Electrokinetic subsurface characterization

Abstract only

Theory and experiment of differential acoustic resonance spectroscopy

Recent advances in Differential Acoustic Resonance Spectroscopy (DARS) techniques have given rise to applications in the field of poromechanics. We report on the experimental demonstration of bulk modulus measurements on poroelastic samples at sonic frequencies (1 kHz) with DARS. Normal mode perturbation is due to scattering of a foreign object (i.e., a rock sample) within an otherwise fluid-filled resonator. The perturbation theory on an elastic object determines its bulk modulus (inverse compressibility). The experimental bulk modulus of medium- to high-permeability (>10 mD) poroelastic samples is in agreement with predictions from quasi-static loading of a porous sphere using the Biot theory. This result demonstrates that pore fluid flow governs the dominant relaxation process of the rock during compression. For low-permeability samples (<10 mD), pressure equilibration via slow wave diffusion is limited, and only qualitative agreement is found between the upper bound (Gassmann undrained modulus) and the lower bound (volume-weighted compressibilities of the two constituents). DARS experiments, in conjunction with the poroelastic theory presented here, allow one to infer such rock physical properties as the effective bulk modulus at sonic frequencies

The new method to characterize the gas emissions during torrefaction real-time

\u3cp\u3eLarge scale torrefaction encounters difficulties in processing different types of biomass of various properties and morphologies to guarantee and maintain the required quality. Ideally, torrefaction runs auto-thermally. Key issue here is that the heat production, which is generated by combusting the produced torr-gas (gas which is released from the biomass during torrefaction), should be sufficient for the torrefaction process including losses from the system. It is essential to know the heating value of the torr-gas as a function of the torrefaction process for each biomass type (e.g. temperature and residence time). One of the problems is that this heating value cannot be determined in real-time by analyzing the gas composition because of the slow processing time of gas analysis equipment. A second drawback of gas analysis chromatography is that the tars present in the torr-gas condense and thus clog and contaminate the measurement equipment. To be able to measure this heating value in real-time, along with the development of the final product quality, a new method is proposed which to some extent resembles the actual torrefaction system in which a sample material is brought to the actual torrefaction condition while the temperature distribution in the sample is being recorded along with the generated thermal power of the ablated torr-gas.\u3c/p\u3

Moisture penetration in oak during sinusoidal humidity fluctuations studied by NMR

\u3cp\u3eMost natural fluctuations in relative humidity are cyclic of nature, e.g. daily or seasonal. During these fluctuations, hygroscopic materials exchange moisture with the surrounding air. The penetration of moisture into the material depends on the frequency of the fluctuation, but also on the transport characteristics of the material. Here we present an experimental study on the penetration depth of moisture in oak during sinusoidal relative humidity fluctuations, covering a wide range of frequencies. Using nuclear magnetic resonance, we show that the amplitude in moisture content decreases exponentially from the exposed surface. The slope of the decay on a logarithmic scale provides the diffusion coefficient in the three principal directions of wood (longitudinal, radial, tangential), which are in good agreement with literature values. Furthermore, we show the influence of the moisture content range on the decay in amplitude by performing experiments in different relative humidity ranges. Numerical experiments are performed to assess the dependence of moisture penetration on different model parameters.\u3c/p\u3

Seismoelectric interface electromagnetic wave characteristics for the finite offset Vertical Seismoelectric Profiling configuration:Theoretical modeling and experiment verification

\u3cp\u3eThe seismoelectric interface electromagnetic characteristics have been studied for the finite offset Vertical Seismoelectric Profiling (VSEP) configuration. The approach consists of theoretical modeling and laboratory verification. The results show that the wave variation characteristics along the finite offset measurement line are markedly different from those along the zero-offset line. More interestingly, the wave characteristics for both configurations can be satisfactorily explained by the electric dipole model for the seismoelectric interface wave radiation. Besides, the experiment confirms the modeling result based on the seismoelectric coupling theory and validates the VSEP technique as an effective method for subsurface interface delineation.\u3c/p\u3

The frequency dependence of hygro-expansive scaling of oak

\u3cp\u3eFluctuations in the ambient relative humidity are often cyclic of nature, composed of a wide variety of frequencies. Variations may be as fast as one minute or as slow as a complete season. A wooden object exposed to these fluctuations exchanges moisture with the air, resulting in a change in the local moisture content of the wood. Since moisture penetration is affected by the timescale of the changes in ambient conditions, so are processes caused by moisture content variations. Moisture content variations cause dimensional changes of a wooden object, which, if mechanically restrained, lead to a buildup of stresses and ultimately to damage. It is therefore important to predict the frequency behavior of moisture content and expansion in wood. In the presented study, experiments are conducted in which the moisture content and expansion of oak cubes with different sizes is measured during sinusoidal relative humidity fluctuations with different frequencies. The amplitude in moisture content and expansion is shown to be simply scalable based on sample size only. Derived diffusion coefficients are in agreement with literature values, although the experimental frequency behavior is shown to deviate qualitatively from diffusive frequency behavior.\u3c/p\u3

An automated fracture trace detection technique using the complex shearlet transform

\u3cp\u3eRepresenting fractures explicitly using a discrete fracture network (DFN) approach is often necessary to model the complex physics that govern thermo-hydro-mechanical-chemical processes (THMC) in porous media. DFNs find applications in modelling geothermal heat recovery, hydrocarbon exploitation, and groundwater flow. It is advantageous to construct DFNs from the photogrammetry of fractured outcrop analogues as the DFNs would capture realistic, fracture network properties. Recent advances in drone photogrammetry have greatly simplified the process of acquiring outcrop images, and there is a remarkable increase in the volume of image data that can be routinely generated. However, manually digitizing fracture traces is time-consuming and inevitably subject to interpreter bias. Additionally, variations in interpretation style can result in different fracture network geometries, which, may then influence modelling results depending on the use case of the fracture study. In this paper, an automated fracture trace detection technique is introduced. The method consists of ridge detection using the complex shearlet transform coupled with post-processing algorithms that threshold, skeletonize, and vectorize fracture traces. The technique is applied to the task of automatic trace extraction at varying scales of rock discontinuities, ranging from 10° to 102m. We present automatic trace extraction results from three different fractured outcrop settings. The results indicate that the automated approach enables the extraction of fracture patterns at a volume beyond what is manually feasible. Comparative analysis of automatically extracted results with manual interpretations demonstrates that the method can eliminate the subjectivity that is typically associated with manual interpretation. The proposed method augments the process of characterizing rock fractures from outcrops.\u3c/p\u3

A numerical study on the effect of anisotropy on hydraulic fractures

\u3cbr/\u3eIn this paper, we present a two-dimensional numerical model for modelling of hydraulic fracturing in anisotropic media. The numerical model is based on extended finite element method. The saturated porous medium is modelled using Biot’s theory of poroelasticity. An enhanced local pressure model is used for modelling the pressure within the fracture, taking into account the external fluid injection and the leak-off. Directional dependence of all the rock properties, both elastic and flow related, is taken into account. A combination of the Tsai–Hill failure criterion and Camacho–Ortiz propagation criterion is proposed to determine the fracture propagation. We study the impact on fracture propagation (in both magnitude and direction) due to anisotropies induced by various parameters, namely ultimate tensile strength, Young’s modulus, permeability and overburden pressure. The influence of several combinations of all these anisotropies along with different grain orientations and initial fracture directions on the fracture propagation direction is studied. Different regimes are identified where the fracture propagation direction is controlled by the degree of material anisotropy instead of the stress anisotropy.\u3cbr/\u3e\u3cbr/\u3e\u3cbr/\u3eKeywords\u3cbr/\u3eRock anisotropy Transverse isotropy Hydraulic fracturing Porous media Extended finite element metho

On the numerical simulation of crack interaction in hydraulic fracturing

In this paper, we apply the enhanced local pressure (ELP) model to study crack interaction in hydraulic fracturing. The method is based on the extended finite element method (X-FEM) where the pressure and the displacement fields are assumed to be discontinuous over the fracture exploiting the partition of unity property of finite element shape functions. The material is fully saturated and Darcy’s law describes the fluid flow in the material. The fracture process is described by a cohesive traction-separation law, whereas the pressure in the fracture is included by an additional degree of freedom. Interaction of a hydraulic fracture with a natural fracture is considered by assuming multiple discontinuities in the domain. The model is able to capture several processes, such as fracture arrest on the natural fracture, or hydraulic fractures that cross the natural fracture. Fluid is able to flow from the hydraulic fracture into the natural fracture. Two numerical criteria are implemented to determine whether or not the fracture is crossing or if fluid diversion occurs. Computational results showing the performance of the model and the effectiveness of the two criteria are presented. The influence of the angle between a hydraulic fracture and a natural fracture on the interaction behaviour is compared with experimental results and theoretical data