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Results
Dilation of Shales under Triaxial Compression
Ding, Xiang (China University of Petroleum-Beijing) | Wang, Zhiyin (China University of Petroleum-Beijing) | Zhang, Guangqing (China University of Petroleum-Beijing) | Li, Shiyuan (China University of Petroleum-Beijing) | Lin, Qing (China University of Petroleum-Beijing)
Abstract: Though dilation of shales plays an important role in petroleum engineering, the research of the dilatancy of shales is currently rare. As the confining pressures and the initial porosities of rocks control the dilatancy, a conceptual partition of different dilation zones is proposed. In order to obtain the critical confining pressure of shales at given porosity, a group of shale specimens are subjected to compression tests in the lab. Combining with the dilatancy index, a power function is used to fit the relationship between dilatancy indices calculated by experimental results and confining pressures. The dilatancy indices of shales are found to be decreased with increasing confining pressures as expected. Then a critical confining pressure beyond which the shale specimens would stop dilation is predicted by the power function. And finally some different opinions about the dilation process of rocks are also discussed. So as to get the critical curves in confining pressure verse porosity chart which is crucial in dilation prediction of shales, some more shale specimens should be carried to compression tests in the future. Introduction Dilatancy may be described as the change in volume associated with shear distortion of an element in the material (Vermeer and Borst, 1984). The dilation angle was first introduced to characterize the dilatant property of a granular material by Bent Hansen in 1958. The dilatancy affects the loading-carrying capacity and the spread of the plastic zones (Cox et al., 1961; Zaadnoordijk, 1983; Vermeer and Borst. 1984). Dilatancy is a crucial factor in the stability design at shallow depths, where confinement pressure is low (Reynolds, 1985). The importance of dilatancy on earthquakes is also studied (Frank, 1965; Scholz, 1968; Zoback et al., 1975; Nur, 1975). The dilatancy influences the hydraulic behavior in rocks (Zoback and Byerlee, 1975; Zhang et al., 1994; Peach and Spiers, 1996; Zhu and Wong, 1999; Heiland and Raab, 2001; Simpson et al., 2001; Tang et al., 2002) which may be used in hydraulic fracturing in petroleum industry. Islam mentions the potential applications of dilatancy in numerical modeling of borehole stability (Islam and Skalle, 2010), sand production analysis, reservoir compaction, stress arching, network fracturing, and underground storage of CO2 (Islam et al., 2013).
- Europe (0.94)
- North America > United States > Texas (0.46)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
Permeability Evolution and Proppant Compaction in Artificial Fractures on Green River Shale
Li, Xiang (EMS Energy Institute and G3 Center, The Pennsylvania State University) | Feng, Zijun (EMS Energy Institute and G3 Center, The Pennsylvania State University, Taiyuan University of Technology) | Han, Gang (Aramco Services Company) | Elsworth, Derek (EMS Energy Institute and G3 Center, The Pennsylvania State University) | Marone, Chris (EMS Energy Institute and G3 Center, The Pennsylvania State University) | Saffer, Demian (EMS Energy Institute and G3 Center, The Pennsylvania State University) | Cheon, Dae-Sung (Geologic Environment Division, KIGAM)
Abstract: Through laboratory experiments with artificial propped fractures in Green River Shale, this paper compares the evolution of permeability for native CH4 and with those of sorbing CO2, slightly sorbing N2 and non-sorbing He, as a function of pore pressure. The findings from these experiments help to understand proppant embedment and fracture diagenesis in shales. Experiments were conducted on 1inch diameter, 2inch long split cylindrical samples sandwiched with proppant at a constant confining stress of 20 MPa and with varied pore pressure โ increases in pore pressure represent concomitant decreases in effective stress. Permeability and sorption characteristics are measured by pulse transient methods. To explore the effect of swelling and embedment on fracture surface geometry, we measure the evolution of transport characteristics for different proppant geometries (single layer vs. multi-layer), gas saturation, and sample variance. In order to simulate both production and enhanced gas recovery processes, both injection and depletion cases are investigated. For both strongly- (CO2, CH4) and slightly-adsorptive gases (N2) the permeability first decreases when gas pressure increases because of swelling. It then increases beyond the Langmuir threshold due to the over-riding influence of effective stresses. Due to its highest adsorptive affinity, CO2 returns the lowest permeability among these three gas permeants. Compared to the case of a mono-layer propped sample, the sample with four layers exhibits less swelling as implied by its elevated k/k0 ratio. Interestingly the duration of gas exposure and saturation tested here which is up to ~20hrs does not have a significant influence on permeability for either adsorptive or non-adsorptive gases. Permeabilities recovered from both injection and depletion cycles generally overlap each other and are repeatable with little hysteresis. This suggests the dominant role of reversible swelling over irreversible embedment. Permeability variance between different samples is of the order of ~1.5 - 2 times but with repeatable trends and order of magnitude parity. Gas permeant composition and related swelling effects exert important influences on the permeability evolution of shales under nominally in situ conditions.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
Abstract: A geomechanical analysis of underground coal gasification (UCG) requires integrated modeling in complex multidisciplinary areas of coupled fluid flow, rock and cavity mechanics. In this study, the results from an existing thermal and multi-phase fluid flow simulator are imported into a geomechanical module to solve for the vertical displacement and the stress variation around a propagating cavity induced by a coal gasification process. A Controlled Retracting Injection Point (CRIP) well configuration is applied in the thermal reservoir simulator to model chemical reactions and geochemistry. The high temperature nature of the UCG process as well as creation of void space within the rock mass continuum require the use of efficient rock constitutive models. Three different constitutive models are investigated in this study: linear elastic, hyperbolic, and elasto-plastic. We have then applied these constitutive models on a case study to compare the results. The displacement of a coal layer and the surrounding blocks as well as stress arching in regions away from the cavity can efficiently be captured by the use of all the above constitutive models; however, a stress analysis around the induced cavity necessitates implementation of a constitutive model which can efficiently capture the shear softening after the rock failure. Introduction Coal accounts for almost 30% of global primary energy consumption, which makes it the second largest primary energy source in the world after oil. Coal is also the largest provider of electricity. Over 40% of global power production derives from coal [1]. Coal's significant position in the global energy mix is mainly because it is abundant, low-cost, and the most wide-spread fossil fuel in the world [2]. Despite a decline in the coal demand growth in 2014, International Energy Agency's forecast shows the growth of almost 1% per year through 2020 [1]. Moreover, based on the 2013 survey of World Energy Council, coal consumption is forecast to increase over 50% to 2030. This rise will be mainly due to the escalating electricity rates in the developing countries [2]. Therefore, more coal extraction is necessary to meet the global future energy demand.
- Asia (0.68)
- North America > United States > Texas (0.46)
- North America > Canada > Alberta (0.29)
- Geology > Rock Type > Sedimentary Rock > Organic-Rich Rock > Coal (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
Abstract: Violent gas outbursts are one of the most severe hazards in underground mining. When outbursts occur, a large amount of coal and gas is suddenly and violently ejected into the roadway and working area with the possibility of serious hazard and injury. Recent studies have shown that the physical behavior responsible for the energetic failure of coal is entirely consistent with coal viewed as a dual porosity-dual permeability-dual stiffness continuum where strength is proportional to effective stresses, and where effective stresses are controlled by both the pore pressure and varying stress field. Gas desorption driven by overstress is highlighted in this study as the key factor responsible for the increase in pore pressure close to the working face, and implicated together with elevated stress level, permeability evolution and drainage conditions in the triggering of outbursts. In this work, we incorporate the likely mass rates of desorption driven by an increase in abutment stress and mediated by permeability evolution to define the rates and distributions of gas pressure changes. The changing pattern of pressure redistribution is identified, and parametric studies are then performed to investigate all the key factors that influence the redistribution of pore pressure with respect to the deformation of the coal seam. Permeability evolution in the overstressed zone is determined by the evolution of porosity, which is attributed to both the change in effective stresses in the abutment and sorption-induced strain. This model is capable of predicting the potential risk ahead of the working face during mining and can be adapted to different conditions in terms of varying mechanical factors, coal properties and mining methods. Introduction Gas outbursts in coal mining are defined as the instantaneous and violent ejection of brittle coal, rock and a massive volume of gas, potentially damaging mining machinery, underground support and causing physical injury to personnel. It has been recognized as one of the most severe hazards in underground mining since its early reporting in the 1850s. More than 30,000 outbursts have occurred historically, of which reportedly the most disastrous accident caused 187 deaths in the Piast area of Poland in the Nowa Ruda Colliery, in 1941 (Lama and Bodziony, 1998). Nineteen outbursts driven by both of methane and carbon dioxide have been recorded in the Collinsville area, Australia since the first case killing seven men in 1954 (Harvey, 2002). Although the frequency of occurrence of outbursts has reduced in recent years with the development of technology and improved mining methods, outbursts remain a dangerous phenomenon in the global underground mining industry.
- Geology > Rock Type > Sedimentary Rock > Organic-Rich Rock > Coal (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
Abstract: Based on the assumption of Vertical Transverse Isotropy (VTI), we investigated the ultrasonic velocity and anisotropy of organic-rich chalks (ORC). These chalks, from the late Cretaceous Ghareb and Mishash formations in Israel, have a wide range of porosity (0 โ 45 PU), high kerogen content (5 โ 15 wt% TOC) and include sections with varying degrees of thermal maturation. Cores were extracted both from the Shefela basin of central Israel and two wells in the southern Golan Heights. We measured ultrasonic velocities of P and S waves in different directions using the ultrasonic transmission method, whilst adjusting effective (confining) pressure levels. The elastic constants were then calculated from the measured velocities and densities, as well as Thomsen anisotropy parameters. Compared to previous studies of organic-rich shale source rocks, which exhibit strong acoustic anisotropy, these organic-rich chalks display weak, yet still measureable transverse anisotropy. Our results present significant differences between the immature and early-maturation stage samples. In our samples, it had become evident that microcracks are present during the early-maturation stage, yet are absent throughout the immature stage. An additional distinction between the maturation stages reveals itself while examining the elastic constants, showing a much stiffer nature in the early-mature sample, which could also be attributed to compaction and lower porosity. Introduction Accelerating interest to develop unconventional oil and gas fields in the late Cretaceous Ghareb and Mishash formations in Israel motivates our laboratory work to measure and describe the seismic anisotropy of these rocks. These Late Cretaceous formations consist mainly of fine-grained carbonates with high concentrations of organic matter. The lithology of these rocks is mostly chalk, with occurrences of cherts, marls, porcelanites and phosphorites. These target layers show sedimentary layering and complex network of organic matter and minerals. Evidence of mechanical and petrophysical anisotropy had been recognized in previous studies of immature ORC from the Shefela basin [1], [2]. As far as we know, however, it is the first time that acoustic anisotropy of these formations is being measured. It is generally believed that at the field scale seismic anisotropy is influenced by three major factors: (1) interlayering of lithologies, or laminae, of contrasting elastic properties on scale much finer than the propagating seismic wavelength (2) preferred orientation of minerals and (3) stress-induced fractures and microcracks that show preferred alignment [3].
- Asia > Middle East > Israel (0.76)
- North America > United States (0.69)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.35)
- Geology > Geological Subdiscipline > Economic Geology > Petroleum Geology (0.34)
Mechanical Behavior and Microstructure Development in Consolidation of Nominally Dry Granular Salt
Ding, J. (Texas A&M University) | Chester, F. M. (Texas A&M University) | Chester, J. S. (Texas A&M University) | Zhu, C. (Georgia Institute of Technology) | Arson, C. (Georgia Institute of Technology)
Abstract: Uniaxial consolidation of granular salt is carried out to study the mechanical behavior and fabric development in a material that deforms by microscopic brittle and intracrystalline-plastic processes. Dry granular salt is sieved to produce well-sorted size fractions. The granular salt is consolidated in a heated cell at axial stresses up to 90 MPa and temperatures of 100 - 200 ยฐC to document stress-consolidation relationships and microstructural development. Polished and chemically-etched petrographic sections of salt samples prior to and after deformation at 150 ยฐC are studied using transmitted- and reflected-light optical microscopy. We show that temperature has profound effect on porosity reduction during consolidation. At tested conditions, the dominant deformation mechanism is crystal plasticity; brittle deformation is largely suppressed. Samples consolidated at higher maximum axial stress develop higher overall dislocation densities. The distribution of dislocations, however, is strongly heterogeneous from grain to grain because of the complex grain-scale loading geometries and the distribution of intragranular flaws such as fluid inclusions. Static recrystallization occurs in some highly strained areas, but overall is minor at 150 ยฐC. The experiments help to improve our understanding of consolidation, and serve to guide the fabrication of synthetic rock salt as experimental material, as well as to inform and test constitutive models of deformation of granular salt for engineering needs. Introduction The rheological properties of rock salt have been an important research focus because it is considered aviable geomaterial for engineered repositories of waste and energy resources, such as radioactive waste, oil, and gas (Urai et al., 1986; Carter et al., 1993). Rock salt also is studied as an analog to other geomaterials in that various deformation mechanisms, including cracking, frictional sliding, pressure solution, crystal plasticity, and dynamic recrystallization are easily activated at laboratory conditions (Zhang et al., 2007). Extensive experimental work has been done on rock salt to determine constitutive relationships and guide numerical modeling (e.g., Watanabe and Peach, 2002; Ter Heege, et al., 2005; Zhu and Arson, 2015). Both natural and synthetic rock salt have been used for experimental investigations; artificially prepared rock salt is of higher purity and can be fabricated in a way that best serves parametric studies (e.g., Carter and Hansen, 1983; Schenk and Urai, 2004; Bourcier et al., 2013). Consolidation of granular salt is a common method used to produce synthetic rock salt samples; however, salt is highly sensitive to moisture so it is critical to have a good control of the humidity of the environment in which nominally-dry salt samples are produced, stored, and processed. Maintaining a consistently dry environment during all stages of handling salt samples can be challenging.
Abstract: This paper is devoted to micromechanical modeling of time dependent behavior of claystones by means of a two-steps homogenization procedure. Two material scales are considered; at the mesoscopic scale, the material is described as a clay matrix reinforced by linear elastic inclusions. At the microscopic scale, the clay matrix is supposed as a porous medium composed of a solid phase which is described by the classical Drucker-Prager criterion and spherical pores. The macroscopic plastic criterion is determined by the modified secant method (Shen et al., 2013); it takes into account simultaneously the effect of pores and mineral inclusions. This criterion is then used as the yield function for the description of the viscoplastic deformation inside the clay matrix. The micro-macro model is proposed to study the elastoviscoplastic behavior of Callovo-Oxfordian argillites. Both the instantaneous and delayed behavior are considered within the unique model. Comparisons between numerical simulations and experimental data show that the proposed model is able to reproduce the main features of the mechanical behavior of this material. Introduction The study of short and long-term behavior of claystones is of significant interest in many engineering applications such as the geological disposal of radioactive waste, the exploitation of shale gas and acid gas sequestration. In the context of the storage of radioactive waste, the purpose to study the time-dependent behavior is to understand and ultimately be able to model what happens in time around the coating of a gallery under the effect of creep of rock. Usually, the time-dependent inelastic behavior of materials is described by the viscoplastic theory (Simo and Hughes, 1998) where the time-dependent deformation is attributed to the inherent viscous effect of material. This approach provide a mathematical description of the time-dependent behavior. For geomaterials, the overstress concept of (Perzyna, 1963) is used, in most viscoplastic models, for the definition of viscoplastic deformation such in (Huang et al., 2014). Recently, based on extensive laboratory investigations, the time-dependent deformation is associated to the progressive evolution of microstructure i.e., the time-dependent deformation is considered as the macroscopic consequence of progressive degradation of material in microscopic scale (Shao et al., 2003) and (Pietruszczak et al., 2004). For example, the physical mechanisms of this phenomena can be: subcritical propagation of microcracks in hard rocks, pore collapse in chalk rocks (Xie, 2005) or dissolution process due to chemical-mechanical coupling (Lydzba et al, 2007). In our study, the time-dependent deformation is attributed to viscous effect inside the clay matrix based on the fundamental theory of viscoplasticity (Perzyna, 1963).
- Europe > France (0.47)
- North America > United States > Texas (0.28)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock (1.00)
- Geology > Mineral > Silicate (1.00)
Abstract: Presented herein are laboratory gas migration experiments conducted on samples of tuff with varying lithologies mounted within a triaxial core holder. A pressurized gas mixture standard comprised of known concentrations of argon (Ar), xenon (Xe), nitrogen (N2) and sulfur hexafluoride (SF6 used as a tracer) was used based on previous field gas migration studies. The gas mix is applied at known pressure to the upstream side of the samples to induce flow through the pore spaces and/or across fracture surfaces and the gases are detected in real-time on the downstream side using a quadrupole mass spectrometer (QMS). Downstream detection under vacuum is possible by precise metering of the gas mixture through a leak valve with active feedback control. Arrival times and time-variant concentrations of the applied gases downstream are collected for comparison between samples. We intend to determine transport properties of noble gases and SF6, and hypothesize that transport properties vary due to solubility and water content. The parameters derived from this work will provide valuable insight into the three-dimensional structure of damage zones, including fracture networks, the production of temporally variable signatures, and the methods to best detect underground nuclear explosion signatures. Introduction The detection, location, and identification of suspected nuclear explosions are United States security priorities. Currently, global nuclear explosion monitoring relies on networks of sensors to provide rapid detection, location, and discrimination of suspect events. In some cases, such as underground nuclear explosions (UNEs), prompt signals may not provide sufficiently exact locations or explosion details and the timing and amplitude of non-prompt signals may be difficult to predict algorithmically. Therefore, there is a need to conduct research into improving the post-facto identification of suspect events with emphasis on non-prompt signals. Non-prompt signal analysis can lead to an improved understanding of the detected source to aid in data-informed discrimination (e.g., explosion vs. earthquake or nuclear explosion vs. reactor). The Underground Nuclear Explosion Signatures Experiment (UNESE) is a nonproliferation effort created to address science and research and development aspects associated with nuclear explosion verification and source analysis based on non-prompt signals. Scientific knowledge and capabilities developed via UNESE will enhance U.S. capabilities in detecting, locating, and identifying UNEs, with particular focus on non-prompt signals. UNESE R&D results will be applicable in current and future verification and nonproliferation contexts, and as a detection-based deterrent factor for potential proliferant nations or in transparency efforts.
- Geology > Geological Subdiscipline (1.00)
- Geology > Rock Type (0.68)
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (0.54)
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.47)
Abstract: In Western China, mining area has the characteristics of thin bedrock and shallow-buried. The mine excavation and deep mineral production will change the stability of initial rock mass structure and the in-situ stress field. The rock fractures are fully developed and even result in a completely broken state of rock. To study the volume effect on the water-sand seepage characteristics in broken rocks, the seepage experiments were performed by a new seepage system of water-sand mixture, which was designed based on the CMT5305 electronic universal testing machine. The results showed that when the initial porosity of broken rock kept constant, the sand filterability of broken rock increased with the thickness of rock sample; the influence of the thickness of rock sample on the quantity of filtering sand increased with the initial porosity of broken rock; the filterability of broken rock increased to a constant value with the rock sample thickness; the influence of rock sample thickness on the water seepage was not obvious, and the initial porosity had a greater effect on the water-sandstone seepage characteristics than the rock sample thickness. Introduction With the development of deep coal mining, more complicated mechanics problems appeared. The original structure and stress state of rock mass are changed by the process of mine excavation and deep mine mineral production [1]. It causes the rock mass fully fractured and even completely broken. The study on surrounding rock stability, disaster prevention and other scientific problems are more difficult [2]. The scholars of civil, oil, mining and other industries pay attention to the rock seepage characteristics and a series of studies on the seepage characteristics of fractured rock and fluid-solid coupling have been reported [3]. The water isolating properties of fragment rock mass depend on the seepage stability of water and sediment. Compared to the intact rock, fractured rock has the characteristics of higher porosity and permeability, and easily causes rock engineering disasters. In coal mining engineering, water inrush and flooding accidents usually occurred in water cut zones [3-5]. For example, during the water inrush disaster of Zhao Zhuang coal mine in June 2, 1984, the maximum water inrush amount is 2053m3/min [6]. The seepage characteristic of fractured rock is an important factor for the safety of coal mining. Scholars investigated the seepage characteristics from the aspects of particle diameter, porosity and stress distribution [7, 8]. Li Shuncai etc. [9, 10] performed the experimental research on the seepage characteristics of saturated fractured sandstone with time. Wang Luzhen etc. [11, 12] studied the effect of loading process on permeability of coal particle with size distribution.
- Asia > China (0.50)
- North America > United States > Texas (0.28)
Multidisciplinary Interpretation of a Tight Gas Reservoir to Understand Its Production Behavior, Northwestern Africa. A Change of an Old Paradigm Model.
Stockhausen, Harald W. (Repsol Exploration geohazard team) | Cavero, Jose A. (Repsol G&G Technical Quality Direction) | Corallo, Francesco (Repsol Exploration Algeria-Algeria)
Abstract: This paper presents a multidisciplinary study where geology, sedimentology, geomechanics, geophysics, petrophysics and production disciplines allowed understand the production behavior of a tight gas field interpreted originally as natural fractured reservoir. The goal was find areas with good remaining gas potential; by understanding the main factors (natural fractures, rock facies, diagenesis, porosity, rock mechanics, etc.) that controls the field production behavior. To accomplish this objective, workflow was built, starting by describing jointly selected cores with good set of logs and laboratory analysis, including updated concepts, new ideas and innovative vision. The cumulative production map showed a tendency aligned with the regional deposition trend, and not with the fault/fractures alignments, as original interpreted. The conventional cores and well log images interpretation, showed a low fracture density (less than 0.6 fractures/m), that supported a new view, where the glacial facies channels and its corresponding progradation deltas, played an important role controlling the best distribution of reservoir properties making sense of well production, fluid distribution, regardless hydraulic fracture behavior and presence of faults. Having studied this reservoir with new concepts, honoring well data and including the synergy of different approach and ideas, change the previous paradigm, allowing a better understanding of its behavior. Introduction Understanding the geological and fluid factors that controls the production of a reservoir, is the key to successfully develop and produce the hydrocarbon reserves from a field in an efficient and economical manner. For complex reservoirs (naturally fracture reservoirs, high pressure, structurally complex reservoirs or reservoirs that require hydraulic fracture stimulation), the interaction of the different parameters (geologically and fluids) and the lack of data, make them more difficult to understand and characterized them, leading to a geological and dynamical model that is not accurate and with a lot of assumptions, that some of them could be wrong. The last one is the case that we want to present in this paper, a gas reservoir originally interpreted as a naturally fractured reservoir, with a low permeability matrix, which required hydraulic fracture stimulation to accomplish an economical breakeven.
- Africa (0.87)
- North America > United States > Texas (0.29)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Sedimentary Geology > Depositional Environment > Continental Environment (0.37)