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international journal
Considerations For Discrete Modeling of Rock Cutting
Mendoza, J.A. (U.S. Department of Energy, National Energy Technology Laboratory, University of Pittsburgh) | Gamwo, I.K. (U.S. Department of Energy, National Energy Technology Laboratory) | Zhang, W. (U.S. Department of Energy, National Energy Technology Laboratory) | Lin, J.S. (U.S. Department of Energy, National Energy Technology Laboratory, University of Pittsburgh)
ABSTRACT We have shown previously that the mechanics of rock cutting, as posed in a laboratory scratch test, could be reasonably modeled using the finite element method-- at least for slab cutting cases where a cutter had the same width as that of the rock sample [1]. This study further expanded research into the modeling of groove cutting by a single polycrystalline diamond compact cutter. The cutter modeled had a width narrower than that of a rock sample, and the rocks being cut went beyond the immediate area of contact with a cutter, both laterally and along the depth. To provide an accurate estimate of the cutting force, it was essential to capture the volume of cut. It turned out that the volume of cut was sensitive to the mesh size used in a finite element modeling. This study thus first investigated the mesh-size conditions. Also affecting the results of an analysis was the number of elements in contact with a cutter. After establishing preliminary guidelines, numerical experiments were conducted on groove cutting using both rectangular and disc cutters. Finally, a circular cutting model, albeit preliminary, was developed. 1. INTRODUCTION In an effort to establish the modeling framework of rock cutting within the finite element method, we previously modeled laboratory rock scratch tests (FEM) [1]. We demonstrated that the Lagrangian FEM approach was capable of modeling the rock fragmentation progression from crack initiation, to chip formation, to the interactions of chips, cutter, and the rock sample. In particular, we were able to duplicate two failure modes as observed in the laboratory tests: the ductile failure for shallow cuts and the brittle failure for deep cuts. The cutting modeled was slab cutting, in which the width of a cutter was the same as that of the rock sample. The problem was essentially a two dimensional one. In an effort to model the cutting action of a drilling bit, this study expanded research into the modeling of groove cutting by a single polycrystalline diamond compact cutter. The cutter had a narrower width and cut beyond the immediate contact area, both laterally and along the depth. A clear difference between slab cutting and groove cutting was the lateral cutting of the latter, resulting in a larger specific energy in groove cutting [2, 3]. To provide an accurate estimate of the cutting force, it was essential to capture the volume of cut. It turned out that the volume of cut was sensitive to the mesh size used in a finite element modeling. This study thus first investigated the mesh-size conditions. Also affecting the results of an analysis was the number of elements in contact with a cutter. After establishing preliminary guidelines, numerical experiments were conducted on groove cutting using both rectangular and disc cutters. Finally, a circular cutting model, albeit preliminary, was also developed. 2. MODELING CONSIDERATIONS The core considerations of the finite element modeling effort were: ยท The bottom, right, and left surfaces of the rock piece are treated as โnon-reflectingโ boundaries, which allow stress waves to be dissipated instead of being reflected,
- North America > United States (1.00)
- Europe (1.00)
ABSTRACT This paper presents the numerical modeling of granular debris flow using a mixed continuum-discontinuum approach. The discontinuum approach is based on the Discontinuous Deformation Analysis (DDA) for the modeling of granular debris flows which are found to be the major type of the debris flow in Taiwan. On the other hand, the continuum approach from solving the one-dimensional shallow water equations for the modeling of fluid flow with the incorporation of the DDA is conducted. Differing from the conventional continuum-based method which lacks of the mechanics for describing the behavior of the stony debris flow or the granular flow, the proposed method with the combination of continuum- and discontinuumapproaches is capable of exploring the initiation and mechanism of the granular debris flow to further understand the debris flow behavior. 1. INTRODUCTION Occasional rainfall, steep relief, and adequate debris flow materials are three major components to form a debris flow event in a potential debris-flow creek. The topographic, geological and hydrologic characteristics of Taiwan are corresponding to the three components of occurrence of debris flows. Taiwan is, therefore, frequently beset by debris flow problems during typhoon and heavy rainfall. These fast-moving flows accompanying mud and rock are capable of destroying houses and lives, washing out roads and bridges, or obstructing streams and roadways. To mitigate and manage hazards induced by debris flows, it is necessary to model the debris flow route and deposition process. It is widely recognized that most shallow landslides in Taiwan occurred as a result of heavy rainfall and consequent pore pressure increases in the near subsurface. Historically typhoon events with highintensity, long-duration rainfall often triggered shallow, rapidly moving landslides, i.e. debris flows, resulting in casualties and property damage in the past decades. A number of studies have demonstrated that rainfallinduced landslides can be transformed into debris flows as they move downslope [1, 2]. In the past decades, the hazards of debris flow triggered by earthquake activities and heavy rainfalls occur frequently in Taiwan as well as in China. Especially, the typhoon events with gradually increased rainfall intensity in recently due to global warming often caused severe property damage and inflicted heavy casualties. A debris flow is a flowing mixture of water-saturated debris that moves downslope under the force of gravity. Debris flows consists of materials varying in size from clay to blocks with several tens of meters in maximum dimension. To understand the mechanical behaviour of the debris flow, the limit equilibrium concept using infinite slope theory based on continuum solid mechanics were used to develop the critical formula. On the contrary, from the fluid mechanics point of view, debris flows are inherently non-Newtonian flows in which the pseudo-visco-plastic fluid model was often adopted [3, 4]. Along the past decades, the hazards of debris flow triggered by earthquake activities and heavy rainfalls occur frequently in Taiwan, as shown in Fig. 1(a) and (b). Especially after the 921 earthquake in 1999, the 8-8 typhoon event in 2009, and the mudflow disaster in Gansu province in 2010, large-scale debris flow hazards occurred in Taiwan and China without forewarning. It stimulates the study for exploring the initiation and mechanism of the granular debris flow to further understand the debris flow behavior.
- Asia > Taiwan (1.00)
- Asia > China > Gansu Province (0.24)
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (1.00)
- Geology > Geological Subdiscipline > Geomechanics (0.91)
ABSTRACT Rock bolts have been widely used as a primary support system to stabilize rock masses around tunnels, underground mine galleries, slopes and others structures. To model the interaction between fully grouted bolts and the rock mass a numerical procedure is developed called โenriched finite element method (EFEM)?. Conceptually if a solid finite element is intersected by a grouted rock bolt, it becomes an โenriched? element. Nodes of enriched elements have additional degrees of freedom which are used to determine displacements and stresses in the bolt. Stiffness of enriched elements is formulated based on properties of the rock mass, bolt rod and grout, orientation of the bolt and borehole diameter. This paper quantitatively evaluates bolt performance in different shapes of underground openings viz circular, rectangular and D-shaped, using the proposed enriched finite element method (EFEM) combined with elasto-plastic behaviour of rock mass and grout material. In addition, a comparative study of bolt performance is also presented considering both coupled and decoupled behaviour of rock bolts. INTRODUCTION Rock bolts have been widely used as a primary support system to stabilize slopes, hydro dams and underground structures such as tunnels and mine workings and others structure made in rock masses. The term โrock boltโ is defined in geomechanics as a form of mechanical support that is inserted into the rock mass with the primary objective of increasing its stiffness and/or strength with respect to tensile or shear loads. In general, rock bolts reinforce rock masses through restraining the deformation within rock masses and reduces the yield region around the excavation boundary. During the last four decades, different types of rock bolts have been practiced, out of which fully grouted active/passive bolts were the most common types. For a fully grouted passive rock bolt installed in deformable rock masses, a neutral point exists on the bolt, where shear stress at the interface between the bolt and grout material vanishes. Based on neutral point concepts, shear stresses and axial loads developed along a bolt rod are analytically formulated by many researchers. Bolt grout interactions around a circular tunnel in Hoek-Brown medium have been formulated analytically considering a bolt density factor. Considering different approaches to bolt performance Stille presented a closed form elasto-plastic analytical solutions of grouted bolts. Based on the shear lag model (SLM), Cai et al. derived an analytical solution of rock bolts for describing the interaction behaviours of rock bolt, grout material and rock mass. Brady and Lorig. numerically analyzed the interactions of bolt grout in Mohr Coulomb media using the finite difference method (FDM) technique. In addition, numerous studies have been published on the analytical solution of stresses and displacements around a circular tunnel considering elasto-plastic rock mass with Mohr-Coulomb yield criterion. Elwi and Hrudey and d?Avila et al., proposed embedded finite element method for reinforcing curved layers and concrete structures respectively. Finite Element Method (FEM) and/or FDM based procedures are also developed for the analysis of the said problem and have been presented in many references for solving geotechnical problems.
ABSTRACT Heat extraction from deep, hot and tight rocks for energy production is based on forced water circulation through fractures. This process will cause changes in fracture aperture distribution and may also induce seismicity. To account for these, it is necessary to consider the role of poro-mechanical, thermal and geochemical process on intact rock and fracture deformation. A model is developed in this work for three-dimensional analysis of intersecting natural and hydraulic fractures. The model considers the geometric nonlinearity of the joint deformation in shear and closure. Temporal variations of injection/extraction rate and pressure-dependent leak-off are also considered. The model uses a coupled finite element/boundary element method to solve for stress, fracture displacements as well as fracture and matrix fluid pressure. The displacement discontinuity method is used to model the mechanical response of the fractured media while considering the non-linear joint response. Examples are provided to show the impact of joint interactions on potential for slip, fracture permeability, and the flow path and their dependence on the in-situ stresses conditions and the injection rate. Poroelastic effects on joint deformation are also highlighted. The model shows to be versatile for studying the unconventional reservoir stimulation and understanding of induced micro-seismicity. INTRODUCTION Whilst designing geothermal reservoirs, the impedance factor, water loss rate, and availability of an adequately large heat exchange surface between the rock and the circulating fluid are considered to control the economic viability of heat extraction operation. As the fractures are the major pathway for fluid flow and heat exchange, analysis of their spatial-temporal behavior has been the focus of many investigations, which have shown that coupled poro-mechanical, thermal, and geochemical process have a large influence on fracture permeability evolution. Generally, predicting the impact of the interactions of these processes in natural and man-made fracture requires numerical simulation. Two approaches can be used for this purpose, a statistical fracture network approach in which the reservoir is simulated using a system of fractured rock blocks, and a deterministic fracture modeling approach wherein the major fractures are directly modeled. These fractures often are distinguished by direct imaging of the wellbore and geological/geophysical studies. During past decades, various numerical models in each category have been developed and used in reservoir simulators. For example, Bruel introduced a simplified thermo-poroelastic method for simulating a circulation test in Soultz-sous-Forets geothermal project in Rhine Garben, France, without explicit consideration of flow through the rock. Wessling et al. simulated water injection into a fracture using 2.D-ROCMAS finite element software which has a coupled flow-geomechanic capability. Mathias et al. modeled the problem without coupling between hydrological and mechanical processes and assumed a constant total stress and fracture aperture during injection/extraction cycles. Swenson et al. developed a finite-element model to solved the problem using a 2D finite element method by assuming 1D fluid flow and constant joint stiffness. The displacement discontinuity method has proven to be particularly effective for the class of problems involving a finite number of discrete fractures within the circulation system.
- North America > United States (1.00)
- Europe > France > Grand Est > Bas-Rhin > Soultz-sous-Forรชts (0.25)
Simulation of Rock Fracturing Using Particle Flow Modeling: Phase I - Model Development And Calibration
Ding, Xiaobin (Department of Civil Engineering and Engineering Mechanics, University of Arizona) | Zhang, Lianyang (Department of Civil Engineering and Engineering Mechanics, University of Arizona)
ABSTRACT This paper presents part of the results for the first phase of the research on simulating rock fracturing with particle flow modeling, using the three dimensional Particle Flow Code (PFC3D). The first phase work focuses on development and calibration of the numerical model based on the micro structure and laboratory mechanical test data (unconfined compressive strength, tensile strength and stress-strain curves) of real rocks. The results show that although the unconfined compressive strength can be well simulated using the standard PFC3D model, the tensile strength is over predicted and leads to unconfined compressive strength to tensile strength (UCS/T) ratios significantly lower than the laboratory test value. Two different methods have been used to improve the simulation results and increase the UCS/T ratio. The contact bond release model which simulates the pre-existent micro cracks in rock can double the UCS/T ratio from the standard PFC3D model but still under-predicts the UCS/T ratio. The near sphere clump particle model containing 50% clump particles only slightly improve the simulation results. Other methods and/or a combination of different methods need to be studied in order to develop a PFC3D model which can correctly simulate the mechanical behavior of real rocks. INTRODUCTION
- North America > United States > Arizona (0.30)
- Asia > Middle East > Turkey (0.28)
- Research Report > New Finding (0.36)
- Research Report > Experimental Study (0.34)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (0.70)
- Reservoir Description and Dynamics > Reservoir Characterization > Geologic modeling (0.64)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Naturally-fractured reservoirs (0.47)
ABSTRACT The recent advancements in fracturing technology have generated more interest in pursuing novel techniques such as simultaneous frac jobs, zip-fracs and multi-stage fracturing. The diagnostic data from these treatments also suggests complex, multi-stranded fracture zones as a result of interaction between different competing fracture strands. Explanation and prediction of fracture patterns in these cases are not possible without a deep understanding of the mechanism behind the competition between different fracture tips. A stability analysis for hydraulic fracturing growth and its implementation using extended finite element methods are presented in this paper. Several examples are provided to show the significance of stability analyses in fracturing treatments. The fracture propagation modeling demonstrated using this numerical method could be the key to determining efficient perforation and spacing in hydraulic fracture design analysis, as well as evaluating novel techniques such as sequential or simultaneous fracturing within a single or multiple horizontal wellbores. 1. INTRODUCTION Multistage hydraulic fracturing in long horizontal wells has become a common practice for economic production in extremely low permeability hydrocarbon reservoirs. For each stage of fracturing, several perforations are performed to predetermine/limit fractures, initiation point. The presence of several initiation points for fracture growth leads to the question about how fractures will start to grow in perforated zones and the possibility of their interactions. Additionally, the role of preexisting natural fracture on the formation of multistranded hydraulic fractures is observed and widely accepted [1, 2, 3, 4]. In this paper, it has been tried to propose a frame to determine whether fractures grow simultaneously or only one or a group of them is growing and how neighbor fractures interact with each other during the treatment. Recent serious social concerns regarding the drinking water pollution after fracturing jobs also requires a good understanding and predictive tools to address these concerns and also avoid any environmental catastrophe in future. Problems of interactive fractures arise naturally in many cases, such as thermally induced parallel fractures in brittle solids which are uniformly cooled or heated on their straight edges [5, 6]. In these investigations, it has been observed that due to the fractures interactions, critical states are formed, at which a given fracture growth pattern may become energetically less favorable, thus gives a way to a new regime or set of fracture tips to grow [7]. Multiple fractures problems have also been observed in the geological processes as well. Dike swarm are consisting of hundreds of parallel dikes. Jin and Johnson [8] tried to model simultaneous steady-state propagation of a series of parallel dikes. However, in the case that several fracture tips have reached rock toughness, a stability analysis will determine the fracture configuration path that leads to the maximum decrease in the total potential energy. Following the second law of thermodynamics, we know that [7]: among all admissible variations in the crack lengths which correspond to a given variation in the load parameter, the ones which minimize the total potential energy produce the most stable state, and hence are the actual ones.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Structural Geology > Tectonics > Plate Tectonics (0.48)
- Well Completion > Hydraulic Fracturing > Multistage fracturing (1.00)
- Well Completion > Completion Installation and Operations > Perforating (1.00)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Naturally-fractured reservoirs (1.00)
- Well Drilling > Drilling Operations > Directional drilling (0.88)
ABSTRACT This paper deals with a time stepping method for the finite element simulation of two phase flow hydraulic and mechanical (H2M) coupled processes in porous media, which is a common phenomenon in geological applications such as CO2 storage facilities. The computation task arising from the numerical modeling of H2M coupled processes in such real geological application is intensive. Therefore, the high performance computing is of interest to the corresponding researchers. In the present study, we present a time stepping method with PI (proportional and integral feedback) automatic control to improve the computation efficiency of the modeling of H2M coupled processes. We apply the PI control to solve the nonlinear coupled partial differential equations with a first order finite difference scheme for time discretization and the Picard method for linearization. The efficiency of the present method is demonstrated by applying it to a CO storage benchmark. 1. INTRODUCTION To reduce anthropogenic greenhouse gas emissions into the atmosphere, the carbon dioxide capture and storage (CCS) concept is introduced by some researchers as an emerging transition technology [1,2]. The study of CCS is therefore under active consideration recently. According to various studies, deep saline aquifers provide the most substantial carbon dioxide storage capacity [3,4,5,6,7], and are often located near possible CO2 sources such as coal-fired power plants. To ascertain migration and trapping of CO2 in the formations and assess the capacity and the safety (possible leakage) of the reservoir, the numerical simulation of injection and spreading of carbon dioxide in the underground is essential for understanding the physical and chemical processes at different length and time scales. In the numerical analysis of time dependent thermo-hydraulic processes in porous media, the time stepping is a crucial issue for numerical stability and computational efficiency. Practically, the fixed time step size does not often satisfy the stability and efficiency requirements in solving problems that exhibit complexity in geometry and nonlinearity in material properties. Therefore, adaptive time stepping methods including high-order integration have been developed and are widely applied [8]. Among the available adaptive time stepping methods, the well-known techniques for prediction of the time step size h are e.g. Courant number approach based on Courant-Friedrichs- Lewy condition [9] for the finite difference method, primary variable based prediction (e.g. those presented in ref. [10,11] and local error control methods (cf.[8,12,13]). The local error control methods especially those based on theoretical control ideas are problem independent for any numerical methods for ODEs [8,12,13,14]. For nonlinear equations, the theory based automatic controls such as P (proportional feedback) or PI (proportional and integral feedback) permit stable and efficient time stepping [8,14] for numerical solver. The present work is subjected to apply the adaptive time stepping with automatic control for the finite element modeling of two phase flow hydraulic and mechanical coupled processes in CO2 storage facilities. To this purpose, we present an approach of PI (proportional and integral feedback) [18] automatic time stepping for modeling the problems with different coupled physical processes. Within the context of the presented time stepping approach, each process uses the time step size predicted by the PI control itself to guarantee the stability of the simulation of each process under coupling.
- Europe (1.00)
- North America > United States > California (0.28)
Pick Profile Effect On Linear Cutting of Sandstone
Yao, Q.Y. (School of Mechanical and Manufacturing Engineering, the University of New South Wales) | Mostafavi, S.S. (School of Mechanical and Manufacturing Engineering, the University of New South Wales) | Bao, R.B. (School of Mechanical and Manufacturing Engineering, the University of New South Wales) | Zhang, L.C. (School of Mechanical and Manufacturing Engineering, the University of New South Wales) | Lunn, J. (Bradken) | Melmeth, C. (Bradken)
ABSTRACT Linear cutting of Helidon sandstone was conducted to investigate the effect of pick geometries and pick orientations on pick performance, using a conical pick (C1) and a pyramid pick with different orientations (P1: a pyramidal surface in parallel with the rock surface to cut; and P3: a pyramidal surface in 45 degrees with the rock surface to cut). It was found that the pyramid pick P1 can produce the largest fragments and consumes the least specific energy. With increasing the depth of cut, the specific energy decreases for all picks. It was also found that in all the cases there is a critical speed of cut at which the specific energy is minimal. The study concluded that pick profile does affect the pick cutting performance and that P1 is the most desirable orientation. 1. INTRODUCTION Pointed picks have been widely used in the mechanical excavation of rocks, where the performance of a pick is often assessed by the specific energy consumed. It was reported that the coarseness of the chips is descendingly related to the specific energy [1]. Investigations based on linear rock cutting tests revealed that the texture coefficient and feldspar content of sandstones influenced the rock cuttability and the assessment by specific energy is highly reliable [2]. It has been demonstrated [3] that when using conical picks, changing the included angle of the pick could improve the cutting efficiency or cutting performance in terms of pick life, dust generation and energy consumptions. A comparable experimental study with sharp and blunt picks [4] showed that dust generation could be related to specific energy. However, the previous studies have been on the cutting conditions using conical picks with different included angles [3]. The aim of this paper is to investigate the geometry and orientation effect involving both pyramidal and conical picks [5-6]. 2. EXPERIMENT SETUP Fig.1 shows the experimental setup of the linear cutting tests. A pick was held static, while the rock sample fixed on the machine table approached the pick horizontally to realize the cutting. The cutting forces were measured by the dynamometers attached to the pick holder. Two types of cutting picks were used, which are conical (C1) and pyramid (P1 and P3). Picks P1 and P3 have the same geometry and dimensions but their orientations relative to the rock sample surface were different, as illustrated in Fig.2, where P1 had a pyramidal surface in parallel with the rock surface to cut (see section B-B) and P3 had its pyramidal surfaces in 45 degrees with the rock surface to cut (see section C-C). The rock sample for the tests was Helidon sandstone from a local quarry in Brisbane Australia whose dimensions were Length ร Width ร Height = 1700mm ร 450mm ร 450mm. Some properties of the sandstone are listed in Table 1 below. 3. RESULTS AND DISCUSSION Effect of pick profile on cutting mechanisms and chip formation. Fig.3 schematically illustrates the cutting process, where ? stands for the half included angle of a pick, a is the angle of attack, ร is the clearance angle and h is the depth of cut. ,
Deconvolving CO2-Enhanced Coalbed Methane Processes In Subbituminous Coals
Kumar, Hemant (Energy and Mineral Engineering, EMS Energy Institute and G3 Center, Pennsylvania State University) | Elsworth, Derek (Energy and Mineral Engineering, EMS Energy Institute and G3 Center, Pennsylvania State University) | Mathews, Jonathan P. (Energy and Mineral Engineering, EMS Energy Institute and G3 Center, Pennsylvania State University) | Liu, Jishan (School of Mechanical Engineering, University of Western Australia) | Pone, Denis (ConocoPhillips)
ABSTRACT The evolution of permeability in CO2-enhanced coalbed methane (ECBM) recovery involves dynamic changes in coal shrinkage/swelling with the reduction/increase in gas sorption/desorption. Injection of CO2 changes local pore pressures and induces related matrix volume strains, modulated in part by the mechanical boundary conditions; changes in gas saturation and pressure induce changes in permeability. Typically the recovery of methane induces shrinkage and the injection of CO2 induces swelling โ concomitantly permeability decreases where net swelling results and increases where net shrinkage is present. However, permeabilities are also impacted by other important physical phenomena, including water saturations and the sequence of sweeps by different gases used in ECBM. These may be N2 or CO2. To address these issues we report experimental measurements of permeability evolution in subbituminous coal cores from the San Juan basin infiltrated by He, CH4 and CO2 under varying pore pressure at constant applied stresses. Experiments are completed with variable water saturations, gas saturations and effective stresses as key parameters modulating permeability evolution. For a subbituminous coal the presence of moisture reduces the sorption capacity, swells the coal volume and offers resistance in flow of other fluids (capillary forces) hence lowering permeability magnitudes. Reduction in permeability with sorption of CH4 and CO2 is lower in wet samples than dry. However, absolute values of permeability for dry samples are higher than the moist (9% moisture content). Swelling induced by sorption of CH4 and CO2 in the coal matrix likely causes aperture reduction including cleat closure. Experimental observations indicate that the magnitude of swelling increases with pore pressure. Preliminary test results demonstrate that the permeability of subbituminous coal (San Juan basin) does not show a significant change with pore pressure for a non-sorbing gas (He), implying the relative stiffness of these coals. 1. INTRODUCTION Active coalbed methane projects in the San Juan basin exploit the high methane content of the deep and unmineable (2500-5000 feet deep) bituminous to subbituminous coals and cleat-aided permeability. It has been the top producer of coalbed methane in the US and is a favorable site for geo-sequestration of CO2 with concurrent production of natural gas [1-2]. Injection of CO2 in unmineable coal seams provide โvalue-addedโ sequestration with multiple benefits such as enhanced coalbed methane recovery with lower net-cost [3]. The CO2 and CH4 sorption capacity in some dry low-rank coals has been determined to be as high as 10:1 [4]. Coalbeds are commonly self-sourcing and low permeability (on the order of fractions of milliDarcies (10-16 m2) gas reservoirs. Coalbed methane is being recovered by means of reservoir pressure depletion [5]. Various studies on laboratory and pilot plant scales suggest that geomechanical processes involved during the ECBM recovery process affect the evolution of permeability and hence production [6], [7]. Coal shrinkage and swelling with gas desorption/adsorption has important influence on the evolution of permeability [8-11]. Coal swells with adsorption of CO2 and develops compactive stresses if mechanically constrained [12-15]. Coal swelling has been implicated in observed reductions in permeabilities during ECBM operations at pilot plant scale [16-18].
- Geology > Rock Type > Sedimentary Rock > Organic-Rich Rock > Coal (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Oceania > Australia > Queensland > Central Highlands > Bowen Basin (0.99)
- North America > United States > Wyoming > Powder River Basin (0.99)
- North America > United States > New Mexico > San Juan Basin (0.99)
- (4 more...)
Permeability Evolution In Fractured Anthracite: the Roles of Crack Geometry And Water-content
Wang, Shugang (Department of Energy and Mineral Engineering and G3 Center, Pennsylvania State University) | Elsworth, Derek (Department of Energy and Mineral Engineering and G3 Center, Pennsylvania State University) | Liu, Jishan (School of Mechanical Engineering, The University of Western Australia)
ABSTRACT We report laboratory experiments that examine the roles of crack geometry and water-content on the evolution of permeability in fractured coals. Experiments are conducted on 2.5 cm diameter, 5 cm long cylindrical anthracite samples from Pennsylvania. To explore the permeability evolution due to coal swelling, we conduct experiments on a sample containing multiple embedded cracks and a fully cracked sample, under both dried and water saturated conditions. Under constant total stress and with pore pressure increases, we find the presence of low gas pressure permeability reduction for the intact sample under dried condition, and the absence of permeability reduction for the split sample under dried condition. This observation is congruent with the need for connected bridges to be present within the split sample to cause the observed swelling-induced reduction in permeability. Under water saturated condition, the initial permeabilities for all gases are nearly two orders lower compared with those under dried condition, and all permeabilities increase with increasing pore pressure for both samples. Results suggest that the presence of water in the samples prevents coals from swelling. We also find the sorption capacities and swelling strains are significantly small for water saturated samples. 1. INTRODUCTION Gas flow and transport in coal seams is significantly different from that of other rock types because of the phenomena of gas sorption and coal swelling. The relative roles of stress level, gas pressure, gas composition, fracture geometry of coal, and water-content are intimately connected to the processes of gas sorption, diffusion, transport, and coal swelling. Experimental effort has been applied to investigate gas permeability and its evolution in coals. Permeabilities of coal to sorbing gases such as CH4 and CO2 are known to be lower than permeabilities to nonsorbing or lightly sorbing gases such as argon and nitrogen [1-4]. Permeabilities may decrease by as much as five orders of magnitude for confining pressures increasing from 0.1 to 70 MPa [1, 5]. Under constant total stress, sorbing gas permeability decreases with increasing pore pressure due to coal swelling [3, 4, 6-8], and increases with decreasing pore pressure due to matrix shrinkage [9-12]. Rebound pressure, which corresponds to the minimum permeability, has been observed for CO2 injection at 1.7 MPa [13], and at 7 MPa [14, 15]. Pemeability is also influenced by both the presence of water and the magnitude of water saturation [16]. The sorption capacities of coal to CH4 and CO2 have been explored using a variety of measurement methods. Experiments have shown that CO2 is adsorbed preferentially relative to CH4 in most instances, and the ratios of the sorption capacities (in molar units) are between 1.15 and 3.16 [4, 17- 24]. The presence of water reduces the sorption capacity to gases by around 30% [4, 25-27], due to the competition between water molecules and the sorbing gas for sorption sites on the coal surface [28, 29]. Previous studies have shown that coal swells when exposed to CH4, and CO2, with volumetric strain ranging from 0.1% to 15%, under pressures up to 20 MPa and temperatures up to 55C [3,4, 6, 7, 9, 10, 30-34].
- North America > United States > Pennsylvania (0.34)
- North America > United States > Alabama (0.31)
- Research Report > New Finding (0.48)
- Research Report > Experimental Study (0.34)
- Geology > Rock Type > Sedimentary Rock > Organic-Rich Rock > Coal (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- North America > United States > Alabama > Black Warrior Basin (0.99)
- Europe > United Kingdom > Northumberland Basin (0.99)
- Asia > China > Qinshui Basin (0.99)