Porosity In Sandstones Research Articles

Controls on composition and diagenesis of wave- and river-dominated deltas: impacts on reservoir properties. An example from the La Anita Formation (Argentina)

Wave-, tidal-, and fluvial-dominated deltas are important hydrocarbon reservoirs worldwide, and it is well known that depositional facies types, detrital composition and diagenetic products are key aspects when evaluating potential reservoir properties. Moreover, detrital composition and diagenetic products are controlled by external forces such as tectonism, climate, and depositional conditions.The Upper Cretaceous La Anita Formation (Austral-Magallanes Basin, Argentina) is a deltaic succession that shows a clear vertical variation in the relative role of wave, tidal and fluvial processes. The formation was accumulated during the foreland stage of the basin under relatively warm climate conditions. A detailed compositional and diagenetic analysis for these deposits is provided to evaluate the role of the controlling factors on the detrital and authigenic composition, and the impact on reservoir properties.Samples were analyzed by conventional petrography, X-ray diffraction and scanning electron microscopy to obtain a full spectrum of its detrital, authigenic and diagenetic composition. The analyzed sandstones were classified as lithic arenites which come from the fold-thrust belt region and from the magmatic arc located toward the westward. The clay mineral composition is characterized by variable proportions of illite, smectite, kaolinite and mixed-layer illite-smectite with the sporadic presence of chlorite. Stratigraphic variations in the abundance of authigenic kaolinite and smectite indicate a change in the climatic conditions during the depositions of the unit evolving from tropical to more temperate conditions. Diagenetic processes affecting the porosity are considered as controlled mainly by the lithological nature of the deposits and depositional facies type. Sandstone porosity is mainly primary in origin, moderate to good in abundance and show micropore to mesopore as the main pore-sizes.This work enhances that the diagenetic products are heavily controlled by the dominant depositional process. The presence of diagenetic products as quartz overgrowth and calcite and hematite cement reduces pore spaces and affects the reservoir properties. The development of these types of cement is conditioned by the relative dominance of wave and river processes. Under wave-dominated conditions, quartz tends to develop thick overgrowths. Conversely, under river-dominated conditions quartz overgrowths are thinner and calcite and hematite cementation patches are developed.

A coupled elastic constitutive model for high porosity sandstone

An elastic constitutive model is developed to predict the mechanical unloading response for high porosity sandstone. The resulting model is sufficiently complex to represent known mechanical response, yet reasonably straightforward to calibrate. A mechanical dataset from a suite of true triaxial tests on Castlegate sandstone is used to develop, calibrate, and validate the model, which incorporates the experimentally observed evolutions of the elastic bulk and shear moduli with stress and plastic strain. Furthermore, to properly represent the hydrostatic unloading response, a coupled term is required, in which the plastic volume strain (accumulated during both hydrostatic and deviatoric loading) modifies the nonlinear stress dependence of the bulk modulus. Omission of this coupling term causes significant over prediction of the plastic volume strain and bulk modulus after complete unloading. However, the coupled term is not required to model deviatoric response and shear modulus evolution. While a suite of 28 tests was used to develop the model, only one carefully controlled test is required to calibrate the model for a chosen high porosity sandstone.

Quantifying occurrence of deformation bands in sandstone as a function of structural and petrophysical factors and their impact on reservoir quality: an example from outcrop analog of Productive Series (Pliocene), South Caspian Basin

Deformation bands (DB) are known to influence porosity and permeability in sandstones. This study aims to predict the occurrence of DB and to quantify their impact on reservoir properties based on field measurements in the steeply dipping limb of a kilometer-scale fold in Yasamal Valley, western South Caspian Basin. An integrated approach of characterizing bands and their effect on reservoir properties included measurements of natural gamma radioactivity and permeability using portable tools, along with bed dip and the count of DB across distinct facies. A set of core analyses was performed on outcrop plugs with and without bands to estimate the alteration of rock properties at the pore scale. Interpretation of outcrop gamma-ray data indicates the absence of bands in Balakhany sandstones containing shale volume greater than 18% for unconsolidated and 32% for calcite-rich facies. A high amount of calcite cement appears to increase the number of DB. A poor, positive trend between bed dip and DB concentration was identified. We show that net to gross, defined as the thickness fraction of sandstone bound by mudstones, is among the parameters controlling the occurrence of bands. Samples containing a single DB show a 33% and 3% decrease in permeability and porosity, respectively, relative to the host rock. We reveal a new set of lithological and petrophysical factors influencing DB occurrence. This study offers a direct tool that can be applied in subsurface reservoir analogs to predict the occurrence and concentration of DB and estimate their influence on rock properties.

Diverse effects of intragranular fractures on reservoir properties, diagenesis, and gas migration: Insight from Permian tight sandstone in the Hangjinqi area, north Ordos Basin

As petroleum industry tilts towards unconventional petroleum resources, the significance of fractures in exploration and development becomes increasingly prominent. In this study, thin sections and scanning electron microscopic (SEM) observation, porosity and permeability measurement, mercury injection porosimeter (MIP) and computerized tomography (CT) analysis were carried out to investigate the morphology and development characteristics of intragranular fractures, and their effects on physical properties, reservoir diagenesis, and gas migration of the He-1 Member tight sandstones in the Hangjinqi area. Results show that the tight sandstones have intergranular and intragranular dissolved pores, intercrystalline pores and heterogeneous distributed intragranular fractures. The average porosity and permeability of the tight sandstones with intragranular fractures are 12.76% and 4.70 × 10−3 μm2, while those lack of fractures are 6.06% and 0.574 × 10−3 μm2, respectively. The fracture density increases with the enlargement of clastic-grain size and the content of rigid grains. Results indicate that the intragranular fractures greatly enhance permeability, but they have limited direct contribution to total porosity. And the sandstones rich in intragranular fractures have a more homogeneous and connective pore structure with larger radius of pore throat. Intragranular fractures are beneficial to the latter feldspar and debris dissolution caused by organic acids derived from hydrocarbon generation in coal-bearing strata. The dissolved pores directly enhance the porosity of tight sandstones compared to intragranular fractures, and together they form a percolation network to increase permeability. Faults mainly control the lateral petroleum migration from the south to the north, and intragranular fractures affect the local accumulation of gas. This study helps to illustrate the influences of intragranular fractures on reservoir properties, diagenetic difference, and gas migration of the He-1 Member tight sandstones in the Hangjinqi area, and can guide the evaluation and exploration of tight sandstone reservoirs in the Ordos Basin and other basins with similar settings around the world.

Sandstone reservoir quality in light of depositional and diagenetic processes of the Messinian Qawasim Formation, onshore Nile Delta, Egypt

• Sandstone in the upper part of the Qawasim Formation is the main reservoir in the El Tamad field. • Primarily, the sandstone is arenite in composition, well sorted with sub-rounded to rounded grains. • Diagenesis enhanced the reservoir quality of the sandstone with low-cement content, poor compaction and weak fracturing. • Clastic rocks of the Qawasim Formation were deposited in a shelf environment as stacked barrier-island bars. • Carbonate rocks of the Qawasim Formation were deposited in a shelf lagoon with open circulation. The Nile Delta region is the main gas-producing province in Egypt. Late Miocene sediments are the target of drilling in the onshore Nile Delta where it has the potentiality to store and produce hydrocarbons. Five wells of the Messinian Qawasim Formation at El Tamad Field were selected for the present study. Detailed petrographic investigations were carried out to reveal the depositional and diagenetic features of the Qawasim Formation rock units. Two main facies were distinguished in the studied formation: clastic and carbonate facies. The clastic rocks include conglomerate, sandstone and shale, while the carbonate rocks include limestone and dolostone. Depositional, diagenetic and petrophysical parameters of the sandstones of the Qawasim Formation reflect that this rock unit an excellent reservoir for hydrocarbon. These microfacies display depositional features such as well sorting, open packing and poor cementation in both coarse and fine-grained microfacies. The porosity of sandstone was enhanced by diagenetic processes such as dolomitization of calcite cement and low compaction that resulted in grain cracking, while the dissolution of feldspars had a subordinate impact on increasing porosity due to their low abundance and the resultant authigenic clay minerals.

Investigating the Frequency Spectrum Characteristic of Stress Wave under Multistage Loading Stress

The stability of rock mass is destroyed by natural or human activities and leads to stress redistribution, causing the rock mass in a certain stressful environment. This study conducted a small disturbance impact tests on sandstone bar under loading stress by modified split Hopkinson pressure bar (SHPB). The results show that the reflection and transmission characteristics of stress wave are affected due to the loading stress changes in the sandstone porosity. The loading stress has a specific effect on the frequency spectrum distribution of the stress wave. The frequency spectrum curve has gone through the three stages, a gradual increase, then rapid attenuation, and finally a smooth development with the frequency increasing, and its dominant frequencies are mainly concentrated in 0∼2 kHz. The loading stress has a significant influence on the variation tendency of the dominant frequency. The dominant frequency experiences a slow increase and then tends to be stable, and the total energy of the frequency band shows a fast attenuation and then a gentle development, and its stress boundary point is σ/σc = 30%. The total energy attenuates as a first‐order exponential function and its attenuation rate shows an exponential‐linear function with the increasing loading stress, the farther away from the shock end, the faster the total energy attenuation is. The sandstone can filter the high‐frequency wave and the low‐frequency wave can penetrate rock media better. The closer the distance to the impact source, the greater the total energy of the frequency band. The frequency band energy is mainly concentrated in 0∼36.62 kHz, the higher the frequency of the frequency band is, the smaller the energy ratio is. Therefore, those conclusions can provide a reference for the evolution analysis of the stress wave spectrum in an excavated rock mass.

Simulation of ductile grain deformation and the porosity loss predicted model of sandstone during compaction based on grain packing texture

The content of ductile grains and grain packing texture have great influence on the quality of sandstone, especially tight sandstone. A new simulation method for ductile grain deformation during compaction in sandstone under triaxial stress servo based on the discrete element method (DEM) was established. The micro-mechanical parameters for the simulation were obtained using physical experimental data in the literature. The numerical simulation results of single ductile grains and sandstone with ductile grains confirmed the reliability of the simulation method, where the effect of grain crushing, cementation and dissolution on porosity evolution were not considered. The simulation and physical experiment results show the porosity of sandstone decreases linearly with the increase of ductile grain content. The relationship between porosity and effective stress can be predicted using exponential model, when the grain packing texture was one-component nature packing texture dominated and ductile grain content was less than 50%. When the grain packing texture changes to binary packing and the content of ductile grains was less than 50%, the porosity evolution curve show some fluctuations, and the monitoring compaction curve showed that the relationship between effective stress and porosity can not be represented by exponential model. The void ratio changes of sandstone with binary packing texture during compaction can be predicted based on grain packing texture, and the correlation coefficient between the predicted and simulated void ratio changes (△e) were high, the average errors accounts less than 6% of the average of void ratio changes (△e) when the content of ductile grains were less than 25%. The errors between predicted and simulated void ratio changes (△e) based on packing texture increases with the increase of ductile grain content. When the content of ductile grains exceeded 50%, the exponential model was no longer applicable for sandstone, while the logarithmic model is better. In addition, the proposed simulation method is significant for the study of porosity evolution process and the quantitative evaluation of the relative importance of its diagenesis in sandstone having ductile grains.

New filtration parameters from X-ray computed tomography for tight rock images

New parameters are proposed to evaluate the filtration properties of rocks obtained on the basis of 3D interpretation of images from X-ray computed tomography. The analyzed parameters are: global average pore connectivity, average blind pore connectivity, blind pore coefficient per object and blind pore coefficient per branch. The 3D pore space from computed X-ray tomography must be subjected to a process of pore space transformation into a skeleton. Then, the presented parameters can be evaluated, taking into consideration the pore channels (branches), pore channel connection points (junctions) and blind pores (pore without connection to the other pore). The calculations were made for low porosity sandstones, mudstones, limestones, and dolomites which differ in terms of age and depth of present deposition. The global average pore connectivity reflects the degree of development of the pore space in which the formation fluid can flow. The higher the global average pore connectivity, the most complex the pore structure can be expected. The higher the parameter of the average blind pore connectivity, the worse are the filtration properties of the rock. The higher the concentration of blind pore coefficient per object or branch, the worse the filtration properties of the rock. Moreover, new parameters were compared with the Euler characteristic and coordination number, revealing a high consistency.

Fracture mechanism of tight sandstone under high and complex 3-D stress compression: Insights from acoustic emission

Tight sandstone gas (TSG) reservoirs are developed in large buried depths and complex geo-stress field environments. During exploitation, the surrounding rock of a drill well is often damaged or can collapse under artificial engineering disturbances. Understanding the failure mechanism of tight sandstone under high and complex three-dimensional (3-D) stress states is essential for the safe and efficient exploitation of TSG. In this study, using the stress Lode angle ( θ σ ) as a variable, failure experiments of low porosity sandstone specimens under various 3-D stress paths are performed, and the mechanical responses (e.g., stress–strain behavior, strength, fracture pattern, and acoustic emission characteristics) are analyzed. The results show that as θ σ increases, the strength of the specimen as well as the deviatoric stress required its failure decrease linearly, whereas its brittleness increases. The failure of the specimens is primarily due to numerous micro tensile cracks and a few macro shear cracks. θ σ significantly affects the cracking mode during failure. Acoustic emission (AE) parameters show that the failure process can be categorized into three stages within the time-to-failure window, among which Stage 2 (acceleration stage) can be regarded as the precursor stage of the ultimate failure of the specimen. The descriptive statistical results of AE energy show that uniaxial stress, hydrostatic stress, and true triaxial stress compression impose different effects on the damage mode of the specimens. Under the high 3-D stresses, the multiple fracture surfaces formed inside the specimen are intertwined to present several “X”-shaped fracture pairs. These findings facilitate the understanding of the failure mechanism of rock mass surrounding the wellbore, and of significance in stability designing in well trajectory of TSG reservoir actual development. • Stress Lode angles significantly affect cracking mode of tight sandstone. • Uniaxial stress, hydrostatic stress, and true triaxial stress compression impose different effects on damage mode of tight sandstone. • Precursor stage of tight sandstone ultimate failure under complex three-dimensional stress conditions can be identified through acoustic emission signals.

Petrography and reservoir quality controls in shallow transitional marine Cretaceous-Paleogene deposits in the Dahomey Basin, Nigeria

Upper Mesozoic and Early Cenozoic sedimentary rocks are of great significance to the petroleum geology of the Eastern Dahomey Basin as they are valuable prospects for hydrocarbon reservoirs. Diagenesis and reservoir quality (porosity and permeability) controls are deduced from core samples recovered from six shallow exploration wells in the onshore area of the Eastern Dahomey Basin using petrographic, petrophysical and mineralogical tools. The studied lithostratigraphic intervals consist of limestone, calcareous siliciclastic sandstone (Ewekoro and Araromi Formations), quartz arenite and clay-matrix supported sandstone (Afowo Formation) which were deposited within a shallow marine depositional environment.Siliciclastic samples of the Ewekoro Formation consist predominantly of calcareous sandstone and limestone, which are classified as packstone and wackestone. The porosity values in the sandstones range from 5 to 36% whereas permeability ranges from 0.006 to 109 mD. Ewekoro Limestone porosity ranges from 7 to 32% and permeability ranges from 0.003 to 13 mD. Samples of the Araromi Formation exhibit a porosity of 4 to 45% with permeability between <0.001 and 1764 mD in sandstones whereas a single limestone sample has 8% porosity and permeability of 0.03 mD. Porosity in quartz arenitic sandstones of the Afowo Formation ranges from 15 to 47% with permeability of 0.05 to >10,000 mD, while in the clay-matrix supported sandstones, the values range from 18 to 42% and 0.05 to 5 mD for porosity and permeability, respectively. The main pore-occluding authigenic minerals are carbonate cements (Fe-dolomite, Fe-calcite, siderite and dolomite), iron oxide, kaolinite, and illite-smectite mixed layer clays. The best reservoir quality is observed in the quartz arenitic sandstones of the Araromi and Afowo Formations, except for the low permeability, clay-matrix supported sandstone facies.The most significant controlling factors in sandstones are clay mineral matrix contents and the presence of carbonate cements. Fe-calcite, Fe-dolomite and kaolinite formation occurs during early diagenesis in the studied lithologies and has helped to reduce the effect of compaction. The results of this study indicate fair to good reservoir quality in the shallow marine sedimentary rocks of the Eastern Dahomey Basin, which could be targeted in further exploration campaigns in the region.

Comparison of Petrophysical Properties of Porous Rocks Using NMR, Micro-CT, and Fluid Flow Simulations

Micro-computed tomography (micro-CT) is increasingly utilized to image the pore network and to derive petrophysical properties in combination with modelling software. The effect of micro-CT image resolution and size on the accuracy of the derived petrophysical properties is addressed in this study using a relatively homogenous sandstone and a heterogenous, highly porous bioclastic limestone. Standard laboratory procedures including NMR (nuclear magnetic resonance) analysis, micro-CT analysis at different image resolutions and sizes and pore-scale flow simulations were used to determine and compare petrophysical properties. NMR-derived pore-size distribution (PSD) was comparable to the micro-CT-derived PSD at a resolution of 7 µm for both the rock types. Porosity was higher using the water saturation method as compared to the NMR method in both rocks. The resolution did not show a significant effect on the porosity of the homogeneous sandstone, but porosity in the heterogeneous limestone varies depending on the location of the sub-sample. The transport regime in the sandstone was derived by simulations and changed with the resolution of the micro-CT image. The transport regime in the sandstone was advection-dominated at higher image resolution and diffusion-dominated when using a lower image resolution. In contrast, advection was the dominant transport regime for the limestone based on simulations using higher and lower image resolutions. Simulation-derived permeability for a 400 Voxel3 image at 7 µm resolution in the Berea sandstone matched laboratory results, although local heterogeneity within the rock plays an integral role in the permeability estimation within the sub-sampled images. The simulation-derived permeability was highly variable in the Mount Gambier limestone depending on the image size and resolution with the closest value to a laboratory result simulated with an image resolution of 2.5 µm and a size of 300 Voxel3. Overall, the study demonstrates the need to decide on micro-CT parameters depending on the type of petrophysical property of interest and the degree of heterogeneity within the rock types.

Ore-controlling structures of sandstone-hosted uranium deposit in the southwestern Ordos Basin: Revealed from seismic and gravity data

We present the seismic and 3-D gravity study to define the ore-controlling structures of sandstone-hosted uranium deposits in the southwestern Ordos Basin, the second largest sedimentary basin in China. The instantaneous amplitude and frequency are extracted from seismic reflection data, and the seismic impedance is obtained by joint inversion of well-log and post-stack seismic data. Results of the seismic attributes and inversion show that the uranium-bearing deposits exhibit low instantaneous frequencies, low instantaneous amplitudes and medium–low seismic impedances. The scratch analysis method is employed on the residual gravity anomaly generated from the gravity bouguer anomaly by using the potential field separation. The edge coefficient image of the residual gravity anomaly shows that the fault structures are composed of NE-trending faults in the west and EW-trending faults in the east of the study area. The 3-D gravity inversion constrained by the seismic data was performed to reveal that uranium-bearing deposits are characterized by low gravity anomaly, which is related to the larger porosity of the uranium-bearing sandstones. Geophysical results show that the uranium-bearing deposits are controlled by flower structures, EW-trending normal faults and NW-trending thrust faults. Flower structures are produced by the tectonic inversion as a result of the uplift and NE-directed growth of the Tibetan Plateau. Ore-controlling faults cut crossing the floor of the ore-bearing strata as channels, along which the ore-forming fluids move upward and are enriched in the sandstone strata with high permeability and porosity in Luohe Formation. In addition, the roof and floor of the ore-bearing strata in Luohe Formation are characterized by high impedance and gravity anomaly, which has a sealing effect on uranium deposits. Then we predicted the thickness of the sandstones in Luohe Formation, as the sandstone is a prerequisite for mineralization.

Experiment and Mechanism Investigation on Freezing-Thawing of Sandstone with Different Water Contents

Freezing-thawing cycles seriously affect the safety of underground engineering in cold regions. At present, most research studies focus on the effect of number and freezing temperature on freezing-thawing cycles. As another important factor, the mechanism of rock mass water content affecting freezing-thawing is less studied. This paper studied the influence of the water content on mechanical property, microstructure, and acoustic emission characteristics of sandstone. The results indicated that the uniaxial compressive strength (UCS) and elastic modulus (E) of sandstone after 20 freezing-thawing cycles decreased as the water content increased. However, the decreasing rate of UCS gradually decreased, while the decreasing rate of E gradually increased. Furthermore, the empirical formulas of UCS and E about water content were obtained. The porosity and plasticity of sandstone after 20 freezing-thawing cycles increased as the water content increased. The empirical formulas of UCS and E about water content were obtained. The porosity and plasticity of sandstone after 20 freezing-thawing cycles increased as the water content increased. The decreasing trend of UCS with porosity was the same as that of UCS with water content. The failure form of sandstone gradually changed from splitting failure to shear failure. The results of the acoustic emission test showed that the stress-strain curves combined with acoustic emission ring counting could reveal the damage evolution process of sandstone during loading.

Ionic Surface Dielectric Properties Distribution on Reservoir Sandstone

The petrophysical and dielectric properties for both carbonate and saturated sandstone with monovalent and divalent electrolyte they are accurately modeling of an isotropic dielectric properties has been the major research area in oil and gas industries for effective sweep efficiency. The reservoir petrophysical properties consist of cation and anion exchange capacity on a specific area and the sandstone porosity. The transportation of the ions is a charge carrier that mediates conduction in the pore fluids under the electrical double layers that exist between the minerals and the pore fluids interface. The dielectric anisotropic and the frequency-dependent behavior of reservoir sandstone with the minerals will be fully elucidated, it was revealed from the result obtained the effect of the anisotropic dielectric properties on the reservoir sandstone based on the influx of NaCl electrolyte modify the wettability of rock formation from oil-wet to water-wet at 9000 and 11000 ppm concentration with the aids of the electromagnetic field. The resistivity index of the reservoir sandstone reduces with the increase of electrolyte to the system.

Experimental investigation and prediction model for UCS loss of unsaturated sandstones under freeze-thaw action

Sandstone is widely distributed in cold regions and the freeze-thaw deterioration of them has caused many geological engineering disasters. As an important and direct index of frost resistance, the strength loss of sandstones under freeze-thaw actions should be investigated to provide a guidance for the stability assessment of geological engineering. In this research, the UCS (Uniaxial compressive strength) loss of six typical sandstones with different water contents after 0, 20, 40 and 60 freeze-thaw cycles was measured in the laboratory. The experimental results indicated that the freeze-thaw damage was more serious in sandstones containing high water contents, and the critical saturations for causing a significant loss of UCS under freeze-thaw were 60%-80% for these sandstones. Below this critical saturation, the UCS loss of the sandstones was mainly caused by water weakening rather than freeze-thaw damage. Besides, a developed strength prediction model was proposed by combining the exponential decay function and multiple linear regression method. The initial porosity, elastic modulus and tensile strength of fresh sandstones were a good parameter combination to accurately determine the decay constant in this developed model. The main novelty of this model is that it can accurately and easily estimate the UCS loss of sandstones after any freeze-thaw cycle only using the initial parameters of fresh sandstones, but it does not need to perform freeze-thaw and mechanical strength experiments. This study not only provides an accurate prediction model of UCS under freeze-thaw, but also makes a contribution to better understanding the frost resistance mechanism of sandstones.

Visualizations of Berea sandstone pores using neutron tomography

Sandstone is a reservoir rock commonly found by oil and gas companies. Oil and gas usually trap inside the sandstone pores. A Destructive-Testing (DT) method or chemical solution method is usually used to measure the porosities of the sandstone sample. On the other hand, neutron computed tomography (NCT) can visualise and quantify all the porosities of the sandstone non-destructively. Neutron tomography is an imaging technique that employs neutron generated by a nuclear research reactor. The NCT produces cross-sectional images of the object that was used to visualise the Barea sandstone porosities distribution. The results show that each rock sample possesses connected and concentrated pores in the middle part of the rock with total porosity of ± 20%.

Catalog of Coefficients for Estimating Bulk and Shear Moduli as a Function of Lithology

The purpose of this paper is to present correlation coefficients for a variety of rock types that can be used in a suitable petroelastic model (PEM). The correlation coefficients for different rock types facilitate the application of a petroelastic model in reservoir flow models. By combining the correlation coefficients and the PEM, it is possible to obtain low-cost estimates of reservoir geophysical attributes. The rock types include dolomite, limestone, high porosity sandstone, poorly consolidated sandstone, tight gas sandstone, and well consolidated Gulf Coast sandstone.

Estimating Reservoir Properties from 3D Seismic Attributes Using Simultaneous Prestack Inversion: A Case Study of Lufeng Oil Field, South China Sea

Summary Lufeng oil field was discovered in the 1980s in the Pearl River Mouth Basin (PRMB) of the South China Sea and has entered the stage of secondary oil recovery. One major problem that has restricted the subsequent exploration of the oil field is the unclear regional reservoir and caprock distribution, because practitioners have been using post-stack attributes and acoustic impedance inversion to analyze the distribution of sandstone (reservoir) and mudstone (caprock). In current geophysical research on reservoirs and caprocks, prestack inversion has been widely used because of its advantage over post-stack inversion. However, the accuracy of using single P/S-wave velocity ratio (VP/VS) or density (Vden) inverted by prestack to predict lithology is still insufficient. In this study, we created a new attribute VRDEN, the sum of VP/VS and the weighted Vden, to capture the lithology variation of the reservoir. We integrated 3D seismic data and well log data and applied simultaneous prestack inversion and multiattribute regression analysis to determine reservoir properties, such as sand thickness, effective porosity, and distribution of sandstone and mudstone of the Lufeng oil field. Then, we calculated the new attribute VRDEN from VP/VS and Vden obtained from simultaneous prestack inversion to determine the lithology variation. The multiattribute regression analysis, combining prestack attributes and post-stack attributes, indicates the effective porosity and sand volume in the Enping Formation, which contains the main oil-bearing reservoirs in the Lufeng oil field. Results show that when the sandstone thickness is greater than 12.5 m, the prediction error of VRDEN is the lowest compared with VP/VS and Vden. In En-2 member of the Lower Enping Formation, medium- to high-porosity (14 to 17%) sandstone (19.5 m thickness) is widely distributed in the west and middle of the study area, with an area of nearly 300 km2. The high-porosity sand zones stretch from the east of the Lower Enping Formation to the west of the Upper Enping Formation, which is the result of westward progradation of the braided delta. Our workflow used a novel seismic attribute VRDEN in the simultaneous prestack inversion and multiattribute regression process to provide a more predictive spatial distribution of reservoir-nonreservoir features. The improved reservoir understanding will allow more efficient exploitation of the Lufeng oil field, and the improved workflow will facilitate exploration of other oil fields in the world.

Multi-Objective Optimization of CO2 Sequestration in Heterogeneous Saline Aquifers under Geological Uncertainty

This paper presents a Pareto-based multi-objective optimization for operating CO2 sequestration with a multi-well system under geological uncertainty; the optimal well allocation, i.e., the optimal allocation of CO2 rates at injection wells, is obtained when there is minimum operation pressure as well as maximum sequestration efficiency. The distance-based generalized sensitivity analysis evaluates the influence of geological uncertainty on the amount of CO2 sequestration through four injection wells at 3D heterogeneous saline aquifers. The spatial properties significantly influencing the trapping volume, in descending order of influence, are mean sandstone porosity, mean sandstone permeability, shale volume ratio, and the Dykstra–Parsons coefficient of permeability. This confirms the importance of storable capacity and heterogeneity in quantitatively analyzing the trapping mechanisms. Multi-objective optimization involves the use of two aquifer models relevant to heterogeneity; one is highly heterogeneous and the other is less so. The optimal well allocations converge to non-dominated solutions and result in a large injection through one specific well, which generates the wide spread of a highly mobile CO2 plume. As the aquifer becomes heterogeneous with a large shale volume and a high Dykstra–Parsons coefficient, the trapping performances of the combined structural and residual sequestration plateau relatively early. The results discuss the effects of spatial heterogeneity on achieving CO2 geological storage, and they provide an operation strategy including multi-objective optimization.

Study on the Evolution Law of Sandstone Porosity under Soaking-Drying Cycles

In view of the problem whereby the rock masses in water-level fluctuation zones of reservoir banks are susceptible to deterioration due to soaking-drying cycles, the sandstone of a typical landslide in the Three Gorges Reservoir area is selected as the test object to determine the ion concentration of an immersion solution and rock sample via periodic water-rock microscopic images, quantitative back analysis of the porosity changes caused by the dissolution of rock minerals under soaking-drying cycles, and evaluation of the sandstone porosity evolution. The results show that, 1) in the first two soaking-drying cycles, the generation rate of each ion is relatively high, and after 6 soaking-drying cycles, the ion generation rate tends to remain stable. 2) The volume reduction in calcite is the largest, followed by that in albite and potash feldspar. 3) Small porosity changes result in a considerable decrease in the compressive strength. 4) Sandstone porosity increase leads to an increase in rock pores, which is manifested as a deterioration of the macroscopic rock mechanical properties. Relevant research results could provide a reliable data basis for the study of large-scale reservoir bank slope protection and geological disaster evaluation and prediction.