Pore-throat Ratio Research Articles

Fine classification of ultra-low permeability reservoirs around the Placanticline of Daqing oilfield (PR of China)

Under SY/T 6285-2011 classification standard in China, reservoirs of which permeability ranging from 1 × 10−3μm2 to 10 × 10−3μm2 are divided into ultra-low permeability reservoir. While development practice of ultra-low permeability reservoirs around the Placanticline of Daqing shows that, water-breakthrough time and development effect of different reservoir types are greatly different. Besides the permeability values, reservoir fracture development is another important factor affecting development effect of ultra-low permeability reservoirs. Therefore, based on geologic characteristics of 63 blocks in ultra-low permeability reservoirs around the Placanticline of Daqing oilfield, using fracture parameters as primary classification index, matrix permeability, average throat radius, pore-throat ratio, movable fluid saturation and fluid mobility as main classification indexes, the method combined with grey correlation analysis, multi-parameter cluster analysis, dynamic-static consistency check and comprehensive evaluation is established. The classification evaluation criteria for two groups and four sub-groups has been determined, and the classification results of each block are verified. The results show that this method solves the problem of classification difficulty caused by classification parameters of individual blocks falling in different classification areas and realizes the goal of rational classification of 63 developed ultra-low permeability blocks. At the same time, it has the characteristics of simple calculation and high accuracy, which provides a powerful basis for the efficient and fine classification of ultra-low permeability blocks with similar geological characteristics.

Characteristics of microscopic pore-throat structure of tight oil reservoirs in Sichuan Basin measured by rate-controlled mercury injection

Abstract Based on the results of rate-controlled mercury-injection experiments, the microscopic pore-throat structure characteristics of tight sandstone in Sha-1 Section and tight limestone in Da’anzhai Section of Sichuan Basin were quantitatively characterized. The results show that the pore radius distribution characteristics of tight oil reservoirs are similar. The main distribution is between 100~190 μm, and the average pore radius is 160 μm. While the distribution of the throat radius of tight sandstone and limestone is quite different, the distribution of the throat of sandstone samples is relatively concentrated, and the distribution of the throat of limestone samples is relatively sparse. There is a good positive correlation between the average throat radius and permeability, but the correlation between fractal dimension and permeability is not obvious. This indicates that the permeability is mainly affected by the radius of the throat. The pore-throat ratio in tight oil reservoirs is relatively large, and the resistance to seepage is greater during development. Therefore, during the development of tight oil, measures should be taken to increase the radius of the throat, reduce the ratio of pore radius to pore-throat radius, and improve the seepage capacity of the reservoir, thereby improving the development of tight oil.

Effects of seasonal water-level fluctuation on soil pore structure in the Three Gorges Reservoir, China

Inundation of the Three Gorges Reservoir has created a 30-m water-level fluctuation zone with seasonal hydrological alternations of submergence and exposure, which may greatly affect soil properties and bank stability. The aim of this study was to investigate the response of soil pore structure to seasonal water-level fluctuation in the reservoir, and particularly, the hydrological change of wetting and drying cycles. Soil pore structure was visualized with industrial X-ray computed tomography and digital image analysis techniques. The results showed that soil total porosity (> 100 μm), total pore number, total throat number, and mean throat surface area increased significantly under wetting and drying cycles. Soil porosity, pore number and throat number within each size class increased in the course of wetting and drying cycles. The coordination number, degree of anisotropy and fractal dimension were indicating an increase. In contrast, the mean shape factor, pore-throat ratio, and Euler-Poincare number decreased due to wetting and drying cycles. These illustrated that the wetting and drying cycles made soil pore structure become more porous, continuous, heterogeneous and complex. It can thus be deduced that the water-level fluctuation would modify soil porosity, pore size distribution, and pore morphology in the Three Gorges Reservoir, which may have profound implications for soil processes, soil functions, and bank stability.

Characteristics of microscopic pore structure and its influence on spontaneous imbibition of tight gas reservoir in the Ordos Basin, China

Spontaneous imbibition is an important mechanism governing the tight reservoir production performance, especially in the early stage of production after hydraulic fracturing. The microscopic pore structure of reservoirs is an important factor affecting imbibition. In this study, high-pressure mercury injection (HPMI), rate-controlled mercury injection, and low-temperature nitrogen adsorption (LP-N2GA) were preformed to study pore structure characteristics and fluid distribution in samples from the Ordos Basin, China. Nuclear magnetic resonance (NMR) and imbibition experiments were used to investigate the fluid distribution in porous media during imbibition. The porosity of samples ranges from 11.378% to 12.356%, with an average of 12.034%. The permeability of samples ranges from 0.034 mD to 0.056 mD, with an average of 0.048 mD. Results show that the recovery ranges from 68% to 94.81%, with an average of 81.09%. The reservoir is dominated by open flat slit micropores and mesopores. Sample pore volume ranges from 0.0155 ml/g to 0.0193 ml/g, with an average of 0.01687 ml/g. Micropores and mesopores less than 50 nm provide most of the pore volume. Pore radius and pore-throat ratio were obtained from experiments. Imbibition occurs primarily in pores ranging from 0.1 μm to 1 μm in radius, followed by pores ranging from 1 μm to 10 μm. The effects of porosity, permeability, average pore radius, average throat radius, average pore-throat ratio, specific surface area, and pore volume on imbibition recovery were statistically analyzed. Grey relational analysis was applied to sequence the imbibition influencing factors and determine the most influential factor. Results demonstrate that imbibition recovery is the most affected by average pore radius, followed by permeability, porosity, average pore-throat ratio, median radius, average throat radius, and specific surface area.

Fractal and Multifractal Characteristics of Pore Throats in the Bakken Shale

To evaluate pore structures of the Bakken Shale, which is one of the most important factors that affect petrophysical properties, high-pressure mercury intrusion was employed in this study. Pore structures such as pore-throat size, pore-throat ratio, and fractal attributes are investigated in this major shale play. Pore-throat size from 3.6 to 200 um is widely distributed in these shale samples. Accordingly, pore-throat size distributions demonstrate the multimodal behavior within the samples. The whole pore-throat network can be divided into four clusters: one set of large pores, two transitional/intermediate pore groups, and one set of smaller pores. The fractal analysis revealed that fractal dimensions decrease as the pore-throat size decreases. The multifractal analysis demonstrated that as the maturity of the shale samples increases, pore-throat size distributions would become more uniform and pore structures tend to become more homogeneous. The results are compared to our previous results obtained from nitrogen gas adsorption for further verifications of fractal behavior. Finally, although fractal analysis of mercury intrusion and nitrogen gas adsorption were comparable, the results of multifractal analysis from these two methods were not identical.

Quantitative Analysis of Micron-Scale and Nano-Scale Pore Throat Characteristics of Tight Sandstone Using Matlab

Based on micro-scale casting thin sections, nano-scale SEM images, and the pore distribution map identified through a binary image in Matlab, the pore size distribution and pore throat coordination number of the strata of Upper Paleozoic He8 section tight sandstone in the southeastern Ordos Basin were quantitatively analyzed with the above experimental data. In combination with a high-pressure mercury injection experiment, the pore throat distribution, the pore throat ratio, and the relationships between the characteristics, parameters, and pore permeability were investigated clearly. The results show that the tight sandstone pore space in the study area is dominated by micron-sized intergranular pores, dissolved pores, and intragranular pores. The nano-scale pore throat consisted of clay minerals, intercrystalline pores, and the flake intergranular pores of overgrowth quartz grains. Kaolinite and illite intercrystalline pores occupy the pore space below 600 nm, while the ones above 800 nm are mainly dominated by the intergranular pores of overgrowth quartz grains, and the 600–800 nm ones are transitional zones. The permeability of tight sandstone increases with the average pore throat radius, sorting coefficient, median pore throat radius, and average pore throat number. The porosity is positively correlated with the average pore radius and the average pore throat coordination number, and negatively correlated with the median pore throat radius.

Characteristics of waterflood at low rate in low permeability sandstones based on the CT scanning

ABSTRACTIt is reported that the flooding rate in low permeability sandstones is low and the oil recovery is hard to increase after water breakthrough. Understanding characteristics of waterflood is hence important for the recovery improvement. In this work, flooding tests on low permeability sandstones were conducted. The corresponding flooding characteristics were investigated by means of CT scanning and Nuclear Magnetic Resonance. Effects of irreducible water and different rates were also discussed in detail. Experimental results reveal a piston-like displacement at a low rate in low permeability samples. The saturation profile is steep and almost vertical to the forward direction. The results at a low rate confirm that once water broke through, increasing the flooding rate or flooding time can hardly reduce the remaining oil inside the sample. It is probably due to the high pore-throat ratio proven by rate-controlled mercury. Results also confirm that the presence of initial water enhanced sweep efficiency substantially. On one hand, because water had previously occupied the small pores, the subsequent oil can only invade relatively large pores and became more movable. On the other hand, stable collars can not form due to the steep front, which may suppress the snap-off.

Evaluation of the Percolation Sensitivity of Loose Sandstone Using Digital Core Technology

Background:The integrity of the extracted core in loose sandstone gas reservoirs is poor, and because hydration and collapse easily occur, it is difficult to evaluate the sensitivity characteristics accurately by the traditional core flooding experiments.Objectives:We instead investigate the stress sensitivity and water sensitivity of the formation water soaking time using digital core technology.Methods:We take the core of a loose sandstone gas reservoir as a research object and begin by scanning the core samples with a CT scanner. A three-dimensional image of the core can be obtained, the digital information extracted, the pore structure of the porous media mapped directly to the network, and a digital core established using the principles of fractal geometry. The three-dimensional pore network model can also be extracted. Next, we can compare and correct the results calculated by the model based on the real core experimental results, and an objective and effective digital core model can be obtained.Results and Conclusion:Finally, we can calculate the different effective stress, pore throat parameters (pore throat radius, shape factor, coordination number, pore-throat ratio) and relative permeability of different formation water injury times. The research results demonstrate that in sandstone gas reservoir development, as the effective stress continuously increases, the rock pore-throat parameters continue to decrease, and the permeability of the reservoir rock ultimately declines by more than 43.2%. Clay minerals will expand after the edge and bottom water intrude into the reservoir and soak it for a long time: the pore throat is significantly narrowed within 30 days, while after 30 days more, the pore throat undergoes any only slight further changes, and the final permeability decline of the reservoir rock is up to 5.7%. The research results provide important basic petrophysical data for the development of loose sandstone gas reservoirs which, in turn, provide a scientific basis for formulating a reasonable gas production rate in a gas reservoir.

Effect of pore structure on displacement efficiency and oil-cluster morphology by using micro computed tomography (μCT) technique

Low recovery in mature oil fields, mostly in a high-water-cut stage, requires advanced methods to enhance oil recovery. However, incomplete understanding of the mechanisms of low oil recovery in pore-scale limits the application of proposed methods in industry. Digital core technology and high-resolution micro computed tomography (μ CT) scanning were deployed to investigate effects of pore structure and permeability scales on oil displacement efficiency and remaining oil-cluster morphology visually and quantitatively. In the study, brine displacing oil experiments were conducted on four naturally water-wet sandstone samples with different permeability scales. A μCT equipment with a resolution of 3.78 μm was used to image the samples at different water flooding stages by injecting brine water with different PV (pore volume): 0 (irreducible water saturation), 1, 5, 15, and 50PV. The complexity of the pore structures of four samples were quantified by fractal dimension analysis from μCT images. The order of pore structure from simple to complex is samples 1, 2, 3, and 4. Then the distributions of cluster pattern of reminding oil were analyzed. Based on the cluster pattern distributions, microscopic remaining oil was classified into five categories by using shape factor and Euler number. Results show that large connected oil clusters can be more easily broken into smaller segments in high permeability samples. Sample 1 has the largest pore connectivity and shows the biggest change of distribution pattern. The clusters are concentrated on one to two distribution patterns. If the permeability gets lower and pore structure becomes complex, all distribution patterns coexist. Also, the lower permeability samples show a smaller displacement efficiency at all displacing stages. A large coordination number improves the displacement efficiency, but the large tortuosity and pore-throat ratio reduce the displacement efficiency. Results of this study can enrich the understandings of the low oil recovery and can inspire innovative methods to further tap those remaining oil.

Micro pore and throat characteristics and origin of tight sandstone reservoirs: A case study of the Triassic Chang 6 and Chang 8 members in Longdong area, Ordos Basin, NW China

Abstract The microstructure differences of the Triassic Chang 6 and Chang 8 members tight reservoirs in the Longdong area of Ordos Basin were compared by means of cast thin sections, scanning electron microscope, X-ray diffraction, and constant rate mercury injection. Their pore evolution models were established, and the effects of main diagenesis on densification were examined. The throat is the main factor controlling the physical properties of the Chang 6 and Chang 8 members reservoirs: The lower the permeability, the smaller and the more concentrated the throat radius and the larger the proportion of the throats in the effective storage space. There are several obvious differences between Chang 6 and Chang 8 members: (1) with the increase of permeability, the contribution of the relative large throats to the permeability in the Chang 8 member reservoir is more than that in the Chang 6 member reservoir; (2) the control effect on pore-throat ratio of the nano-throats in the Chang 6 member reservoir is more significant. The sedimentary action determines the primary pore structure of the Chang 6 and Chang 8 members sand bodies, and the diagenesis is the main factor controlling the densification of the reservoirs. Because of the difference in rock fabrics and the chlorite content of Chang 6 and Chang 8, the strong compaction resulted in less porosity reduction (17%) of the Chang 81 reservoir with larger buried depth and larger ground temperature than the Chang 63 reservoir (19%). The siliceous, calcareous and clay minerals cement filling the pores and blocking the pore throat, which is the key factor causing the big differences between the reservoir permeability of Chang 6 and Chang 8 members.

A Finite Element Analysis of Pore Throat Ratio on the Characteristics of Flow Structure through Low Permeability Reservoir

Present research work focus on the two–dimensional axisymmetric incompressible flow of a Newtonian fluid through Pore–Throat tube. Different model parameters are employed to investigate the influence of inertia and impact of various pore throat ratios on flow structure, pressure differential, stress distribution and friction factor. A time–marching finite element method is used to analyse the numerical solutions through flow structure via streamline distribution, pressure distribution, speed and stress. A time–marching Taylor–Galerkin/Pressure–Correction procedure in semi–implicit form is employed to achieve the steady–state solutions. Predicted numerical results demonstrate Pore–Throat ratio have vital impact on the flow field distribution.

Characterization of micro-nano pore networks in shale oil reservoirs of Paleogene Shahejie Formation in Dongying Sag of Bohai Bay Basin, East China

Abstract For typical blocky, laminated and bedded mudrock samples from the Paleogene Shahejie Formation in the Dongying Sag of Bohai Bay Basin, this work systematically focuses on their structure characterization of multiple micro-nano pore networks. A use of mercury injection capillary pressure (MICP) documented the presence of multiple μm-nm pore networks, and obtained their respective porosity, permeability and tortuosity. Different sample sizes (500-841 μm GRI fractions, 1 cm-sized cubes, and 2.54 cm in diameter and 2-3 cm in height core plugs) and approaches (low-pressure N 2 gas physisorption, GRI matrix permeability, MICP, helium pycnometry, and pulse decay permeameter) were used to measure pore size distribution, porosity and permeability. The average porosity and matrix permeability determined from MICP are (6.31±1.64)% and (27.4±31.1)×10 -9 μm 2 , the pore throat diameter of pores is mainly around 5 nm, and the median pore throat diameter based on 50% of final cumulative volume is (8.20±3.01) nm in shale. The pore-throat ratios decrease with a decrease of pore size diameter. Moreover, the permeability of shale samples with lamination is nearly 20 times larger than matrix permeability. The geometrical tortuosity of the nano-scale 2.8−10.0 nm pore networks is 8.44 in these shales, which indicates a poor connectivity of matrix pore network and low flow capability. Overall, the variable and limited pore connectivity of shale samples will affect hydrocarbon preservation and recovery.

Experimental study of bubble breakup process in non-Newtonian fluid in 3-D pore-throat microchannels

Foam regeneration, an important step in foam profile control and foam flooding, is determined by the process of large dispersed mother bubbles breaking into small ones. The bubble behaviors in 3-D pore-throat microchannels were investigated in different polymer solutions by a high-speed digital camera. The initial bubble slug was generated through a co-flowing geometry and then flowed through a microfluidic constriction. Both breakup and non-breakup were observed at the operating conditions of Ca ranging from 0.000068 to 0.0067, and the breakup mechanisms can be divided into two different types: single bubble snap-off and multi-bubbles pinch-off. Emphases are given to the influences of pore-throat structures, capillary number and continuous phase viscoelasticity on the size variation of the daughter bubbles. The average daughter bubble size decreases with the increase of the pore-throat ratio or elasticity of solution. Furthermore, the change of the average size of daughter bubbles with capillary number could be scaled with a power law in breakup type Ι, while in breakup type ΙΙ, the mother bubble size becomes another decisive factor.

Numerical simulation of migration characteristics of the two-phase interface in water–gas displacement

The process of water–gas displacement in water–drive gas reservoirs has been always an important research topic. However, knowledge about the flow patterns of water–gas flow under different conditions is insufficient at present. To investigate the variation of the geometry and position of the water–gas interface as well as the water flooding efficiency with time during water–gas displacement, the distribution law of water and gas at the microcosmic scale is simulated and analysed based on the geometric model of a single pore channel abstracted from the parallel tube bundle model and continuous tube bundle model of a porous medium reservoir, with the help of laminar two-phase flow and the level set module of COMSOL Multiphysics. The simulation results show the following: (1) Under the combined effects of the boundary layer effect, interfacial tension and pore diameter changes in water–gas displacement, the shape of the water–gas interface changes from a tongue-shape to a U-shape, during which a series of interface shapes, including a piston shape, finger shape, W-shape and Ω-shape, are observed. In the pore channel, the area of the water phase below the pore channel axial is larger than that above the pore channel axial. (2) The pore-throat ratio has a considerable impact on the shape and location of the water–gas interface as well as the displacement efficiency. When the water–gas displacement is from a small pore-channel to a large one, the displacement efficiency increases as the pore-throat ratio decreases. On the contrary, if the water–gas displacement is from a large pore channel to a small one, the displacement efficiency increases as the pore-throat ratio increases. Numerical simulation can be used to study the shape and location of the water–gas interface as well as the displacement efficiency in pore channels with different scales of diameters.

Fractal analysis of flow resistance in random porous media based on the staggered pore-throat model

Flow resistance in porous media is a hot and difficult problem due to its importance and the complexity of the fluid flow in porous media. To improve the aligned pore-throat model, a more reasonable staggered pore-throat model is established. Fractal is employed in random porous media and the analytical flow resistance formulation is derived. The formulation is the function of tortuosity, porosity, pore-throat ratio, interference coefficient and so on. It’s found that the staggered pore-throat model and the original pore-throat model have their own applicable ranges of Reynolds number and porosity respectively. The interference coefficient is negligible at low Reynolds number, while considerable at high Reynolds number. The value of the interference coefficient c is determined to be 0.8.

Investigation on bubble snap-off in 3-D pore-throat micro-structures

Abstract Bubble behaviors in the single and cascaded three-dimensional (3-D) pore-throat micromodels were studied at the operating ranging from 0.0007 to 0.0249. It was found that the average size of daughter bubbles decreased with the rising of the pore-throat ratio, length of throat and mother bubble velocity. The variation of the average size of daughter bubbles with capillary number could be scaled with a power law. In addition, the bubble breakup mainly occurred in the first pore-throat structure and the size of the daughter bubbles maintained almost stable.

Determination of water-lock critical value of low-permeability sandstones based on digital core

Research and development of water lock inhibiting measures is very crucial in verifying the link mechanism between the internal factors of water lock and its extent of damage. Based on conventional water-lock physics experiments, however, only the consequence of macro water lock damage can be investigated, while the microscopic mechanism cannot be studied. In this paper, 3D digital cores of low-permeability sandstones were prepared by means of high-resolution micro-CT scan, and their equivalent pore network model was built as well. Virtual “imbibition” experiments controlled by capillary force were carried out by using pore-scale flow simulation. Then the link mechanism between the microscopic internal factors (e.g. wettability, water saturation and pore–throat structure parameters) and the water-lock damage degree was discussed. It is shown that the damage degree of water lock reduces gradually as the wettability transits from water wet to gas wet. Therefore, the water lock damage can be reduced effectively and gas-well productivity can be improved so long as the capillary environment is changed from strong water wettability to weak gas wettability. The more different the initial water saturation is from the irreducible water saturation, the more serious the water lock damage is. The damage degree of water lock is in a negative correlation with the coordinate number, but a positive correlation with the pore–throat ratio. Based on the existing research results, water lock tends to form in the formations composed of medium-sized throats. It is concluded that there is a critical throat radius, at which the water lock is the most serious.

Influence of pore structure parameters on flow characteristics based on a digital rock and the pore network model

Many approaches have been developed to study the role that pore structure parameters play in flow characteristics. In this paper, regular network models are used to study the influence of the porosity, particle size, pore throat ratio, coordination number, shape factor and pore throat orientation on the absolute permeability. Among them, every parameter has obvious effect except shape factor when single-phase fluid flows in the models. Then the random pore network model is developed to analyze the relative permeability curve with a changing pore throat ratio, coordination number and shape factor. When the water saturation is constant, the oil relative permeability is higher for models that have a higher coordination number or have a lower pore throat ratio, and the wet phase has low permeability because of the corners. In order to study the dominant factor of the pore structure for two-phase flow further, a digital core, based on CT scanning, is constructed to firstly research the capillary influence of internal structure that determines whether the fluid can flow at a certain pressure difference.

Experimental study on the pore structure characteristics of tight sandstone reservoirs in Upper Triassic Ordos Basin China

Tight sandstone reservoirs typically show a wide pore size distribution, which ranges from several nanometers to several hundred micrometers, requiring a combination of several techniques to properly characterize the pore structure characteristics. In this article, scanning electron microscopy, nitrogen gas adsorption, pressure-controlled porosimetry, and rate-controlled porosimetry were applied to investigate the pore systems of five tight samples of Yanchang Formation in Upper Triassic Ordos Basin China. Pore throat types and shapes were qualitatively identified and classified by scanning electron microscopy and nitrogen gas adsorption, and pore size distribution was calculated by combination of nitrogen gas adsorption, pressure-controlled porosimetry, and rate-controlled porosimetry, which is proposed as a new method to obtain the overall pore structure characteristics of tight sandstone reservoirs; then analyzing microscopic pore structure controls of permeability stress sensitivity of tight sandstone reservoirs. Results indicate that three typical pore types exist in tight sandstone reservoirs, which are interparticle pores, grain dissolution pores and micro cracks, and pore shapes contain sheet and bent sheet, cylindrical, and bottle neck shapes. Pore size distribution develops from nano-micrometer scale completely in tight sandstone reservoir, nitrogen adsorption experiments can accurately characterize nanometer scale pore size distribution mainly ranging 2–50 nm; nano-micrometer scale pore throats distribution characteristics are quantitatively analyzed by pressure-controlled porosimetry and rate-controlled porosimetry, pore throats radius ranges 10 nm–40 µm using the former, pores radius ranges mainly 80–300 µm, throats radius ranges 100 nm–5 µm and pore throat ratio difference is bigger by the latter, and the pore throat structures determine the typical reservoir characteristics in Ordos basin. The emphasis of study on mechanism of permeability stress sensitivity currently is microscopic pore throat structures instead of macroscopic permeability, indicating that the strongest is micro cracks and neck pores, intergranular cylindrical pores and intragranular dissolution pores is the weakest. This is significant to the development of tight sandstone reservoirs.

Effect of interfacial layer on water flow in nanochannels: Lattice Boltzmann simulations

A novel interfacial model was proposed to understand water flow mechanism in nanochannels. Based on our pore-throat nanochannel model, the effect of interfacial layer on water flow in nanochannels was quantitatively studied using Lattice Boltzmann method (LBM). It is found that both the permeability of nanochannel and water velocity in the nanochannel dramatically decrease with increasing the thickness of interfacial layer. The permeability of nanochannel with pore radius of 10nm decreases by about three orders of magnitude when the thickness of interfacial layer is changed from 0nm to 3nm gradually. Furthermore, it has been demonstrated that the cross-section shape has a great effect on the water flow inside nanochannel and the effect of interfacial layer on the permeability of nanochannel has a close relationship with cross-section shape when the pore size is smaller than 12nm. Besides, both pore-throat ratio and throat length can greatly affect water flow in nanochannels, and the influence of interfacial layer on water flow in nanochannels becomes more evident with increasing pore-throat ratio and throat length. Our theoretical results provide a simple and effective method to study the flow phenomena in nano-porous media, particularly to quantitatively study the interfacial layer effect in nano-porous media.