Permeability determination of organic-rich shales is still a major challenge. Uncertainty in this estimate involves several factors. Two significant ones are the occurrence of gas adsorption which can severely limit gas transport in the pores and understanding the physical chemistry issues of the pore's surface area estimation when using various gases. In this study, we reported our experimental results of permeability measurement on several unconventional shale samples, and investigated the effect of gas type, pore pressure, effective stress and sample orientation on the measured permeabilities.Permeability of shale samples is measured using the complex pressure transient technique. Three different gases, argon, nitrogen, and carbon dioxide, are used as permeating fluid through the samples. Experiments are conducted isothermally at various pore and confining pressures that maintain a constant net effective stress. Generally, samples have higher measured permeabilities when using nitrogen as pore fluid rather than using argon. The discrepancy was attributed to different adsorption potentials between argon and nitrogen: Argon has a similar sorption potential to methane while nitrogen's sorption potential is relatively weak. As expected, the measured permeability of all samples decreases when the pore pressure increases reflecting the reduction in the gas slippage effect. Samples from the same whole core display permeability anisotropy: Horizontal plugs cut parallel to bedding have a higher measured permeability, which is in the range of microdarcy, while the permeability of vertical plugs cut perpendicular to bedding is in the range of nanodarcy. This anisotropy behavior is believed to be caused by the fractures contained within the horizontal samples. The measured permeability is observed to decease with increasing effective stress acting on the samples. This reduction behaves differently: Permeability decreases very slowly when the increasing effective stress is resulted from the decrease of the pore pressure. The enhanced Klinkenberg effect due to the decreasing pore pressure compensates at least partly the permeability reduction resulting from increasing effective stress. However, permeability reduces dramatically when the effective stress increases because of the increasing confining pressure. In this case, the flow channels may be reduced or even closed, thus blocking the flow of gas.
Accurate determination of organic-rich shale permeability is still a major challenge. Various methods have been proposed to measure the permeability on core plugs or crushed samples under various stress conditions using different fluids. Permeability obtained from core plugs and their crushed samples could differ by two orders of magnitude, potentially painting very different views of the reservoirs and resulting in differences in asset development workflows. This situation only reinforces the need for considerable additional focused work to quantify tight rock permeability and better understand the measurement method dependence. This paper presents the experimental comparison of three different unsteady-state transient methods for measuring the permeability of organic-rich shale plugs: pressure build-up, pulse-decay and oscillating pulse techniques. Permeability measurements are conducted isothermally using nitrogen gas on core plugs from the Barnett, Eagle Ford, Marcellus and Mancos formations at the same confining pressure and pore pressure. These plugs differ in mineralogy, total organic carbon (TOC), nuclear magnetic resonance (NMR) and helium porosity. Fractures are observed through the horizontal samples along the gas flow direction, while vertical samples do not have fractures. For each plug, the permeability tests begin with the pressure build-up measurement, and are followed sequentially by the pulse-decay and oscillating pulse methods whenever applicable. The plugs are then cut into smaller sizes for continuation of permeability tests and investigation of the permeability dependence on the plug size. For the samples analyzed, the permeability measured from the pulse-decay method is essentially identical to that from the oscillating pulse method. Compared to these two methods, the pressure buildup test generally gives a relatively higher value when requiring an independent porosity measurement to compute the permeability, while it gives a relatively lower value when no porosity is needed. However, the permeability difference among these methods is generally small. This indicates that the pressure build-up test can be used to perform permeability measurement on large core plugs that cannot be tested using the pulse-decay or oscillating pulse methods. There is no trend observed on the permeability dependence on the plug sizes used. Depending on fracture distribution and connectivity, the measured permeability of horizontal samples is randomly affected by the sample sizes, indicating multiple samples with various sizes may be required to obtain a representative permeability value for horizontal plugs. Vertical plugs show little permeability dependence on the sample sizes. Consequently, a small vertical plug can be used for very tight rocks enabling quicker matrix permeability measurements.
In this study an extensive experimental program has been carried out in order to characterize the mechanical behavior of two weak sandstone formations of an offshore field for application to sand production modeling. The experimental tests included Scratch tests, Triaxial tests and Advanced thick wall cylinder tests (ATWC) where the sand production initiation and the cumulative sand produced were registered. Numerical simulations of experimental tests were then performed using an advanced elasto-plastic constitutive model. Triaxial tests simulations allowed calibrating the constitutive model parameters. These parameters were employed for the numerical simulation of the ATWC in order to determine the equivalent plastic strain threshold required to the onset of sand production observed in laboratory for sanding assessment. Results obtained highlight the importance to use a realistic representation of the rock behavior focusing on post-yield behavior in order to build confidence in model predictions.
Unconventional shale mechanical parameters are important for many applications such as reservoir stress-state determination, horizontal drilling and hydraulic fracturing design among other engineering design parameters. These parameters include Young's modulus, Poisson's ratio, cohesion, angle of internal friction, and unconfined compressive strength. Their determination is commonly performed on single-stage triaxial (SST) compression tests using three or more core samples at various confining pressures. For unconventional shale, this has been a practical bottleneck - it is very difficult to drill multiple plugs with good quality from a whole core because of its brittleness and complexity. An alternative procedure is the multi-stage triaxial (MST) compression test, which requires only one plug to be tested. However, its main problem lies in the practical difficulty in determining the failure envelope of the sample. Judgment must be made regarding the stress-strain state "immediately prior to failure". It is not uncommon that a wrong estimation of the failure state occurs when interpreting these stress-strain curves. The MST test uses one plug, which in itself could be a significant advantage. This paper presents a robust modified method of the MST compression test that accurately determines the imminent rock failure through continuously monitoring the radial deformation. To test and validate the method, Berea sandstone and Mancos shale plugs are selected to perform a series of the SST and MST tests. For each rock type, at least four plugs are used, which are cut from the same whole core or have similar lithofacies. Experimental results show that the Mohr-Coulomb failure envelope can be generated perfectly from both the SST and MST tests. However, the failure parameters derived from the MST tests are lower than those from the SST tests. The difference may be attributed to the intrinsic heterogeneity of rock samples or the damage accumulated from the early stages in a MST test. The proposed MST method is an efficient alternative to generate the Mohr-Coulomb failure envelope when the availability of core samples with good quality is limited, especially in the exploration and development of unconventional reservoirs.
Resumen El estudio de la CNT en la Transición ha generado una considerable bibliografía, especialmente en los últimos quince años, que ha venido a llenar el vacío historiográfico previo. Sin embargo, a pesar de los progresos realizados, el estado de la cuestión dista de ser satisfactorio. Los estudios insisten en la presentación de la CNT como una organización eminentemente anarquista, orillando y discriminando su componente sindical y de clase. El presente texto realiza un repaso del estado de la cuestión y sus condicionantes, a la vez que plantea una serie de propuestas de análisis que permitan avanzar en el estudio y la identificación de la central anarcosindicalista como una organización esencialmente obrera. Palabras clave CNT, anarcosindicalismo, movimiento obrero, transición, historiografía
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