An experiment on self‐potential (SP) and strain changes during elastic deformation and fracture of rock samples was conducted by means of a biaxial servo‐control loading‐machine and a detecting array of strain gauges and electrodes. Some correlated and widespread pulse‐like transient variations in self‐potential and strain‐field appeared at some special situations, including sudden loading/ unloading, stick‐slip and fracture of rocks. The results indicate that though sometimes the stress ‐ is rather high, there is no SP anomaly occurring if the stress change‐rate γ = dσ/dt equals zero or keeps positive or negative constants. On the contrary, at the moment changes suddenly the pulse‐like SP change γV will always arise no matter the increment Δ;γ is positive or negative. The positive or negative sign of Δγ gives opposite effect on ΔV. The anomalous amplitude of ‐V is proportional to that of Δγ. The observed clear temporal and spatial self‐potential changes at the moment of sudden stress‐rate change and rock fracture mean that it is possible to detect the precursory and co‐seismic electric and corresponding strain variations if the above‐mentioned conditions can be met near a seismogenic zone. It is a support to the seismic precursory research of self‐potential and strain.
Summary
Networks of large pores connected by narrower throats (pore networks) are essential inputs into network models that are routinely used to predict transport properties from digital rock images. Extracting pore networks from microcomputed-tomography (micro-CT) images of rocks involves a number of steps: filtering, segmentation, skeletonization, and others. Because of the amount of clay and its distribution, the segmentation of micro-CT images is not trivial, and different algorithms exist for achieving this. Similarly, several methods are available for skeletonizing the segmented images and for extracting the pore networks. The nonuniqueness of these processes raises questions about the predictive power of network models. In the present work, we evaluate the effects of these processes on the computed petrophysical and multiphase-flow properties of reservoir-rock samples.
By use of micro-CT images of reservoir sandstones, we first apply three different segmentation algorithms and assess the effects of the different algorithms on estimated porosity, amount of clay, and clay distribution. Single-phase properties are computed directly on the segmented images and compared with experimental data. Next, we extract skeletons from the segmented images by use of three different algorithms. On the pore networks generated from the different skeletons, we simulate two-phase oil/ water and three-phase gas/oil/water displacements by use of a quasistatic pore-network model.
Analysis of the segmentation results shows differences in the amount of clay, in the total porosity, and in the computed singlephase properties. Simulated results show that there are differences in the network-predicted single-phase properties as well. However, predicted multiphase-transport properties from the different networks are in good agreement. This indicates that the topology of the pore space is well preserved in the extracted skeleton. Comparison of the computed capillary pressure and relative permeability curves for all networks with available experimental data shows good agreements.
By use of a segmentation that captures porosity and microporosity, we show that the extracted networks can be used to reliably predict multiphase-transport properties, irrespective of the algorithms used.
Olivine is a key constituent in the silicate Earth; its composition and texture informs petrogenetic understanding of numerous rock types. Here we develop a quantitative and reproducible method to measure olivine composition in three dimensions without destructive analysis, meaning full textural context is maintained. The olivine solid solution between forsterite and fayalite was measured using a combination of three-dimensional (3D) X-ray imaging techniques, 2D back scattered electron imaging, and spot-analyses using wavelength dispersive electron probe microanalysis. The linear attenuation coefficient of natural crystals across a range of forsterite content from ~73-91 mol% were confirmed to scale linearly with composition using 53, 60 and 70 kV monochromatic beams at I12-JEEP beamline, Diamond Light Source utilising the helical fly-scan acquisition. A polychromatic X-ray source was used to scan the same crystals, which yielded image contrast equivalent to measuring the mol% of forsterite with an accuracy <1.0 %. X-ray tomography can now provide fully integrated textural and chemical analysis of natural samples containing olivine, which will support 3D and 3D+time petrologic modelling. The study has revealed >3 mm domains within a large crystal of San Carlos forsterite that vary by ~2 Fo mol%. This offers a solution to an outstanding question of inter-laboratory standardisation, and also demonstrates the utility of 3D, non-destructive, chemical measurement. To our knowledge, this study is the first to describe the application of XMT to quantitative chemical measurement across a mineral solid solution. Our approach may be expanded to calculate the chemistry of other mineral systems in 3D, depending upon the number, chemistry and density of end-members.
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