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The low-medium maturity shale oil in China has large resource potential. The occurrence state of such resource is mainly retained hydrocarbon and unconverted organic substance, making the in-situ conversion process a requisite technology. Meanwhile, the naturally fractured shale reservoir has multiple types of pore space, making the hydrothermal coupling process more complex. In this paper, a coupled thermal-reactive compositional flow model and numerical solution method is proposed. Specifically, the apparent permeability of fractured shale oil reservoir is derived based on fractal theory. Besides, the multistage parallel chemical kinetic reactions during in-situ conversion process and evolution of reservoir fluid composition and shale porosity are incorporated. Then the accuracy of the proposed model is tested through numerical cases. Finally, the impact of key operation parameters and fractal parameters on shale oil exploitation is analyzed, such as heating temperature, heating spacing, pore size fractal dimension, fracture length fractal dimension and tortuosity fractal dimension. The proposed model is capable of accurate and efficient prediction of the fractured low-medium maturity shale oil reservoir production. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study on the equivalent permeability of a fractured rock. A series of two-dimensional discrete fracture network (DFN) models for the calculation of equivalent permeability are generated based on discrete element method (DEM). A sufficient large "parent" DFN model is generated based on the data obtained from a site investigation result of Three Gorges Project in China. Smaller DFN models are extracted from the large "parent" DFN model to calculate the equivalent permeability with an interval of rotation angle of 1 5 ∘ . Fluid flow through fractures in both horizontal and vertical directions is simulated. The results show that when the side length of DFN models are larger than 40 m, the equivalent permeability of both k x and k y become stable, indicating that a DFN model size of 40 m can be approximated as a representative elementary volume (REV) for those studied rocks. Penetration ellipses are fitted using the least square method on the basis of the calculated equivalent permeability tensor and the main seepage directions of this fractured rock were determined as 63–67 ∘ . Fractal characteristics of DFN models are analyzed with box-counting method by changing the fracture trace length and fracture density, and the results show that equivalent permeability exhibits a logarithmic increasing trend with the increment of fractal dimension. [ABSTRACT FROM AUTHOR]

The utilization of phosphorus slag (PHS) to replace the fly ash in the construction of hydraulic projects has attracted a growing attention in China. In this study, the influence of PHS fineness and content on cement hydration, mechanical strength, permeability as well as the pore structure and fractal dimension ( D s) of concrete have been discussed. The results indicate that the PHS addition retards the cement hydration and hence decreases the hydration heat within three days. The incorporation of PHS with a Blaine specific surface area of 505 m2/kg could participate in the early pozzolanic reaction and consequently offsets the retarding effect to some extent. The incorporation of 20–40 wt.% PHS declines the early strength of concrete, but this reduction effect on strength can be eliminated to some degrees by mechanically grinding the PHS. The compressive strengths of concrete added with PHS with a high fineness of 505 m2/kg (abbreviated as PHS-H) are about 16.0–20.6% higher at three days and 8.9–11.0% higher at 180 days compared that of the control concrete. The contribution of PHS-H to the pore structure refinement is more significant than that of PHS with a low fineness of 302 m2/kg (abbreviated as PHS-L) at various ages because PHS-H is of much higher reactivity and can consume more Ca(OH)2 than PHS-L which leads to a denser microstructure and a lower chloride diffusion coefficient ( D RCM ). The incorporation of PHS decreases the D s at three days, whereas the concrete incorporated with PHS has much higher D s than that of control one at late age. The D RCM value increases with increasing the porosity and the most probable aperture, while D s has a more significant effect on D RCM than the porosity and the most probable aperture. The concrete added with 20 wt.% PHS-H exhibits the highest D s and the lowest D RCM value at long-term age among the five concrete mixtures in this work. [ABSTRACT FROM AUTHOR]

Dam concrete suffers from serious abrasion damages in southwestern China, the abrasion resistance of concrete is therefore one of the most important factors determining the reliability even the safety of the dams. In the present work, the effects of fly and/or silica fume on the mechanical properties, drying shrinkage, as well the cracking and abrasion resistance of concrete were investigated, then the pore structures of concrete added with fly ash and silica fume and the pore surface fractal dimensions ( D s ) were determined by the mercury intrusion porosimetry (MIP) method and a fractal model, respectively. Finally, the relationships between the abrasion resistance of concrete and the porosity as well as the D s were revealed and discussed. The results indicate that silica fume significantly increases the drying shrinkage especially at early age, while the incorporation of fly ash and silica fume together can decrease the early plastic shrinkage-induced cracking risk and the final shrinkage to some degrees. Besides, the utilization of 5 wt.% silica fume and 20 wt.% fly ash together increases the abrasion resistance and mechanical property by about 4–9% at various ages. In addition, the compressive strength and the abrasion resistance of concrete are linearly correlated with the concrete porosity and the D s . Both the fly ash and silica fume could decrease the porosity of concrete and increase the D s , therefore the concrete containing fly ash and silica fume have desired mechanical property and abrasion resistance. Moreover, D s has a more profound effect on the abrasion resistance of concrete than the concrete porosity. The addition of 5 wt.% silica fume and 20 wt.% fly ash together is suitable for the concretes used for wearing surfaces in terms of mechanical property, cracking and abrasion resistance. [ABSTRACT FROM AUTHOR]

To better understand the nanopore characteristics and their effects on methane adsorption capacity of shales, we performed fractal analysis of nine shale samples collected from the fifth member of Upper Triassic Xujiahe Formation in the Sichuan Basin, southwest China. N 2 adsorption results show that shales have different adsorption characteristics at relative pressure of 0–0.5 and 0.5–1. Two fractal dimensions D 1 and D 2 were calculated using the Frenkel–Halsey–Hill (FHH) equation. Results show that the methane adsorption capacity increases with the increase of D 1 and D 2 , of which D 1 has a more significant influence on adsorption capacity than D 2 . Further studies indicate that D 1 represents the pore surface fractal characteristics caused by the irregularity of shale surface, whereas D 2 represents the pore structure fractal characteristics, which is mainly affected by shale components (e.g. TOC, clay minerals) and pore parameters (e.g. average pore diameter, micropores content). A higher D 1 corresponds to a more irregular pore surface, which provides more space for methane adsorption. While a higher D 2 indicates a more complex pore structure and a stronger capillary condensation action on the pore surface, which in turn enhances the methane adsorption capacity. [ABSTRACT FROM AUTHOR]

Fractal dimension is an important parameter in the evaluation of tight reservoirs. For an outcrop section of the Nenjiang formation in the Songliao Basin, China, the pore structure and pore fractal characteristics of shale parasequences were investigated using fractal theory. In addition, factors causing pore structure changes were analyzed using the results of low-temperature nitrogen adsorption and scanning electron microscope (SEM) experiments. Conducive to gas migration and secondary pores development such as dissolution, results showed that nanoscale pores dominated by fracture-like morphology and consequent good internal connectivity were observed in each pore size section within the target layer. Each parasequence is characterized by a sequential upward decrease of average pore size and an upward increase of total pore volume, with an increasing number of pores from 2 nm to 50 nm. Pores are isolated from each other, with poor connectivity and relatively complex composition of brittle minerals and clay minerals. Main components of the brittle minerals, quartz and feldspar, occur in 20–50% and higher clay mineral content ranging from 50% to 70%. In the parasequence cycle, clay mineral gradually decreases while the brittle mineral content increases. Fractal dimension is negatively correlated with clay mineral content and positively correlated with brittle mineral (quartz and feldspar) content. The fractal dimension calculated by the imaging method and the FHH method shows an upward increasing tendency in each of the parasequence cycles. This is as a result of different phenomena, varied sediment hydrodynamic forces leading to particle size differences and increased brittle minerals resulting in microcracks, therefore, the fractal dimension of the large pores (imaging method) increases upward in the parasequence. Simultaneously, with increased content and accompanied dissolution of brittle minerals causing an increase of small pores from base to top of the parasequence, the fractal dimension of the small pores (FHH method) grows. [ABSTRACT FROM AUTHOR]

Lucaogou tight oil reservoir, located in the Junggar Basin, Northwest of China, is one of the typical tight oil reservoirs. Complex lithology leads to a wide pore size distribution (PSD), ranging from several nanometers to hundreds of micrometers. To better understand PSD and fractal features of Lucaogou tight oil reservoir, the experiment methods including scanning electron microscope (SEM), rate-controlled mercury injection (RMI) and pressure-controlled mercury injection (PMI) were performed on the six samples with different lithology. The results indicate that four types of pores exist in Lucaogou tight oil reservoir, including dissolution pores, clay dominated pores, microfractures and inter-granular pores. A combination of PMI and RMI was proposed to calculate the overall PSD of tight oil reservoirs, the overall pore radius of Lucaogou tight oil reservoir ranges from 3.6 nm to 500 μ m. The fractal analysis was carried out based on the PMI data. Fractal dimension (Fd) values varied between 2.843 and 2.913 with a mean value of 2.88. Fd increases with a decrease of quartz content and an increase of clay mineral content. Samples from tight oil reservoirs with smaller average pore radius have stronger complexity of pore structure. Fractal dimension shows negative correlations with porosity and permeability. In addition, fractal characteristics of different tight reservoirs were compared and analyzed. [ABSTRACT FROM AUTHOR]