Digital Core Model Research Articles

A METHOD TO SELECT REPRESENTATIVE ROCK SAMPLES FOR DIGITAL CORE MODELING

X-ray computed tomography (CT) scanning method is the most accurate method to construct digital core, which can reflect the microscopic pore structure of real cores; therefore, it is widely used and researched by experts and scholars all over the world. However, there are few reports about how to select the CT scan core samples at present, and the current practice is to make CT scan samples by visually observing rocks or core columns to select a region that is considered representative or interesting, which can lead to a large difference between the selected sample and the whole rock and a digital core that cannot represent the real rock as a whole. In order to construct the digital cores that can reflect the whole rock structure and reservoir properties, combining with fractal theory, a scientific and reasonable method was proposed to select representative rock samples for digital core modeling. First of all, a core column is scanned by X-ray CT at a certain resolution and CT gray scale images are obtained and stored in the order of scan. Secondly, the fractal dimension (FD) of each image is calculated by box-counting method, and the calculated porosity of each image is achieved by the existing formula. Then, according to the size of the digital core to be constructed, the CT gray scale images are grouped, and the average FD and the average porosity of each combination are calculated by the derived equations. Finally, based on the proposed criteria the best image combination is selected and the preferred sample is determined accordingly. At the same time, a facile experiment was conducted to test the effectiveness of this method. The experimental results show that there are some errors between the subjectively selected cores and the long core in terms of permeability and porosity, and the petrophysical parameters of the core selected by the proposed method are close to those of the long core; as a consequence, the validity of this method was verified and it is feasible and practical to select the representative rock samples for digital core modeling by this method.

Research on the equivalence between digital core and rock physics models

In this paper, we calculate the elastic modulus of 3D digital cores using the finite element method, systematically study the equivalence between the digital core model and various rock physics models, and carefully analyze the conditions of the equivalence relationships. The influences of the pore aspect ratio and consolidation coefficient on the equivalence relationships are also further refined. Theoretical analysis indicates that the finite element simulation based on the digital core is equivalent to the boundary theory and Gassmann model. For pure sandstones, effective medium theory models (SCA and DEM) and the digital core models are equivalent in cases when the pore aspect ratio is within a certain range, and dry frame models (Nur and Pride model) and the digital core model are equivalent in cases when the consolidation coefficient is a specific value. According to the equivalence relationships, the comparison of the elastic modulus results of the effective medium theory and digital rock physics is an effective approach for predicting the pore aspect ratio. Furthermore, the traditional digital core models with two components (pores and matrix) are extended to multiple minerals to more precisely characterize the features and mineral compositions of rocks in underground reservoirs. This paper studies the effects of shale content on the elastic modulus in shaly sandstones. When structural shale is present in the sandstone, the elastic modulus of the digital cores are in a reasonable agreement with the DEM model. However, when dispersed shale is present in the sandstone, the Hill model cannot describe the changes in the stiffness of the pore space precisely. Digital rock physics describes the rock features such as pore aspect ratio, consolidation coefficient and rock stiffness. Therefore, digital core technology can, to some extent, replace the theoretical rock physics models because the results are more accurate than those of the theoretical models.

Influence of stress sensitivity on microscopic pore structure and fluid flow in porous media

At present, large quantities of experiments related to stress sensitivities on the macroscale have been conducted. Pore structure is complicated, especially in unconventional reservoirs of tight sandstone. As a result, stress sensitivity has a great influence on changing the pore space. This is of great significance for enhancing oil recovery to systematically study the stress sensitivity at the pore scale. In this paper, based on CT scanning technology, which can capture the real pore space characteristics of the core samples, and combined with a digital core and pore network model, the relationships between the effective stress and the pore structure are obtained. First, the theory and method of constructing the digital core and extracting the pore network model according to CT scanning are introduced. The 3D core image could be obtained after CT scanning, and the digital core is established after filtering and segmentation. The rock structure characteristics are obtained from analyzing the geometry-topology structure of the extracted pore network model, and the fluid flow analysis is conducted by numerical flow simulation. Second, the stress sensitivity experiments and analysis are conducted. Stress sensitivity experiments are performed in the carbon fiber core holder, and the core is scanned under a series of pressures. Digital cores and pore network models under a series of pressures are obtained, and the corresponding pore structure characteristics and fluid flow laws are analyzed. Last, these methods are employed in middle-high permeability sandstone and tight sandstone. The stress sensitivities of these sandstones are achieved and compared. In this paper, the stress sensitivity evaluation method at pore-scale is developed, which has the practical application of oilfield development.

Developing a new NMR-based permeability model for fractured carbonate gas reservoirs

Fractured gas-bearing carbonates are dual porosity media characterized by their complex pore structure and heterogeneity. In reservoir evaluation, the pore fluids could accurately be characterized by nuclear magnetic resonance (NMR) well logging. However, the NMR-based permeability models of fractured carbonates are limited due to lack of understanding of how fractures affect NMR T2 spectrum. Based on the inevitable influence of fractures on reservoir permeability conditions, the current research investigates the effects of different fracture parameters on the resultant T2 spectrums and permeability values of the fractured gas-bearing carbonates. Firstly, a carbonate core sample having no fractures was scanned by CT imaging and its digital model was reconstructed by digital core algorithms. Secondly, fracture having various parameters dip angle, length, aperture and density were inserted into that digital core model to represent different fractured carbonate rock. In the next step, random walk simulation and lattice Boltzmann method were respectively used to simulate corresponding T2 spectrums and permeabilities of these models. We introduced a new kind of spectrum called ‘T2 spectrum of fractures’ or ‘T2f spectrum’ and used it to study effects of different fracture parameters on the NMR T2 spectrum. Finally, based on the quantitative analysis, a novel permeability model was presented for the fractured carbonates. Combined with nuclear magnetic resonance well logging data, this novel approach was used in the interpretation process of well logs. Compared to the existing models of permeability, this new model better matched the real-field permeabilities and greatly improved the accuracy of permeability interpretation in the studied fractured carbonate gas reservoir.

Digital core and pore network model reconstruction based on random fractal theory

Microscopic structures of porous media are mainly reconstructed using physical experimental methods and numerical reconstruction methods. However, their accuracy is closely related to the resolution of real rock core images and polishing techniques of core chips amongst others, which significantly limit the development of micro-network flow simulation. In this paper, we proposed a new method for digital reconstruction of rock cores on the basis of the random fractal theory. First, we quantitatively describe and identify the multiple fractal dimension and self-similar interval of CT scanning data of 10 sets of rock cores based on the fractal characteristics of microscopic pore structures. Then, we derive a set of fractal expressions of the random probability density function, mean, and variance based on the random distribution theory, and apply the direct sampling method of continuously distributed random variables to obtain the porous media pore data satisfying multi-fractal characteristics. The results show that the rock core data constructed using our method are in a good agreement with the original experimental data, and better than those obtained from single fractal dimension and other distribution characteristics. Furthermore, we analyzed the size distribution of microscopic pores of 3 real rock cores obtained using mercury intrusion porosimetry based on the multi-fractal theory, and determined the multi-fractal interval and dimension of their microscopic pore distribution. We applied our method to design the pore structure and the throat structure of porous media and established a three-dimensional PNM. At last, the network model simulator, which simulates the oil-water flow through the interconnected pore model, proves to be a powerful investigative tool to study the nature of fluid flow at a pore scale level.

Finite difference modeling of ultrasonic propagation (coda waves) in digital porous cores with un-split convolutional PML and rotated staggered grid

Abstract Ultrasonic wave propagation in heterogeneous porous cores under laboratory studies is an extremely complex process involved with strong scattering by microscale heterogeneous structures. The resulting coda waves, as an index to measure scattering attenuation, are recorded as continuous waveforms in the tail portion of wavetrains. Because of the contamination of reflections from the side ends and reverberations between the sample surfaces, it is difficult to extract pure coda waves from ultrasonic measurements for the estimation of the P - and S -coda attenuation quality factors. Comparisons of numerical and experimental ultrasonic wave propagation in heterogeneous porous cores can give important insight into understanding the effect of boundary reflections on the P - and S -codas in the laboratory experiment. It challenges numerical modeling techniques by three major issues: the creation of a digital core model to map heterogeneous rock properties in detail, the perfect simulation with a controllable and accurate absorbing boundary, and overcoming the numerical dispersions resulting from high-frequency propagation and strong heterogeneity in material. A rotated staggered-grid finite-difference method of Biot's poroelastic equations is presented with an unsplit convolutional perfectly matched layer (CPML) absorbing boundary to simulate poroelastic wave propagation in isotropic and fluid-saturated porous media. The contamination of boundary reflections on coda waves is controlled by the CPML absorbing coefficients for the comparison between numerical and experimental ultrasonic waveforms. Numerical examples with a digital porous core demonstrate that the boundary reflections contaminate coda waves seriously, causing much larger coda quality factors and thus underestimating scattering attenuation.

Digital core modeling from irregular grains

Digital cores are the basis to virtual rock physical experiments. Present modeling methods are insufficient to modeling cores to meet the requirements of virtual rock physical experiments. Different cores of sandstones can be modeled with different input arguments by process based method. The revised process based method adopts irregular grains in the sedimentation process. Eight digital core modeling experiments with the revised process based method are implemented. It is shown that, to a certain extent, varying the input arguments in succession can result to cores with continuously varied macro‐property.

Numerical simulation of non-Archie electrophysical property of saturated rock with lattice Boltzmann method

The electrophysical property of saturated rocks is very important for reservoir identification and evaluation. In this paper, the lattice Boltzmann method (LBM) was used to study the electrophysical property of rock saturated with fluid because of its advantages over conventional numerical approaches in handling complex pore geometry and boundary conditions. The digital core model was constructed through the accumulation of matrix grains based on their radius distribution obtained by the measurements of core samples. The flow of electrical current through the core model saturated with oil and water was simulated on the mesoscopic scale to reveal the non-Archie relationship between resistivity index and water saturation (I-Sw). The results from LBM simulation and laboratory measurements demonstrated that the I-Sw relation in the range of low water saturation was generally not a straight line in the log-log coordinates as described by the Archie equation. We thus developed a new equation based on numerical simulation and physical experiments. This new equation was used to fit the data from laboratory core measurements and previously published data. Determination of fluid saturation and reservoir evaluation could be significantly improved by using the new equation.