Lian, C. J. (Shandong University of Science and Technology) | Hou, J. Z. (Shandong University of Science and Technology) | Gao, G. L. (Taian Taishuo Strata Control Science and Technology Co. Ltd.) | Wang, G. (Taian Taishuo Strata Control Science and Technology Co. Ltd.) | Song, W. T. (Henan Polytechnic University)
ABSTRACT: It has wide distributions and large recoverable reserves of Jurassic period coal seam in China. It is difficult to maintain stability of the development roadways with long service term for Jurassic strata because there are abundant argillaceous rocks and some minerals in the high argillaceous rocks will be expanding while meeting with water. The fractures of the roadways develop well under high stress and are suitable to filling with grouting. But the poor cementing performance of cement with argillaceous rock as well as a heavy water filtration rate of cement slurry had resulted in failures of many engineering cases adopting cement grouting to reinforce this kind of roadways. In this paper, according to characters of the high argillaceous rock in Jurassic strata, a marlaceous inorganic grouting material which possesses the well cementing performance with argillaceous rocks and little filtration rate was introduced; and the grouting reinforcement mechanism, construction technique and engineering application effect about it was clarified. It will be of great significance for reinforcement and maintenance of the development roadways with high argillaceous rocks.
According to statistics, 60% of the proved coal reserves in China distributes in Early-Middle Jurassic period of northern North China, southern Northeast China and Northwest China, along with late Jurassic to early Cretaceous period of Northeast China and east Inner Mongolia. The period of coal forming is short and argillaceous rocks are abundant in Jurassic strata. Moreover, there are quite a few expanded minerals in some strata. So the roadways are easily to be deformed and damaged when affected by mining-induced stress. In addition, this kind of soft rock roadways has an obvious time effect. For the development roadways with long service term, serious deformations are frequently observed and part of the roadways has suffered deformation and maintenance time after time. The stability support of the development roadways really need much cost.
A series of laboratory diametrical compression tests was performed on Brisbane tuff disc specimens to investigate their mode-I fracture toughness response to static and cyclic loading, as a function of the applied load. Both the static and cyclic loading tests were carried out on Cracked Chevron Notched Brazilian Disc (CCNBD) rock specimens. In the tests described herein, the reduction in fracture toughness under dynamic cyclic loading was found to be up to 48% of the static fracture toughness. The experimental results showed continuous irreversible damage accumulation at the tip of the chevron notched crack in the dynamic cyclic loading tests, strongly influenced by the loading amplitude. Contrary to the static tests, the cyclic tests produced much more crushed material in front of the tip of the chevron notched crack. It was concluded that cyclic loading causes many micro-cracks, resulting in an extensive Fracture Process Zone (FPZ) in front of pre-existing cracks in rocks.
The use of underground facilities in rock for purposes such as transportation tunnels, power station caverns, radioactive waste repositories, and water and gas storage is increasing. The excavation of such spaces results in a change in the in situ stress distribution; these changes alter the mechanical parameters of the rock mass, including its strength, deformability and, in particular, its permeability through the network of stress-relief cracks that develop. The Excavation Damaged Zone (EDZ) is the disturbed rock zone around an underground opening following excavation. The occurrence of (macroscopic) fracturing around the perimeter of a tunnel could be a combination of damage caused by the excavation process, and damage caused by stress concentrations around the tunnel opening; in turn caused by seasonal cyclic variations of temperature and traffic-induced cyclic loading. The mechanical behavior of rock, and pre-existing or newly-formed cracks under static loading, has been thoroughly investigated. However, the response of rock to the cyclic, repetitive stresses resulting from dynamic loading has been generally neglected, with the exception of a few rather limited studies [1, 2, 3, 4, 5]. The effect of the fatigue on rock failure is still the subject of much research in fracture mechanics [6, 7]. Rock fatigue has also been studied in the field of rock cutting. An early attempt to assess the effect of cyclic loading on rocks due to the action of mechanical cutters using static and percussive tools was made by Roxborough . In the last decade, Hood and Alehossein , described a novel method of rock cutting using oscillating disc cutting (ODC) technology. They found that the force required to cut hard rock was reduced up to 60 to 70% using ODC technology compared with that required using conventional techniques. For quasi-brittle materials such as concrete and rock, cracking is the major cause of material failure in many cases. The most fundamental parameter in fracture mechanics is the fracture toughness, which describes the resistance of a material to crack propagation. As a result, assessment of the resistance to crack propagation is crucial in understanding the behavior of structures involving brittle materials.
Izadi, Ghazal (Department of Energy and Mineral Engineering, EMS Energy Institute and G3 Center, Pennsylvania State University) | Elsworth, Derek (Department of Energy and Mineral Engineering, EMS Energy Institute and G3 Center, Pennsylvania State University)
We explore the complex interaction of coupled thermal, hydraulic, mechanical and chemical (THMC) processes that influence the evolution of EGS reservoirs in general, and in particular with reference to strong, low-permeability reservoirs with or without relic fracturing. We define and describe dominant behaviors that evolve with the evolution of the reservoir: from short-term stimulation through mid-term production and culminating in long-term decline. These include short-term response where effective stresses and thermal quenching dominate the behavior of the reservoir and are influenced by the local structure in the rock at the scale of a few meters and in particular the form and orientation of pre-existing fractures. Typical behaviors include the reduction of local mean stresses and the development of shear fracturing principally on pre-existing fractures but also the creation of fresh fractures and new reactive and heat-transfer surface area. Continuum models are useful analogs to represent the principal features of this intermediate-term production response. Reaction fronts may propagate through the reservoir and impact the evolution of permeability in surprising ways when considered together with the influence of the effective and thermal stress state. Finally, the long-term decline of the reservoir may be observed as flow-rates may build and the potential for the development of fluid and thermal short-circuiting as pathways grow. Finally, we apply a model with static-dynamic frictional strength-drop to evaluate the rate and severity of triggered seismicity. The changing stress state is calculated from the pore pressure, thermal drawdown and chemical effects in a coupled THMC model with dual porosity. In the rate-state friction model, we vary crack length from 1m to 1000m and examine the intrinsic scaling of energy release. Energy release increases with the cube of crack length, the square of stress drop and linearly with rock mass stiffness. Seismic activity is concentrated around the near-wellbore injection region. It is earliest for closely spaced fractures in reservoir rocks where the thermal drawdown of stress is largest at early times but results in numerous low-magnitude events. For closely spaced fractures (~0.1 m) near-injection failure develops in the short term (<1 month) and for more widely-spaced fractures (~10 m) it is delayed (>7 years) and pushed further out into the reservoir. Changes in energy release generate moment magnitudes which vary from -2 to 2 for small to large fractures. These observations are used to define the evolution of spatial seismicity within the reservoir and its migration with production, dependent on the mobilization of relic fractures. To reduce the energy release of single large pre-existing fractures we explore the role of thermally-induced micro-fractures as a mechanism to reduce stored strain energy. By allowing the development of micro-fractures in the system, more accumulated energy and deformation is released aseismically, thereby reducing the number of large events. These models are used to define the evolution of seismicity with the progress of stimulation and then production within the reservoir.
There is empirical evidence that within EGS reservoirs, injection/production-induced poroelastic stresses may promote frictional sliding on pre-existing faults with large seismic events tending to take place on developed or active faults (Majer et al., 2007; Segall, 1989).
In the following work the method of unequal stress state estimation of rock massive is numerically researched. This estimation is carried out by the fracture interval cross section area dependence on loading pressure of its walls through the solid enclosure. The algorithm of evaluation of test data as well as questions of method practical realization is examined. INTRODUCTION
Typical method of rock stress state estimation with the help of hydraulic fracture involves the loading of borehole walls and crack edges by fracturing fluid pressure. In the calculation model shut-in pressure (PS) is used. This pressure is registered as soon as the pumps are shut down. In this model the fracture opening pressure on borehole outline in recycling of loading is also used (Pr). In permeable rock mass value of PS depends on pressure the fluid leakage into the environment, the volume of which is unknown in general cases. Another problem consists in fluid filtration into crack. This leads to poor estimation of Pr as well as stress determination by classical method . To prevent contact between the fracture fluid and the rock mass a solid enclosure is applied, that excludes the possibility of PS measurement and leads to the necessity of changing to another parameter. Methods based on measurement of different oriented fractures pressure are practically developed. According to the way of creating a fracture they are divided into:
HTPF method (Hydraulic Tests on Pre-existing Fracture)  which uses natural rock jointing when it is well-expressed;
DF method ( Double Sleeve Fracturing Method)  in which radio symmetrical loading of borehole walls through the solid enclosure first forms the fracture in the direction of maximum compression ability, the fracture further increase by redistribution of stresses – the second fracture normal to the first one;
SF method (Single Sleeve Fracture Method)  in which several fractures of different orientation are created using the directional single-axle borehole outline, which expand perpendicular to its axis ;
The experimental research shows that in irregular compression field the shape of forming cracks have more complicated character than is allowed in the methods mentioned above. Fractures created by using SF method in the direction not coincident with the principle stress, divert from the datum plane to a deflected position . It is also shown that in such a field the main condition of DF method doesn’t work i.e. forming fractures are not orthogonal to each other . When the fractures are reopened, it leads to uncontrolled interaction of fractures as well as to significant error in stresses estimation, which increases with the augmentation of external field irregularity . We propose another approach to the problem . The idea consists is applying the solid enclosure to the deformational characteristics research of the bottom-hole zone, containing extended hydraulic fractures formed by the classical method . In the process of repeated loading, the crack is partially opened along hydraulic fracturing plane where the ambience has zero tension strength. The length of open section L (L<L0) increases step-by-step that has an influence the rock mass stress state in the borehole environ and on the value of crack width size on its outline and cross-sectional area.
This paper employs coal mine blast-induced ground motions to interpret microseismic phenomena that from time to time may be induced by petroleum resource development. Reservoir depletion, flooding, etc may produce microseismic ground motions that lead to concern on the part of those who feel the ground motion. These concerns can be addressed by comparison to other similar ground motions, such as the blast-induced motions described herein, and their effect on residential structures. These similar motions were found not to cause even cosmetic cracking of the weak wall covering of a residential structure. Furthermore ground motion-induced crack response was found to have an order of magnitude less effect than climatological changes associated with the passage of a weather front.
The following caution from the web site “Induced Earthquake Bibliography: Oil and Gas Production Induced Earthquake References” is always important in any article such as this. "The very fact that someone has studied the possibility of induced seismicity at a given site or related to a given activity does not mean that activity or site has really induced seismicity. It certainly does not mean that the activity has "caused damaging earthquakes." Some human activities do induce microseismic events that cause damage in limited locations, but most induced micro-seismicity is not damaging. This paper presents measured response of a structure to motions producing peak particle velocities of 10 to 19 mm/s at 5 to 21 Hz, which can be employed to interpret microseismic phenomena. These ground motions were produced by surface coal mining. They are compared to motions produced by two micro seismic phenomena, gas field depletion and hydraulic fracture. Ground motions produced by the gas field depletion were found to be similar to those produced by coal mine blasting. Since the ground motions were found to be similar, response of an instrumented test house to the coal mine blastinduced ground motions can then be assumed to be similar to that induced by the gas field depletion. The paper begins with a description of the ground motions produced by gas field depletion and hydraulic fracturing. These ground motions are then compared to those produced by the coal mine blasting adjacent to the instrumented test house. Single degree of freedom response spectra, which account for both amplitude and frequency content, are found to be helpful in this comparison. Response of the test house and the instrumentation employed to monitor crack response are then described. Crack responses to dynamic, ground motion- induced excitation are compared with those produced by long term climatological effects through use of micro-meter displacement transducers. The dynamic responses were found to be far less than those produced by the passage of a weather front.
2. GROUND MOTIONS PRODUCED BY OIL FIELD OPERATION AND HYDRALUIC FRACTURING
Van Eck et al’s 2006  description of the motions produced by general operation of the Roswinkel gas field in the Netherlands allows development of a response spectrum for the production induced ground motions.
Controlled blasting techniques are used to control overbreak and to aid in the stability of the remaining rock formation. The less competent the rock mass itself is, the more care has to be taken in avoiding damage. Presplitting is one of the most common methods which is used in many open pit mining and surface blast design. The purpose of presplitting is to form a fracture plane across which the radial cracks from the production blast cannot travel. Presplitting should be thought of as a protective measure to keep the final wall from being damaged by the production blasting. The purpose of this study is to investigate of effect of presplitting on the generation of a smooth wall in a rock domain under a surface blast process. The 2D distinct element code was used for simulation of presplitting in a rock slope. The blast load history as a function of time applied to the inner wall of each blasthole. Important parameters that were considered in the analysis were stress tensor and fracturing pattern. The blast loading magnitude and blasthole spacing were found to be very significant in the final results.
Drilling and blasting continues to be an important method of block production and block splitting. Drill and blast technique has a disadvantage that sometimes it produces cracks in uncontrolled manner and also produces micro cracks in the block as well as in remaining rock, if not carefully carried out. Recovery by this method is low as compared to other methods. Therefore, attempts have been made to develop controlled growth of crack in the desired direction. The control of fractures in undamaged brittle materials is of considerable interest in several practical applications including rock fragmentation and overbreak control in mining [1–3]. One way of achieving controlled crack growth along specific directions and inhibit growth along other directions is to generate stress concentrations along those preferred directions. Several researchers have suggested a number of methods for achieving fracture plane control by means of blasting. Fourney et al.  suggested a blasting method which utilizes a ligamented split-tube charge holder. Nakagawa et al.  examined the effectiveness of the guide hole technique by model experiments using acrylic resin plates and concrete blocks having a charge hole and circular guide holes. Katsuyama et al.  suggested a controlled blasting method using a sleeve with slits in a borehole. Mohanty [7,8] suggested a fracture plane control technique using satellite holes on either side of the central pressurized hole, and demonstrated its use through laboratory experiments and field trials in rock. Nakamura et al.  suggested a new blasting method for achieving crack control by utilizing a charge holder with two-wedge-shaped air cavities. Nakamura  performed model experiments to examine the effectiveness of the guide hole with notches. Cho et al.  performed experiments using a notched charge hole to visualize fracturing and gas flow due to detonation ofexplosives
This research study investigates the cracking processes associated with inclusion pairs of varying shape, orientation and inclusion materials. Specifically, this study summarizes a series of uniaxial compression tests on gypsum specimens with varying inclusion pair configurations. The inclusions consisted of differing materials, of contrasting Young’s Modulus (higher and lower than the matrix), shapes (hexagon, diamond, ellipse), and relative pair orientations (bridging angle). Similar cracking sequences were seen in the newly introduced inclusion pairs as in previous studies. Slightly increased debonding (usually corresponding to increased interface shearing) occurred as inclusion pairs with inclined interfaces were introduced. Coalescence behavior trended from indirect or no coalescence, to direct shear coalescence, to combined direct tensile-shear coalescence as the inclusion bridging angle was increased, similar to past studies on circular and square inclusion pairs and flaw pairs. Also, the coalescence related to inclusion interface inclination and bridging angles resembled the actual coalescence of flaw pairs with similar inclination and bridging angles.
The cracking processes in a brittle material consisting of a matrix with inclusions are important mechanisms for both natural materials (rocks) as well as synthetic composite materials (e.g. concrete). There have been many past studies regarding the cracking processes in brittle materials, which contain pre-existing cracks (called flaws) both analytically [1, 2], as well as experimentally [2, 3, 4]. Also, the cracking processes in brittle materials, which contain inclusions have been studied both analytically [6, 7, 8] and experimentally [7, 9, 10, 11, 12]. Only recently have experiments been performed with the technology capable of capturing high speed imagery to fully describe the crack propagation and coalescence behavior in a brittle material. The majority of the previous research performed on brittle materials with inclusions investigated the fracturing patterns associated with circular or rectangular (square) inclusions. The present research was conducted to develop a more detailed description of the coalescence patterns of uniaxially loaded gypsum specimens with inclusion pairs of varying shape, stiffness and orientation. Emphasis was placed on the coalescence behavior associated with the effects of varying these inclusion pair configurations.
2. PREVIOUS STUDIES
2.1 Flaw Coalescence Studies Amongst the many experimental studies regarding flaws in brittle materials, experimental work done by Wong and Einstein  is particularly significant because it incorporated the use of a high speed camera to follow crack propagation and coalescence. One of the most important contributions of Wong and Einstein’s study was a proposed set of coalescence categories for different co-planar and stepped flaw pairs (Figure 2.1). These coalescence patterns will later serve as a basis for comparing the coalescence patterns seen in brittle materials containing inclusions pairs.
2.2 Inclusion Coalescence Studies Extending on the macro-scale flaw testing techniques used in the Massachusetts Institute of Technology (MIT) Rock Mechanics Laboratory, brittle material with inclusions was investigated with high speed imagery by Janeiro and Einstein . That study tested 1” single square, circle, diamond, and hexagon inclusion shapes as well as 1/2" circular and square inclusions with varying inclusion material stiffness (Figure 2.2).
Fragmentation mechanism in sphere indentation was explored numerically using the discrete element code PFC3D®. The fracturing sequences were analyzed in a face-centered cubic (FCC) crystal structure and a randomly generated particle assembly with material properties similar to those of hard rocks such as granite. In the crystal structure, development of the Hertzian cone crack and the damaged zone underneath the indenter was followed by nucleation of a full penny-shaped median crack. In the randomly generated sample, the Hertzian cone crack was no longer evident. Half penny-shaped radial cracks instead of the median crack formed. Depending on the state from which unloading of the indenter started, a deep saucer-shaped lateral crack may initiate and propagate in both the crystal structure and the random packing. Locations of the lateral crack initiation were observed to be at the side instead of the base of the damaged zone. The tensile stress field developed near the free surface during the unloading stage was responsible for the formation of the lateral cracks.
Fragmentation mechanism in sphere indentation was explored numerically in this paper using the discrete element code PFC3D® .The fracturing sequences were analyzed in a face-centered cubic (FCC) crystal structure and a randomly generated particle assembly with material properties similar to those of hard rocks such as granite. Special attention has been paid to investigate the effect of unloading on the fracture mechanics of the lateral cracks. Indentation fracture mechanics has been extensively studied since Hertz in 1880s, in particular for materials such as glasses, metals and ceramics [3-9]. While the fracturing mechanisms of the Hertzian cone crack and the radial and median cracks are well established, in studies focusing on material testing, lateral cracks have been usually regarded as secondary and received little attention . Development of lateral cracks is nevertheless the main mechanism of material removal in mechanical excavation in rocks. Fundamental understanding of how lateral cracks initiate and propagate is therefore critical to improving the drilling efficiency and bit design. Lawn and Swain  observed the development of lateral cracks in the unloading stage of Knoop and Vickers indentation tests in soda-lime glass. Based on the Boussinesq stress field for a point load, they described the following idealized sequence of events for indentation in brittle materials with a sharp indenter, i.e., a tool of wedge, conical or pyramidal shape. When the indenter gets into contact with the surface, irreversible deformation around the contact point is first produced. As the indentation load exceeds a threshold, a median crack initiates below the contact point at the elastoplastic boundary and propagates downward stably. When the indenter is removed, the median crack first closes, but may grow sideways to reach the surface and become a half penny-shaped crack on complete unloading. The residual stress field due to mismatch in strain recovery in the plastic zone and the surrounding elastic matrix may lead to development of the lateral cracks. The lateral cracks may initiate beneath the plastic zone and propagate towards the free surface.
Digital image correlation was used to identify the process zone by measuring incremental displacement contours and profiles at different crack configurations. Smooth boundary specimens of Berea sandstone, average grain size of 0.2 mm, were studied in the three-point bend test, such that fracture occurred within a specimen without a macroscopic stress concentrator. The experimental results were related to the incremental displacement field due to the initiation and propagation of a mode I fracture. The change in crack opening displacement between two different crack configurations provided the basis to interpret the process-zone length, which was estimated to be 5 – 7 mm or about 30 times the average grain size. The length remained more or less constant (5 – 7 mm) with crack propagation in the post-peak region.
Rock and other quasi-brittle materials may exhibit a significant process zone due to microcracking, and the evaluation of its size becomes critical in deciding the applicability of linear fracture mechanics . In the context of a Dugdale-Barenblatt model, the process zone can be interpreted as a cohesive zone, where closing tractions act on the separating surfaces. Due to the influence of crack tortuosity or unbroken ligaments, closing tractions may be viewed as a function of separation. The length and/or width of the process zone is often considered to be a material property . Methods to estimate process-zone size include direct and indirect approaches. The direct approaches include techniques such as acoustic emission (AE) and optical methods, to obtain direct experimental observations of the process zone during crack propagation. For example, the locations of AE events that occur around peak load during fracture have been used to characterize the dimensions of the process zone in different types of rock . Also, information about the process zone can be extracted from electronic speckle pattern interferometry . For the indirect approach, characteristics of the process zone (e.g. length, critical opening displacement) are found through parametric fitting of experimental results from the global response, such as load-deflection or load-crack mouth opening displacement (CMOD), without directly measuring the local fracture processes in the specimen. However, both approaches have limitations. In the direct methods, for instance, AE cannot provide any information about crack displacements, and optical approaches are sensitive to environmental noise that results in a difficulty to observe the fracture process after peak load. Indirect methods do not measure the local displacements surrounding the crack, and the optimization is based on the global response of the specimen. Furthermore, the most serious issue for the indirect method is that the optimization usually does not yield identical results for the same given material due to the different weight assigned to the experimental data. There is very little direct information about the local displacements at the region where the crack tip is propagating. A relatively new technique called digital image correlation (DIC) is gaining in popularity in experimental mechanics. DIC is an approach that can be viewed as a modern form of the particle tracking technique, where “sub-image” movements are traced.