Throat Size Distribution Research Articles

Changes in pore structures and porosity-permeability evolution of saline-lacustrine carbonate reservoir triggered by fresh water-rock reaction

Salt dissolution occurs frequently during the drilling, water-flooding development and other operations of saline-lacustrine carbonate reservoirs, greatly changing the petrophysical properties of rocks. To understand the salt dissolution behavior and its effect on the rock structures, core-flooding and crushed sample soaking experiments are conducted on saline-lacustrine carbonate samples. Results indicate that ion concentration from core flooding experiments increases first and then decreases fast with time proceeding, while it has a positive e-based logarithmic relation with soaking time from sample soaking experiments. According to ion analysis, the main dissolved salt minerals include halite, anhydrite and glauberite. With the size increase of crushed samples, the dissolved mass is decreased, having a positive logarithmic relation with sample specific surface, and sample specific surface is logarithmically increased with soaking time. Rock capillary pressure curves get lower, and pore throat size distributions become wider than those before dissolution of salt minerals. Salt dissolution mainly affects the pore microstructure with size ranging from 0.01 to 10 μm, while having little effect on nanopores <0.01 μm. Salt dissolution could make the clay swelling and fine migration, inhibiting the increment in porosity and permeability. In addition, rock permeability has a power relation with porosity during the dissolution, and a permeability-growth-controlling threshold porosity is observed, which indicates the transformation point of dissolution area from pore bodies to throats. Furthermore, a mathematical model for characterization of rock porosity and permeability evolution relationship is derived, which allows for rock microstructure, salt composition and dissolution, and clay composition and swelling behavior.

Characterization of the pore-throat size of tight oil reservoirs and its control on reservoir physical properties: A case study of the Triassic tight sandstone of the sediment gravity flow in the Ordos Basin, China

Tight oil is a significant component of unconventional oil and gas resources. The pore structures of tight oil reservoirs determine the seepage law, which is a significant factor in evaluating reservoir quality. However, because of the strong heterogeneity of the pore structure in tight oil reservoirs, it is difficult to characterize the pore structure by conventional methods. Therefore, on the basis of conventional reservoirs research methods, combined with high-pressure mercury intrusion (HPMI) and constant-pressure mercury intrusion (CPMI), the pore-throat size distribution of tight sandstone reservoirs and its control on reservoir physical properties were studied. The main pore types in the study area were residual intergranular pores, secondary intragranular pores, intercrystalline micropores and microfractures, and throat types were sheet-like and tube throats. The pore-throat size distribution characterized by HPMI ranges from 0.0025 to 2.79 μm, and pore-throats larger than 10 μm occurred less frequently. The pore size distribution characterized by CPMI was between 90 and 350 μm, and the peak value distribution of the pore radius of the different samples was relatively concentrated, while the throat size distribution was between 0.12 and 6 μm, indicating that the throat radius of the different samples showed a strong heterogeneity, which also led to a strong heterogeneity of pore-throat radius ratio. Although HPMI and CPMI had their own limitations in characterizing the pore-throat size distribution, their combination could effectively characterize the whole pore-throat size distribution of tight reservoirs, which ranged from 0.0025 to 350 μm. The permeability of the tight sandstone reservoirs is primarily determined by the relatively wider pore-throats with small proportions, and the permeability decreased as the proportion of narrower pore-throats increased. The nanoscale pore throats (radius < 0.1 μm) were widely developed in the tight reservoirs, which had little influence on permeability, but these pore throats could be used as oil and gas storage spaces and played an important role in improving reservoir porosity.

The effect of thermal maturity on porosity development in the Upper Devonian –Lower Mississippian Woodford Shale, Permian Basin, US: Insights into the role of silica nanospheres and microcrystalline quartz on porosity preservation

Organic-matter pores are considered to be the dominant contributor to total porosity in many shale reservoirs, and consequently much attention has been paid to investigate the factors influencing their generation, especially thermal maturity. Recent studies have shown that in addition to maturation of organic matter, other diagenetic processes also play important roles in evolution of pore systems, including compaction, mineral dissolution, and quartz cementation. This study investigates the effect of thermal maturity on porosity development in the siliceous Woodford Shale by integrating multiple techniques including helium pycnometry, low pressure gas (N2) adsorption, mercury injection capillary pressure, and field emission scanning electron microscopy (SEM) analyses. We compare samples from two long cores from Woodford Shale at different levels of thermal maturity, one in the early oil window (average calculated Ro = 0.69%) and the other in the wet gas window (average calculated Ro = 1.34%).Porosity, pore size, type, and volume vary systematically with the thermal maturity increasing from early oil window to wet gas window. Porosity measured by helium pycnometry averages 4.1% for early oil window samples, and increased by approximately 50% for the wet gas window samples (average porosity =6.3%). Both pore size and pore throat size distributions display decreasing trends from early oil window to wet gas window. Petrographic examination of SEM images shows that pores developed between clay platelets dominate in the early oil window samples, and secondary organic-matter pores are the dominant pore type in wet gas window samples. Porosity shows no obvious correlation to TOC content in both oil window and gas window samples, but correlates positive to silica content, suggesting that silica content largely controls variations in porosity. Both silica nanospheres and microcrystalline quartz were observed and appear to be significant in preserving pores from compaction by forming rigid frameworks that prevent pores from collapsing. Generation of secondary organic-matter pores and preservation of primary pores by silica nanospheres account for the net porosity increase from early oil window to wet gas window.

Study of the evolution of transport properties induced by additive processing sand mold using X-ray computed tomography

Abstract Accurate characterization of the mass transport properties of additively processed sand molds is essential in order to achieve reproducibility of the produced castings and control of gas defects in foundry industries. The present work highlights the potential use of X-ray micro-computed tomography (μ-CT) to characterize the evolution of permeability and some major microstructural features of such additively processed sand molds. The evolution of mass transport properties of sand mold samples under specific processing conditions met in additive manufacturing and its influence on the porosity, the permeability, the tortuosity, and the pore and throat size distributions were characterized from 3D images provided by X-Ray μ-CT. The obtained results showed that the mass transport properties of additively processed sand molds can be closely predicted by using non-destructive in situ methods, such that improvements to the casting can be made to create more optimized 3D printed structures for foundry applications.

The Impacts of Nano-Micrometer Pore Structure on the Gas Migration and Accumulation in Tight Sandstone Gas Reservoirs

The uncertainties between reservoir quality and gas migration and accumulation in tight sandstone gas reservoirs are intrinsically attributed to complex microscopic pore structures. Integrated analysis including the physical simulation experiment of gas migration and accumulation, X-ray computed tomography (X-CT), and casting thin section (CTS) were conducted on core plug samples collected from the Upper Paleozoic Permian Lower Shihezi and Shanxi tight sandstone of the Daniudi area in the Ordos Basin to investigate the impacts of pore structure on the gas migration and accumulation. Physical simulation suggested that the gas flows in migration in tight sandstone reservoirs were characterized by deviated-Darcy linear flow and non-linear flow regimes. Minimum and stable migration pressure square gradients determined by application of apparent permeability were employed as key parameters to describe gas flow. Pore structure characterization revealed that the tight sandstone reservoir was characterized by wide pore and throat size distributions and poor pore-throat connectivity. The pore–throat combinations could be divided into three types, including the macropore and coarse throat dominant reservoir, full-pore and full-throat form, and meso-small pore and fine throat dominant form. Comparative analyses indicated that pore and throat radii determined the gas flow regimes by controlling the minimum and stable migration pressure gradients. Gas accumulation capacity was dominated by the connected effective porosity, and the gas accumulation process was controlled by the cumulative effective porosity contribution from macropores to micropores. Variations in pore structures resulted in differences in gas migration and accumulation of tight sandstone reservoirs. The macropore and coarse throat-dominant and the full-pore and full-throat reservoirs exhibited greater gas migration and accumulation potentials than the small pore and fine throat dominate form.

Three-dimensional fingering structures in immiscible flow at the crossover from viscous to capillary fingering

Fluid displacement experiments were performed with immiscible fluids in a packed bed of glass spheres. The three-dimensional (3D) structure of the fingering pattern was visualized by computer tomography (CT) for a range of capillary numbers (Ca) between 5.22 × 10−7 and 1.04 × 10−4 at the viscosity ratio of log M = −2.203 in the absence of the influence of buoyancy force. Based on the 3D CT images, the characteristics of the fingering pattern at the crossover from viscous fingering (VF) to capillary fingering (CF) were investigated quantitatively. The structure of the fingering pattern changed gradually from streak-like for VF to more compact for CF with a decrease in Ca. Associated with the crossover from VF to CF, saturation of the invading phase (IP) and the fractal number of the structure both increased. The invaded pore size and invaded throat size distributions were also estimated for all values of Ca. The invaded pore size distribution was similar for all Ca values, but a clear shift in the throat size distribution was observed with the crossover from VF to CF.

Pore connectivity characterization of shale using integrated wood’s metal impregnation, microscopy, tomography, tracer mapping and porosimetry

The pore connectivity of tight shale reservoirs plays an essential role in the movement of shale gas and oil, however, the characteristics of connected pores in shale with a multi-scale and coupled pore-fracture system are poorly constrained. Working with typical American (Barnett and Eagle Ford) and Chinese (Longmaxi) shale samples in 2D/3D spaces at nano- to mm-scales, connective pores were intruded with a molten alloy (Wood’s metal; WM) under a temperature of ~85 °C and high pressure (60, 300, and 600 MPa) conditions. After solidification of the alloy at room temperature, polished sections were used to map WM components by field emission-scanning electron microscopy (SEM), micro- and nano-X-ray tomography and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). These tests were supplemented with mercury intrusion porosimetry (MIP) for pore-fracture throat size distribution. The shale matrix is generally characterized by low pore connectivity; however, the extent of connectivity within μm-sized and dispersed organic matter (OM) particles is high, with the observed WM-filled pore space ranging from 10% to 70% (averaged at 43%) for the Barnett Shale sample. The grain-edge fractures are important channels to connect multiple OM-hosted pore systems dispersed in shale matrix. Our work illustrates that shales exhibit a dual-connectivity behavior, with the effective porosity decreasing sharply as the distance from the sample boundary increases; the good pore connectivity zone away from the edge of sample is 500 μm under a pressure of 600 MPa for the Barnett Shale sample.

Pore size distribution, their geometry and connectivity in deeply buried Paleogene Es1 sandstone reservoir, Nanpu Sag, East China

The study of pore characteristics is of great importance in reservoir evaluation, especially in deeply buried sandstone. It controls the storage mechanism and reservoir fluid properties of the permeable horizons. The first member of Eocene Shahejie Formation (Es1) sandstone is classified as feldspathic litharenite and lithic arkose. The present research investigates the pore characteristics and reservoir features of the deeply buried sandstone reservoir of Es1 member of Shahejie Formation. The techniques including thin-section petrography, mercury injection capillary pressure (MICP), scanning electron microscopy and laser scanning confocal microscope images were used to demarcate the pores including primary intergranular pores and secondary intergranular, intragranular, dissolution and fracture pores. Mercury injection test and routine core analysis were led to demarcate the pore network characteristics of the studied reservoir. Pore size and pore throat size distribution are acquired from mercury injection test. Porosity values range from 0.5% to 30%, and permeability ranges 0.006–7000 mD. Pore radii of coarse-grained sandstone and fine-grained sandstone range from 0.2 to > 4 µm and 1 nm to 1.60 µm, respectively, by MICP analysis. The mineral composition also plays an important role in protecting the pores with pressure from failure. Fractured sandstone and coarse-grained sandstone consist of large and interconnected pores that enhance the reservoir porosity and permeability, whereas fine-grained sandstone and siltstone consist of numerous pores but not well interconnected, and so they consist of high porosity with low permeability.

Pore throat size distribution and oiliness of tight sands-A case study of the Southern Songliao Basin, China

In this study, several methods such as mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR) for characterizing the pore throat size distribution of tight sandstone reservoirs were selected. The differences of mercury injection parameters such as maximum interconnected pore throat radius, median radius, mercury injection curve, and pore throat distribution curve of oil-bearing and oil-free samples were compared. By designing the algorithm program and using the mercury injection data, the coefficient of converting the T2 relaxation time to the pore-throat radius was corrected, and the pore throat distribution of different oil saturation samples were analyzed. Environmental scanning electron microscope (ESEM) was also used to observe the microscopic pores and the oil in them. Using the above data, we systematically analyzed the control of pore throat distribution on the oiliness. It is found that the oiliness depends on the maximum interconnected pore throat radius, and the oil saturation depends on the oil migration front pressure and pore volume. The oiliness of tight sand has good relationship with pore throat size distribution, and the better reservoir petrophysical properties result in better oiliness. Statistics on the pore throat size distribution of oil-bearing tight sands were carried out and main intervals of pore throat size distribution of oil-bearing reservoirs were given in this paper. This understanding provides a basis for searching sweet spots in the future exploration of the tight oil.

Multiscale Digital Porous Rock Reconstruction Using Template Matching

AbstractRocks are heterogeneous multiscale porous media: two rock samples with identical bulk properties can vary widely in microstructure. The advent of digital rock technology and modern 3‐D printing provides new opportunities to replicate rocks. However, the inherent trade‐off between imaging resolution and sample size limits the scales over which microstructure and macrostructure can be identified and related to each other. Here, we develop a multiscale digital rock construction strategy by combining X‐ray computed microtomography and focused‐ion beam (FIB)‐scanning electron microscope (SEM) images, and we apply the technique to a tight sandstone. The computed tomography (CT) scanning images characterize macroscale pore structures, while the FIB‐SEM images capture microscale pore textures. The FIB‐SEM images are then coupled to CT images via a template‐matching algorithm and superposition. Bulk properties, including porosity and pore and throat size distribution, can be recovered with this approach. Permeability prediction with a pore network model for the largest connected pore network are 3 orders and 1 order of magnitude greater than the bulk rock measured value using the CT‐only and the SEM‐CT coupled images, respectively.

Grid density overlapping hierarchical algorithm for clustering of carbonate reservoir rock types: A case from Mishrif Formation of West Qurna-1 oilfield, Iraq

The primary goal of this study is to solve a major challenge of reservoir rock typing in quantitatively understanding the relationship between geological and petrophysical properties of carbonate reservoir. This paper first presents an improved overlapping index based on the grid density to define the overlapping relationship between different clusters and a grid density overlapping hierarchical algorithm (GDOH) for clustering of carbonate reservoir rock types. GDOH clustering algorithm is a hybrid type which integrates the gird, density, overlapping index and the hierarchical clustering method using the number of grid divisions and the threshold value of the overlapping index as the key parameters. Taking the Mishrif Formation of West Qurna Oilfield-1 as a case, the GDOH clustering algorithm is employed for clustering of carbonate reservoir rock types. As a result, ten geological rock types are identified according to the lithological type and diagenetic type using the thin section data. We finally got four reservoir rock types using the GDOH clustering algorithm with the overlapping index of core porosity and the permeability data as the metric. These reservoir rock types are different in reservoir qualities and porosity-permeability relationships. Each reservoir rock type also shows a similar set of capillary pressure curves and a typical and unique set of pore throat size distribution. The clustering result gives us some clues to understand the control of depositional and diagenesis features on the variation of reservoir rock types in reservoir quality and porosity-permeability relationship. Compared with the model-based petrophysical rock-typing techniques such as Winland r35, FZI and FZI-Star (FZI*), GDOH algorithm shows some apparent advantages in the evaluation of reservoir quality and prediction of permeability in three dimensions based on geologic principles in addition to understanding the relationship between reservoir rock type and petrophysical static and dynamic rock type.

Pore‐Throat Combination Types and Gas‐Water Relative Permeability Responses of Tight Gas Sandstone Reservoirs in the Zizhou Area of East Ordos Basin, China

With the aim of better understanding the tight gas reservoirs in the Zizhou area of east Ordos Basin, a total of 222 samples were collected from 50 wells for a series of experiments. In this study, three pore‐throat combination types in sandstones were revealed and confirmed to play a controlling role in the distribution of throat size and the characteristics of gas‐water relative permeability. The type‐I sandstones are dominated by intercrystalline micropores connected by cluster throats, of which the distribution curves of throat size are narrow and have a strong single peak (peak ratio &gt;30%). The pores in the type‐II sandstones dominantly consist of secondary dissolution pores and intercrystalline micropores, and throats mainly occur as slice‐shaped throats along cleavages between rigid grain margins and cluster throats in clay cement. The distribution curves of throat size for the type‐II sandstones show a bimodal distribution with a substantial low‐value region between the peaks (peak ratio &lt;15%). Primary intergranular pores and secondary intergranular pores are mainly found in type‐III samples, which are connected by various throats. The throat size distribution curves of type‐III sandstones show a nearly normal distribution with low kurtosis (peak ratio &lt;10%), and the micro‐scale throat radii (&gt;0.5 (μm) constitute a large proportion. From type‐I to type‐III sandstones, the irreducible water saturation (Swo) decreased; furthermore, the slope of the curves of Krw/Krg in two‐phase saturation zone decreased and the two‐phase saturation zone increased, indicating that the gas relative flow ability increased. Variations of the permeability exist in sandstones with different pore‐throat combination types, which indicate the type‐III sandstones are better reservoirs, followed by type‐II sandstones and type‐I sandstones. As an important factor affecting the reservoir quality, the pore‐throat combination type in sandstones is the cumulative expression of lithology and diagenetic modifications with strong heterogeneity.

Effects of micropores on geometric, topological and transport properties of pore systems for low-permeability porous media

Pore systems become very complex in heterogeneous reservoirs such as low-permeability and tight sandstones for macropores and micropores both exist in such reservoirs. Although the large pores may control the reservoir quality and rock properties, micropores always can affect the geometric and transport properties of the pore systems. In this paper, thus, such an effect is studied comprehensively. As such, low- and high-resolution computed tomography (CT) scanning experiments were carried out on the same low-permeability sandstone in order to obtain the low- and high-resolution digital rock (LRDR and HRDR) images, respectively. The impacts of micropores on properties of pore systems were investigated based on digital rock analysis of the LRDR and HRDR images for two samples. In this study, the micro-porosity is defined as the pores and throats which can be covered by the HRDR images but cannot be covered by the LRDR images. The results show that there are better connectivity, smaller tortuosity and higher heterogeneity for the HRDR image, compared to the LRDR image of the same rock. The fractal analysis of pore space indicates that the surface of the pore space with micropores becomes rougher. Moreover, we found that the micropore space plays a more significant role in the throats than the pores by comparing the pore and throat size distributions of the LRDR and HRDR images. In addition, the micropores can lead to a decrease in the average shape factor and smoothness of the boundary of pores and throats. It was also found that micro-porosity not only increases the average coordination number but also increases the number of isolated pores according to the coordination number distributions. Finally, the results of single- and two-phase flow simulation show that the absolute permeability and formation factor become larger and smaller, respectively, for the pore systems with micropores. This result reveals that micropores can improve the transportability and connectivity of the pore systems.

Characterization of Pore Throat Size Distribution in Tight Sandstones with Nuclear Magnetic Resonance and High-Pressure Mercury Intrusion

Characterization of pore throat size distribution (PTSD) in tight sandstones is of substantial significance for tight sandstone reservoirs evaluation. High-pressure mercury intrusion (HPMI) and nuclear magnetic resonance (NMR) are the effective methods for characterizing PTSD of reservoirs. NMR T2 spectra is usually converted to mercury intrusion capillary pressure for PTSD characterization. However, the conversion is challenging in tight sandstones due to tiny pore throat sizes. In this paper, the linear conversion method and the nonlinear conversion method are investigated, and the error minimization method and the least square method are proposed to calculate the conversion coefficients of the linear conversion method and the nonlinear conversion method, respectively. Finally, the advantages and disadvantages of these two different conversion methods are discussed and compared with field case study. The research results show that the average linear conversion coefficients of the 20 tight sandstone core plugs collected from Yanchang Formation, Ordos Basin of China is 0.0133 μm/ms; the average nonlinear conversion coefficient is 0.0093 μm/ms and the average nonlinear conversion exponent is 0.725. Although PTSD converted from NMR spectra by the nonlinear conversion method is wider than that obtained from linear conversion method, the nonlinear conversion method can retain the characteristic of bi-modal distribution in PTSD.

Tracing mercury entrapment in porous cement paste after mercury intrusion test by X-ray computed tomography and implications for pore structure characterization

Mercury entrapment in a porous material after mercury intrusion porosimetry (MIP) test is widely reported, yet the characterization of the entrapped mercury remains unclear. This study reported an experimental investigation on the entrapped mercury in a porous cement paste with/without surface-conformance control determined by X-ray computed tomography (X-CT). MIP tests were performed under three different maximum intrusion pressures. Results show that the surface-conformance control can substantially depress the first rise of throat size distributions and lower the threshold throat sizes. The entrapped mercury drops can be easily traced by X-CT due to the high mass attenuation coefficient of mercury. Both the entrapped and released mercury drops only occupy the limited volume fraction of the intruded pores. The surface conformance can decrease the mercury volume released from the pores. The findings of this study provide insights in better pore structure characterization of porous cement-based materials.

Granite microporosity changes due to fracturing and alteration: secondary mineral phases as proxies for porosity and permeability estimation

Abstract. Several alteration facies of fractured Lipnice granite are studied in detail on borehole samples by means of mercury intrusion porosimetry, polarized and fluorescent light microscopy, and microprobe chemical analyses. The goal is to describe the granite void space geometry in the vicinity of fractures with alteration halos and to link specific geometries with simply detectable parameters to facilitate quick estimation of porosity and permeability based on, for example, drill cuttings. The core of the study is the results of porosity and throat size distribution analyses on 21 specimens representing unique combinations of fracture-related structures within six different alteration facies basically differing in secondary phyllosilicate chemistry and porosity structure. Based on a simple model to calculate permeability from the measured porosities and throat size distributions, the difference in permeability between the fresh granite and the most fractured and altered granite is 5 orders of magnitude. Our observations suggest that the porosity, the size of connections and the proportion of crack porosity increase with fracture density, while precipitation of iron-rich infills as well as of fine-grained secondary phyllosilicates acts in the opposite way. Different styles and intensities of such end-member agents shape the final void space geometry and imply various combinations of storage, transport and retardation capacity for specific structures. This study also shows the possibility to use standard mercury intrusion porosimetry with advanced experimental settings and data treatment to distinguish important differences in void space geometry within a span of a few percent of porosity.

Exponential Capillary Pressure Functions in Sedimentary Rocks

The Brooks-Corey power-law capillary pressure model is commonly imposed on core analysis data without verifying the validity of its underlying assumptions. The Brooks-Corey model, originally developed to model the pressure head during the drainage of soil, is only valid at low wetting phase saturations. However, such models are often applied in petroleum production simulations and may lead to erroneous recovery factors when the saturation range of interest is far from the end points. We demonstrate that exponential models work much better for capillary pressure compared to the Brooks-Corey model over a wide saturation range. Mercury injection porosimetry, petrographic image analysis, and magnetic resonance studies suggest that the pore and throat size distribution in many rocks are log-normally distributed. This fact was previously employed to calculate the capillary pressure function as a function of saturation for pore size distributions described by atruncated log-normal distribution. Employing a Taylor series expansion, we simplify the random fractal capillary pressure model of Hunt to Pc = exp(a − bS), where S is the wetting phase saturation, and a and b characteristic of the porous medium. An extensive dataset of seventeen centrifuge capillary pressure measurements were used in this research to demonstrate the merit of the new method. For both sandstones and carbonates, the logarithm of capillary pressure showed a linear relationship with saturation as observed by magnetic resonance imaging centrifuge capillary pressure measurements over a wide saturation range. This work demonstrates that: (a) in semi-log plots of capillary pressure as a function of saturation, capillary pressurewill vary linearly over a wide saturation range, (b) such a plot as described in (a) will show the uni-or bimodal pore size distribution of the rock, (c) the exponential capillary pressure function simplifies analytical modelsthat use the capillary pressure function, for example oil recovery models for fractured reservoirs.

Effects of microscopic pore structure heterogeneity on the distribution and morphology of remaining oil

Abstract Waterflooding experiments were performed using Micro-CT on four cores of different pore structures from Donghe sandstone reservoirs in the Tarim Basin. The water, oil and grains were accurately separated by the advanced image processing technology, the pore network model was established, and parameters such as the number of throats and the throat size distribution were calculated to characterize the microscopic heterogeneity of pore structure, the flow of oil phase during displacement, and the morphology and distribution of remaining oil after displacement. The cores with the same macroscopic porosity-permeability have great differences in microscopic heterogeneity of pore structure. Both macro porosity-permeability and micro heterogeneity of pore structure have an influence on the migration of oil phase and the morphology and distribution of remaining oil. When the heterogeneity is strong, the water phase will preferentially flow through the dominant paths and the remaining oil clusters will be formed in the small pores. The more the number of oil clusters (droplets) formed during displacement process, the smaller the average volume of cluster is, and the remaining oil is dominated by the cluster continuous phase with high saturation. The weaker the heterogeneity, the higher the pore sweep efficiency is, and the remaining oil clusters are mainly trapped in the form of non-continuous phase. The distribution and morphology of micro remaining oil are related to the absolute permeability, capillary number and micro-heterogeneity. So, the identification plate of microscopic residual oil continuity distribution established on this basis can describe the relationship between these three factors and distribution of remaining oil and identify the continuity of the remaining oil distribution accurately.

Comprehensive characterization of pore and throat system for tight sandstone reservoirs and associated permeability determination method using SEM, rate-controlled mercury and high pressure mercury

Unconventional resource formations like tight sandstone reservoirs have complex pore and throat structure that are hard to characterize in micro scale. In this research, the pore and throat system of tight sandstones was characterized comprehensively, analysed qualitatively using scanning electron microscopy (SEM) techniques and quantitatively through combined high pressure mercury (HPM) and rate-controlled mercury (RCM) methods. First, the throat, distinguished from the pore, was divided in several types for tight sandstones and visualized through SEM imaging. Then, an integrated approach was proposed to combine the results from both HPM and RCM measurements, resulting in overall size distributions of the pore and the throat with scales from nanometer to micrometer. Both overall size distribution using the new combining method represented the pore and the throat more accurately in wider ranges. The results also showed that for tight sandstone core samples with different permeability, the difference of pore size distribution was less obvious than that of the throat. Therefore, the quantitative characterization of the throat size distribution was ultimately employed in developing a new permeability model together with physical considerations. The results demonstrated that the permeability of tight sandstones was in general depend on the size distribution of relatively larger throat, denoted as the macrothroat. However, for extreme low permeability (<0.1mD) formation, the small throats like microthroats and mesothroats mainly contributed to the permeability since the contents of macrothroats are really low.

Reservoir characteristics and hydrocarbon accumulation of the glauconitic sandstone in the Tarapoa Block, Oriente Basin, Ecuador

Although Oriente basin has explored throughout the past fifty years, little is known about the reservoir quality and hydrocarbon accumulation of the Cretaceous Upper T glauconitic sandstones. Until recently, the Upper T has drawn attention due to the commercial oil discovery. Howerer, exploration of glauconitic sandstones is complicated and reservoir quality is difficult to evaluate based on logging and seismic data.The glauconitic sandstones developed in open marine environments, characterized by a framework of glauconite and quartz grains. Diagenesis includes compation, silica overgrowth, feldspars dissolution, iron carbonate cementation. Its porosity ranges 5–20% and less than 500 md permeability. The dominant pore type is intergranular, especially residual intergranular. Glauconitic sandstones have bimodal pore throat size distributions and complex pore geometries.The Upper T glauconitic sandstones are capped by a thin veneer of fossiliferous limestones and laminated to burrowed shales. These large-area matured source rocks close to the glauconitic sandstones are probably the source kitchen in the slope of Oriente Basin. The oil pools are very little relation to structure. A large area of continuous developed glauconitic sandstone reservoirs and adjacent source rocks form an excellent source–reservoir–cap assemblage. Differ from previous views of long-distance migration from the southwest Oriente Basin, we propose that volume flow and diffusive flow are the primary migration and accumulation mechanism for the Upper T reservoir in Tarapoa Block.