Throat Radius Research Articles

Sedimentological and diagenetic effects on petrophysical characteristics and hydraulic flow units of Zhujiang sandstones in the Pearl River Mouth Basin, south China Sea

The Zhu-Ⅲ sag is a key oil and gas rich basin. The Zhujiang Formation is characterized by strong heterogeneity, a changeable sedimentary environment and complex diagenesis, which presents great challenges in the identification of the reservoir sweet spot. Previous researchers have almost no study on it. Therefore, finding high-quality reservoir units is the top priority. In this study, we focus on classifying and evaluating the reservoir units, finding high-quality units and providing exploration recommendations. We carried out core observations and description, X-ray diffraction, cast thin-section observations, electron microscopy observations and mercury intrusion experiments. We used the normal probability distribution model to divide the hydraulic flow unit into six types and determined their hydraulic flow capacity. We summarized the depositional model of the study area through core observations and well analysis. By combining the data on the hydraulic flow units, pore throat radius, sedimentary background, diagenesis and reservoir physical properties, we classified six rock facies (RFs). We found that there is good correspondence between rock facies, hydraulic flow units and sedimentary environments. The results indicate that the quality of the RF-1, RF-2 and RF-3 reservoirs are good, while the quality of the reservoir in RF-4, RF-5 and RF-6 are poor. We conclude that RF-2 mainly corresponds to oil layers and oil-water layers, which are favourable target layers for exploration. This research is the first time to study the sedimentation and diagenesis and combine them to evaluate the reservoir physical properties and flow units in the South China Sea comprehensively.

A study of transfer learning in digital rock properties measurement

The measurement of physical parameters of porous rock, which constitute reservoirs, is an essential part of hydrocarbon exploration. Typically, the measurement of these physical parameters is carried out through core analysis in a laboratory, which requires considerable time and high costs. Another approach involves using digital rock models, where the physical parameters are calculated through image processing and numerical simulations. However, this method also requires a significant amount of time for estimating the physical parameters of each rock sample. Machine learning, specifically convolutional neural network (CNN) algorithms, has been developed as an alternative method for estimating the physical parameters of porous rock in a shorter time frame. The advancement of CNN, particularly through transfer learning using pre-trained models, has contributed to rapid prediction capabilities. However, not all pre-trained models are suitable for estimating the physical parameters of porous rock. In this study, transfer learning was applied to estimate parameters of sandstones such as porosity, specific surface area, average grain size, average coordination number, and average throat radius. Six types of pre-trained models were utilized: ResNet152, DenseNet201, Xception, InceptionV3, InceptionResNetV2, and MobileNetV2. The results of this study indicate that the DenseNet201 model achieved the best performance with an error rate of 2.11%. Overall, this study highlights the potential of transfer learning to ultimately lead to more efficient and effective computation.

Research on Fractal Characteristics and Influencing Factors of Pore-Throats in Tight Sandstone Reservoirs: A Case Study of Chang 6 of the Upper Triassic Yanchang Formation in Huaqing Area, Ordos Basin, China

In recent years, tight sandstone oil and gas have been an important area for unconventional oil and gas exploration and development in China. It is of great significance to clarify the pore-throat structure characteristics of tight sandstone reservoirs to guide production practices. This study takes the tight sandstone of the sixth member of the Yanchang Formation in the Huaqing area, Ordos Basin, as an example, based on experimental methods such as high-pressure mercury intrusion, cast thin sections and scanning electron microscopy. At the same time, the pore-throat structure of tight sandstone reservoirs is divided into three types using the tube-bundle and spherical fractal models. The corresponding pore and throat radius distribution, pore-throat combination mode and influencing factors of various pore-throats are studied. The results show that the fractal dimension of type I pore-throats is the smallest, and the distribution of their pore-throat radii is the most uniform. They are dominated by intercrystalline pores and dissolution pores with tube-bundle throats and small pores with small throats. Type II pore-throats have the largest fractal dimension and the worst pore-throat uniformity. They are dominated by residual primary intergranular pores with necked throats and large pores with small throats. The type III pore-throat fractal dimension is in the middle, mainly composed of residual dissolved intergranular pores with pore-reduced throats, sheet-like and curved sheet-like throats, and large pores with large throats. The influence of different pore-throat combinations on the reservoir is reflected in the different characteristics of mercury injection parameters. The main influencing factors for the differences in the fractal dimensions of different pore-throats are diagenesis, rock composition and pore-throat combination type. Diagenesis and rock composition, in turn, affect the type and development degree of pore-throats, as well as the combination of pore-throats. The purpose of this study was to clarify the internal connection modes of different homogeneous pore-throats and their influencing factors, enrich the theoretical basis for the study of tight sandstone reservoirs and provide theoretical guidance for their exploration and development.

Driving Forces of Natural Gas Flow and Gas–Water Distribution Patterns in Tight Gas Reservoirs: A Case Study of NX Gas Field in the Offshore Xihu Depression, East China

The driving forces behind gas flow and migration, as well as the associated gas–water distribution patterns in tight gas reservoirs, are not only closely related to the formation mechanisms of “sweet spots”, but also serve as crucial geological foundations for the development of efficient modes and optimal well placement. In this work, three methods, namely, critical gas column height driven by buoyancy, critical pore throat radius driven by buoyancy, and gas–water distribution attitude, were used to quantitatively evaluate the critical conditions for buoyancy and overpressure to get gas flowing in the tight sandstone gas field. In light of the geological background, the driving forces of gas flow/migration and gas–water distribution patterns were comprehensively analyzed. On this basis of the origins of overpressure driving gas flow/migration were identified by using multiple empirical methods, the evolution of overpressure and characteristics of gas–water distribution driven by overpressure were studied by using PetroMod_2014 simulation software. The results show that the four main gas-bearing layers in the NX tight sandstone gas reservoir differ widely in gas flow/migration dynamics and gas–water distribution patterns. Gas accumulation in the H3b layer is influenced by both buoyancy and overpressure. Subsequently, buoyancy leads to the differentiation of gas from water based on density and the formation of edge water. Furthermore, the distribution area of the gas reservoir is determined by the presence of an anticline trap. In contrast, in H3a, H4b and H5a gas layers, buoyancy is not sufficient to overcome the capillary force to make the gas migrate during and after accumulation, and the driving force of gas flow is the overpressure formed by fluid volume expansion during hydrocarbon generation of Pinghu Formation source rocks. Because buoyancy is not the driving force of natural gas flow, H3a, H4b and H5a layers have gas and water in the same layer and produced together, and no boundary and bottom water, where the anticlinal trap does not control the distribution of gas and water, and gas source faults control the boundary of the gas reservoir. These understandings not only significantly expand the gas-bearing target of H3a, H4b and H5a gas layers delineated in the buoyancy driving pattern but also provide an important geological basis for the formulation of an efficient development plan by class and grade for the NX tight sandstone gas field.

Cylindrical wormholes and electromagnetic field

In this manuscript, we study the construction of static cylindrically symmetric thin shell wormhole models with an electric charge in the background of f(R,T) gravity. We utilized “cut and paste” technique to match two identical spacetimes across the separation hypersurface Σ. In this gravity, f is a generic function of the Ricci scalar (R) and the trace of the stress-energy tensor (T), as the gravitational sources. We continue our systematic investigation by using specific f(R,T) model with quadratic corrections in R. In this scenario, we assume that the isotropic perfect fluid adequately represents the matter sector, implying that matter at the shell is sustained by generalized Chaplygin gas. Using generalized Chaplygin gas, to ensure a consistent matching of the two spacetimes across the separation hypersurface Σ, suitable boundary conditions and constraints are imposed. These conditions guarantee the continuity of the metric coefficients, the electric charge distribution, and other relevant quantities at the thin shell within the framework of the quadratic f(R,T) gravity. The effects of mass, equation of state parameters, and electric charge are examined on the stability of static wormhole models for the widest possible range of model parameters. The positive value of the second derivative of potential at throat radius is used to characterize the stability requirement which is indicated via plots. We study the presence of charge which might affect the stability region of constructed wormhole. It is suggested that a charged wormhole model might be more stable than an uncharged one.

Microscopic Pore Structure Characteristics and Fluid Mobility in Tight Reservoirs: A Case Study of the Chang 7 Member in the Western Xin’anbian Area of the Ordos Basin, China

This research addresses the limited understanding of movable fluid occurrence characteristics in the Chang 7 reservoir by employing mercury injection capillary pressure, constant-rate mercury injections, and nuclear magnetic resonance methods. This study investigates the microscopic pore structure characteristics and movable fluids in the tight reservoir of the western Xin’an region, located in the Ordos Basin. The finding reveals that as permeability decreases, the distribution of the throat radius becomes more concentrated in the low-value area, resulting in a narrow distribution range with high curve peaks. Conversely, with an increasing permeability, the distribution range expands towards the high-value area while the curve peak decreases. This research underscores the significance of the throat radius, especially the main flow throat radius, in constraining the permeability of rock samples. Furthermore, this study highlights a stronger correlation between permeability and movable fluid saturation than porosity. This finding emphasizes the importance of considering movable fluid saturation when assessing reservoir characteristics. Notably, the throat radius plays a crucial role in influencing the occurrence characteristics of movable fluids, with a smaller throat radii posing hindrances to fluid flow in the reservoir. Additionally, the presence of clay minerals in the reservoir leads to pore segmentation and increased fluid flow resistance, ultimately reducing the saturation of movable fluids. I must be understood that these factors are essential for developing and producing reservoirs with similar characteristics. In conclusion, the insights gained from this study hold considerable theoretical value and provide essential references for developing and producing reservoirs with tight characteristics, particularly in the western Xin’an region of the Ordos Basin.

Three-dimensional wormhole with cosmic string effects on eigenvalue solution of non-relativistic quantum particles

In this paper, we explore the quantum system of non-relativistic particles in a unique scenario: a circularly symmetric and static three-dimensional wormhole space-time accompanied by cosmic strings. We focus on a specific case where the redshift function varPhi (r) to be zero and defining the shape function as A(r)=frac{b}{r^2}. After establishing this background space-time, we investigate the behavior of a harmonic oscillator within the same wormhole context. By doing so, we observe the effects of the cosmic string and wormhole throat radius on the eigenvalue solution of the oscillator’s eigenvalue problem. The primary finding is that these cosmic features lead to modifications in the energy spectrum and wave functions of the system, breaking the degeneracy of energy levels that would typically be present in a more conventional setting. As a particular case, we present the specific energy level E_{1,ell } and the corresponding wave function psi _{1,ell }, which are associated with the ground state of the quantum system. These results highlight the fascinating and unique properties of the harmonic oscillator in the background of a circularly symmetric, static wormhole space-time with cosmic strings.

Application of unsupervised learning and deep learning for rock type prediction and petrophysical characterization using multi-scale data

This study integrates well log data, routine core analyses, microcomputed X-ray tomography (μCT) images, and sedimentary petrography to accurately characterize and evaluate the carbonate reservoirs of the Barra Velha formation (Aptian) of the Santos Basin within Brazilian pre-salt region. In these carbonate reservoirs, the porous system is extremely diverse and variable, making it challenging to establish rock typing with comparable petrophysical properties. Based on this integrated study, the reservoir sequences were characterized and a precise definition of four reservoir rock types (RRTs) was performed by integrating the petrophysical values from the plugs and their corresponding well log data of two cored wells using K-means unsupervised classification algorithm. The classification results were combined with various conventional techniques to evaluate the quality and geological characteristics of the studied sequence. This evaluation encompassed different parameters such as flow and storage capacity, reservoir quality index, flow zone indicator, pore spaces interpretation, and average pore and throat radius. The study involved a detailed analysis of thin sections to identify various facies, including shrubstones, reworked, and spherulitestone, and to classify various forms of porosity such as interparticle, intraparticle, intercrystalline, vug, moldic, fracture, and growth framework porosity. Pore Network Modeling from μCT analysis of plugs was used specifically for the characterization of pores and throats of plug samples from each RRT. These datasets were utilized as supporting evidence to offer a more accurate and inclusive knowledge of reservoir quality. The study aimed to develop predictive models by implementing deep learning and machine learning algorithms trained on well log data to estimate plug porosity and rock type. Two deep learning models, ResNet and 1D CNN, were trained and evaluated for plug porosity prediction, with the 1D CNN model showing superior performance. Additionally, the XGBoost algorithm was applied to predict rock type, achieving high accuracy on both the training and validation datasets. The predicted results were compared with actual data to evaluate the effectiveness of the models and were then utilized to estimate plug permeability values. The results demonstrate the potential of deep learning and machine learning approaches in reservoir characterization and management, enabling the evaluation of subsurface reservoir properties even with incomplete datasets, which could lead to an improved understanding of the reservoir properties and better management of the reservoir. This integrated study provides deeper insight into the complex reservoir properties and can help improve decision-making processes and optimize management and production strategies in the challenging pre-salt carbonate reservoirs or similar complex reservoirs.

Model-independent traversable wormholes from baryon acoustic oscillations

In this paper, we investigate the model-independent traversable wormholes from baryon acoustic oscillations. Firstly, we place the statistical constraints on the average dark energy equation of state ωav by only using BAO data. Subsequently, two specific wormhole solutions are obtained, i.e, the cases of the constant redshift function and a special choice for the shape function. For the first case, we analyze the traversabilities of the wormhole configuration, and for the second case, we find that one can construct theoretically a traversable wormhole with infinitesimal amounts of average null energy condition violating phantom fluid. Furthermore, we perform the stability analysis for the first case, and find that the stable equilibrium configurations may increase for increasing values of the throat radius of the wormhole in the cases of a positive and a negative surface energy density. It is worth noting that the obtained wormhole solutions are static and spherically symmetrical metric, and that we assume ωav to be a constant between different redshifts when placing constraints, hence, these wormhole solutions can be interpreted as stable and static phantom wormholes configurations at some certain redshift which lies in the range [0.32, 2.34].

Effects of [formula omitted]-dimensional circularly symmetric and static traversable wormhole with cosmic string on oscillator field

In this study, we examine the relativistic quantum dynamics of an oscillator field described by the Klein–Gordon equation in a circularly symmetric and static (1+2)-dimensional traversable wormhole space–time with cosmic strings. We derive the radial equation for the Klein–Gordon oscillator in this wormhole background and solved it using special functions. For specific cases, we present the ground state energy level E1,ℓ± and wave function ψ1,ℓ of the oscillator field, subject to a constraint on the oscillator frequency ω1,ℓ. In fact, it is shown that the eigenvalue solution of the oscillator field is influenced by both the cosmic string parameter α and the wormhole throat radius a, leading to shifts in the energy levels and wave functions.

Quantitative characterization of critical reservoir physical properties of tight oil charging in the third member of the Shahejie Formation in the Gaobei Slope of Nanpu Sag, Bohai Bay Basin

Unconventional petroleum occupies an important position in the world energy structure. Tight oil is an important field of unconventional oil and gas exploration and development, and the physical property condition of a tight oil reservoir is an important determinant of its commercial production. The application of different geological and engineering strategies to quantitative characterization of the aspect of hydrocarbon charge vis-à-vis reservoir physical properties is critical and essential to thoroughly understand the formation and accumulation mechanism and help in achieving the optimum producibility of tight oil resources. This study focuses on the tight oil of the third member of the Shahejie Formation (Es3) region in the Gaobei Slope of the Nanpu Sag. The upper and lower physical limits of charging were quantitatively deduced and explained characterized by a combination of geological analysis, analytical testing, and numerical simulation of force balance; the formation mechanism of the tight oil reservoir is primarily constrained by its permeability. The inversion phenomenon of oil and water in the tight oil Es3 reservoir occurs at a burial depth of 3900 m. The pore type of the reservoir is mainly primary in nature, with a positive correlation between the porosity and the permeability. The upper limit of tight oil charging is quantitatively characterized by porosity approximately equal to 10%, the corresponding permeability being 1 × 10−3 μm2. When the reservoir's physical properties exceed the upper limit of tight oil charging, the reservoir mainly develops a water layer. This is mainly caused by the boundary effect of tight reservoirs, where the pore throat radius corresponding to the critical physical property is equivalent to the boundary layer thickness of oil in the reservoir pore, making it difficult for boundary fluids to flow in the pore space at this time. On the other hand, based on comprehensive analysis of the minimum flow hole roar radius, the irreducible water saturation, and drilling result statistical methods, the lower limit of the tight oil charging in Es3 reservoir is characterized quantitatively by the porosity of about 2%–4%, wherein the pore throat radius is approximately equal to 0.04 μm. The lower limit of tight oil charging varies from one place to another, mainly attributed to factors such as interfacial tension, hydrocarbon generation dynamics, kerogen type, and basin temperature. The charging process of tight oil complies with the principle of force balance, whereas the pressure exerted the generated hydrocarbon is counterbalanced by the capillary force of the pore space.

Existence of wormhole solutions in [formula omitted] gravity under non-commutative geometries

In this paper, we have systematically discussed the existence of the spherically symmetric wormhole solutions in the framework of f(Q,T) gravity under two interesting non-commutative geometries such as Gaussian and Lorentzian distributions of the string theory. Also, to find the solutions, we consider two f(Q,T) models such as linear f(Q,T)=αQ+βT and non-linear f(Q,T)=Q+λQ2+ηT models in our study. We obtained analytic and numerical solutions for the above models in the presence of both non-commutative distributions. We discussed wormhole solutions analytically for the first model and numerically for the second model and graphically showed their behaviors with the appropriate choice of free parameters. We noticed that the obtained shape function is compatible with the flare-out conditions under asymptotic background. Further, we checked energy conditions at the wormhole throat with throat radius r0 and found that NEC is violated for both models under non-commutative background. At last, we examine the gravitational lensing phenomenon for the precise wormhole model and determine that the deflection angle diverges at the wormhole throat.

Characterization of Flow Parameters in Shale Nano-Porous Media Using Pore Network Model: A Field Example from Shale Oil Reservoir in Songliao Basin, China

The pore-throat radius of the shale oil reservoir is extremely small, and it is difficult to accurately obtain the absolute permeability and oil–water two-phase relative permeability of the actual oil reservoir through conventional core experiments. However, these parameters are very important for reservoir numerical simulation. In this paper, a method for characterizing flow parameters based on a pore network model that considers differential pressure flow and diffusion flow is proposed. Firstly, a digital core was reconstructed using focused ion beam scanning electron microscopy (FIB-SEM) from the Gulong shale reservoir in the Songliao Basin, China, and a pore network model was extracted. Secondly, quasi-static single-phase flow and two-phase flow equations considering diffusion were established in the pore network model. Finally, pore-throat parameters, absolute permeability, and oil–water two-phase permeability curves were calculated, respectively. The results show that the pore-throat distribution of the Gulong shale reservoir is mainly concentrated in the nanometer scale; the mean pore radius is 87 nm, the mean throat radius is 41 nm, and the mean coordination number is 3.97. The calculated permeability considering diffusion is 0.000124 mD, which is approximately twice the permeability calculated without considering diffusion. The irreducible water saturation of the Gulong shale reservoir is approximately 0.4, and the residual oil saturation is approximately 0.35. The method proposed in this paper can provide an important approach for characterizing the flow parameters of similar shale oil reservoirs.

Multiscale Fine Characterization of a Coal Pore–Fracture System Based on SEM, CT, and NMR in the Jingbian Block, Ordos Basin

To achieve an accurate and comprehensive characterization of the multiscale pore–fracture characteristics of Permian coal in the Jingbian Block, Ordos Basin, a combination of scanning electron microscopy (SEM), X-ray computed tomography (CT), and nuclear magnetic resonance (NMR) techniques was utilized. With these experiments, the mineral composition, pore size distribution (PSD), porosity, and connectivity of pores in coal samples were characterized through qualitative and quantitative methods. The results show that the SEM experiments enabled qualitative identification of pores and mineral types. The coal samples primarily contained gas pores, cell pores, intercrystalline pores, and moldic pores, and clay minerals were the predominant fracture fillings. The 3D reconstruction of the CT experiments shows that the pores and fractures generally expand horizontally, while the minerals show obvious bedding expansion characteristics. Moreover, the estimation of full-size porosity in coal samples can be achieved by combining CT and NMR experiments. The full-size porosity of samples G11-5-1, G11-5-6, G11-5-9, and G11-5-11 was 8.93%, 9.11%, 10.45%, and 11.63%, respectively. The connectivity differences are primarily determined by the throat development degree and the connected pore–fracture count. Samples with more connected pores and larger throat radii exhibit excellent connectivity.

Synergistic mechanism of CTAB and nonionic polyacrylamide on pore structure of kaolinite filter cake

Filtration rate and water content of filter cake are decisive indicators in tailings solid–liquid separation processes, with the pore structure of the filter cake playing crucial roles in determining these outcomes. This work aims to investigate the effect of different filter aids on filter cake characteristic and to elucidate the mechanism of surface wettability regulation and flocculation on the pore structure of the filter cake using X-ray microtomography (XMT) to capture the porous structure of the filter cake. The results indicated that the filtration rate of the kaolinite suspension reached 12.25 × 10 -3 mL/(s·cm2) when the dose of cetyl trimethyl ammonium bromide (CTAB) was 0.3 mmol/L and nonionic polyacrylamide (NPAM) was 40 g/t. In comparison to the case without CTAB, the water content of the kaolinite filter cake decreased from 40.21% to 34.91%. Analysis of the filter cake structure demonstrated that the percentage of isolated pores in the filter cake, obtained via CTAB pretreatment followed by NPAM flocculation, was reduced to 10.12%, with uniform pore space development, good homogeneity, and an increased pore throat coordination number. These enhancements effectively improved the connectivity of the filter cake pores and created more pathways for water migration during the filtration process. Concurrently, the pore throat radius ratio in the filter cake decreased from 1.15 to 1.10, minimizing capillary resistance during water migration, resulting in a significant increase in filtration speed and efficacy.

Modeling Wormholes Generated by Dark Matter Galactic Halos in f(R)$f(R)$ Modified Gravity

AbstractWormholes (WHs) are hypothetical topologically non‐trivial spacetime structures that can be freely traversed by observers and connect two asymptotic regions or infinities. From the current theoretical development, the prospect of their existence is challenging but cannot be excluded. In this paper, generalized Ellis–Bronikov (GEB) traversable WH geometries for static and spherically symmetric spacetime in the background of gravity is explored. First, the Tsujikawa‐like model and the shape function for the GEB model is considered, which depend on a sequence of simple Lorentzian WHs with two parameters: a free even integer exponent, n, besides the throat radius, r0. One also consider that these WHs are generated by dark matter galactic halos (DMGHs), based on the three most common phenomenological models, viz., Navarro–Frenk–White (NFW), Thomas–Fermi (TF), and pseudo‐isothermal (PI). In this concern, the satisfaction of the energy conditions (ECs) which are dependent on the dark matter (DM) models, viz., dominant energy condition (DEC) and strong energy condition (SEC) and those which are not dependent viz., null energy condition (NEC) and WEC at the WH throat and its neighborhood is investigated. Finally, the presence of exotic matter is confirmed by the violation of the NEC in all cases, revealing the supremacy and physical acceptability to support the existence of the WHs and making them compatible and traversable in Tsujikawa's‐like model.

Pore Structure and Permeability Characterization of Tight Sandstone Reservoirs: From a Multiscale Perspective

Due to the influence of a sedimentary environment, sandstone characteristics, diagenesis, and geological structure, the complexity and heterogeneity of the pore structure in tight sandstone reservoirs act as key barriers for accurate characterization of the influence of different pore microparameters on reservoir physical properties. This paper obtained different scale microscopic pore–throat parameters through mercury intrusion porosimetry (MIP) and digital core reconstruction models. According to the connectivity of different scale pores, connected pore structure parameters, and pore fractal dimension, the pore structure characteristics of tight sandstone reservoirs were evaluated. Subsequently, through partial correlation analysis, the contribution rate of different scale connected pore parameters to permeability and porosity was clarified. Then, a multiparameter fitting equation for the absolute permeability and porosity of the rock was obtained through multiple regression analysis. The analysis results show that (1) the connected pores of tight sandstone reservoirs are mainly mesopores, with the pore radius distribution between 1 and 3 μm, and throat radius distribution between 0 and 2 μm. (2) The complexity of the pore structure in tight sandstone reservoirs is most strongly correlated with the fractal dimension of the pore structure at the mesoporous scale. (3) The pore radius and throat length at the mesoporous scale have the strongest correlation with the absolute permeability of the rock, and the pore radius at the mesoporous scale has the strongest correlation with the porosity of the rock. (4) The multiparameter fitting equation established by multiple regression analysis quantitatively and qualitatively analyzed the impact of microscopic parameters of the pore–throat structure at different scales on reservoir properties, achieving the purpose of predicting the absolute permeability and porosity of the tight sandstone reservoir. It provides guidance for the study of the pore structure and permeability characteristics of tight sandstones.

Quantifying the Microdistribution of Different Fluid Types in the Multiscale Pore Throat Structure of Low-Permeability Conglomerates

The volumes of different fluid types and the pore throat structure in low-permeability reservoirs are crucial parameters for understanding flow mechanisms and predicting production performance. The previously applied methods for distinguishing different fluid types are mainly based on T2 spectra under saturated, centrifugal, and heat-treated conditions. However, for rocks with different mineral compositions and pore structures, the critical temperature between capillary-bound fluid (CAF) and clay-bound fluid (CLF) differs from 60 to 100 °C. Moreover, the alteration of temperature can cause the transformation of clay minerals and a change in a nanopore structure. Herein, an improved method that integrates nuclear magnetic resonance (NMR) tests (n-dodecane-saturated, centrifugal, and spontaneous imbibition treatment conditions) and multicycle mercury intrusion porosimetry, was proposed for quantitatively evaluating the microdistributions of different fluid types, classifying the pore throat system, and analyzing the relationship between pore throat structure and different fluid types in low-permeability conglomerates. Based on the fractal characteristics and mercury intrusion/extrusion characteristics of pore throats at different scales, the pore throat system in these conglomerates was divided into macropore throats (>1 μm), mesopore throats (0.2–1 μm), and micropore throats (<0.2 μm). Macropore throats are connected to intergranular pores (>100 ms); mesopore throats are mostly connected to intergranular pores and intragranular dissolved pores (>10 ms); and micropore throats are connected to intragranular dissolved pores (<10 ms) and clay-related pores (<3 ms). The proportions of movable fluid, CAF, and CLF in the low-permeability conglomerates are 20.4–57.4%, 6.67–32.2%, and 36.0–47.4%, respectively. The movable fluids are mainly distributed in macropore throats and mesopore throats. CAF mainly exists in the micropore throats (pore throat radius <0.2 μm and pore size <10 ms). CLF is mainly distributed in pores with T2 < 3 ms, that is clay-related pores. The improved method of fluid typing is suitable for low-permeability or tight clastic reservoirs (excluding shale).

Pore-morphology-based pore structure characterization for various porous media

Quantitative characterization of pore structure plays an important role in predicting properties of porous media and it has significant applications in energy and chemical engineering. Mercury injection capillary pressure experiments and gas adsorption experiments are the major pore structure characterization techniques, but they are time-consuming, costly, and destructive. In this paper, a simple pore-morphology-based method is developed to simulate the mercury injection and gas pore condensation in CT image for obtaining the pore structure characteristics. The main idea of the developed algorithm is to utilize some of the morphological operations to capture the pore geometry and topology, and then combine with the invasion phase connectivity analysis to implement the mercury injection and gas pore condensation. The algorithm is designed in several ways with an improved computational speed and characterization accuracy. The algorithm is user-friendly and can be applied on 2D and 3D images. Various porous media are used to assess the performance of the algorithm and the characteristics of pore structures. The results are compared to the available results in the literature. Findings show that the algorithm can reliably quantitatively characterize the pore structure for a wide variety of porous media. The general pore radius distribution, pore throat radius distribution, and strict throat radius distribution involved in pore structure characterization are clarified. The characterization of heterogeneous pore structures and the effects of pore roughness on capillary pressure are discussed.

Modeling Permeability Using Advanced White-Box Machine Learning Technique: Application to a Heterogeneous Carbonate Reservoir.

From exploration to production, the permeability of reservoir rocks is essential for various stages of all types of hydrocarbon field development. In the absence of costly reservoir rock samples, having a reliable correlation to predict rock permeability in the zone(s) of interest is crucial. To predict permeability conventionally, petrophysical rock typing is done. This method divides the reservoir into zones of similar petrophysical properties, and the permeability correlation for each zone is independently developed. The challenge of this approach is that the success depends upon the reservoir's complexity and heterogeneity and the methods and parameters used for rock typing. As a result, in the case of heterogeneous reservoirs, conventional rock typing methods and indices fail to predict the permeability accurately. The target area is a heterogeneous carbonate reservoir in southwestern Iran with a permeability range of 0.1-127.0 md. In this work, two approaches were used. First, based on permeability, porosity, the radius of pore throats at mercury saturation of 35% (r35), and connate water saturation (Swc) as inputs of K-nearest neighbors, the reservoir was classified into two petrophysical zones, and then, permeability for each zone was estimated. Due to the heterogeneous nature of the formation, the predicted permeability results needed to be more accurate. In the second part, we applied novel machine learning algorithms, modified group modeling data handling (GMDH), and genetic programming (GP) to develop one universal permeability equation for the whole reservoir of interest as a function of porosity, the radius of pore throats at mercury saturation of 35% (r35), and connate water saturation (Swc). The novelty of the current approach is that despite being universal, the models developed using GP and GMDH performed substantially better than zone-specific permeability, index-based empirical, or data-driven models used in the literature, such as FZI and Winland. The predicted permeability using GMDH and GP resulted in accurate prediction with R2 of 0.99 and 0.95, respectively, in the heterogeneous reservoir of interest. Moreover, as this study aimed to develop an explainable model, different parameter importance analyses were also applied to the developed permeability models, and r35 was found to be the most impactful feature.