shale interactions is described. Outcrop Pierre Shale is prepared for test by an initial drainage stage, in order to reduce the chemical potential of the pore water to a value more representative of buried rock. This enhances swelling when the shale is subsequently placed in contact with deionised water. Unlike vapour phase equilibration of chemical potentials, the stresses applied during drainage compact the shale. This is thought to destroy some of the cementation within the shale, though to nowhere near the extent that occurs when shales are reconstituted from ground material. The wellbore radius is measured during swelling by means of a caliper, and results obtained using several drilling muds are reported in order to demonstrate the shale to stresses applied during drainage is analyzed using poroelacticity theory, assuming isotropy of the shale. This leads to estimates of the shale mechanical properties which are in line with literature values.

Effects of Strain Rate and Confining Pressure on the Deformation and Failure of Shale

Summary Previous work on shale mechanical properties has focused on the slow deformation rates appropriate to wellbore deformation. Deformation of shale under a drill bit occurs at a very high rate, and the failure properties of the rock under these conditions are crucial in determining bit performance and in extracting lithology and pore-pressure information from drilling parameters. Triaxial tests were performed on two nonswelling shales under a wide range of strain rates and confining and pore pressures. At low strain rates, when fluid is relatively free to move within the shale, shale deformation and failure are governed by effective stress or pressure (i.e., total confining pressure minus pore pressure), as is the case for ordinary rock. If the pore pressure in the shale is high, increasing the strain rate beyond about 0.1 %/sec causes large increases in the strength and ductility of the shale. Total pressure begins to influence the strength. At high strain rates, the influence of effective pressure decreases, except when it is very low (i.e., when pore pressure is very high); ductility then rises rapidly. This behavior is opposite that expected in ordinary rocks. This paper briefly discusses the reasons for these phenomena and their impact on wellbore and drilling problems.

Micromechanical Characterization of Fluid/Shale Interactions by Means of Nanoindentation

SummaryMitigation and prevention of shale-formation damage caused by hydraulic-fracturing fluid/rock interactions play an important role in well-production stability and subsequent refracturing design. In this paper, the effect of converting typically hydrophilic fractured surfaces to hydrophobic ones on fluid-induced softening of shales was investigated. Specifically, nanoindentation was used to characterize changes in the mechanical properties of shale samples after different surface treatments. A thin layer of octadecyltrimethoxysilane (OTMS) coating was deposited on the initially hydrophilic surface of shale, followed by inundation in water for certain periods of time to allow for fluid/rock interactions. Nanoindentation testing was then conducted on the treated shales to characterize their hardness, Young's modulus, and fracture toughness to examine the alteration of shale's mechanical properties caused by fluid/rock interactions and to check whether hydrophobic coating can mitigate shale-softening. Results from nanoindentation testing were analyzed by a newly proposed clay-matrix criterion for data screening. Different rock-surface treatments lead to changes in rock properties. Both the hardness and Young's modulus of the treated samples converge and stabilize at relatively large depths. Samples with hydrophobic surfaces exhibit a much lower degree of softening, as reflected by their better mechanical properties (e.g., 40% increase in hardness, 25% increase in Young's modulus, and 35% increase in fracture toughness), compared with untreated shale samples. The results of this study also demonstrate that the continuous-stiffness-measurement (CSM) method and the repeated loading method for nanoindentation loading yield similar ranges of micromechanical properties of the bulk shale. However, the CSM method, if combined with the newly proposed clay-matrix-based criterion for data screening can better define and characterize fluid/shale interactions or softening of shales.

Oil Shale Properties by Split Cylinder Method

The mechanical properties of oil shale are being investigated to provide the basic information necessary to develop and optimize procedures for modifying the rock in place, as required by diverse production techniques. A method is devised where Poisson's ratio, Young's modulus, and ultimate tensile strength along the bedding planes can be obtained on oil shale samples (oil shale are slightly anisotropic) from a split cylinder test. Stress and vertical and horizontal strain measurements on 40 oil shale samples from the Tipton Member of Wyoming's Green River Formation were used to calculate mechanical properties. The determined mechanical properties show strong linear relationships to the volume of organic matter in the oil shale but little relationship to the major matrix mineral, dolomite. Relationships developed are sufficiently precise to provide useful estimates of oil shale's mechanical properties from the organic volume.