Comparison of the Pore Structure of Ultralow-Permeability Reservoirs Between the East and West Subsags of the Lishui Sag Using Constant-Rate Mercury Injection

In this study, the differences in reservoir parameters, such as pore radius, throat radius, and pore-throat ratio, between the east and west subsags of the Lishui Sag are analyzed by using data obtained from a constant-rate mercury injection experiment. Furthermore, the quality of the reservoirs in the two subsags is systematically evaluated. Results show that the throat radius of the Lishui west subsag is larger than that of the east subsag, and this parameter has a positive correlation with reservoir quality. However, the pore-throat ratio of the east subsag is larger than that of the west subsag, which has an inverse relationship with reservoir quality. The main reasons for this reservoir difference can be attributed to sedimentation and diagenesis. The sedimentary facies types of the Lishui east subsag are the fan delta, shore lake, shallow lake, and shore shallow lake;their sandstone composition maturity is low;the clay mineral content is high;and the rock has undergone strong diagenesis. Therefore, the physical conditions of the reservoir are poor. However, the sandstones in the Lishui west subsag have weak cementation and compaction, mainly with an intergranular pore structure type, which leads to good connectivity between pores. Therefore, the storage performance and seepage capacity of the Lishui west subsag are better than those of the east subsag;the west subsag is the main area of oil and gas accumulation, as confirmed in the process of exploration and development.

Pore throat structure heterogeneity and its effect on gas-phase seepage capacity in tight sandstone reservoirs:A case study from the Triassic Yanchang Formation,Ordos Basin

The microscopic heterogeneity of pore-throat structures in tight sandstone is a crucial parameter for understanding the transport mechanism of fluid flow.In this work,we firstly developed the new procedure to characterize the pore size distribution(PSD)and throat size distribution(TSD)by combining the nuclear magnetic resonance(NMR),cast thin section(CTS),and constant-rate mercury injection(CRMI)tests,and used the permeability estimated model to verify the full-scale PSD and TSD.Then,we respectively analyzed the fractal feature of the pore and throat,and characterized the heterogeneity of pores and throats.Finally,we elaborated the effect of the pore and throat heterogeneity on the gas-phase seepage capacity base on the analysis of the simple capillary tube model and gas-flooding experiment.The results showed that(1)The PSD and TSD of the tight sandstone sample ranged from 0.01 to 10 mm and from 0.1 to 57 mm,respectively,mainly contributed by the micropores and mesopores.Meanwhile,the permeability estimated by the PSD and TSD was consistent with the experimental permeability,and relative error was lower than 8%.(2)The PSD and TSD exhibited multifractal characteristics,and singularity strength range,Δα,could be used as the indicator for characterizing the heterogeneity of pore and throat.Furthermore,the throat of the sample showed stronger heterogeneity than that the pore.(3)The throats played an important role for the fluid transport in the tight sandstone,and the effect of the throat heterogeneity on the gas-phase seepage capacity was different under the lower and higher injection pressure.The macropores and micropores maybe respectively become the preferential migration pathways at the lower and higher injection pressure.In the end,the identification plate was established in our paper,and could be described the relationship among the throat heterogeneity,injection pressure,permeability and flow path of the gas phase in the tight sandstone.

Microscopic pore characteristics of andesite and implication from mineral content:A case study in Huoshiling Formation of Changling fault depression,Songliao Basin

In order to study the microscopic pore characteristics of andesite reservoir and the effect of mineral content on the andesite pore,this study takes the andesite of the Huoshiling Formation in Longfengshan of Changling fault depression,Songliao Basin as a subject.The andesite reservoir space was discerned through the observation of cores and casting thin sections.Besides,the pore size distribution of andesites and their mineral contents were quantitatively characterized by high-pressure mercury injection,nitrogen adsorption and XRD,respectively.The results show that:(1)There are various types of reservoir space in andesites,including vesicles,amygdule,intergranular pores,matrix dissolution pores and dissolution pores of amygdala,and three types of fractures including dissolution,structural and explosion fractures.(2)The pore size distribution of andesite is complex.The main pore size of andesite is mid-pore(10-20 nm)with some large-pores(>50 nm).Mid-pore and large-pore provide the main specific surface area,which are the main space for gas storage.(3)The andesite reservoir space in the study area is mainly controlled by dissolution,as supported by the relationship between the change of mineral content and porosity evolution.The porosity of andesites decreases with the increase of quartz and chlorite content,but increases with the increase of soluble mineral,e.g.,feldspar content.

Characteristics and classification of paleozoic tight reservoirs in the central uplift of the South Yellow Sea Basin

Several sets of Paleozoic tight reservoirs are developed in the Central Uplift of the South Yellow Sea Basin.A qualitative analysis of the microscopic pore structure of the tight reservoir rocks was carried out through cast thin slice and scanning electron microscopic image observation.Based on reservoir pet-rophysical properties,thirty core samples in the Central Uplift of the South Yellow Sea Basin were selected for high-pressure mercury intrusion(HPMI)analysis,which was then combined with fractal calculation to classify and evaluate the tight reservoirs.The analysis of the HPMI curves and related parameters shows that the Paleozoic tight reservoirs can be divided into three types:Type-A,Type-B and Type-C.Type-A sandstone reservoirs contain pores with size mostly ranging between 0.01 and 0.1 mm,followed by pores with size range of 0.001-0.01 m m,and relatively fewer pores larger than 0.1 m m.The Type-B reservoirs are carbonate rocks with extremely heterogeneous pore size distribution,which is closely related to the development of dissolution pores and microfractures.Type-C sandstone reservoirs are dominated by nanopores and submicron pores that distribute more heterogeneously than pores in Type-A reservoirs.The pore distribution in sandstone reservoirs shows significant fractal characteristics and is closely related to the pore size.The heterogeneity of nanopore distribution has a negative cor-relation with porosity and median pressure and a relatively weak correlation with permeability.Our study has important implications for petroleum exploration in the South Yellow Sea Basin.

Multi-Scale Pore Structure Heterogeneity in Tuff Reservoirs Investigated with Multi-Experimental Method and Fractal Dimensions in Chang 7 Formation,Southern Ordos Basin

The tight tuff reservoir is an unusual type of unconventional reservoir with strong heterogeneity.However,there is a lack of research on the microscopic pore structure that causes the heterogeneity of tuff reservoirs.Using the Chang 7 Formation in Ordos Basin,China as a case study,carbon-dioxide gas adsorption,nitrogen gas adsorption and high-pressure mercury injection are integrated to investigate the multi-scale pore structure characteristics of tuff reservoirs.Meanwhile,the fractal dimension is introduced to characterize the complexity of pore structure in tuff reservoirs.By this multi-experimental method,the quantitative characterizations of the full-range pore size distribution of four tuff types were obtained and compared in the size ranges of micropores,mesopores and macropores.Fractal dimension curves derived from full-range pores are divided into six segments as D1,D2,D3,D4,D5 and D6 corresponding to fractal characteristics of micropores,smaller mesopores,larger mesopores,smaller macropores,medium macropores and larger macropores,respectively.The macropore volume,average macropore radius and fractal dimension D5 significantly control petrophysical properties.The larger macropore volume,average macropore radius and D5 correspond to favorable pore structure and good reservoir quality,which provides new indexes for the tuff reservoir evaluation.This study enriches the understanding of the heterogeneity of pore structures and contributes to unconventional oil and gas exploration and development.

Analysis and Characterization of Pore System and Grain Sizes of Carbonate Rocks from Southern Lebanon

Carbonate rocks are common in many parts of the world, including the Eastern Mediterranean, where they host significant groundwater supplies and are widely used as building and ornamental stones. The porosity of carbonate rocks plays a critical role in fluid storage and retrieval. The pore structure connectivity, in particular, controls many properties of geological formations, as well as the relationships between the properties of individual minerals and the bulk properties of the rock. To study the relationships between porosity, rock properties, pore structure, pore size, and their impact on reservoir characteristics, 46 carbonate rock samples were collected from four stratigraphic sections exposed near Sidon, South Lebanon. The studied carbonate rocks are related to marine deposits of different ages(e.g., Upper Cretaceous, Eocene, and Upper Miocene). In order to understand the pore connectivity, the MICP(mercury injection capillary pressure) technique was conducted on ten representative samples. Results from the SEM analysis indicate the dominance of very fine and fine pore sizes, with various categories ranging in diameter from 0.1 to10 μm. The MICP data revealed that the pore throat radii vary widely from 0.001 to 1.4 μm, and that all samples are dominated by micropore throats. The grain size analysis indicated that the studied rocks have significant amounts of silt-and clay-size grains with respect to the coarser ‘sand-size’ particles, suggesting a high proportion of microporosity. Obtained results, such as the poorly-sorted nature of grains, high microporosity, and the high percentage of micropore throats, justify the observed low mean hydraulic radius, the high entry pressure, and the very low permeability of the studied samples. These results suggest that the carbonate rocks near Sidon(south of Lebanon) are possibly classified as non-reservoir facies.

Understanding pore characteristics through core-based petrographic and petrophysical analysis in a heterogeneous carbonate reservoir:A case study from the Mumbai Offshore Basin,India

Carbonate rocks exhibit complex and heterogeneous pore structures;such heterogeneity is manifested by the occurrence of a wide variety of pore types with different sizes and geometries as a result of depositional and diagenetic processes.These complications substantially increase the uncertainty of predicted rock hydraulic parameters because samples with comparable porosities might have very different permeability values.In this study,small-scale characterisation of porosity and permeability in heterogeneous Eocene limestone samples from the Bassein Formation of the B-X structure of the MK Field in Mumbai Offshore Basin,India,was carried out,employing an integrated framework that in-corporates thin-section petrography,routine core analysis,mercury injection capillary pressure and nuclear magnetic resonance data.The pore characteristics of these carbonates range from poor to excellent.The studied samples exhibited large ranges of porosity,permeability and other associated petrophysical attributes.The pore types,as well as their orientations and connectivity,are the primary factors causing the heterogeneity.Because of the complexity of the pore networks,a simple lithofacies classification alone would have been insufficient to link porosity and permeability.The reservoir char-acteristics in the study area are strongly linked to the development and/or destruction of reservoir porosity-permeability during different phases of diagenesis.Twenty-four carbonate core samples from the limestone unit were studied and classified into microfacies and pore type classes,producing an accurate assessment of reservoir attributes.The comprehensive workflow incorporates the pore volume distributions and pore throat attributes for each rock type.Three carbonate microfacies were identified by petrographic analysis and their petrophysical characteristics,such as porosity,permeability,pore throat size,pore volume and fluid flow factors,were measured.The study demonstrates how macro-porosity,mesoporosity and microporosity are associated with various rock types and how they affect permeability and cementation exponents.The results of this study provide a comprehensive experi-mental framework for geological and geophysical interpretation that can be applied to identify potential reservoir facies and strengthen our understanding of heterogeneous carbonates.The framework can also be used to guide reservoir evaluation of similar heterogeneous formations in other areas.

Influence of sedimentation and diagenesis on reservoir physical properties:a case study of the Funing Formation,Subei Basin,eastern China

The sandstone of the third member of the Funing Formation(E1f3)in the northern slope zone of the Gaoyou Sag has the typical characteristics of high porosity and ultralow permeability,which makes it difficult for oil to flow.In this study,the lithological characteristics,sedimentary facies,diagenetic characteristics,pore struc-ture,and seepage ability of this sandstone are characterized in detail.Correlation analysis is used to reveal the reason for the sandstone high porosity-low permeability phenom-enon in the study area.The results indicate that this phenomenon is controlled mainly by the following three factors:1)the sedimentary environment is the initial affecting factor,whereby the deposition of a large number of fine-grained materials reduces the primary pores of sandstone.2)The Funing Formation has undergone strong compaction and cementation,which have led to the removal of most of the primary pores and a reduction in size of the throat channels.3)Owing to fluid activity during the later stage of diagenesis,sandstone underwent intense dissolution and a large number of particles(feldspar and lithic debris)formed many dissolution pores(accounting for nearly 60%of the total pore space).Among these factors,dissolution has contributed the most to the development of high porosity-low permeability phenomenon.This is mainly attributed to the inhomogeneous dissolution process,whereby the degree of particle dissolution(e.g.feldspar)exceeds that of cementing minerals(clay and carbonate minerals).The secondary dissolution pores have increased the porosity of sandstone in the study area;however,the pore connectivity(permeability)has not been significantly improved,thus resulting in the special high porosity-low permeability characteristics of this sandstone.

Characterization of the pore system in an over-mature marine shale reservoir:A case study of a successful shale gas well in Southern Sichuan Basin,China

During the past two years the shale gas exploration in Southern Sichuan basin received some exciting achievements.Data of a new appraisal well showed that the gas producrtions of vertical well and horizontal well are^1.5×104 m3/day/well(with maximum^3.5×104 m3/day/well)and^12.5×104 m3/day/well(with maximum^40×104 m3/day/well),respectively,indicating a good gas potential in this area.Eight core samples from the reservoir were investigated by using a carbon sulfur analyzer,microphotometry,x-ray diffractometry,field-emission scanning electron microscopy(FE-SEM),mercury injection porosimetry(MIP),and low-pressure nitrogen adsorption to obtain a better understanding of the reservoir characteristics of the Upper OrdovicianeLower Silurian organic-rich shale.Results show that the total organic carbon(TOC)content ranges from 0.5%to 5.9%,whereas the equivalent vitrinite reflectance(VRr)is between 2.8%and 3.0%.Pores in the studied samples were observed in three modes of occurrence,namely,interparticle pores,intraparticle pores,and intraparticle organic pores.The total porosity(P)ranges from 1.6%to 5.3%,and MIP data sets suggest that pores with throats larger than 20 nm contribute little to the pore volume.Low-pressure N2 adsorption isotherms indicate that the total specific surface area(SBET)ranges from 9.6 m2/g to 18.9 m2/g,and the pore volume(V)ranges from 0.011 cm3/g to 0.020 cm3/g.The plot of dV/dW versus W shows that the fine mesopores(pore size(BJH)<4 nm)mainly contribute to the pore volume.The P,SBET,and V show a good positive correlation with TOC and a weak positive correlation with the total clay mineral content,thus indicating that the nanopores are mainly generated by the decomposition of organic matter.The reservoir characteristics of the Upper OrdovicianeLower Silurian organic-rich shale are comparable with commercial shale gas plays in North America.The sample gas contents with TOC>2%are more than 3.0 m3/ton.The observation can be a good reference for the future exploration and evaluation of reservoir in this area.

Quantitative characterization of pore structure of the CarboniferousePermian tight sandstone gas reservoirs in eastern Linqing depression by using NMR technique

Micro-nano scale pores can accurately and fastly be measured by the nuclear magnetic resonance(NMR)technique,which provides a new method to quantitatively characterize pore structures in tight sandstone.Based on the method of calibration of mercury pressure data for NMR T_(2) spectrum,for the measurement inaccuracy due to the mercury saturation less than 100%in tight sandstone,the mercury pressure curve and T_(2) spectrum is used to cumulate from the maximum pore on the right boundary to the small pores in the left,the range of pore-throat radius measured by the mercury injection in the leftward cumulative curve is selected as a comparable interval of NMR pore-throat radius,and the longitudinal interpolation method and the least square method are utilized to construct the distribution curve of pore-throat radius transformed by T_(2) spectrum.The modified method is used to obtain NMR T_(2) spectrum,conversion coefficient of pore-throat radius and pore-throat radius distribution of the Carboniferous-Permian tight sandstone gas reservoirs in the eastern Linqing depression,and characteristics of reservoir pore structures are quantitatively investigated;in addition,in combination with analysis of thin section and scanning electron microscopy,the reservoir effectiveness and cause of the pore structure variability in the tight sandstone are also well studied.The results show that the NMR pore-throat radius curve obtained by the modified method has a high consistency with the mercury injection curve,and the NMR test accuracy of tight sandstone is significantly improved.In the study area,the pore-throat radius of the Carboniferous-Permian tight sandstone mainly ranges from 0.002 to 2 mm,the pore is generally submicro-nano scale,but the pore-throat radius distribution of different types of sandstone varies significantly.The lithic quartz sandstone is rich in siliceous matter and poor in plastic detritus and matrix,generally dominated by submicro-scale pore-throats including micro-scale porethroats;lithic feldspar sandstone and quartz-rich feldspar lithic sandstone are rich in quartz and poor in plastic detritus and matrix,dominated by submicro-nano scale pore-throats(nano-scale pore-throats predominantly);the lithic fragment-rich feldspar lithic sandstone and lithic sandstone are poor in quartz and rich in plastic detritus and matrix,mainly dominated by nano-scale pore-throats smaller than 0.05 mm.Micropetrographic components are key factors to control pore structure difference and reservoir effectiveness,and the reservoir quality may be macroscopically controlled by sedimentary microfacies;the lithic quartz sandstones of coarse-and fine-grained point bar/riverbed microfacies are the most favorable reservoirs;the lithic feldspar sandstone of fine-grained point bar microfacies,the quartzrich feldspar lithic sandstone of fine-grained distributary channel and barrier bar microfacies are relatively favorable reservoirs,while both lithic fragment-rich feldspar lithic sandstone and lithic sandstone of tidal-flat facies are ineffective reservoirs with very poor porosity and permeability.

Investigations on the relationship among the porosity,permeability and pore throat size of transition zone samples in carbonate reservoirs using multiple regression analysis,artificial neural network and adaptive neuro-fuzzy interface system

Finding an accurate method for estimating permeability aside from well logs has been a difficult task for many years.The most commonly used methods targeted towards regression technique to understand the correlation between pore throat radii,porosity and permeability are Winland and Pittman equation approaches.While these methods are very common among petrophysicists,they do not give a good prediction in certain cases.Consequently,this paper investigates the relationship among porosity,permeability,and pore throat radii using three methods such as multiple regression analysis,artificial neural network(ANN),and adaptive neuro-fuzzy inference system(ANFIS)for application in transition zone permeability modeling.Firstly,a comprehensive mercury injection capillary pressure(MICP)test was conducted using 228 transition zone carbonate core samples from a field located in the Middle-East region.Multiple regression analysis was later performed to estimate the permeability using pore throat and porosity measurement.For the ANN,a two-layer feed-forward neural network with sigmoid hidden neurons and a linear output neuron was used.The technique involves training,validation,and testing of input/output data.However,for the ANFIS method,a hybrid optimization consisting of least-square and backpropagation gradient descent methods with a subtractive clustering technique was used.The ANFIS combines both the artificial neural network and fuzzy logic inference system(FIS)for the training,validation,and testing of input/output data.The results show that the best correlation for the multiple regression technique is achieved for pore throat radii with 35%mercury saturation(R35).However,for both the ANN and ANFIS techniques,pore throat radii with 55%mercury saturation(R55)gives the best result.Both the ANN and ANFIS are later found to be more effective and efficient and thus recommended as compared with the multiple regression technique commonly used in the industry.