Constant-rate Mercury Injection Research Articles

Study on the Coupling Law between Pore-Scale Fluid Flow Capacity and Pore-Throat Configuration in Tight Sandstone Reservoirs

As for the tight sandstones, pore-scale fluid flow ability is one of the most fundamental parameters that determine potential development capacity when compared to conventional reservoirs due to relatively tiny spaces. The determination of pore-throat structures and their impact on fluid flow behavior could improve the forecast ability for development. Therefore, the main purpose of this paper is to study the effects of differences in microscopic pore structures on moveable fluid saturation in tight sandstone reservoirs. Casting thin sections (CTS), scanning electron microscope (SEM), constant-rate mercury injection (CRMI), nuclear magnetic resonance (NMR), etc. are used as experimental methods to qualitatively describe the differences in accumulation space types and to quantitatively compare and analyze the differences in microscopic pore structures between reservoirs for Chang 6 formation in the Jiyuan and Heshui areas. The different characteristic parameters are chosen to analyze their correlations with moveable fluid saturation. Thus, as a result, due to good correlations with moveable fluid saturation, parameters such as throat radius, pore-throat radius ratio, and pore-throat amount allocation ratio are chosen to characterize the differences in microscopic pore structures between reservoirs in those areas. Differences in throat radius, pore-throat radius ratio, and a pore-throat amount allocation ratio of microscopic pore structure of Chang 6 members in these two areas have led to differences in moveable fluid saturation to some extent. This research provided the evidence about the coupling law between pore-scale fluid flow capacity and pore-throat configuration in tight sandstone reservoirs, and the results can provide theoretical guidance for field development of oil and gas fields.

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 μm and from 0.1 to 57 μm, 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.

Why high porosity but low-production wells occur in carbonate reservoirs: An explanation from the perspective of pore structure

High porosity but low-production wells exist in the Leikoupo Formation in the Sichuan Basin, China. The unclear explanation for this phenomenon has led to the failure of fracture development. We selected 15 cores from the study area based on the oil test data to investigate the cause from the pore structure perspective. The pore structures were studied via a variety of petrophysical experiments, which include conventional physical property analysis, casting thin section, nuclear magnetic resonance technique, and constant-rate mercury injection. The results indicate that the main explanation for the high porosity but low-production wells is that the larger pore-to-throat ratio makes for poor pore connectivity and more large pores and vugs are trapped. Movable fluid saturation can be significantly influenced by microscopic pore structure rather than physical parameters, which is positively affected by throat radius and negatively correlated with the pore-throat radius ratio, relative sorting coefficient, and tortuosity of the throat. The results provide a basis for the exploration of carbonate reservoirs from a microscopic perspective.

Rate-Controlled Mercury Injection Experiments to Characterize Pore Space Geometry of Berea Sandstone

Interpretation of the relationship between heterogeneity and the flow in porous media is very important in increasing the recovery factor for an oil or gas reservoir. Capillarity for instance, controls fluids static distribution in a reservoir prior to production and remaining hydrocarbons after production commences. Therefore, capillary pressure data are used by petroleum engineers, geologists, and petrophysicists to evaluate production characteristics of petroleum accumulations. Conventional pressure-controlled mercury porosimetry produces an overall capillary pressure curve and pore throat size distribution data that provide little information about the porous medium structure and pore geometry. The present study provides information on three capillary pressure curves obtained from rate-controlled mercury injection porosimetry; one describes the larger pore spaces or pore bodies of a rock, another describes the smaller pores or pore throats that connect the larger pores, and a final curve which corresponds to the overall capillary pressure curve obtained from the conventional pressure-controlled mercury injection. An experimental constant-rate mercury injection apparatus was constructed that consists of a piston displacement pump, a computer controlled stepper motor drive and a core sample cell designed to minimize dead volume. The apparatus was placed in a glass chamber and subjected to an air bath to maintain a constant temperature of 27o C throughout the experiments. Then constant rate mercury injection experiments were performed on three Berea Sandstone core plugs. Results show that volume-controlled or rate-controlled porosimetry provides considerably more detailed data and information on heterogeneity and the statistical nature of pore space structure than the conventional pressure-controlled porosimetry as pressure fluctuations with time reveal menisci locations in pore bodies and pore throats. Moreover, pore size distributions based on volume-accessed pores and pore radii were obtained from the pressure versus saturation relationship.

Study of Fractal and Multifractal Features of Pore Structure in Tight Sandstone Reservoirs of the Permian Lucaogou Formation, Jimsar Sag, Junggar Basin, Northwest China.

The primary factor impacting the tight sandstone reservoirs and fluid flow capacity represents the pore-throat structure. On the basis of petrophysical characteristics test, scanning electron microscopy (SEM), and casting thin-section examination of tight sandstone reservoir specimens from the Permian Lucaogou Formation in Jimsar Sag, Junggar Basin., the full-size pore-throat parameters and distribution characteristics were determined by constant-rate mercury injection (CRMI) analysis. Using fractal theory and multifractal theory, the pore architectures of sandstone pores are analyzed. Mercury intrusion capillary pressure (MICP) is used to compute the dimensions of fractals using various fractal models and multifractal characteristics. Analysis is done on the relationships between tight sandstone pore architectures and fractal and multifractal characteristics. According to the research, a network of tightly packed sandstone pores may be assessed using the dimensions of fractals computed from a 3D capillary model. When displacement pressure is increased, the dimensions of fractals rise; when permeability, pore-throat diameter, and variable coefficient are increased, it falls. Tight sandstone pores exhibit multifractal features, according to the multifractal analysis, and multifractal parameters may depict the size, concentration, and asymmetry of the pore size distribution (PSD). Sandstone’s PSD is comparable when its multifractal parameters (Δα, Δf, α0, α1, α2) are identical. Pore diameters of tight sandstone are positively connected with information dimensions D1 and correlation dimensions D2, and information dimensions D1 have a greater impact on PSD than correlation dimensions D2. Additionally, the 3D capillary model’s dimensions of fractals and D1 exhibit a substantial negative association. These findings play a significant guiding role in the quantitative characterization of unconventional reservoir pore structures. The multifractal technique is effective to define the heterogeneity of the sandstone pore system and to differentiate between distinct PSD in heterogeneity.

Pore structure analysis of tight reservoirs in He-8 Member of Upper Paleozoic in the southwestern Ordos Basin, China

Quantitative characterization of the pore structure in tight reservoir can provide basic parameters for the evaluation of the reservoir quality. By using the cast thin-section petrography, scanning electron microscopy, mercury injection capillary pressure, and constant-rate mercury injection, we have evaluated the distribution characteristics of pore throat and its contribution to permeability. In addition, we have established the relationship between different pore-throat structure parameters and reservoir physical, which provides theoretical support for reservoir evaluation of the He-8 Member in the study area. The results indicate that the pore structure exerts a great control on the quality of the reservoir. Large pore throats (>0.15 μm) are the main contributors to the high seepage capacity of the reservoir. Throat radius, throat distribution, and pore-throat ratio are the key factors controlling the quality of tight reservoir. The He-8 Member of the Upper Paleozoic in the study area is dominated by tight reservoirs, accompanied by a small number of conventional reservoirs.

Heterogeneity of pore-throat structures and petrophysical properties of tight sandstone and its influence on seepage: A case study of sandstone in the Yanchang Formation, Ordos Basin, China

Tight sandstone reservoirs are characterized by poor petrophysical properties, strong pore-throat structures, and heterogeneous permeability, which prevent oilfield water flooding. We take the tight sandstone reservoir of the Yanchang Formation in Ordos Basin as an example, select samples from the Chang 6 and Chang 8 members in the HX block for constant-rate mercury injection and single-core and combined-core model water-flooding tests, and discuss the effects of heterogeneous pore-throat structures and petrophysical properties on the seepage of the tight sandstone. Constant-rate mercury injection tests found that the permeability of the tight sandstone was controlled by the throats. Compared with the pore structures, the throat structures were more heterogeneous, and the samples with more heterogeneous throats were more permeable. Therefore, in addition to the morphology of the pore-throat structures, the heterogeneity of the throats also was used to evaluate the seepage. When applying the water-flooding tests with the combined-core model, the water absorption capacity of the high-permeability core with a high permeability contrast (7.5) was greater than that of the high-permeability core with a low permeability contrast (3.6). Moreover, by comparing the results from the water-flooding experiment for the same core between a single-core and the combined-core model, the loss of the oil displacement efficiency (70.7%) of the low-permeability core in the combined-core model with a high permeability contrast (7.5) was much higher than that (29.8%) of the low-permeability core in the combined-core model, which had a low permeability contrast (3.6). The water absorption percentage and the recovery of the low-permeability core in the combined-core model with a high permeability contrast were lower, indicating that after water flooding, the remaining oil in the low-permeability reservoir in the area with a strong permeability heterogeneity was enriched. Such enrichment may increase the exploitation potential of tight reservoirs.

Dynamic track model of miscible CO2 geological utilizations with complex microscopic pore-throat structures

Monitoring field-scale CO2 geological utilizations is of paramount importance but challenging due to the complexities in microscopic heterogeneity. In this paper, complex geological characterizations and fluid properties are specifically analysed and a prediction model is developed, which is capable to track the dynamic behaviour of miscible CO2 multiphase flow in microscopically-heterogeneous porous media. More specifically, first, pore-throat sizes and distributions were characterised from the constant-rate mercury injection and the CO2-displacement seepage resistance was evaluated from a capillary bundle model. The differences of seepage resistance caused from the throat changing and coupling diffusion-dissolution effects and viscosity-resistance reductions were specifically studied in the process of continuous miscible CO2 displacement. Accordingly, a comprehensive mathematical model was developed with the time-node analysis method and validated through comparison with the experimental results. The leading edge of CO2 displacement as well as the timing of gas breakthrough and displacement completion are determined to be different with varying throat sizes. It is further found that the gas breakthrough time and the time required to complete the displacement are reduced with increasing throat size and their differences also decrease. Moreover, the interval area of injection and production wells could be characterised as pure CO2, diffusion and pure oil zones, whose positions could be dynamically tracked from the recovery performance. The calculated oil recovery and gas-oil ratio from the developed model share good agreement with experimental results, with deviations of 2.2 and 0.7%. Such the validated mathematical model is then applied to successfully predict the dynamic performance of an actual reservoir (H3).

Study on the pore structure, fluid mobility, and oiliness of the lacustrine organic-rich shale affected by volcanic ash from the Permian Lucaogou Formation in the Santanghu Basin, Northwest China

Lacustrine organic-rich shale with extremely heterogeneous pore structure is widespread in the second member of the Permian Lucaogou Formation (P2l2) of the Santanghu Basin. However, information on the pore structure, fluid mobility, and oiliness characteristics is still lacking, as well as the interaction between them in the organic-rich shales remains controversial. To address these problems, this study analyzed the influencing factors of pore structure, oiliness, and fluid mobility of the P2l2 shale and constrained the lower limit of movable fluid pore throat radius based on thin sections, routine core analysis, whole rock X-ray diffraction (XRD), scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), nuclear magnetic resonance (NMR), constant-rate mercury injection (CMI), and nano-CT scanning data. The P2l2 shale was characterized by fine-grained minerals of felsic (tuffaceous material) and carbonate (dolomite), with poor reservoir physical properties and strong oil-bearing heterogeneity. Micro- and nano-scale pores are the main type of storage space. Among them, most of the microfractures emit green to yellow hydrocarbon fluorescence regardless of whether they are filled or not, as do the edges of dolomite grains. While the edges of the felsic minerals are characterized by stronger orange yellow fluorescence of organic matter. According to the morphology and parameters of HPMI and NMR curves, the pore structure can be divided into four types. From type 1 to type 4 pore structure, the maximum mercury saturation decreases from 95 to 47 % (avg. 70 %), indicating poor pore connectivity, and the pore throat radius that mainly contributes to permeability is > 0.04 μm. Meantime, the NMR T2 spectrum gradually transitions from right unimodal to left unimodal behavior, with a gradual decrease in the saturation of movable fluid and a transition of the mineral type from carbonate to felsic. Fluid mobility is not only influenced by macropores, but also related to the content of small pores (T2 < 1 ms) and the mineral type. On this basis, the lower limit of the movable fluid pore throat radius in the P2l2 organic-rich shale is finally determined to be 40 nm.

Controlling factors of matrix acidizing potential of low permeability clastic reservoir in faulted lacustrine basin

Based on constant rate mercury injection experiment, casting thin section identification, scanning electron microscope observation, clay mineral X-ray and rock specific surface area analysis, the controlling factors of matrix acidizing potential of low-permeability sandstone reservoir in fault depression lacustrine basin were determined from three aspects: pore filling, throat filling and pore-throat combination characteristics. It is concluded that provenance controls the plane partition of reservoir pore fillings. Burial depth controls the longitudinal zoning of key filling material in reservoir throat. The difference of rock structure in sedimentary facies - microfacies controlling zone leads to the change of pore-throat assemblage pattern. The matrix acidizing scheme of low permeability sandstone reservoir in fault depression basin can be formulated according to the law of “provenance zoning, buried depth zoning and being controlled by microfacies in zone”, and the implementation scheme of pre-acid, main acid and post-acid can be put forward respectively. This method can effectively promote the integration process of exploration and development of low permeability clastic reservoir in mature exploration area.

Impacts of pore-throat spaces on movable fluid: Implications for understanding the tight oil exploitation process

In this study, a series of experiments were carried out on tight sandstone samples from the Yangchang Formation in the Ordos Basin, China to investigate their pore-throat spaces and fluid flow capacity, including nuclear magnetic resonance (NMR) test, constant-rate mercury injection (CRMI), pore-casted thin section analysis, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The pore-throat space was divided into two main types based on CRMI and NMR results: the throat-dominant space and the intergranular pore-dominant space, and the volume proportions was calculated. Proportions of different pore-throat spaces do not affect the total reservoir space, and the proportion of the throat-dominant space plays an important role in controlling the complexity of the pore-throat structure. The intergranular pore-dominant space contributes the main permeability, which affects the fluid flow capacity of tight sandstone through the large throats connecting intergranular pores. Fluid flow capacity in the intergranular pore-dominant space is related to the configuration of pores and throats, a higher proportion of the short conduit throats exist means a more homogeneous pore-throat structure, bound water is redistributed on the pore-throat surface to form a thin water film after centrifugation, leading to a higher proportion of movable fluid. The change of throat volume proportion to the total pore-throat volume with pore-throat radius was studied based on CRMI. With the increase of the pore-throat radius, the proportion of the throat volume first decreases in a fluctuating manner and then starts to increase rapidly at a certain radius (rt-p). The rt-p value is believed to determine the fluid flow capacity of the tight sandstone. This study has certain reference significance for understanding the fluid flow during the exploitation process of tight sandstone.

A Comparative Study on Microscopic Characteristics of Volcanic Reservoirs in the Carboniferous Kalagang and Haerjiawu Formations in the Santanghu Basin, China

Self-jetting high-yield oil flow was obtained from Ma 67 and Ma 36 wells drilled in the volcanic reservoirs of the Haerjiawu Formation in the Santanghu Basin, China. This has shifted the prospectors’ attention to the Haerjiawu Formation from the Kalagang Formation, which is generally considered to have favorable physical properties. To further explain the geological reasons why oil flow can jet itself from the volcanic rocks in the Haerjiawu Formation with poor physical properties, this study carries out a systematic comparison on the microscopic pore structures of volcanic rocks through unconventional tests such as low-temperature nitrogen adsorption, high-pressure mercury injection, and constant-rate mercury injection based on the analyses of physical properties and minerals. The results obtained are as follows. The volcanic rocks of the Kalagang Formation have relatively high pore permeability. However, their micropores have a wide distribution range of pore size and feature highly meandering structures and strong heterogeneity. Meanwhile, small pore throats connect large pores in the volcanic rocks, resulting in a relatively high pore/throat ratio. All these are conducive to the occurrence of tight oil and gas but unfavorable for the flow of oil and gas. The volcanic rocks in the Haerjiawu Formation have relatively low volcanic permeability. However, small pores connect large pore throats in the volcanic rocks; thus, leading to a relatively low pore/throat ratio. Meanwhile, the volcanic rocks feature low meandering structures, strong homogeneity, and high connectivity. All these are favorable for the formation of tight oil and gas reservoirs. These assessment results also indicate that the assessment indices of tight volcanic reservoirs should not only include porosity and permeability. Instead, more attention should be paid to the microscopic pore structures, and it is necessary to analyze the charging and flow of tight oil from the configuration of pores and pore throats. This study not only explains the geological factors of the wells with self-jetting high-yield oil flow in the Haerjiawu Formation from the perspective of microscopic pore structures but also provides a new idea and comparison method for the assessment of tight reservoirs in other areas.

Quality characterization of tight sandstone reservoirs in the Yanchang Formation of the Honghe oilfield, Ordos Basin, central China

Reservoir quality is one of the important geological factors controlling the development of tight oil in the Honghe oilfield, Ordos Basin, northwestern China. Analyses of core, well logging, mud logging and geophysical data as well as thin sections (casting and fluorescence) were combined with testing methods (such as grain size analysis, constant-rate mercury injection, and scanning electron microscopy) to characterize the micropore–throat development in reservoirs of the eighth member of Yanchang Formation in the oilfield. From the perspective of sedimentation and diagenesis, the mechanisms causing reservoir quality difference were explored and a method for characterizing reservoir quality difference and distribution was proposed. The results show that complex and diverse pore–throat configurations and multi-scale throat development are the microscale manifestations of reservoir quality differences in the member. Three types of pore–throat combinations are recognized, including intergranular pore-wide lamellar throat, intergranular and intragranular pore-wide lamellar throat, and intergranular/clustered micropore-wide lamellar throat. Different diagenesis processes and intensities under the control of sedimentary conditions determine the differential development of the reservoirs. Diagenetic facies are the indicators of reservoir quality. Diagenetic facies with chlorite cementation-moderate dissolution indicates reservoirs with the most ideal physical properties for hydrocarbon accumulation, while that with moderate calcite and kaolinite cementations are usually observed in reservoirs with less ideal physical properties. Reservoirs with the worst physical properties often correlate with diagenetic facies with strong calcite cementation and compaction facies. A multi-level constrained method under the control of sedimentation and diagenetic facies is proposed for characterizing tight reservoir quality difference in the member. The spatial distribution of sedimentary elements is analyzed through sedimentary configurations, the diagenetic facies distribution is constrained by sedimentary elements and the reservoir quality distribution is predicted with constraint of diagenetic facies. It suggests that the high-quality reservoirs in the member occur vertically in the middle of thick channel sandstone, while poor reservoirs occur in the upper or lower parts of the channel sandstone due to intensive compaction and cementation. Laterally, the quality of reservoirs enhances along the channels with a change in shape to lens or strips along the middle and downstream sections.

The Evolutionary Characteristics of Reservoir Microstructure under Long-Term Waterflooding Development and Its Fractal Description

Generally, long-term waterflooding development often leads to the change of reservoir pore and clay mineral composition, which results in the change of permeability and wettability. In order to explore the relationship between core micropore structure and water cut, based on physical simulation experiments and fractal theory, we proposed a fractal evolutionary model to describe the evolution characteristics of microstructure of long-term water driving reservoirs. In this paper, core pore structure by SEM was first conducted to analyze the change of core pore structure before and after waterflooding under the conditions of magnification of 200 times, 800 times, and 2000 times, respectively. Then, conventional and constant rate mercury injection tests were combined to perform the comparative analysis of core structural parameters before and after waterflooding. Finally, a micropore-throat structure evolution model of core was established. Research shows that the connectivity of larger pores becomes better after long-term water driving, the degree of heterogeneity weakens, and the micro heterogeneity of small pores becomes stronger and stronger. The throat characteristics change in a complex manner, the radius tends to increase, and the sorting becomes better, while the connectivity of small throat changes complex. In general, the heterogeneity of throat increases with the time of water injection in reservoirs with low porosity and permeability. On the basis of fractal theory and variation characteristics of rock pore structure in water driving reservoirs, we have established a micropore-throat structure evolution model of core pore-throat characteristics. This fractal evolution model quantitatively characterized the complexity and evolution law of pore structure and clarified the relationship between fractal dimension of core pore structure and water cut under different stages of water driving.

Numerical Simulation Study of Pore-Throat Evolution of Upper Paleozoic in Ordos Basin, China

The parameters of grain size, contents of silica, kaolinite, hydromica, calcite, and a geological time of tight sandstone reservoirs in Upper Paleozoic in Ordos basin were researched thoroughly, and the impact of the diagenetic evolution process of different sandstone types on porosity and throats was analyzed, based on the quantitative statistics from thin sections, measurements of porosity and permeability, and conventional and constant-rate mercury injection tests. We not only build the evolution of porosity through process-oriented numerical simulations during the geological time but also establish effect-oriented numerical simulations between porosity and different diagenesis parameters. Furthermore, we set up a fitting relationship between diagenetic factors and pore throats in different gas-bearing reservoirs. Differentiation results in the evolution of porosity and a pore-throat system of sandstone types have clear characteristics, such as lithic quartz sandstones of the He 8 Member in the Sulige area and quartz sandstones of the Shan 2 Member in the Yulin area. The fitting results show that the main factors influencing the evolution of porosity and a pore-throat system are grain size and siliceous cement, which can also be validated by the measured data on two gas-bearing intervals. The results are important to a deep understanding of the relationship between the reservoir continuing to experience porosity and permeability evolution and the timing of petroleum charging into the reservoir and can also be applied elsewhere as a quick means in high grading areas of risks during field development.

Pore throat structure and classification of Paleogene tight reservoirs in Jiyang depression, Bohai Bay Basin, China

The pore throat structure characteristics of Paleogene tight sandstone and sandy conglomerate in the Jiyang depression are studied using cast thin section, conventional mercury injection, constant rate mercury injection and micro CT scanning data, and a reservoir classification scheme based on pore throat structure parameters is established. The material composition and structural characteristics of tight reservoirs are analyzed by casting thin section data. The pore throat structure characteristics of tight reservoirs are studied by conventional mercury injection, constant rate mercury injection and micro CT scanning. Ten pore throat structure parameters are analyzed by cluster analysis. Based on the classification results and oil test results, the classification scheme of Paleogene tight reservoirs is established. The Paleogene tight reservoirs in the Jiyang depression have the characteristics of macropores and microthroats, with pores in micron scale, throats in nano-submicron scale, and wide variation of ratio of pore radius to throat radius. The permeability of the tight reservoir is controlled by throat radius, the smaller the difference between pore radius and throat radius, and the more uniform the pore throat size, the higher the permeability will be. The lower limits of average pore throat radius for the tight sandstone and tight sandy conglomerate to produce industrial oil flow without fracturing are 0.6 μm and 0.8 μm, respectively. Reservoirs that can produce industrial oil flow only after fracturing have an average pore-throat radius between 0.2–0.6 μm, and reservoirs with average pore throat radius less than 0.2 μm are ineffective reservoirs under the current fracturing techniques. Different types of tight sandstone and sandy conglomerate reservoirs are classified and evaluated, which are well applied in exploratory evaluation.

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.

Diagenetic alteration, pore-throat network, and reservoir quality of tight gas sandstone reservoirs: A case study of the upper Paleozoic sequence in the northern Tianhuan depression in the Ordos Basin, China

The Sulige gas field in the Ordos Basin is the largest gas field in China. The studied area, the northern Tianhuan depression, is located west of the gas field and is considered as a potential gas-producing area. The complex diagenetic alteration history and its effects on the pore-throat network and reservoir quality of the tight gas sandstones are explained. The upper Paleozoic first member of the Shanxi Formation and eighth member of the Shihezi Formation tight gas reservoir rocks are composed of coarse- to medium-grained sandstones and are dominated by sublitharenites. The most important diagenetic factor for porosity reduction was compaction, followed by cementation. Diagenetic mineral phases include quartz overgrowth, calcite cement and replacement, and kaolinite cement and replacement, with minor heulandite and chlorite. Pressure dissolution of quartz grains and quartzite fragments is the main source of silica for the early and late quartz overgrowth cementation (formed mainly from 120°C to 140°C). The hydrolysis and alteration of volcanic material and mica flakes released some silicon, calcium, and aluminum ions. Calcium and carbon in calcite cements could be derived from surrounding mudstones. Calcite replaced the quartz overgrowths. The formation of kaolinite, as cement and replacements, was controlled by the interactions of silica, aluminum, pH, and temperature (140°C). Because of deformation of detrital components (mica flakes and ductile rock fragments) and diagenetic replacement, clear negative correlations between reservoir quality and contents of the diagenetic minerals are lacking. A complex and isolated pore-throat network was created by strong diagenetic transformation. Based on analysis of thin sections, three-dimensional x-ray micro–computed tomography, and constant-rate mercury injection data, the samples with relatively good reservoir quality (porosity >10% and permeability >0.5 md) are characterized by dissolution pores and primary intergranular pores and commonly contain more and larger pore throats both in volume (counts >1800 and size >0.02 per unit volume) and diameter (>1.20 μm) than the samples with poor reservoir quality. The intercrystalline pores of kaolinite are very widespread and considered favorable for oil and gas accumulation because some asphalt was found within these pores. This study may be used, by analogy, to better understand diagenetic factors controlling the formation of tight gas sandstone reservoirs and predict the development of pore-throat systems.

Pore–throat structure and fractal characteristics of tight sandstones in Yanchang Formation, Ordos Basin

Pore–throat structure is a key factor that influences the storage and fluid flow capacity of tight sandstone reservoirs. By performing petro-physical characteristic tests, scanning electron microscopy, and casting thin sections, the main pore types of tight sandstone reservoir samples collected from the Upper Triassic Yanchang Formation in the Baibao area of the Southwest Ordos Basin in China were identified to be dissolved pores, intergranular pores, intercrystalline pores, and micro-cracks. Meanwhile, the size of the pore–throat was widely distributed, whereas its shape varied. Given the advantages and disadvantages of various technical methods, high-pressure mercury injection (HPMI) and nuclear magnetic resonance (NMR) technology were used to study the distribution characteristics of a full-size pore–throat. All 10 samples showed bimodal and unimodal distribution characteristics, and their distribution characteristics were consistent with the results of the petro-physical characteristics, high-pressure mercury injection, and movable fluid saturation tests. The throat distribution characteristics of the reservoir were studied by applying constant-rate mercury injection (CRMI), and the throat radius ranged from 0.43 μm to 9.76 μm with an average of 3.34 μm.The fractal features of the pore–throat and throat were quantitatively characterized by CRMI. The fractal curves of the pore–throat showed self-similarities, and the fractal dimension D ranged between 2.6297 and 2.9529 with an average of 2.7865. The fractal curves of the throats were broken down into two segments, and the fractal dimensions corresponded to two ranges, namely, the small (Dt1) and large throats (Dt2) fractal dimensions. The average of fractal dimension Dt1 was 2.9170, and the average of fractal dimension Dt2 was higher than that of Dt1 with an average of 2.9828, thereby indicating the complex structure and strong heterogeneity of the former. The fractal dimensions D, Dt1, and Dt2 were negatively correlated with permeability, movable fluid saturation, average throat radius, clay mineral content, and laumontite content, positively correlated with quartz content, and showed poor correlation with feldspar. Compared with Dt2, the fractal dimension Dt1 had a higher correlation with pore–throat parameters and mineral content. Given the higher self-similarity of small throats, their distribution is considered highly homogeneous.

Characteristics of pore structure of tight gas reservoir and its influence on fluid distribution during fracturing

Micro-pore structural characteristics are complex in tight gas reservoirs, and it affects the fluid flow. Hydraulic fracturing is the key technique for economic gas production from tight reservoirs. In the fracturing process, fracturing fluid imbibes into reservoir matrix driven by capillary force and affects the dynamic distribution of reservoir fluid. It is thus of great interests to investigate the micro-pore structural characteristics, and its relationship with the dynamic distribution of fluid. In this study, cores from the Yanchang tight sandstone reservoirs in the Ordos basin were selected to evaluate the micro-pore structural characteristics and dynamic distribution of fluid. High-pressure mercury injection (HPMI), constant-rate mercury injection (CRMI), and low-pressure nitrogen gas adsorption (LP-N2GA) are used to evaluate the structural characteristics of micro-pores. The results show that average pore radius of samples ranges from 131.25 μm to 191.10 μm, with an average value of 170.42 μm, and the average throat radius ranges from 1.15 μm to 4.07 μm, with an average value of 2.489 μm. The pores are mainly closed small pores, and some H3-type crack-type pores are present. Imbibition, centrifugation experiment and nuclear magnetic resonance (NMR) techniques were used to study the dynamic distribution of fluid. The results show that imbibition recovery ranges from 40.04% to 87.29%, with an average of 62.14%, and movable fluid saturation ranges from 48.20% to 94.74%, with an average of 76.61%. The fluid during imbibition and the movable fluid during saturation were primarily distributed in mesopores and macropores. Results of statistical analysis suggests that the pore-throat structure is among the key factors impacting the imbibition recovery and movable fluid saturation.