Simulation Study on the Effect of Coal Seam Hydraulic Fracturing to Increase Permeability

Hydraulic fracturing is mainly used to improve coal seam permeability, extract coal seam gas and prevent coal-gas outburst. However, in the aspect of anti-reflection effect, it is judged by the method of field inspection, and there is no good prediction method before the engineering application. In this work, PFC2D software is used to simulate the process of coal seam hydraulic fracturing, and the results are introduced into the mathematical coupling model of the interaction between coal seam deformation and gas flow for analysis. The results show that the particles are moving towards the direction of the minimum principal stress, the moving speed of the particles at the crack tip increases, and the crack will finally expand towards the direction of the maximum principal stress. The gas pressure decline rate and permeability increase rate of the fractured model are both higher than that of the unfractured model. With the increase of extraction time, the decrease rate of gas pressure and the increase rate of permeability both decrease rapidly and gradually approach 0. The longer the hydraulic fracturing time is, the more complex the fracture network is, and the faster the gas pressure drops. However, the difference between the horizontal and vertical gas pressure drops will become larger and larger, and the impact of fracturing on gas drainage effect will gradually become smaller. With the increase of fracturing time, the difference of permeability increase at different monitoring points becomes larger and larger, but with the increase of gas drainage time, the difference will gradually decrease. The higher the initial void pressure is, the faster the gas pressure drops and the greater the permeability increase. However, the influence of the initial void pressure on the permeability increase will gradually become smaller. The research results could provide certain guidance for the prediction of anti-reflection effect of hydraulic fracturing in underground coal mines.

Distribution and origin of anomalously high permeability zones in Weizhou formation, Weizhou 12-X oilfield, Weixinan Sag, China

In order to study the dominant seepage channel of the third member of Weizhou formation (Ew3) in Weizhou 12-X oilfield, Weixinan Sag, Beibu Gulf Basin, and tap the potential of remaining oil. The distribution and causes of the anomalously high permeability zones in Weizhou Formation were studied by using conventional core physical property analysis, scanning electron microscope, laser particle size analysis, X-ray diffraction and thin section microscopic identification. As the results show, vertically,there are three anomalously high permeability zones in the A11, A12 and A21 micro-stage of the middle diagenetic stage, with the depth range of 2300 m ~ 2400 m, 2400 m ~ 2600 m, 2600 m ~ 2900 m respectively. Grain size, sorting, dissolution and early emplacement of hydrocarbons are the main causes of anomalously high permeability zones. Although both grain size and sorting affect porosity and permeability, the effect of grain size on permeability is stronger than sorting, and sorting has a stronger effect on porosity than grain size. Magmatic hydrothermal and organic acid promote dissolution and concomitant porosity and permeability increase by dissolving unstable minerals. The early emplacement of hydrocarbons retard the cementation and accompanying porosity and permeability reduction by reducing the water-rock ratio. Finally, sandstone reservoirs in the Ew3 are characterized by anomalously high permeability zones.

Slip Gas Flow and Heat Transfer in Confined Porous Media With Different Shape Cylinders

With the recent advances in micro devices, an accurate gas flow and heat transfer analysis become more relevant considering the slip effect. A micro-scale, multiple-relaxation-time (MRT) lattice Boltzmann method with double distribution function approach is used to simulate flow and heat transfer through circular- and diamond-shaped cylinders at the porescale level. The velocity slip and temperature jump are captured at the boundaries using a non-equilibrium extrapolation scheme with the counter-extrapolation method. A pore-scale domain of micro-cylinders comprised of circle and diamond shape are studied. It is found that the permeability increases linearly with an increase in Knudsen number for both circular- and diamond-shaped cylinders. However, the permeability increase for circular obstacle is larger than that of the diamond one. A larger surface area for diamond cylinder will offer more resistance to flow, hence resulting in lower values. For heat transfer, the Nusselt number shows an increase with increasing Reynolds number, however, it decreases with the increase in porosity. Nusselt number values are found to be higher for a circular obstacle. A variable boundary gradient for circular obstacle could be a possible explanation for this difference.

Effect of a heterogeneity on tensile failure: interaction between fractures in a limestone

<p>Not all rocks are perfect. Frequently heterogeneities will be present, either in the form of pre-existing fractures, or in the form of sealed fractures. To date, investigation of sample heterogeneity, specifically tensile strength and strength anisotropy has focused on layered rocks, such as shales, sandstones and gneisses. Data is lacking on the effect of single planar heterogeneities, such as pre-existing fractures or stylolites, even though these frequently occur in geo-energy settings.</p><p>We have performed Brazilian Disc tests on limestone samples containing planar heterogeneities, investigating Brazilian test Strength (BtS) and the effects of orientation on strength. We used prefractured Indiana limestone to represent a planar heterogeneity without cohesion and Treuchtlinger Marmor samples with central stylolites to represent a planar heterogeneity of unknown strength (as an example of a sealed fracture). The planar discontinuity was set at different rotation angles of approximately 0–20–30–45–60–90⁰, where 90⁰ (steep angle) is parallel to the principal loading direction, and 0⁰ (low angle) to the horizontal axis of the sample. All experiments were filmed, and where possible Particle Image Velocimetry was used to determine internal particle motion. Moreover, we used a 2D Comsol model in which we simplified the stylolite surface as a sinusoid. The model was used to qualitatively determine how i) a different period of the sinusoid and ii) relative strength of sinusoid/matrix affect the results.</p><p>Our results show that all imperfect samples are weaker than intact samples. The 2D Comsol model indicates that the qualitative results remain unaffected by changing the period (assumed to be representative of roughness) of the cohesive heterogeneity, nor by the relative strength contrast: the location of the first fracture remains unaffected. For both heterogeneity types, the fracture patterns can be divided into four categories, with two clear endmembers, and a more diffusive subdivision in between.</p><p>For a cohesion-less heterogeneity:</p><ul><li>steep angles lead to frictional sliding along the interface, and only a small hypothesized permeability increase.</li> <li>Intermediate angles lead to a combination of tensile failure of the matrix and sliding along the interface, where for steeper angles more new fractures form which follow the path of the existing fracture.</li> <li>Low angles lead to closure of the old fracture and new tensile failure.</li> </ul><p>For a cohesive heterogeneity of unknown cohesion:</p><ul><li>Steep angles lead to intensive failure of the heterogeneous zone, attributed to the presence of a stress concentrator.</li> <li>Intermediate angles lead to partial failure along the heterogeneous zone, and the formation of new fractures in the matrix, potentially instigated by mode II failure to accommodate motion.</li> <li>Low angles lead to the formation of a new fracture plus opening within the heterogeneous zone.</li> </ul><p>These results imply that hydrofracture (i.e. creating tensile stresses) of a stylolite-rich zone will lead to more fractures than fractures in a homogeneous zone, where the orientation of the stylolites and bedding will control the orientation of the permeable pathways.</p>

Strength and permeability evolution of andesite during benchtop acid dissolution experiments: implications for volcanic systems

<p>Acidic crater lakes are common features of subaerial volcanic systems; indeed, research suggests the existence of over 700 volcanic lakes around the world. Their persistence requires a regular input of water (e.g., meteoric water) at a rate that exceeds the migration of fluid from the system—for example, due to evaporation or fluid flow through the porous edifice.  Flank aquifers and fumarole fields may similarly be strongly acidic environments.</p><p>In order to explore the evolution of the physical and mechanical properties of an andesite under these field-relevant chemical conditions, we performed batch reaction experiments over timescales from 1 day to 4 months. The experiments involved immersion of a suite of samples in a solution of sulfuric acid (0.125 M; pH ∼0.6). Periodically, samples were removed and their physical and mechanical properties measured. We observe a progressive loss of sample mass, along with a general increase in porosity. We attribute this to the dissolution of plagioclase,  accompanied by the generation of a microporous diktytaxitic groundmass due to glass dissolution.</p><p>Plagioclase phenocrysts are seen to undergo progressive pseudomorphic replacement by an amorphous phase enriched in silica and depleted in other, relatively more soluble, cations (Na, Ca, and Al). In the first phase of dissolution (i.e. between 1 and 10 days), this process appears to be confined to preexisting fractures within the plagioclase phenocrysts. Ultimately, however, these phenocrysts tend toward entire replacement by amorphous silica. We do not observe evidence of induced dissolution or alteration in the other mineral constituents of the material: pyroxene, cristobalite, and titanomagnetite, specifically.</p><p>Examining the required Klinkenberg corrections during permeability measurements, we quantitatively demonstrate that the relative aperture of flow pathways increases with progressive acid immersion, by as much as a factor of five. We propose that the dissolution process results in the widening of pore throats and the improvement of pore connectivity, with the effect of increasing permeability by over an order of magnitude relative to the initial measurements. Compressive strength of our samples was also decreased, insofar as porosity tends to increase.</p><p>We highlight broader implications of the observed permeability increase and strength reduction for volcanic systems including induced flank failure and related hazards, improved efficiency of volatile migration, and attendant eruption implications.</p>

Experimental Investigation of Permeability Evolution on Sandstone in Triaxial and Long-Term Dissolution Experiment

The temporal permeability and damage evolutions of low-permeability sandstone cores during triaxial and long-term dissolution experiments were measured using a triaxial-flow system. Three triaxial experiments were performed on sandstone cores having initial permeability ranging from 78 × 10 − 18 m 2 to 120 × 10 − 18 m 2 . Two sets of long-term dissolution experiments were conducted on cracked sandstone cores. All dissolution experiments were performed at room temperature and using a 10 g/L H2SO4 and 0.2 g/L H2O2 input solution. Permeability evolution was determined using Darcy’s law. The cores experienced an average increase of 25% in permeability in the dissolution experiment and 900%~1500% increase at the end of the experiment. The dissolution was fairly homogeneous during the long-term experiments whether on the 1 mm scale or the 10 μm scale. The relationship between damage and permeability was speculated and its correlation coefficient has been proved to be close to 1. These results suggest that hydraulic fracturing works well in permeability increase in low-permeability sandstone reservoir.

Air permeability and sound absorption coefficient changes from ultrasonic treatment in a cross section of Malas (Homalium foetidum)

We investigated the effect of ultrasonic treatment on Malas (Homalium foetidum) gas permeability and sound absorption coefficient using the transfer function method. Results showed a longitudinal average Darcy permeability constant of 2.02 (standard deviation SD 0.72) for untreated wood and 6.15 (SD 3.07) for ultrasound-treated wood, a permeability increase of 3.04 times. We also determined the average sound absorption coefficients in the range of 50 to 6.4 kHz and NRC (noise reduction coefficient: average value of sound absorption coefficient value at 250, 500, 1000, and 2000 Hz) of untreated Malas. Those values were 0.23 (SD 0.02) and 0.13 (SD 0.01), respectively, while those of ultrasonic-treated Malas were 0.28 (SD 0.02) and 0.14 (SD 0.02), a 19.74% increase in average sound absorption coefficient.

Neutralization of hexokinase 2-targeting miRNA attenuates the oxidative stress-induced cardiomyocyte apoptosis

Hexokinase 2 (HK2) is a metabolic sensor that couples glycolysis and oxidative phosphorylation of mitochondria by binding to the outer mitochondrial membrane (OMM), and it also has been implicated in induction of apoptotic process by regulating the integrity of OMM. When HK2 detaches from the mitochondria, it triggers permeability increase of the OMM and subsequently facilitates the cytosolic release of cytochrome c, a major apoptosis-inducing factor. According to previous studies, a harsh microenvironment created by ischemic heart disease such as low tissue oxygen and nutrients, and increased reactive oxygen species (ROS) can cause cardiomyocyte apoptosis. Under these conditions, the expression of HK2 in heart significantly decrease and such down-regulation of HK2 was correlated to the increased apoptosis of cardiomyocytes. Therefore, prevention of HK2 down-regulation may salvage cardiomyocytes from apoptosis. MicroRNAs are short, non-coding RNAs that either inhibit transcription of target mRNAs or degrade the targeted mRNAs via complementary binding to the 3’UTR (untranslated region) of the targeted mRNAs. Since miRNAs are known to be involved in virtually every biological processes, it is reasonable to assume that the expression of HK2 is also regulated by miRNAs. Currently, to my best knowledge, there is no previous study examined the miRNA-mediated regulation of HK2 in cardiomyocytes. Thus, in the present study, miRNA-mediated modulation of HK2 during ROS (H2O2)-induced cardiomyocyte apoptosis was investigated. First, the expression of HK2 in cardiomyocytes exposed to H2O2 was evaluated. H2O2 (500μM) induced cardiomyocyte apoptosis and it also decreased the mitochondrial expression of HK2. Based on miRNA-target prediction databases and empirical data, miR-181a was identified as a HK2-targeting miRNA. To further examine the effect of negative regulation of the selected HK2-targeting miRNA on cardiomyocyte apoptosis, anti-miR-181a, which neutralizes endogenous miR-181a, was utilized. Delivery of anti-miR-181a significantly abrogated the H2O2-induced suppression of HK2 expression and subsequent disruption of mitochondrial membrane potential, improving the survival of cardiomyocytes exposed to H2O2. These findings suggest that miR-181a-mediated down-regulation of HK2 contributes to the apoptosis of cardiomyocytes exposed to ROS. Neutralizing miR-181a can be a viable and effective means to prevent cardiomyocyte from apoptosis in ischemic heart disease.

Deflocculants for refractory castables (review)

Review and analysis of foreign studies aimed at researching the influence of various deflocculants on properties of refractоry castables have been executed. Technological efficiency of deflocculants application had been shown:– ADS/W brands comparison to polyacrylate and sodium threepolyphosphate in alumina-cement-based mixture in terms mass`s humidity reduction of 17 % and 22 % to obtain the required vibration flow (200 mm) and samples` compressive strength increase of 29 % and 56 %; – M-ADS/W brands comparison to ammonium polymethacrylate and sodium hexametaphosphate in low-cement high-alumina castable in terms mass`s vibration flow increase of 5 and 14 %, samples’ compressive strength increase of 23 and 14 % for heat-treated at 110 °C and of 44 and 53 % for fired at 1500 °C; – brand REFPAC MIPORE 20 comparison to Peramin AL200 in low-cement corundum-spinel castable in terms of mass`s vibration flow increase of 5 %, gas permeability increase in 15 times with samples` compressive strength decrease of 30 % and differences in water evaporation temperature, with samples’ size increasing, reduce to 3 °С (against 78 °С); in comparison with sodium threepolyphosphate in medium-cement corundum castable with cement content decrease of 2,5 % — gas permeability increase in 12 times and differences in water evaporation temperature reduce to 4 °С (against 50 °С); in multi-cement corundum castable with cement content decrease of 2,5 % — differences in water evaporation temperature reduce to 10 °C; – brand REFPAC 100 comparison to polycarboxylate esters and sodium threepolyphosphate in terms of differences in flow of “freshly prepared” and “aged” low-cement castables, made using high-purity bauxite and silica fume, reduce to 17 % (against 19 and 41 %); vibration flow of “freshly prepared” and “aged” castables, made using lower quality materials, increase of 98 and 8 %; – brands REFPAC 288:REFPAC 388 at ratio of 50:50 comparison to REFPAC 388 in low-cement corundum-spinel castables at ambient temperature of 5 °C in terms of vibration flow of “freshly prepared” and “aged” masses increase of 4 and 80 % with samples` compressive strength decrease of 7 % after curing and 4 % after drying; at ratio of 75:25 comparison to 62,5:37,5 at 20 °C — terms of vibration flow of “freshly prepared” and “aged” masses increase of 10 and 14 % with samples` compressive strength increase of 16 % after curing and 4 % after drying; – brand REFPAC 500 comparison to Peramin AL 200 and/or Peramin AL 300 in low-cement castables with high-purity materials in terms of time for massive precipitation of calcium aluminate reduce in 10 and 3 times at temperatures of 5 and 20 °С, and also increase in 2 times at 35 °С; – polycarboxylate esters with length side chain and very short side chain in low-cement corundum castable with lower and high-purity silica fume; – brand FS 10 in cement-less corundum castable in terms of CMOR increase of 18 %, but comparison to sodium threepolyphosphate — to its reduction of 10 %.