Existence Of Microfractures Research Articles

Fracture and porosity evolution of coal under uniaxial compression: An in-situ X-ray tomography study

To investigate the meso-structure evolution and fracture mechanism of coal under uniaxial compression, an X-ray three-dimensional scanning microscopy in-situ loading test bench was used to conduct a uniaxial compression test of coal. CT data were obtained under different strain conditions and combined using 3D visualization software. According to the surface porosity under different axial forces shows stages in the Z axis, the 3D reconstruction of coal was divided into three regions to compare its structural evolution law. The results show that the uniaxial compression of coal presents staged fractures, and the distribution characteristics of fractures and pores are closely related to internal stress propagation. With the axial force loading, the internal fractures continued to expand, accompanied by the generation of micro-fractures. The development degree of penetrating fracture and lateral strain in region 1 were always higher than in region 2 and region 3, which is more obvious after the scan D; During the whole process of uniaxial compression, the three regions interacted with each other, and the energy transferred between regions has a “hysteresis”. The failure response of region 1 to axial force was earlier than that in region 2, and region 2 was earlier than region 3. However, the fracture evolution behavior of the same region, the three regions showed consistency; After scan D, the meso-structure changes of region 1 were more significant than the other two regions; The coal was locally deformed and destroyed under the action of axial force, resulting in isolated pores and micro-fractures. The spatial distribution of pores and fractures, the existence of micro-fractures and the interaction among different regions aggravated the uneven distribution of axial force in coal, which has a great influence on the fracture process of coal, so that the failure of coal present stages.

EVALUATION OF GEOTECHNICAL PROPERTIES AND PETROGRAPHIC CHARACTERISTICS OF SANDSTONE, BAGH AREA, AZAD JAMMU AND KASHMIR, PAKISTAN

Sandstones are abundant in the regions of Azad Jammu and Kashmir in northeastern Pakistan. However, physio-mechanical data for project planning and construction of these sandstones, which serve as aggregates and foundations for a variety of infrastructures, are lacking. This research aims to investigate the petrographic and physio-mechanical characteristics of Murree and Kamlial Formation sandstones in the Rawalpindi group. The physio-mechanical parameters included specific gravity, water absorption, unconfined compressive strength (UCS), slake durability, Ultrasonic pulse velocity (UPV), and Brazilian. The findings suggest that the sandstone of the Murree and Kamlial formations can be classified as construction-grade rocks. A comprehensive analysis of physico-mechanical observations of these sandstones shows a statistically significant correlation with point load index, Brazilian tensile strength, and UPV in a saturated condition against unconfined compression strength (UCS). Whereas the specific gravity, water absorption, slake durability index, and UPV in dry conditions found no statistically significant correlation against UCS. By rigorous petrographic research, the texture of quartz %, mineralogy and mineral alteration, the existence of micro-fractures, and mineral size distribution all contribute to the total strength qualities of these sandstones. To comprehend the suitability of Muree and Kamlial Formation sandstones, physio-mechanical characteristics were then correlated with the sequence (Murree sandstone) exposed at the Jena Kor village in Peshawar Basin. Based on the results, the sandstones of the Murree and Kamlial Formations in the study area are found appropriate for construction.

Pressure transient and Blasingame production decline analysis of hydraulic fractured well with induced fractures in composite shale gas reservoirs

Vertical wells large scale fracturing and stimulated reservoir volume (SRV) treatments is an important method to enhance productivity of shale gas wells. Since the existence of micro-fractures in shale reservoirs, micro-fractures are opened and connect with hydraulic fracture after vertical wells large scale fracturing. Of course, some micro-fractures do not connect with hydraulic fracture, which leads that average permeability around wellbore is higher than reservoirs permeability. A radial composite model is used to describe the physical phenomenon with SRV region. Therefore, a semi-analytical model of hydraulic fractured well with induced fractures in composite shale gas reservoirs is established and solved by the Laplace transform, pressure drop superposition, vertex rate balance. Wellbore pressure solution of real domain is obtained by the Stehfest numerical inversion. The simplified model is verified by numerical solution and comparison result gets a good match. According to the difference between pressure and pressure derivative curve, wellbore pressure approximate solution of this model during pseudo-steady flowing regime is obtained. According to the wellbore pressure derivative curve and Blasingame production decline integral derivative curve, the seven flow regimes can be distinguished and curve characteristic of every flow regime is analyzed. The influence of hydraulic fracture conductivity, induced fracture number, Langmuir volume, SRV region radius, permeability ration et al. on wellbore pressure response, Blasingame production decline curve is discussed in detail. This work provides a method to calculate single well controlled reserves with short production time by pressure transient and Blasingame production decline analysis for hydraulic fractured wells with induced fracture.

An Experimental Study on Stress Sensitivity of Tight Sandstones with Different Microfractures

Aiming at the stress sensitivity problem of tight reservoirs with different microfractures, the cores of H oilfield and J oilfield with different microfractures were obtained through the fractures experiment, so as to study the change of gas permeability in tight sandstone core plug during the change of confining pressure. Besides, we use the nuclear magnetic resonance (NMR) spectra of the core before and after saturation to verify whether the core has been successfully fractured. Based on Terzaghi’s effective stress principle, the permeability damage rate (D) and the stress sensitivity coefficient (Ss) are used to evaluate the stress sensitivity of the core, which show consistency in evaluating the stress sensitivity. At the same time, we have studied the petrological characteristics of tight sandstone in detail using thin section (TS) and scanning electron microscope (SEM). The results show that the existence of microfractures is the main factor for the high stress sensitivity of tight sandstone. In addition, because of the small throat of the tight reservoir core, the throat closes when the overlying stress increases. As a result, the tight sandstone pore size is greatly reduced and the permeability is gradually reduced. Therefore, in the development of tight reservoirs, we should not only consider the complex fracture network produced by fracturing, but also pay attention to the permanent damage of reservoirs caused by stress sensitivity.

A semi-analytical model for investigating the productivity of fractured horizontal wells in tight oil reservoirs with micro-fractures

Combining horizontal drilling with hydraulic fracturing has become a well-established technique to improve well productivity in tight oil reservoirs. The existence of micro-fractures in these reservoirs affects the fluid flow process as well as the productivity. However, current models for evaluating the well performance of fractured horizontal wells containing micro-fractures lack accuracy. Hence, the objective of this study is to build an efficient model to analyze well productivity, considering the effects of micro-fractures. A novel flow model for multi-fractured horizontal wells was established, considering the effect of micro-fractures on the fluid flow. Subsequently, we built a reservoir model based on the typical fracture and fluid properties of a tight oil block in the Daqing oil field, China. Good agreement between the prediction results and field data was obtained, indicating that the effect of micro-fractures on well productivity cannot be neglected. Further study reveals that micro-fractures essentially function as a joint connecting reservoir matrix and have a persistent impact on well productivity. Micro-fractures can promote well productivity; however, if the value of micro-fracture permeability increases, this promotional effect will slow down. Well productivity can be considerably improved as an increase in micro-fracture percentage. Finally, a series of sensitivity studies were conducted to identify the effects of critical parameters on well productivity. Results show that reservoir permeability significantly influences well productivity, followed by reservoir effective thickness, micro-fracture percentage, fracture number, hydraulic fracture half-length. Pressure sensitivity coefficient, threshold pressure gradient and micro-fracture permeability have the least effect on well productivity. This work provides a sophisticated and efficient tool to evaluate the productivity of multi-fractured horizontal wells in tight oil reservoirs with micro-fractures.

Laboratory evaluation to assess the effectiveness of inhibitive nano-water-based drilling fluids for Zubair shale formation

While drilling through shale formations, shale hydration, including shale swelling and dispersion, is frequently reported as the main wellbore instability problems, particularly when conventional drilling fluids (water-based) are used. These problems have many adverse effects on the drilling operations resulting in non-productive time. Nanoparticles have been recently introduced as a unique alternative to improve the performance of water-based drilling fluids for shale applications. This paper presents an experimental evaluation to investigate the effectiveness of an inhibitive nano-water-based drilling fluid in reducing the swelling of Zubair shale formation. Well-preserved core samples, which were retrieved from Zubair formation, were characterized using X-ray diffraction, and X-ray fluoresces to quantify the amount of the swelling minerals. Scanning electron microscopy was used to identify the existence of microfractures within the samples. Three different nano-based drilling fluids containing titanium dioxide (TiO2), copper oxide (CuO), and magnesium oxide (MgO) at two different concentrations (0.5% and 1.5% by vol) were evaluated through a set of tests to assess the shale reactivity in the presence of these nano-based fluids. In addition, the effect of these nanoparticles on the rheological and filtration properties was studied. The results showed that the shale samples contain 41.26% silicon dioxide (SiO2) and 22.73% aluminum oxide (Al2O3), indicating the presence of smectite and illite. Based on the reactivity tests, fluids containing CuO at 1.5% by vol outperformed the other fluids in terms of reducing the reactivity, where the expansion rate was reduced by 82.7% compared to the reference sample submerged in fresh water. In addition, the addition of nanoparticles resulted in reducing the plastic viscosity, increasing both the yield point and gel strength, and reducing the fluid loss under low-pressure low-temperature conditions.

Investigation of multiphase fluid imbibition in shale through synchrotron‐based dynamic micro‐CT imaging

AbstractWhile substantial advances were made in pore system characterization in shale in the past decade, understanding of fundamentals of multiphase fluid flow in shale remains limited. The complexity of multiphase fluid flow in shale is related to many factors including nanoscale to submicron‐scale pore systems, differences in mineralogy and associated surface wettability, heterogeneity of these properties at different scales, and possible existence of microfractures. Previous work related to fluid flow, such as permeability measurement, spontaneous imbibition, and wettability analysis, includes many simplified assumptions/hypotheses with limited consideration of small‐scale details. In addition, these studies were mostly focused on single‐phase phenomena. Direct observation of multiphase fluid flow in shale showing small‐scale details is highly desired. In this study, dynamic imaging using synchrotron‐based micro‐CT (computed microtomography) was conducted on two shale samples to investigate multiphase (oil and water) spontaneous imbibition. The results revealed many details of multiphase flow in shale, including pore connectivity, porosity and permeability layered distribution, and wettability of microfractures and pores. At the same time, results illustrate the complexity of multiphase flow in shale and gaps in understanding that need further investigation.

Impact of laminations on physical properties of the Bahariya ormation, Egypt

The type locality of the Bahariya Formation in Egypt is the Bahariya Oasis, which lies about 280 km southwest of Cairo, Egypt. It is composed mainly of sandstones which deposited in fluvio-marine to shallow marine environment. The high grain density values recorded for some parts of the Bahariya Formation is due to the existence of iron oxides and pyrite presented as pore filling and/or cementing materials. The primary sedimentary structures like wavy and zigzag laminations severely affect the majority of rock petrophysical properties. The rock laminations are considered as the main source of reservoir rock anisotropy. The relation between pore specific surface (Spor) and permeability is positively improved in the laminated rock samples rather than in the non-laminated. There is a robust and close relationship between porosity and bulk density for non-laminated samples (R2 = 0.86), while for laminated samples, the relation is very weak due to the presence of iron oxides and pyrite laminas. Permeability anisotropy is highly scattered, but generally it increases with depth. Higher permeability anisotropy supports the horizontal migration of reservoir fluids. Permeability anisotropy is most likely due to the existence of microfractures and/or laminations which are mostly existed in the upper part of the Bahariya Formation. The low values of electrical resistivity are showing by laminated samples in depth more than 3516m. It most likely due to the presence of conductive solids like iron rich cement or matrix and rock pore space architectures. However, the mineral composition of some few laminas is free of iron oxides (depth=3515m). The electrical resistivity anisotropy has high values for laminated samples relative to the non-laminated ones. It increases, in general, with depth as a trend of highly scattering data points. This anisotropy is responding by the magnitude of the magnetic susceptibility anisotropy. The non-laminated samples have low magnetic susceptibility relative to the samples with laminations. The magnetic susceptibility of the Bahariya samples is apparently not related to irreducible water saturation (Swi). The main reason for irreducible water saturation variations is the amount and type of clay minerals inside the rock pore spaces. The clay mode of occurrence is usually pore filling, lining, bridging or grain coating. The displacement pressure (Pd) measured for the Bahariya samples are ranged from 10 up to 5000 Psi. The pore surface area of non-laminated samples shows a close and reliable relationship with displacement pressure (R2 = 0.97), while the pore surface area of the laminated samples has no relation with the displacement pressure (R2 = 0. 09). The samples expose mean hydraulic pore radius (R50 &R35) less than 0.04 µm are considered as bad reservoir or even sealing rocks or permeability barrier irrespective to laminations.

Infill development strategies for Block A of Xushen gas field

Along with the enhancement of oil and gas exploration technologies, a large scale of natural gas reservoir of volcanic rock was discovered deep in the north of Songliao Basin of China. The gas reservoir has several distinct features such as diversified rock types, complex lithology, fast change of lithofacies, existence of micro-fractures, poor rock physical properties and strong heterogeneity, etc. Then a multidisciplinary method was used to evaluate the volcanic rock lithology and lithofacies, validity of matrix and fractures, types of pore space and its connection to factures. Based on above mentioned studies, optimisation was carried out on the development mode, layer series division, well pattern and its interspacing, gas production rate, stable production time and final recovery ratio. Also, a reasonable development programme was made. In order to reduce the risk in this programme, such development strategies which proved to highly efficient and have yielded good results were adopted as the overall deployment and successive implementation, pilot well infilling test in priority at local area, highlighted trace evaluation and timely adjustment. [Received: October 29, 2012; Accepted: July 12, 2013]

Treatment and management of severe osteoporosis

Because there is an extremely high probability of new fractures occurring in cases in which there are pre-existing fractures, it is easy for such patients to develop severe osteoporosis. We therefore must correctly ascertain any existence of either vertebral or hip fractures. High values of bone resorption markers reflect the existence of microfractures, so it is necessary to be careful due to the high risk of new fractures. Kinesitherapy for severe osteoporosis focuses on therapeutic exercise, walking, balance drills, and so forth with the aim of maintaining muscular strength and thus preventing falls. For severe osteoporosis such as senile osteoporosis and glucocorticoid-induced osteoporosis, bisphosphonates are currently the only medication that has been proven to be effective for preventing fractures. For the treatment of disuse osteoporosis stemming from paralysis, etc., the intravenous injection of bisphosphonates is able to suppress the decline in bone mass, but there is still very little evidence of its fracture prevention effect.