Throat Ratio Research Articles

Pore structure and fluid distribution of tight sandstone by the combined use of SEM, MICP and X-ray micro-CT

Tight reservoirs contain considerable hydrocarbon resources, which are characterized by complicated pore structure and heterogeneous fluid distribution in the pore network. In this study, a tight sandstone sample from the Lucaogou Formation in the Jimsar Sag was selected for the characterization of pore structure and two-phase (brine and oil) flow experiment. Interparticle and intraparticle pores were identified and quantified by direct image analyses. Intraparticle pores, commonly smaller than 1 μm, were the dominant contributor of surface area, whereas interparticle pores contributed considerably to pore volume with minor quantity. Pore network model of the selected sample was featured by large pore throat ratio, which was also verified by low efficiency of mercury ejection (≈1.66%). The flow potential of tight reservoir was dominantly controlled by pore-throats with a diameter larger than 2 μm. Based on the results of pore-scale micro-CT flow experiments, the configuration of oil clusters in the pore space was categorized into singlet, multiple, branched and network. The size of oil clusters ranged over five orders of magnitude (10 μm3 – 106 μm3), which followed a power-law distribution. The displacement process of water by oil in the pore space of tight sandstone was controlled by pore structure and external driving force. The well-connected pore network mainly composed of large interparticle pores was the preferential flow path, and a high proportion of singlet oil clusters was attributed to high aspect ratios of the pore network. As the external driving force increased, convergence and divergence of oil flow occurred simultaneously in the pore network. The displacement process of water by oil was suggested as a cooperative pore-filling event.

Response to the Variation of Clay Minerals During ASP Flooding in the Saertu Oilfield in the Songliao Basin

In this paper, the variation of clay minerals and their influence on reservoir physical properties and residual oil before and after ASP flooding are analyzed. The results show that the total amount of clay minerals in reservoirs decreases after ASP flooding in the ultra-high-water-cut-stage reservoirs of the Naner Zone in the Saertu Oilfield, Songliao Basin. Therein, the illite content reduces, while the content of illite smectite mixed-layer and chlorite increases. The content of kaolinite varies greatly. The content of kaolinite decreases in some samples, while it increases in other samples. The clay minerals block the pore throat after ASP flooding. As a result, the pore structure coefficient and the seepage tortuosity increase, the primary intergranular pore throat shrinks, and the pore–throat coordination number decreases. Nevertheless, the dissolution of clay minerals reduces the pore–throat ratio and increases porosity and permeability. The variation of clay minerals after ASP flooding not only intensifies the reservoir heterogeneity but also affects the formation and distribution of residual oil. The residual oil of the oil–clay mixed adsorption state is a newly formed residual oil type related to clay, which accounts for 44.2% of the total residual oil reserves, so it is the main occurrence form of the oil in reservoirs after ASP flooding. Therefore, the exploitation of this type of residual oil has great significance to enhance the oil recovery in ultra-high-water-cut-stage reservoirs.

Interactive mechanism of manufacturing factors on the properties of microbial cementing slurry

In recent years, there has been an increasing interest in microbial self-healing technology in the field of well cementation due to its environmentally friendly, high efficiency, low maintenance cost, and intelligent characteristics. Previous studies have proved that the mechanical strength, setting time, and permeability are the fundamental properties of self-healing cementing slurry which determine the quality of well cementation and the recovery of oil and gas. However, limited research was reported on the effect of manufacturing factors on those basic properties of microbial self-healing cementing slurry in the field of well cementation. In the process of preparation, the air is introduced into the microbial self-healing cementing slurry, causing macropores in the cement stone. Besides, the ammonia produced by microorganisms also results in pores with various size in the microbial cement stone. Those pores which are not conducive to the development of mechanical strength of cement stone might develop into cracks, leading to the failure of well cementation, even causing severely fluids channeling in the wellbore. Therefore, it is of great significance for us to explore the cause of pores in microbial cement stone and choose the proper manufacturing parameters for the field application. In this paper. we studied the influence of manufacturing factors including stirring rate, defoamer dosage and curing ages on the mechanical strength and durability of microbial cement stone. Besides, we explored the interaction mechanism between micro pore structure parameters and macro properties such as compressive strength and permeability of microbial cement stone by the computed tomography technology. The results showed that the number of pores in the microbial cementing slurry was increased with the stirring rate, whereas it was reduced with the increase of defoamer dosage and the extension of curing ages. We also found that there was a poor correlation between the pore structure parameters such as porosity, radius of pores and throats, pore throat ratio and mechanical strength, but a positive correlation between the pore throat ratio and permeability. Moreover, the optimal manufacturing parameters were suggested for the possible field application in this paper. These findings provide a theoretical guidance for the fabrication and application of microbial self-healing cementing slurry in the underground construction buildings besides the well cementation.

Pore Structural Features of Granite under Different Temperatures.

To explore the effects of thermal actions on the pore structural features of granite, scanning electron microscope (SEM) and mercury injection experiments were carried out on granite after thermal treatment (25 °C to 400 °C). The pore structure was investigated from various perspectives, including the capillary pressure curve, the pore–throat ratio, the median saturation pressure, the median pore–throat radius, the porosity, the pore volume, and the pore size distribution. Based on mercury intrusion test data, the Winland model of permeability prediction was modified for a high-temperature tight granite reservoir. The results showed that: (1) As the temperature rose, the mercury injection curve was gradually flattened, and the mercury ejection efficiency gradually increased. Meanwhile, the pore–throat ratio and the median saturation pressure decreased exponentially, and the pore connectivity was enhanced. (2) The median pore–throat radius and the porosity of granite increased exponentially as the temperature increased. Above 200 °C, the median pore–throat radius and the porosity increased substantially. (3) The pore volumes of the transitional pores, mesopores and macropores, and the total pore volume inside the granite, increased as the temperature rose. Especially above 200 °C, the transitional pores and the mesopores were prominently developed, and the pore volumes of the transitional pores and the mesopores took up a significantly greater proportion of the total pore volume. (4) As the temperature rose, the pore size distribution of granite became more extensive, the pore–throat structure was obviously developed, and the pore–throat connectivity was enhanced. (5) The relationship between the micropores’ characteristic parameters and the macro-permeability in engineering was established though a modified Winland model, and the modified Winland model had a better prediction effect. The findings provide a solid basis for rock geothermal mining projects and related geotechnical engineering.

Research on the Enhanced Oil Recovery Technique of Horizontal Well Volume Fracturing and CO2 Huff-n-Puff in Tight Oil Reservoirs.

The low oil productivity of tight oil reservoirs is mainly caused by poor reservoir physical properties such as low porosity and permeability, a small pore and throat ratio, and bad well connectivity, which lead to bad water injection capability and rapid pressure decline for the reservoir. In this paper, an enhanced oil recovery technique for tight oil reservoirs is proposed by combination of horizontal well volume fracturing and CO2 huff-n-puff to improve the reservoir physical properties and increase the flow capability of crude oil. In the initial reservoir development stage, the precise horizontal well trajectory and reservoir volume fracturing scale are designed by analyzing the distribution of sand body thickness and the oily sedimentary facies. The micro-seismic tracking technique is also used to monitor the fracture elongation. When the reservoir energy cannot satisfy the economic limit of oil productivity, the CO2 huff-n-puff technique is applied to increase the reservoir energy quickly. After the precise fracturing technique is used in tight oil Block X, the average oil production rate of six fractured horizontal wells increases by 5 ton (1 ton = 7.33 bbl) at the initial production stage, and the effective oil production increase life lasts for 32 months. When the reservoir energy is supplemented using the CO2 huff-n-puff technique, the oil production rate of pilot experiment well SP-1 increases from 1.9 to 12.8 ton with a cumulative oil increase of 1333.8 ton.

Flow Path Resistance in Heterogeneous Porous Media Recast into a Graph-Theory Problem

This work aims to describe the spatial distribution of flow from characteristics of the underlying pore structure in heterogeneous porous media. Thousands of two-dimensional samples of polydispersed granular media are used to (1) obtain the velocity field via direct numerical simulations, and (2) conceptualize the pore network as a graph in each sample. Analysis of the flow field allows us to distinguish preferential from stagnant flow regions and to quantify how channelized the flow is. Then, the graph’s edges are weighted by geometric attributes of their corresponding pores to find the path of minimum resistance of each sample. Overlap between the preferential flow paths and the predicted minimum resistance path determines the accuracy in individual samples. An evolutionary algorithm is employed to determine the “fittest” weighting scheme (here, the channel’s arc length to pore throat ratio) that maximizes accuracy across the entire dataset while minimizing over-parameterization. Finally, the structural similarity of neighboring edges is analyzed to explain the spatial arrangement of preferential flow within the pore network. We find that connected edges within the preferential flow subnetwork are highly similar, while those within the stagnant flow subnetwork are dissimilar. The contrast in similarity between these regions increases with flow channelization, explaining the structural constraints to local flow. The proposed framework may be used for fast characterization of porous media heterogeneity relative to computationally expensive direct numerical simulations.Article HighlightsA quantitative assessment of flow channeling is proposed that distinguishes pore-scale flow fields into preferential and stagnant flow regions.Geometry and topology of the pore network are used to predict the spatial distribution of fast flow paths from structural data alone.Local disorder of pore networks provides structural constraints for flow separation into preferential v stagnant regions and informs on their velocity contrast.

Breakup Behaviors of Viscoelastic Polymer Droplets in 3-D Pore Throat Structure Microchannel

Polymer solution has extremely extensive applications in many natural and industrial processes, especially in oilfield development field, such as polymer flooding, fracturing and water shut-off. Thus, the study of flow behaviors of polymer in formation pores is significantly important. In this paper, the flow behaviors of the polymer droplets in the 3-D pore throat structure were systematically studied. Additionally, the influencing factors (polymer concentration, molecular weight and pore throat ratio, for instance) were investigated. As the increasing of polymer concentration and molecular weight, the polymer droplets were more difficult to break, which means the critical flow rate decreased and the average sizes of the first daughter droplets (FDD) were longer synchronously. Moreover, with the increase in pore throat ratio, the critical flow rate increased and the length of the FDD decreased. In addition, the prediction models of the length of the FDD with polymer concentration and pore throat ratio were established, respectively. The prediction model revealed that the length of the FDD satisfied an exponential relationship with the polymer concentration and a linear relationship with the pore throat ratio. Finally, the average size of droplets after macroscopic core flooding experiment was 8 μm and 17 μm when the polymer concentration was 0.01% and 0.1%, respectively. The results were consistent with the breakup behaviors of polymer droplets in the microscopic pore throat structure.

Lattice Boltzmann Modeling of the Apparent Viscosity of Thinning–Elastic Fluids in Porous Media

Many non-Newtonian fluids, including polymers, exhibit both shear-thinning and viscoelastic rheological properties. A lattice Boltzmann (LB) model is developed for simulation of the flow of thinning–elastic fluids through porous media. This model applies the Oldroyd-B constitutive equation and the Carreau model, respectively, to account for the viscoelastic and shear-thinning behaviors of the thinning–elastic fluid in porous media. Both rheological features are captured well by this model and are verified against analytical solutions. The thinning-then-thickening viscosity curve of the thinning–elastic fluid observed in experiments is reproduced by the present pore-scale simulations. In addition to the traditional extensional theory, we propose other important mechanisms for the increase in apparent viscosity of viscoelastic fluids at higher shear rates. The mechanisms proposed include the reduction in conductivity due to stagnant fluid, the compressed effective flow region, and larger energy dissipations caused by the viscoelastic instability. We find that the viscoelastic thickening effect is more prominent in porous geometries with a large pore–throat ratio.

Influence of pore structural properties on gas hydrate saturation and permeability: A coupled pore-scale modelling and X-ray computed tomography method

Gas hydrate, as an alternate hydrocarbon source, has attracted significant attention in past decades. A precise estimation of permeability of the gas hydrate-bearing formation is essential for predicting the flow behaviors and the associated gas production performance. In this research, the influence of gas hydrate saturation on pore structural properties and then affect permeability was investigated using a three-dimensional micro X-ray computed tomography dataset that records an experiment of xenon hydrate formation in a sand pack at selected times during the experiment. Unlike the previous work, the goal of this work is to characterize pore space evolution, during gas hydrate formation, using a set of key microscopic pore characteristics, i.e. pore and throat radii, pore throat ratio, coordination number and tortuosity, by applying pore network analysis, on larger and therefore more representative sub-volumes. The same segmented volume of that dataset was used in this work to estimate gas hydrate saturation and permeability, recalculated by the lattice Boltzmann method, on those larger sub-volumes at selected snapshots. Besides, the analysis provides further insights into the links of gas hydrate localities and local pore characteristics and therefore their controls on the permeability. New evidence of semi-quantitative nature emerges that grain-coating gas hydrate formed at low gas hydrate saturations play a crucial role on reducing permeability, while pore-filling and/or cementing gas hydrate become dominating at high gas hydrate saturations, and these can be explained by gas hydrate formation mechanisms.

A Simplified Capillary Bundle Model for CO2-Alternating-Water Injection Using an Equivalent Resistance Method

CO2-alternating-water injection is an effective way of enhancing recovery for low-permeability oil reservoirs. The injection process is one of the essential issues that are facing severe challenges because of the low permeability and poor pore space connectivity. Previous researchers mentioned that water injection ability could be decreased by around 20% after the CO2-flooding; hence, it is necessary to quantify the water injectivity variation during an alternated injection process. In this paper, a CO2 convection-diffusion model is established based on the seepage law of CO2 and dissipation effect. The relationship between the width of miscible flooding and injection time is defined. Besides, an equivalent resistance method is introduced for developing a capillary bundle model for featuring an unequal diameter for CO2 water vapor alternate flooding. CO2-oil and CO2-water interactions are analyzed using the new model. The effects of oil viscosity, pore throat ratio, CO2 slug size, and equivalent permeability of the capillary bundle on water injection are analyzed. The result indicates that water injection ability increases with the rise of CO2 slug size and equivalent permeability of the capillary bundle and decreases with the increase of viscosity and pore throat ratio.

Numerical Investigation on the Cold Flow Field of a Typical Cavity-based Scramjet Combustor with Double Ramp Entry

Abstract The effects of back pressure and cavity L/D ratio on the shock wave structure in the cold flow field of a typical cavity-based scramjet combustor with combined inlet and isolator is investigated numerically in the selected scramjet models. The scramjet with a throat ratio of TR 0.0 and cavity L/D 6.04 was analyzed. To perform such analysis, steady, 2-D RANS was used with SST k-ω. From the analysis, the value of static pressure along the cowl surface, contours of Mach number and pressure were obtained. The scramjet was modeled with different TR 0.1, 0.2, 0.25 and 0.3 with the same cavity L/D 6.04 and different cavity L/D 4.04, 9.04 and 12.04 with the same TR 0.25. All the models were analyzed with the same inlet conditions and the results were obtained. From the analysis, it was observed that the increase in back pressure moves the shock train towards the inlet of the isolator which leads to ‘engine unstart’ after the throat ratio of TR 0.1. Also, it is observed that there is an optimal L/D ratio of the cavity L/D 9.04 which restricts the propagation of high-pressure waves obtained in the combustor.

Analysis of the Imbert downdraft gasifier using a species-transport CFD model including tar-cracking reactions

Primary tar reduction is an effective method to diminish tar concentration in producer gas obtained from a downdraft biomass gasification process. It tends to be more economical than secondary tar reduction in post-gas-cleaning processes. This study used a two-dimensional computational fluid dynamics (CFD) model to analyze the performance of an Imbert downdraft biomass gasifier. The spatial distribution of producer gas compositions in the gasifier was explored. The CFD model was validated, using the same conditions and design, by published experimental data. The effects of the key design parameter, i.e., throat diameter, represented by the ratio of throat to gasifier diameters and the height of air nozzle from the throat, on the gasifier performance were predicted. The tar concentration and cold gas efficiency (CGE) were computed. Although the modeling results indicated that decreasing the throat diameter can decrease the tar content up to 0.005 g Nm−3, the CGE decreased by approximately 7% compared with the scenario with no throat. Decreasing the nozzle-to-throat lengths abated tar and hydrogen composition, but the CGE exhibited fluctuation. Therefore, a compromise between tar reduction and the CGE is required to improve the design of the Imbert downdraft gasifier.

Effect of pore throat structure on micro-scale seepage characteristics of tight gas reservoirs

At present, the effects of pore throat structure on micro-scale seepage characteristics of tight gas reservoirs are less researched, and traditional numerical simulation methods are faced with a great number of challenges in the study of micro-scale flow. In this paper, the flow pattern of tight gas was studied based on the actual temperature and pressure of tight gas reservoir and the characteristic size of reservoir pore throat, and the rationality of tight gas flow was simulated by means of lattice Boltzmann method. Then, considering the influences of micro-scale effect, slippage effect and other factors, a tight gas flow model was established on the basis of LBGK-D2Q9 model, and its calculation results were compared with the analytical solutions and the numerical solutions listed in the literature. Finally, the influential laws of pore throat structure on the micro-scale seepage characteristics of tight gas were discussed. And the following research results were obtained. First, when the pressure is in the range of 3–70 MPa and the temperature is in the range of 293.15–373.15 K, the Knudsen number (Kn) is less than 0.1 and the gas flow is in the pattern of slippage flow and weak continuous flow. And in this case, it is reasonable to adopt the LBGK-D2Q9 model to simulate tight gas flow. Second, the effect of the characteristic size of the flow channel on the Kn is much greater than that of the pressure change. When the pore–throat ratio is constant, the Kn increases slowly along the throat. And its increasing trend gets more obvious with the increase of pore–throat ratio. Third, the presence of the throat makes the non-linear distribution characteristics of the pressure in the pore throat significant, and the pressure drop mainly lies in the throat. And the higher the pore–throat ratio is, the larger the pressure drop range in the throat is. Fourth, the non-linear distribution of pressure decreases the gas flow speed significantly, thus reducing the mass flow rate in the flow channel. In conclusion, the simulation result of the model established in this paper is highly coincident with the analytical solutions and the numerical solutions calculated by DSMC and IP methods in the literature, which verifies that this proposed model is reliable. The research results reveal the importance of “connecting fracture and expanding throat” in the practical development engineering of tight gas reservoirs.

Removal of Contaminant Nanoparticles with $$\hbox {CO}_2$$ Nanobullets at Atmospheric Conditions

As the feature size of semiconductor chips is decreasing down to nanometric scales, cleaning of nanoscale contaminant particles without damaging the fine features puts forth severe technological challenges. Here we introduce a design methodology of a nozzle to generate a beam of supersonic $$\hbox {CO}_2$$ solid nanobullets into the air at atmospheric pressure, which dislodge the contaminant particles by colliding with them. The dry cleaning scheme proposed here does not resort to the chillers, vacuum chamber, and carrier-gas handling system, which conventional dry cleaning systems often required and thus hampered their practical applications. We provide a theoretical framework to select key design parameters, such as the area ratio of the nozzle throat and exit and the supply gas pressure. We experimentally verify the superior capability of our nozzle in generating a $$\hbox {CO}_2$$ aerosol beam under the atmospheric back pressure condition. Additional process parameters including the stand-off distance and the incident angle of the $$\hbox {CO}_2$$ beam are optimized to maximize the cleaning efficiency and minimize the pattern damages. Our work suggests a practical nanoparticle cleaning scheme that is faster and simpler than the conventional dry cleaning methods.

Numerical Simulation of Air Entrainment Performance of a Foam Generator Used for Dust Control in Coal Mines

Mine dust is one of the most serious environmental hazards in the coal mining process. This paper introduces a numerical simulation of a novel foam generator used for dust control in coal mines. The amount of foam generated by this device significantly depends on the amount of air entrainment. Therefore, a computational fluid dynamics (CFD) method was used to study three influencing factors, namely, throat-nozzle distance, mixing throat length, and the contraction angle of the suction chamber. The predicted values by the CFD simulation proved to be in good agreement with the experimental data. The results revealed that the air entrainment reached its maximum when the ratio of throat-nozzle distance to mixing throat length was 2/3. The optimum values of the throat ratio (its length to diameter) and the contraction angle of the suction chamber were obtained at 20 and 5°, respectively. This research provides essential guidance in the geometric parameter design of the self-suction type foam generator, which has the advantage of negating the need for compressed-air pipelines and having high reliability, compared to traditional foam generators.

Extraction and numerical simulation of gas–water flow in low permeability coal reservoirs based on a pore network model

ABSTRACT This study aimed to investigate the effects of pore structure on gas–water permeability in low permeability coal reservoirs. A 3D digital coal model with macropores (>13.85 μm) was constructed by using a μCT225kVFCB test system and Avizo image processing software. An equivalent pore network model (EPNM) was extracted on the basis of the 3D model. PoreFlow was used to simulate the micro-flow process of gas and water in the EPNM. A control variable method was used to analyze the influence of pore structure on relative permeability. Results showed a sensitive zone in the relative permeability of the non-wetting phase in the low permeability coal reservoir. In this zone, the relative permeability of the gas phase showed remarkable variations, and the changes of the pore throat ratio (PTR) were minimal. In addition, a critical throat radius (TR) was established in the low permeability coal reservoir with water saturation (S w) of less than 48.33%. Gas fluidity was poor when TR reached its critical value. The water phase relative permeability (K rw) increased with the increase of PTR. The variation of K rw was minimal as the coordination number (CN) increased. The influence of the pore throat structure on K rw was ranked as PTR>TR>CN.

Characteristics of multi-scale pore structure of coal and its influence on permeability

Due to the uneven distribution of pore size in coal and its wide distribution range, it is difficult to effectively characterize the multi-scale pore structure of coal by a single method. In this paper, the multi-scale pore structure characteristics of coal were analyzed comprehensively by using scanning electron microscope, low-temperature liquid nitrogen adsorption, high-pressure mercury intrusion and constant-rate mercury intrusion. In addition, the effects of metamorphism on the volume and specific surface area of pores in coal were revealed, and the relationships between coal rock permeability and pore structure characteristic parameters were described. And the following research results were obtained. First, with the increase of coal metamorphism, the volume and specific surface area of nanopores in coal decrease first and then increase, and they reach the minimum value when Ro,max is about 1.8%. Second, the pore and throat radii of coal samples are overall in the form of normal distribution. And with the increase of coal metamorphism, the pore radius corresponding to the maximum distribution frequency increases. Third, the samples of low-rank bituminous coal are the highest in throat radius distribution range, connected throat radius and average throat radius. Fourth, the samples of anthracite coal are the lowest in throat radius distribution range and connected throat radius. Fifth, there is a single main peak in the distribution of pore–throat ratios of low- and medium-rank bituminous coal samples, and the pore–throat ratios corresponding to the main peak is relatively low. Sixth, the permeability of coal is in a positive correlation with porosity and an average throat radius, and in a negative correlation with an average pore–throat ratio, but in no obvious correlation with an average pore radius.

An Improved NMR Permeability Model for Macromolecules Flowing in Porous Medium

The extraction of macromolecules from nano-self-assembled material can be used as a laboratory model for enhancing oil recovery in reservoirs. By combining Darcy’s law and Poiseuille equation, an improved nuclear magnetic resonance (NMR) permeability model, suitable for macromolecular flow in mesopores is obtained. The calibration coefficients in the Coates equation are expressed in terms of the physical parameters of pore throat ratio rb/rt, tortuosity, and thickness of bond film in the improved model. The results show that the proportion of irreducible fluid to total fluid obtained through NMR characterization can reflect the variation tendency of irreducible macromolecule and water. By simplifying the pores of the extracted samples, the thickness model of irreducible macromolecule and water is established with the total thicknesses calculated as 1.482 nm, 1.585 nm, 1.674 nm, and 1.834 nm. The corresponding permeability results obtained from the NMR characterization (KNMR) are 7.39 mD, 6.02 mD, 5.27 mD, and 6.25 mD. The permeability results obtained from mercury intrusion experiment (KHG) are 5.10 mD, 4.73 mD, 5.82 mD, and 5.56 mD, and those from the Darcy flow experiment (KD) are 4.1 mD and 5.19 mD. The absolute deviation between KNMR and KHG varies from 0.69 to 2.29 mD, while that between KNMR and KD is 1.58 mD. This method can be applied to the enhanced recovery of shale oil.

Simulation of natural gas hydrate formation skeleton with the mathematical model for the calculation of macro-micro parameters

The quantitative relationship among the four process parameters (bentonite, binders, pressure, and time) of artificial core preparation, two macroscopic physical parameters (permeability and porosity), and three microscopic pore parameters (average pore and throat diameters and pore throat ratio) are investigated through multiple sets of experiments. Further, this research considers the natural gas hydrate formation skeleton in the Mount Elbert Well on Alaska North Slope as the simulated object. The optimal artificial core formula with permeability and porosity similar to the in situ hydrate formation skeleton was optimized by an orthogonal test, the fitting function relationships among these three aspects were obtained, and a relationship model was established.The results demonstrated that the permeability and porosity of the optimal formula are extremely close to the natural formation skeleton, and the microscopic pore morphology is similar to that of the natural cores. The established relationship model can be used to accurately estimate the permeability, porosity, and pore and throat diameters using the four process parameters; in turn, the process parameters also can be estimated. The digital simulation preparation technology of the natural gas hydrate formation skeleton developed in this work can accurately and quickly estimate the macro-micro physical parameters of the artificial hydrate formation skeleton. Further, it can also prepare 60-cm-long cores that can be used in the subsequent experimental study of the physical property response in hydrate formation drilling, playing an important role for the simulation of natural formation skeletons.

The damage mechanisms of fracturing fluid on production in tight gas reservoirs

Hydraulic fracturing is generally required for tight gas formation with low matrix permeability to achieve economic production rate. Hydroxypropyl guar gum (HPG) fracturing is one of most commonly used technology. During fracturing, million gallons of guar gum fluid are pumped downhole to initiate fractures, and lots of fluid would filtrate into formation matrix. The guar gum will be adsorbed in the porous medium of sandstone and decrease the permeability of the reservoir. For improving the production of tight gas well after being stimulated, the adsorption mechanism between the guar gum and sandstone is studied. The action force is analysed through the Flourier transformation infrared spectroscopy (FT-IR), Zeta potential instrument and X-ray photoelectron spectroscopy (XPS). The result indicate that the hydrogen bonding is the key force between the guar gum and sandstone. To observe the adsorption morphology, scanning electron microscope (SEM) and atomic force microscope (AFM) are employed. Nuclear magnetic resonance (NMR) and computer tomography (CT) showed that the adsorption and retention of guar gum in tight gas reservoir resulted in the decrease of pore and throat ratio, the decrease of porosity by 2.7%~3.1%, and most adsorption occurs at grain edges and rock beddings.