Properties Of Shale Research Articles

Mechanical Properties of Shale After CO2 and CO2-Based Fluids Imbibition: Experimental and Modeling Study

Mechanical Properties of Shale After CO2 and CO2-Based Fluids Imbibition: Experimental and Modeling Study

Impact of Brittle Deformation on Pore Structure Evolution in Shale: Samples Collected from Different Fault Positions

Tectonic deformation has an important influence on the pore structure and physical properties of shale, and different parts of the same structural style also have great differences. In this study, eight shale samples were collected from two different faults based on the distance from each fault. Comparative analysis of the micro/nanopore structural characteristics and physical properties of these samples was carried out by scanning electron microscopy, liquid nitrogen adsorption, and carbon dioxide adsorption. The results show that as brittle deformation is enhanced (from far away from the fault to inside the fault), the number of organic pores decreases overall, and the connectivity of microfractures and different pore types increases. The total pore volume and total pore specific surface area of shale both increase, the pore volume of mesopores decreases by 31%, and the macropores increase rapidly by 29%. The storage capacity of shale-related folds is higher than that of shale in faults, which is more conducive to the adsorption of shale gas. The permeability of shale in faults is higher than that of shale-related folds, which is more conducive to the seepage and migration of shale gas. The organic layer structure, which is a unique and very rare microstructure at the shale fault site (under shear), was observed inside the fault. It is believed that the organic matter in the shale at the fault site has transformed from the amorphous state at the initial stage of formation to the orientation of the aromatic lamellae to stretching and extension to an increase in the pore size and the enhancement of connectivity. In addition, inorganic nanoparticles were also observed at the fault site. Both structures are important for the small pores in the fault and the increase in reservoir space. When the fault forms a closed environment or tectonic activity ceases in the later period, the early fault location likely becomes a potential high-quality hydrocarbon generation reservoir. These results help improve the understanding of the physical properties of shale reservoirs and shale gas reservoir migration within fault structures and are of great significance for shale gas exploration, development, and prediction.

Mechanical Properties of Lamellar Shale Considering the Effect of Rock Structure and Hydration from Macroscopic and Microscopic Points of View

It is well known that the effect of shale hydration causes wellbore instability due to water phase invasion of drilling fluid to lamellar shale rich formations. This is because of that the mechanical properties (compressive strength, elastic modulus, etc.) of lamellar shale surrounding the borehole, which is rich in clay minerals, will decrease significantly after hydration. In this study, using the lamellar shale in the continental stratum of the southern Ordos Basin, the mechanical properties of lamellar shale were studied by compression tests considering the effect of lamellar structure and hydration from a macroscopic point of view. In addition, the mechanical mechanism was discussed combined with the CT scanning tests results from a microscopic point of view. The results demonstrate the following points. Lamellar shale has stronger anisotropy than bedding shale, the compressive strength (deviatoric stress) and elastic modulus of lamellar shale are both lower than those of bedding shale, and it is more prone to tension fracture. With the increase in the angle (β) between the lamina and the axial direction from 0° to 90°, the compressive strength of lamellar shale decreases when β < 30° and then increases, the elastic modulus of lamellar shale decreases greatly when β < 30° and then tends to flatten. With the increase in hydration time, the compressive strength and elastic modulus of lamellar shale both gradually decrease, and the rates of their decrements reduce. The mechanical properties of lamellar shale are more affected by hydration than those of bedding shale. The hydration of lamellar shale leads to the formation of new fractures and the expansion of existing fractures in the junction area between the laminae and rock matrix, resulting in easy tension fracture along the laminae of shale.

Research on Evolution Characteristics of Shale Crack Based on Simultaneous Monitoring of Multi-Parameters

Meso-crack evolution mechanism of shale is a key factor affecting the mechanical properties of shale. In order to explore evolution laws of cracks in shale during loading, a meso-crack monitoring system, loading test equipment and an automatic ultrasonic data acquisition system were set up. On this basis, a set of experimental apparatus simultaneous monitoring multi-parameters of shale micro-crack was designed, and destruction experiments of shale samples with different bedding angles were carried out to find out evolution characteristics of cracks. The results show the following: (1) The designed apparatus can monitor ultrasonic, mechanical and video information simultaneously of crack evolution in the entire process of shale destruction under load to provide information for analyzing acoustic and mechanical characteristic responses of crack propagation at key time nodes. (2) With an increase in load, shale will undergo four stages of destruction: crack initiation, propagation, penetration and overall failure. In the course of these stages, acoustic characteristics and mechanical characteristics are in good agreement, which proves the validity of predicting rock mechanical parameters with acoustic data. (3) During the loading process of shale, the main amplitude of acoustic wave is more sensitive than mechanical parameters to the change of rock cracks. Research results have important theoretical reference value for evaluating wall stability of shale gas horizontal well with ultrasonic data.

Modeling the Viscoelastic Behavior of Quartz and Clay Minerals in Shale by Nanoindentation Creep Tests

The viscoelastic behavior of minerals in shales is important in predicting the macroscale creep behavior of heterogeneous bulk shale. In this study, in situ indentation measurements of two major constitutive minerals (i.e., quartz and clay) in Longmaxi Formation shale from the Sichuan Basin, South China, were conducted using a nanoindentation technique and high-resolution optical microscope. Firstly, quartz and clay minerals were identified under an optical microscope based on their morphology, surface features, reflection characteristics, particle shapes, and indentation responses. Three viscoelastic models (i.e., three-element Voigt, Burger’s, and two-dashpot Kelvin models) were then used to fit the creep data for both minerals. Finally, the effects of peak load on the viscoelastic behavior of quartz and clay minerals were investigated. Our results show that the sizes of the residual imprints on clay minerals were larger than that of quartz for a specific peak load. Moreover, the initial creep rates and depths in clay minerals were higher than those in quartz. However, the creep rates of quartz and clay minerals displayed similar trends, which were independent of peak load. In addition, all three viscoelastic models produced good fits to the experimental data. However, due to the poor fit in the initial holding stage of the three-element Voigt model and instability of the two-dashpot Kelvin model, Burger’s model is best in obtaining the regression parameters. The regression results indicate that the viscoelastic parameters obtained by these models are associated with peak load, and that a relatively small peak load is more reliable for the determination of viscoelastic parameters. Furthermore, the regression values for the viscoelastic parameters of clay minerals were lower than those of quartz and the bulk shale, suggesting the former facilitates the viscoelastic deformation of shale. Our study provides a better understanding of the nanoscale viscoelastic properties of shale, which can be used to predict the time-dependent deformation of shale.

The influence of depositional and diagenetic processes on rock electrical properties: A case study of the Longmaxi shale in the Sichuan Basin

Primary depositional and diagenetic processes exert very important influences on shale gas reservoirs. Rock electrical properties are an important basis for making reservoir prediction using the electromagnetic method (EM). However, there is still a lack of understanding about the impact of the sedimentary diagenesis process on shale electrical properties, this study focuses on the impact of diagenesis on rock properties. In this study, rock electrical properties are studied based on electromagnetic experiments. We systematically studied the lithofacies of different depositional paleoenvironments and diagenetic processes, and the influence of diagenetic evolution on the rock electrical properties was discussed, by means of X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FE-SEM), low-temperature nitrogen adsorption (LTNA), and trace element (TE) geochemical analysis. Based on the study of mineral composition, grain assemblages and pore systems, we identified four lithofacies in the Longmaxi Formation: siliceous shale, siliceous-argillaceous mixed shale, silty shale and argillaceous shale. Redox proxies (U/Th, V/Cr, and Ni/Co) indicate that the siliceous shale was deposited in a relatively anoxic and reducing environment, indicating a deep-water shelf depositional paleoenvironment. The siliceous-argillaceous shale, silty shale and argillaceous shale were deposited under a relatively dysoxic-oxic environment, indicating a shallow-water shelf depositional paleoenvironment. The order of resistivity values of the lithofacies within the Longmaxi Formation is 36.78 Ω m (siliceous shale), 66.81 Ω m (siliceous-argillaceous mixed shale), 79.54 Ω m (silty shale), and 107.00 Ω m (argillaceous shale), and the resistivity decreases with an increase in porosity. The siliceous shale has the most abundant authigenic quartz, which filled the primary pores forming a rigid framework during the gas window, inhibited compaction, increased the distribution of organic matter (OM), and enhanced the development of OM pores. The high TOC content and high maturity of siliceous shale at the bottom of Longmaxi Formation make the OM pores more developed. Pyrite, conductive fluid and pore network under the main control of OM pores in shale form a conductive circuit when AC voltage is input, which increases the exchange capacity of cations and leads to the phenomenon of low resistivity. The interconnected OM pore network, both depositionally and diagenetically derived, affects the electrical properties of the Longmaxi shale. This study reveals that electrical properties of shale rocks and its variations can be impacted by the depositional environment and diagenetic processes. This work provides resistivity parameters for the electromagnetic exploration of shale gas under complex terrain conditions and provides a theoretical basis for later interpretation.

Monitoring geological storage of CO2 using a new rock physics model

To mitigate the global warming crisis, one of the effective ways is to capture CO2 at an emitting source and inject it underground in saline aquifers, depleted oil and gas reservoirs, or in coal beds. This process is known as carbon capture and storage (CCS). With CCS, CO2 is considered a waste product that has to be disposed of properly, like sewage and other pollutants. While and after CO2 injection, monitoring of the CO2 storage site is necessary to observe CO2 plume movement and detect potential leakage. For CO2 monitoring, various physical property changes are employed to delineate the plume area and migration pathways with their pros and cons. We introduce a new rock physics model to facilitate the time-lapse estimation of CO2 saturation and possible pressure changes within a CO2 storage reservoir based on physical properties obtained from the prestack seismic inversion. We demonstrate that the CO2 plume delineation, saturation, and pressure changes estimations are possible using a combination of Acoustic Impedance (AI) and P- to S-wave velocity ratio (Vp/Vs) inverted from time-lapse or four-dimensional (4D) seismic. We assumed a scenario over a period of 40 years comprising an initial 25 year injection period. Our results show that monitoring the CO2 plume in terms of extent and saturation can be carried out using our rock physics-derived method. The suggested method, without going into the elastic moduli level, handles the elastic property cubes, which are commonly obtained from the prestack seismic inversion. Pressure changes quantification is also possible within un-cemented sands; however, the stress/cementation coefficient in our proposed model needs further study to relate that with effective stress in various types of sandstones. The three-dimensional (3D) seismic usually covers the area from the reservoir's base to the surface making it possible to detect the CO2 plume's lateral and vertical migration. However, the comparatively low resolution of seismic, the inversion uncertainties, lateral mineral, and shale property variations are some limitations, which warrant consideration. This method can also be applied for the exploration and monitoring of hydrocarbon production.

Anisotropy characterization of the elasticity and energy flow of Longmaxi shale under uniaxial compression

The inherent anisotropy of shale makes it exhibit different mechanical properties when measured in different directions. Generally, the measurements of the anisotropic mechanical properties of shale are generally conducted on cylindrical samples, and the computation of elastic constants is usually an empirical approximation. To accurately estimate the elastic parameters of shale rocks, an iterative method for approaching the crack initiation stress threshold was proposed in this work. Several uniaxial compression experiments were performed on cuboid Longmaxi shale samples with different bedding plane orientations. With a newly presented method, the apparent elastic modulus and Poisson’s ratios and five elastic constants were simultaneously determined. By examining the volumetric strain reversal point, the crack damage stress was obtained for each sample. The results demonstrate that the elastic regime, which was the range in which the elastic parameters were determined, significantly varies with bedding plane orientation. A transverse anisotropy ratio of 1.28 between in-plane modulus E1 and out-of-plane modulus E3 was determined for Longmaxi shale. Additionally, with the obtained elastic parameters, the energy releasing and dissipating behaviors of the samples during uniaxial loading were quantitatively characterized. The energy dissipation was found to rapidly increase once the crack damage stress was reached. However, before rock failure occurred, the total dissipated energy in each sample was very limited, indicating that microcrack propagation and rock damage were quite limited before rock rupture and that the shale samples were essentially destroyed by rapid energy release.

The experimental methods and elastic properties of shale bedding planes materials state-of-the-art review

The identification of physical properties and fracture behavior of shale or hydrocarbon materials has grown to have substantial importance in the last four decades to industry and investigators alike due to its importance in unconventional oil and gas resources. This interest deviates from shale being a transversely isotropic material formed by bedding layers with different orientations and isotropic properties. In this paper, the experimental setup and the mechanical properties of shale materials have been reviewed. The investigator shows that the properties of shale are not unique, and it is highly dependent on, volume, loading type, and geolocation. Furthermore, the investigator utilized different experimental setup methods to identify the elastic properties of shale and the most common methods are Transducers Strain Detection, Strain gauges, Ultrasonic, and Digital image correlation that is shown in detail.

Petrophysical analysis and mudstone lithofacies classification of the HRZ shale, North Slope, Alaska

Mudstone is vertically and laterally heterogeneous in terms of variations in mineral composition, organic carbon content, and petrophysical properties. The purpose of this study is to classify mudstone lithofacies within the High Radioactive Zone (HRZ) Shale on the North Slope, Alaska, based upon mineralogy, total organic carbon (TOC), and petrophysical properties to interpret their distributions and depositional/diagenetic environments. We integrate core data from six wells and well logs from 18 wells to analyze the variations in rock properties in the HRZ Shale. Analysis of cores at multiple scales of resolution by techniques, including X-ray diffraction (XRD), trace element geochemistry, pyrolysis, and scanning electron microscopy (SEM) are integrated with conventional and advanced wireline logging suites (including pulsed neutron spectroscopy [PNS]) to map vertical and lateral variation in mudstone lithofacies across the study area. Results show that the HRZ Shale exhibits a high degree of vertical and lateral heterogeneities in mineralogy, TOC, and petrophysical properties. This shale formation is composed of 12 lithofacies: pyritic organic mixed shale, organic mixed shale, pyritic mixed shale, mixed shale, pyritic organic mudstone, organic mudstone, pyritic grey mudstone, grey mudstone, pyritic organic siliceous shale, organic siliceous shale, pyritic siliceous shale, and siliceous shale. Quartz abundance is generally higher in the west, and clay proportions increase from the southwest to the northeast of the study area. The TOC content is variable across the HRZ Shale, ranging from 1.5 to 15 wt%. The lithofacies are influenced by depositional conditions, diagenesis, redox conditions, organic matter productivity, and preservation.

Numerical Modelling of Shallow Foundations on Expansive Shale: A Case Study of Jamshoro, Pakistan

This study analyzes expansive soil swelling under the shallow foundation using the Plaxis 2D FEM software. Under the impact of changing water conditions, expansive soil undergoes obvious volumetric changes, which often cause differential movements of the shallow foundations resting on it, which may lead to structural damage if no special precautions had been taken during the design process. In previous studies, the swelling of soil was simulated in Plaxis by applying the swelling potential with a positive volumetric strain, without considering water level changes, which proved to be not practical. This study demonstrates the time-dependent deformation (swelling) in expansive soil using the Plaxis 2D user-defined Swelling Rock Model in different soil and parametric conditions. The geotechnical properties of expansive shale were determined in the laboratory and by field testing, using parameters of the collected soil as input for the numerical model. Swelling of soil increased due to the increase of water level and decreased with the load. The heave was minimum in the center of the foundation (20 mm), attaining the maximum value at the corner (16 cm). The increase in the modulus of elasticity does not have much effect on soil swelling compared to soil settlement. The effect of matric suction-related changes on the swelling of soil due to the rise of the water table is not significant compared to the inherent soil swelling behaviour.

Genetic Mechanism of Multi-Stage Carbonate Minerals and its Control on Reservoir Physical Properties in Dolomitic Shales

Genetic Mechanism of Multi-Stage Carbonate Minerals and its Control on Reservoir Physical Properties in Dolomitic Shales

Laminar characteristics of lacustrine organic-rich shales and their significance for shale reservoir formation: A case study of the Paleogene shales in the Dongying Sag, Bohai Bay Basin, China

• Six types of the organic-rich shale laminae are reported in the Dongying Sag. • Discontinuous laminae were formed under high-energy hydrodynamic conditions. • Shale laminae containing favorable reservoir storage space are in the order of wavy laminar > lenticular laminar > weak laminar. • Thick and wavy laminae are the most favorable types for shale oil exploration. Shale oil exploration in China has demonstrated that laminated shales are favorable lithofacies for producing shale oil effectively. It is of great significance to study the type and reservoir properties of shales with different laminar structures. In this study, the laminae of Paleogene organic-rich shales in the Dongying Sag, Bohai Bay Basin were divided into two categories and six types based on thin section petrography, scanning electron microscopy (SEM), and geochemical characterization. The laminar types include thin parallel laminae, thick parallel laminae, wavy laminae, lenticular laminae, weak laminae, and sandy laminae. The first three types of laminae have good continuity and were formed under low-energy hydrodynamic conditions. In comparison, the latter three types are discontinuous and were formed under relatively high-energy hydrodynamic conditions with a strong terrigenous input. Significant differences were observed in the reservoir space types, microcracks, and oil-bearing properties among the different laminae. Analyses of porosity and oil saturation further revealed that laminated shales with high laminar continuity exhibit higher porosity, permeability, and oil saturation values than shales with poor laminar continuity. Shales with thick parallel laminar shale and wavy laminar shale possess favorable petrophysical parameters. Thus, from the perspectives of reservoir and oil-bearing properties, laminated shales with high laminar continuity are better than shales with poor laminar continuity for shale oil exploration in the Dongying Sag. The new findings provide a reference for the optimization and evaluation of favorable continental shale oil reservoirs in eastern China.

ON CO2 SEQUESTRATION: CHANGES IN CO2 ENTRY PRESSURE AND ADSORPTION CAPACITY BY HEAT

Geological sequestration of CO2 requires a deep permeable geological formation where captured CO2 can be stored and an overlying impermeable caprock that prevents buoyant CO2 from leaking upward to the surface. Shale formations are good seals due to their abundancy and typically ultralow permeability. Shale's excellent sealing ability makes it a good caprock formation. The correct evaluation of the CO2 sequestration ability of shale caprocks, also referred to as sealing capacity, must be conducted under high-pressure and high-temperature conditions representative of in situ conditions. The CO2 sequestration quality of shale caprocks, sealing capacity, is usually evaluated through measurements of CO2 capillary entry pressure into shale caprocks. In this work, CO2 capillary entry pressure in different shales under a wide range of temperatures (298-428 K) is measured using an incremental pressure transmission methodology. The impact of temperature on the physicochemical and petrophysical properties of shale is addressed, and their indirect effect on CO2 capillary entry pressure is analyzed. Furthermore, the influence of temperature on interfacial tension between CO2 and shale's pore fluid and contact angle is examined. Also, the effect of temperature on CO2 sorption on clay sites is analyzed. The experimental outcome exhibited that capillary entry pressure increases steadily and appreciably for shales (A and B) up to a certain temperature (around 358.15 K), after which the behavior of capillary entry pressure of CO2 for both shales changes markedly. For shale (A), the increase in measured capillary entry pressure for temperatures above 358.15 K slows down significantly, ranging between 841 and 851 psi compared to its measured value of 835 psi at 358.15 K. As for shale (B), the reaction is completely different, where measured capillary entry pressures of CO2 for temperatures above 358.15 K decreased noticeably as measured capillary entry pressures dropped from 534 psi at 358.15 K to 473 psi at 428.15 K. Results show that temperature has a profound impact on CO2 capillary entry pressure when interacting with shales through the alteration of shale's physicochemical and petrophysical properties. Data suggests that pore throat size could be altered by heat through a phenomenon called pore dilation. It was also clearly shown that higher temperature causes higher interfacial tension, which indeed causes higher capillary entry pressures when CO2 interacts with shale. Moreover, results indicate that high temperature could energize active swelling clays and invoke a transition of inactive clays to active swelling clays. High temperature causes clay swelling and may produce hydrational and expansive stresses in shale, especially when confined, which could induce fissures and microfractures. The creation of fissures and microfractures could enhance shale's average pore radii and permeability, thus reducing the capillary entry pressure. In addition, results suggest that the adsorption capacity of CO2 on clay sites decreases with increasing temperature and that the presence of smectite clay greatly enhances the swelling potential of shale upon exposure to CO2.

Investigation on the evaluation method and influencing factors of cross-scale mechanical properties of shale based on indentation experiment

Low-cost, fast and accurate acquisition of multi-scale mechanical properties of shale is essential to realize the complex hydraulic fracture network and the optimization of multi-scale fracture efficiency of bottom hole rock. The macro-scale indentation test is used to study the mechanical parameters of the meso-scale across the scales. The rapid evaluation method of the shale mechanical parameters across the scales is established, which can be used as a method to obtain the mechanical parameters of shale quickly and accurately. The macro-indentation test experiment is carried out through the elastic contact hypothesis, the macro-micro mechanics theory, and the grid indentation experiment method. Considering the coupling effects of peak indentation fluctuations, contact stiffness(S), and the ratio of elastic work to total work (Wu/Wt), the cross-scale distribution characteristics of shale hardness(H) and elastic modulus(E) are studied. The results showed that the elastic modulus and hardness showed a normal distribution on the whole, and the macroscopic indentation process of the rock was accompanied by the meso-scale indentation. For Longmaxi shale, the elastic modulus and hardness have large discreteness and heterogeneity. The kernel density values of elastic modulus and hardness were studied by using kernel density analysis method. Different factors have different effects on the fluctuation amplitude of elastic modulus and hardness. The fluctuation range of the S is the largest, and the fluctuation range of Wu/Wt is the smallest. At the meso-scale, the dispersion of hardness peak value is larger, and the dispersion of elastic modulus peak value is smaller than that of valley value. The S-Wu/Wt-E-H coupling response law presents a “striped” characteristic. The S-Fm-E-H coupling response law presents a “multi-point” distribution. The Fm-Wu/Wt-E-H coupling response law shows a “wave-like” distribution. The hardness of vertical bedding indentation is greater than that of parallel bedding. There is a good linear correlation between the elastic modulus and hardness. With the increase of the elastic modulus, the hardness increases. The research results are helpful to determine the parameters in the actual design of hydraulic fracturing and improving the rock breaking efficiency.

The impact of supercritical CO2 on the pore structure and storage capacity of shales

Injection of supercritical CO2 (SCCO2) causes significant changes in the petrophysical properties of shales and affects the integrity of geological storage sites. The alteration of mineralogy and pore structure plays a major role in defining the fluid transport in pours media of shale gas during CO2 storage. In this study, a series of SCCO2 treatment experiments were performed on different types of shales to evaluate the alterations of the pore structure and mineralogy. Two types of shales from Eagle Ford and Mancos fields were treated with SCCO2 and analyzed with scanning electron microscope, X-ray diffraction, and low-pressure nitrogen adsorption before and after 30 days at 70 °C and 18 MPa. The experimental results indicated that SCCO2 affected the mineral composition and changed the macropore structure of shales, due to the adsorption-induced expansion effect. The pore structure of clay-rich shales samples was more affected by CO2 treatment compared to quartz-rich shales, due to the dissolution of clay contents in the former, which reduced the overall pore volume by 24% in Eagle Ford shales. Conversely, the development of micro-cracks in Mancos shale surface created new pores and increased the pore volume by more than 13%. The results from nitrogen adsorption isotherms indicated a prominent alteration in the mesopores structure. The specific surface area and total pore volume of Eagle Ford shale reduced by 35.46% and 11.86% respectively after the SCCO2 treatment, while the specific surface area and total pore volume of Mancos shale increased by 27.4% and 25.92% respectively. A positive correlation was reported between the fractal dimension and specific surface area. It appeared that the surface roughness was reduced in Eagle Ford shale and relatively increased in Mancos shales after the treatment. The obtained results suggested that the adsorption capacity of clay-rich shales could be reduced after the CO2 treatment, while quartz-rich shales displayed a uniformed pore size distribution profile, indicating a possible increase in the adsorption capacity. These findings can provide technical support to further understand the effect of CO2 on the pore structure and mineralogical alteration of shale during geological storage.

Organic Petrographic and Geochemical Evaluation of the Black Shale of the Duwi Formation, El Sebaiya, Nile Valley, Egypt

This study evaluates the palynologic, organic, inorganic, and petrographic properties of organic-rich black shale (Mahamid Mine) in the El Sebaiya area, Nile Valley, Egypt. Black shale is composed of quartz (50%), calcite (10%), kaolinite (25%) and montmorillonite (15%). Organic and inorganic analyses revealed that this shale was deposited under oxic to anoxic marine conditions during strong chemical weathering. Black shale has poor to very good organic richness, and poor to fair hydrocarbon potential. Organic petrography indicates that the kerogen is mixed types II/III and III and is immature to marginally mature (%VRo is 0.44 and 0.53). Liptinite macerals consist of alginite, cutinite, and bituminite. The hydrocarbon products to be generated at higher maturity are expected to be oil and gas.

Experimental investigation on the effects of heating-cooling cycles on the physical and mechanical properties of shale

The deep shale gas reservoirs are characterized by great burial depth, high formation pressure, high temperature. During the hydraulic fracturing process, the fracturing fluids will inevitably exchange heat with the surrounding rock, causing the shale in the near-well zone to undergo temperature alternation, the physical and mechanical properties of shale reservoirs are significantly affected, which brings technical difficulties to fracturing optimization design. In this paper, the Longmaxi Formation shale is used to pay attention to the rock physical and mechanical characteristics of the shale undergo temperature alternation, the microstructure and damage and fracture characteristics of the shale undergo temperature alternation are analyzed. The results show that: (1)As the heat treatment temperature increases and the number of heating-cooling cycles increases, the permeability of the shale increases, the P-wave velocity decreases, the tensile strength (TS) decreases, and the brittleness index rises. (2) Heat treatment temperature and the number of heating-cooling cycles have little effect on the triaxial compressive strength (TCS) of shale. However, it has a significant effect on the failure mode of shale. The increase of temperature and the number of heating-cooling cycles will enhance the degree of fracture of the shale failure surface. (3) Based on industrial CT scanning and scanning electron microscopy (SEM) observations, the thermal cracks caused by heating-cooling cycles are mostly inter-granular cracks and inter-layer cracks, which unfold on the surface of the sample and extend to the interior. Heating-cooling cycles also lead to an increase in the diameter of organic pores. The study results can provide a certain theoretical basis for the optimization design of staged fracturing parameters for horizontal wells in the process of deep shale gas development.

Reservoir Characteristics of the Lower Permian Marine-Continental Transitional Shales: Example from the Shanxi Formation and Taiyuan Formation in the Ordos Basin

The pore types and pore structure parameters of the heterogenetic shale will affect the percolation and reservoir properties of shale; therefore, the research on these parameters is very important for shale reservoir evaluation. We used X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), low-pressure CO2 adsorption analysis, mercury injection capillary pressure (MICP), and high-pressure methane adsorption analysis to analyze the characteristics of different pore types and their parameters of the Lower Permian Shanxi Formation and Taiyuan Formation in the Ordos Basin. The influence of different mineral contents on the porosity and pore size is also investigated. The Shanxi Formation (SF) is composed of quartz (average of 38.4%), plagioclase, siderite, Fe-dolomite, calcite, pyrite, and clay minerals (average of 50.1%), while the Taiyuan Formation (TF) is composed of calcite (average of 37%), siderite, Fe-dolomite, quartz, pyrite, and clay minerals (average of 32.3%). The most common types of pores observed in this formation are interparticle pores (InterP pores), intraparticle pores (IntraP pores), interclay pores, intercrystalline pores (InterC pores), organic matter pores (OM pores), and microfractures. CO2 adsorption analysis demonstrates the type I physisorption isotherms, showing microporous solids having comparatively small external surfaces. The similar types of isothermal shapes of the Shanxi Formation (SF) and Taiyuan Formation (TF) suggest that both types have similar pore size distribution (PSD) within the measured pore range by the low-pressure CO2 adsorption experiment. The micropore pore size of the TF is larger than that of the SF. MICP shows the larger pores (>50 nm), and most of the volume was adsorbed by macropores. Methane gas sorption capacity increases with increasing pressure. Clay minerals and quartz played an important role in providing adsorption sites for methane gas. The overall analysis of both formations shows that TF has good reservoir properties than SF.

A multi-continuum model for simulating in-situ conversion process in low-medium maturity shale oil reservoir

In-situ conversion is proposed applicable for low-medium maturity shale oil reservoir. However, parallel chemical kinetic reactions and evolution of shale pores during in-situ conversion make the numerical simulation a challenging problem. Although shale is typical multiscale and heterogeneous media, few models in previous studies take the difference between organic and inorganic system into consideration, which cannot simulate fluid flow accurately. In this paper, a multi-continuum model, considering coupled thermal-reactive compositional flow, is developed to simulate in-situ conversion process in low-medium maturity shale oil reservoir. The reaction of kerogen and hydrocarbon is quantified using kinetic reaction model. The evolution of fluid composition and shale properties are also incorporated. The accuracy of multiple-interacting-continua model and compositional model are demonstrated by comparing with commercial software and analytical solution. Then, the typical hexagon vertical well heating pattern is simulated and the feasibility is evaluated from an economic aspect. Finally, a series of case studies are conducted to investigate the impact of operation parameters on shale oil production. Cited as: Wang, Z., Yao, J., Sun, H, Yan, X., Yang, Y. A multi-continuum model for simulating in-situ conversion process in low-medium maturity shale oil reservoir. Advances in Geo-Energy Research, 2021, 5(4): 456-464, doi: 10.46690/ager.2021.04.10