Core Fracture Research Articles

NMR-Based Analysis of Fluid Occurrence Space and Imbibition Oil Recovery in Gulong Shale

The Gulong shale oil reservoir is situated in freshwater to slightly saline lacustrine basins mainly consisting of a pure shale geological structure, which is quite different from other shale reservoirs around the world. Currently, the development of Gulong shale oil mainly relies on hydraulic fracturing, while the subsequent shut-in period for imbibition has been proven to be an effective method for enhancing shale oil recovery. To clarify the characteristics of the fluid occurrence space and the variation in the fluid occurrence during saltwater imbibition in Gulong shale, this paper carried out porosity and permeability tests on Gulong shale cores and analyzed the fluid occurrence space characteristics and imbibition oil recovery based on nuclear magnetic resonance (NMR). In the porosity and permeability tests, T2 distributions were used to correct the porosity measured by the saturation method to obtain the NMR porosity. Combined with the identification of fractures in shale cores using micro-CT and the analysis of porosity and permeability parameters, it was found that the permeability of the shale cores was related to the development of fractures in the shale cores. Through the testing and analysis of T1-T2 maps of the shale cores before and after saturation with oil, it was found that the shale mainly contained heavy oil, light oil, and clay-bound water, and they were distributed in different regions in the T1-T2 maps. Finally, the T1-T2 maps of the shale cores at different imbibition stages were analyzed, and it was found that saltwater mainly entered the minuscule inorganic pores of clay minerals during the imbibition process and squeezed the larger-sized inorganic pores containing light oil through the hydration expansion effect, thus expelling the light oil from the shale core and achieving the purpose of enhanced oil recovery.

Research and Application of Fast Plugging Method for Fault Zone Formation in Tarim Basin, China

The Silurian strata in the Shunbei No. 5 fault zone have the characteristics of long open holes, easy leakage and complex leakage. In the early stages, plugging technologies and methods such as bridging plugging, cement, chemical consolidation and high-water-loss plugging have poor effects and low plugging efficiency. Plugging slurry directly prepared with drilling fluid has low filtration characteristics, and the main reason is that the plugging material cannot filter quickly after the fluid enters the fracture. Based on the basic principle of fast filtration, the main plugging fluid M-Fluid, the micro-elastic high-strength main plugging agent M-Block and the filling agent Filling-Seal have been developed. In combination with the water-loss and wall-building properties of the circulating drilling fluid after plugging, a fast plugging technology for fractured volcanic rock formation has been established. The laboratory evaluation experiment showed that the filtration rate increased rapidly with the increase of temperature, and the filtration rate was about 0.31~0.79 mL/s, while the filtration rate of the drilling fluid was 0.0067 mL/s under the same conditions. The pressure-bearing capacity of various plugging evaluation methods, such as the simulated fracture of a large-grain sand bed, artificial fracture of small core and full-size core and multi-form fracture of double core, all exceed 5 MPa, and the system has a good plugging effect for complex fractures.

Formation of multi-stage and clustered fractures at 3.6–4.9 km in the Shizigou structure, SW Qaidam basin

In the Eocene Upper Xiaganchaigou Formation from depths of about 3.6–4.9 km in the Shizigou (SZG) Structure in the SW Qaidam basin, core and image logs reveal three fracture assemblages (A-C) comprising opening-mode fractures and faults. Assemblages are defined by structural style (groups of fracture types), and orientation patterns and thus kinematics and separated by relative timing by the unlithified rock style of deformation of assemblage A and fracture crosscutting relations between assemblages B and C. Correlation of fracture kinematics with the structural history of the basin suggests a multi-stage fracture population in the Shizigou Structure arising as a response to Eocene transtension, Miocene transpression, and Quaternary shortening under the influence of the Cenozoic evolution of two boundary faults, the Altyn Tagh Fault (ATF) and the Qimen Tagh-Eastern Kunlun Fault (QTEKF). To clarify the relation, we document formation process of multi-stage fractures in cores and their preferred orientations in log images in five wells. Using image logs from horizontal wells (Shi A and B) at ca. 3.7–3.9 km depth and statistical methods for quantifying spatial arrangement patterns, we identified statistically significant conductive fracture clusters; opening-mode fractures of the assemblage B are significantly hierarchically clustered. Combining fracture mode and their preferred orientations with reconstructed stress history, we propose that the deformation history of the fracture population is kinematically related to the Cenozoic activity of the ATF and the QTEKF. The initial stage of fracture formation is associated with Eocene strike-slip faulting along the ATF, followed by the Early Miocene initial transpressional faulting along the QTEKF and finally intensified shortening since the Quaternary, respectively.

Interface structure and bonding strength of metallurgical bonded aluminum foam sandwich (AFS) fabricated by hot-pressing

This paper proposes the aluminum foam sandwich (AFS) manufacturing process by simplifying the preparation processes and reducing costs by use of hot-pressing, where aluminum plates were used as facesheets, aluminum foam as core, and pure zinc plates as joining layers. The effect of hot-pressing distance on interface formation was first illustrated by the thermodynamic equations and bonding strength was investigated. The results showed that increasing the hot-pressing distance was beneficial to the interfacial element diffusion, but harmful to foam structure. Two failure modes of interface separation and foam core fracture were first traced by digital image correlation (DIC). Cracks and holes appeared in the zinc interlayer was the main reason for the low bonding strength. The AFS obtained by 2 mm hot-pressing distance at 450 °C possessed the highest bonding strength, greater than 2.14 MPa, and the porosity of foam core was held at 74%.

Fracture Identification Using Conventional Logs in Ultra-Low Permeability Sandstone Reservoirs: A Case Study of the Chang 6 Member of the Ordos Basin, China

The identification of reservoir fractures is essential as it is an important factor in the design of a field development plan, which in turn affects the efficiency of hydrocarbon production. Water flooding and water channeling are serious due to the lack of objective understanding of the fracture development pattern in the tight oil reservoirs in the Triassic Change 6 member of Y well area, Ordos Basin, China. In this paper, we observed 104.6 m cores from 20 wells with a number of 150 fractures and an outcrop profile, then analyzed the main controlling factors for core fracture development, and finally established a fracture prediction method using conventional logging data. The results indicate that high-angle fractures accounted for 73.20%, fracture orientations were nearly east–west, fracture spacing between 0–10 cm accounted for 80.51%, fracture openings between 0–0.13 mm accounted for 89.27%, fracture down-cutting depths between 0–20 cm accounted for 80%, and 80.81% was not filled. In addition, we found that thin beds and fine sandstones are prone to develop fractures; Finally, our modified curve rate method was an effective method for fracture prediction. We conclude that fractures have the characteristics of high angle, small spacing, small opening, small down-cutting depth, and less filling. The modified curve change rate method is suitable for fracture prediction in tight sandstone reservoirs in the Triassic Change 6 member of the Y well area, Ordos Basin, China.

Implant–Prosthetic Rehabilitation of Maxillary Lateral Incisor Agenesis with Narrow Diameter Implants and Metal–Ceramic vs. All-Ceramic Single Crowns: A 16-Year Prospective Clinical Study

The main objectives of the present prospective clinical study were to evaluate the marginal bone resorption and the peri-implant soft tissue conditions around narrow diameter implants, as well as the clinical performances of metal–ceramic and all-ceramic single crowns in the implant–prosthetic treatment of maxillary lateral incisors agenesis. Seventy-two patients affected by monolateral or bilateral agenesis of the maxillary lateral incisors were included in the study, and a total of 105 narrow diameter implants were inserted. The final single crowns were fabricated 4 months after the surgical procedures by means of either titanium abutments with Auro-Galvan Crowns (AGC) veneered with feldspathic ceramics or zirconia abutments with all-ceramic (alumina or zirconia) crowns layered with dedicated veneering ceramics. Both marginal bone resorption and soft tissue quality were evaluated after a maximum period of 16 years. Two independent survival curves for patients wearing AGC or all-ceramic crowns were calculated by means of Kaplan–Meier analysis, and a log-rank test was performed in order to compare these curves (p < 0.05). After 16 years of clinical function, a mean marginal bone resorption of 1.2 ± 0.61 mm and good response of peri-implant soft tissues were observed. Descriptive statistics resulted in 80% and 87.1% cumulative success rates for patients provided with AGC prostheses onto titanium abutments and all-ceramic crowns on-to zirconia abutments, respectively. The cumulative survival rate calculated for AGC group was 93.3%, whereas all-ceramic group showed a cumulative survival rate of 95.7%. No statistically significant differences were found with the log-rank tests. The overall success and survival rates calculated were 85% and 95% for all the restorations, respectively. Mucositis, abutment unscrewing, loss of retention, chipping, core fracture, and esthetic failure were the major complications reported. The implant–prosthetic approach can be considered an effective and viable solution to treat cases of maxillary lateral incisor agenesis in the long term.

Relationship between the heterogeneity of low-permeability reservoirs and the dynamic evolution of fractures under uniaxial compression conditions by CT scanning: A case study in the jiyang depression of Bohai Bay Basin, China

Based on an analysis of core fractures and computed tomography scanning by X-ray under uniaxial compression tests with loading and unloading, this study investigated the three-dimensional evolution of fracture initiation and propagation in different heterogeneity conditions of low-permeability reservoirs. Reservoir heterogeneity affected the 3D dynamic development of fractures. With increasing loading value, fractures with strong heterogeneity in low-permeability reservoirs followed the development sequence from shallow to deep depth in the profile. When the fracture tip reached the incompetent layer, propagation stopped. Subsequently, when the deformation continued to increase, the fracture crossed the incompetent layer and initiated a new fracture at the front end. Therefore, the across-bedding and through-going macro fractures formed in conjugate sets or were Y-shaped, with roughly equal spacing. In the plane, the fractures initiated along the pre-existing bedding parallel fracture and further propagated into large fractures. Consequently, they formed a net-shaped fracture from the edge to the center, with a long development period. These fractures intersected to form a complex, interconnected, and high-fracture-density system in three dimensions. Under the same loading value, owing to uniform lithology, the formation of micro-fractures in the weak heterogeneity of low-permeability reservoirs is difficult. Furthermore, long shear fractures were only produced in a relatively short formation period with a low fracture density. In the plane, the fractures stepped in an en-énchelon and terminated with horse-tailing microfractures. Moreover, the strong heterogeneity of low-permeability reservoirs was more favorable for producing fractures. The fracture volume was 13.17% of the total sample volume compared to 0.67% in the weak-heterogeneity reservoir under the same loading compression. Furthermore, a better fracturing effect was achieved by fracturing along the direction >50° with the natural fracture and under the condition of small differential horizontal stresses.

Manufacturing of ceramic cores: From hot injection to 3D printing

With the improvement of aero-engine performance, the preparation of hollow blades of single-crystal superalloys with complex inner cavity cooling structures is becoming increasingly urgent. The ceramic core is the key intermediate part of the preparation and has attracted wide attention. To meet this challenge, new technologies that can make up for the defects of long periods and high costs of fabricating complex structural cores by traditional hot injection technology are needed. Vat photopolymerization 3D printing ceramic technology has been applied to the core field to realize the rapid preparation of complex structural cores. However, the industrial application of this technology still needs further research and improvement. Herein, ceramic cores were prepared using traditional hot injection and vat photopolymerization 3D printing techniques using fused silica, nano-ZrO2, and Al2O3 powders as starting materials. The 3D printed ceramic core has a typical layered structure with a small pore size and low porosity. Because of the layered structure, the pore area is larger than that of the hot injection ceramic core, the leaching performance has little effect (0.0277 g/min for 3D printing cores, 0.298 g/min for hot injection cores). In the X and Y directions, the sintering shrinkage is low (2.7%), but in the Z direction, the shrinkage is large (4.7%). The fracture occurs when the inner layer crack expands and connects with the interlayer crack, forming a stepped fracture in the 3D-printed cores. The bending strength of the 3D printed core at high temperature (1500 °C) is 17.3 MPa. These analyses show that the performance of vat photopolymerization 3D-printed ceramic cores can meet the casting requirements of single crystal superalloy blades, which is a potential technology for the preparation of complex structure ceramic cores. The research mode of 3D printing core technology based on the traditional hot injection process provides an effective new idea for promoting the industrial application of 3D printing core technology.

Experimental behavior of truss‐confined buckling‐restrained braces

AbstractBuckling‐restrained braces (BRBs) can effectively improve the stiffness, strength, and energy dissipation capacity of building structures. The key feature of the proposed truss‐confined BRB (TC‐BRB) is an additional truss confining system attached outside the central steel casing. The truss system is composed of several steel open‐web truss frames, thereby providing the required restraining rigidity and reducing the central casing section size and infilled mortar. The overall self‐weight is decreased compared with a conventional BRB, especially in cases of long‐span and large force capacity BRB applications. This research program was conducted to investigate the hysteresis behavior of the TC‐BRBs in terms of truss member response, buckling resisting ability, and cumulative deformation capacity. Six one‐third‐scale TC‐BRB specimens, each 6.3 m long and with different yield capacities, truss frame shapes and sizes, were tested using cyclically increasing loads with or without an out‐of‐plane end drift. The six specimens eventually failed either from global flexural buckling of the entire restraining system or a tensile fracture of the steel core at a very large axial strain. The experimental results demonstrated that the TC‐BRBs can be designed and fabricated to have a very stable and highly repeatable hysteretic response, meeting the American Institute of Steel Construction acceptance criteria. It provides a valuable database to support the further development of design guidelines for the engineering application of the proposed TC‐BRBs.

Classification and Assessment of Core Fractures in a Post-Fracturing Conglomerate Reservoir Using the AHP–FCE Method

To characterize the hydraulic fracture network of a conglomerate reservoir, a slant core well was drilled aimed to obtain direct information regarding hydraulic fractures through slant core at the conglomerate hydraulic fracturing test site (CHFTS). Core fracture classification was the fundamental issue of the project. In this study, three grade classifications for core fractures were proposed. Comprehensive classification of core fractures was carried out using the analytic hierarchy process (AHP)–fuzzy comprehensive evaluation (FCE) method. Finally, the fracture classification results were validated against numerical simulation. The grade-1 fracture classification included hydraulic fractures, drilling-induced fractures and core cutting-induced fractures. A total of 214 hydraulic fractures were observed. For the grade-2 classification, the hydraulic fractures were divided into 47 tensile fractures and 167 shear fractures. For the grade-3 classification, the shear fractures were subdivided into 45 tensile-shear fractures and 122 compression-shear fractures. Based on the numerical verification of the core fracture classifications, the dataset acquired was applied to analyze the spatial distribution of tensile and shear fractures. Results showed that the tensile fractures were mainly in the near-wellbore area with lateral distances of less than 20–25 m from the wellbore. The shear fractures were mainly in the far-wellbore area with lateral distances of 20–30 m from the wellbore. These results provide a basis for understanding the fracture types, density, and failure mechanisms of post-fracturing conglomerate reservoir.

Laboratory Drilling Test of Core Discing Phenomenon under Low Stress

The phenomenon of core discing has always been considered as a unique sign of high geostress area. With the in-depth study of the discing mechanism, there are various indications that the high geostress is not the main controlling factor of the core discing. Based on the geostress classification standard and engineering cases data, the stress level of complete granite prone to discing is determined in this paper. On this basis, laboratory drilling tests with different drilling speeds under high and low stress conditions are designed. The results show that high stress is not the key factor for core discing. When the drilling speed is fast, even if the confining stress is 0, core discing will occur. When the drilling speed is slow, the core can be taken completely even at high stress levels. Drilling parameters fluctuate with discing, and its peaks correspond to the core fracture location.

Quantitative Investigation on Natural Gas Flooding Characteristics in Tight Oil Cores after Fracturing Based on Nuclear Magnetic Resonance Technique

SummaryTight reservoirs are mainly developed by injecting various gases after fracturing. However, the formed fractures are complex, and different fracture conditions have an important impact on the gas injection effect. In addition, natural gas is considered to be suitable for the development of tight reservoirs in China because of the abundant gas source and no corrosion. For this paper, the natural gas injection experiments were studied by combining mercury intrusion porosimetry (MIP) measurements and nuclear magnetic resonance (NMR) measurements. The method can be used to study the distribution characteristics of core pore structure and the recovery characteristics of oil in different pore spaces.In this work, the tight cores of the Changqing Oil Field were selected for fracturing for the natural gas flooding experiments. At first, the distribution characteristics of the core pore structure were studied based on the MIP and NMR measurements. The conversion relationship between the core pore throat radius and the relaxation time (T2) was decided. The NMR T2 distribution was transformed into the distribution of oil in pore space with different throat radii. Then, the gasflooding experiments were conducted to study the oil recovery law of tight cores with different fracture conditions. Finally, the recovery characteristics of oil in different pore spaces were analyzed based on the NMR results of cores.The results show that the pore throat radius of the core is mainly distributed in the range of 0.001 to 10 μm. The oil is mainly stored in the pore space whose pore throat radius ranges from 0.01 to 1 μm. The natural gas also mainly drives the saturated oil in the pore space with a pore throat radius of 0.01 to 1 μm. The increase in fracture area improves the distribution of oil in the larger pore space. In the process of natural gasflooding, with the increase of gas injection, the oil began to be recovered, and then gas was observed at the end of the core. With the continuous injection of natural gas, the rate of recovering oil gradually slowed down, and finally gas breakthrough occurred. The displacement oil process of the nonfractured core was uniform and slow. However, the oil and gas rapidly flowed along the fracture when the natural gas displaced the oil in the fractured core. The oil in the matrix was poorly recovered. Gasflooding mainly recovered the saturated oil in the matrix of nonfractured cores and the saturated oil in the fracture of fractured cores. As the fracture length increased, the oil recovery became lower and the gas breakthrough occurred earlier. The higher fracture density increased the fracture area, which also increased the oil recovery and caused a more intense gas breakthrough.In this paper, the displacement law of tight oil cores by injecting natural gas and the recovery characteristics of oil in different space pores were illuminated. The results can provide theoretical guidance for the formulation of the natural gas injection development plan in tight reservoirs.

Relationship between the fracture morphology and energy of brown rice during processing

During rice processing, grain fracture is a key problem for industrial production, various forces act on brown rice during working, and compression load is a critical factor for grain fracturing. A developed compression tester was built with optical observation to better understand the brown rice fracturing behavior. The relationship between fracture morphology and energy was researched. A fracture energy mathematics model was established. The relationship between fracture energy, force and deformation was analyzed. Test results indicate that brown rice fracture behavior can be divided into three stages: elastic interaction, plastic deformation, mixed fracture. The fracture energy of brown rice is related to the size and degree of core fracture area. Fracture behavior and energy are affected by brown rice moisture content, and fracture energy is positively correlated with cracks. The drying temperature at 75 °C is a turning point in the fracture energy of brown rice. The effect of compression time T is larger than compression velocity on test sample fracture energy. The study provides in-depth analysis for brown rice fracturing mechanism, for revealing the law of grain fracture energy.

Experimental on physical properties of shale gas under low-velocity seepage and stress sensitivity environment

Shale gas reservoirs have complex pore structures, consisting not only of nanopores within and between organic grains but also of microcracks distributed randomly within the matrix. Although free gas and adsorption gas coexist, free gas is stored in pore spaces, and adsorption gas is stored in organic matter and clay minerals. Due to the presence of organic matter and heterogeneity in shale, the gas transfer mechanism is complicated involving multiple scales. Gas flow in shale does not conform to the characteristics of linear seepage at the low-velocity stage given the occurrence and flow state of different gases under different pressures. Meanwhile, very few studies have been performed on the low-velocity seepage characteristics of different gases in shale. Therefore, it is important to present experimental evidence for the low-velocity seepage law of different gases in shale. The present study uses experimental simulations of gas mass transfer at multi-scales to investigate the low-velocity gas flow and stress sensitivity of shale core samples. The results indicate that the shale matrix has a strong adsorption capacity, and that its helium flow capacity is 1.5–2.0 times greater than that of methane without backpressure. With increasing pore pressure, the combined action of gas adsorbed to the cavity and free gas leads to first an increase and then a decrease in the flow capacity of methane. With a low confining pressure, the permeability decreases exponentially. On the other hand, as the net confining pressure increases, the core fracture closes, causing the gas slip effect to increase permeability. The results of the study may provide valuable guidance in understanding gas flow in shale matrix and in the development of gas reservoirs.

Fracture Characteristics and Distribution in Slant Core from Conglomerate Hydraulic Fracturing Test Site (CHFTS) in Junggar Basin, Northwest China

Hydraulic fracture networks, especially fracture geometry, height growth, and proppant transport within the networks, present a critical influence on productivity evaluation and optimization of fracturing parameters. However, information about hydraulic fracture networks in post-fractured formations is seldom available. In this study, the characteristics (density and orientation) of hydraulic fractures were obtained from field observations of cores taken from conglomerate hydraulic fracturing test site (CHFTS). A large number of fractures were observed in the cores, and systematic fracture description was carried out. The fracture analysis data obtained includes fracture density, fracture depth, fracture orientation, morphology, fracture surface features, apertures, fill, fracture mechanical origin (type), etc. Our results show that 228 hydraulic fractures were intersected in a span of 293.71 m of slant core and composed of irregularly spaced single fractures and fracture swarms. One of the potential sources of the observed fracture swarms is near-wellbore tortuosity. Moreover, for regions far away from the wellbore, reservoir heterogeneity can promote complex hydraulic fracture trajectories. The hydraulic fractures were mainly cross-gravel and high-angle fractures and align with maximum horizontal stress (SHmax) ± 15°. The fracture density, orientations, and types obtained from the core fracture description provided valuable information regarding fracture growth behavior. For the near-wellbore area with a transverse distance of less than 25 m from the hydraulically-fractured wellbore, tensile fractures were dominant. While for the area far away from the wellbore, shear fractures were dominant. Our results provide improved understanding of the spatial hydraulic fracture dimensions, proppant distribution, and mechanism of hydraulic fracture formation. The dataset acquired can also be used to calibrate numerical models and characterize hydraulic fracture geometry and proppant distribution.

Marine karst environment characterization using joint geophysical and geotechnical data

ABSTRACTSubmarine karstic environments are complex and challenging to study. Seismic investigations usually have difficulty getting geological information because of a lack of penetration due to the high reflectivity of the calcareous substratum. To circumvent this problem, we studied how to combine marine electrical resistivity tomography (MERT) with geotechnical data to investigate the porosity structure from the geotechnical to the geophysical scale. We applied the technique to the submarine karstic plateau of Banc de Guérande (Saint‐Nazaire, France), which is mainly composed of hard calcarenite and sandy pockets. We obtained sections of two‐dimensional resistivity models from the MERT data inversion. We used existing geotechnical data on extracted cores at several boreholes close to the MERT profiles using a multi‐sensor core logging (MSCL) bench. We used porosity proxies derived from Archie's law and porosity data from the MSCL inferred from gamma density measurements on the core to combine the data of very different scales (metre for MERT and centimetre for MSCL). The comparison between measurements showed a good similarity between in situ MERT and borehole MSCL data at depths greater than ∼10 m below the seafloor. A larger difference was observed close to the seabed, where the MERT porosity was higher than the MSCL porosity. The extraction of water‐saturated cores and the numerous core fractures could explain this difference near the surface. The results were analysed with respect to the scale difference between geophysical and geotechnical data. The conclusions suggested that the difference between MERT and MSCL porosities could be testified from the local heterogeneity of the soil and indicated whether the surrounding substratum was more porous (and thus fractured or dissolved) than the core or vice versa. The study highlighted the necessity of an excellent collocation of the data to retrieve reliable information from the comparison between geophysical and geotechnical data.

Experimental and numerical study on influencing factors of replacement capacity and slickwater flowback efficiency using pre-CO2 fracturing in tight oil reservoirs

CO2 as pre-fracturing fluid technology, with dual advantages of CO2 fracturing and water fracturing, has broad application prospects in the development of unconventional oil and gas, and it is also one of the important ways of carbon utilization and carbon storage. In this paper, an accurate experiment was designed first time to explore the replacement capacity of CO2 under different discharge pressure gradients using cores with different sizes. The fracturing fluid flowback law and enhance oil recovery ability were also studied by the self-designed experiment. Before the test, the core was split axially from the center and the core fracture was filled with mixed 40–70 mesh quartz sand and AB glue to simulate the actual formation fracture. Then a dual-permeability compositional model was established to explore the field scale fracturing fluid flowback. The performances of slickwater fracturing and pre-CO2 fracturing were compared. The effects of flowback pressure gradient, CO2 injection volume, and soaking time on the flowback efficiency and oil recovery were also analyzed. The results show that the efficiency of CO2 replacement of oil increases with the increase of flowback pressure difference and is 15% higher than that of slickwater. CO2 pre-fracturing can significantly improve the flowback efficiency of slickwater and enhance oil recovery. As the pressure gradient increases, the flowback efficiency of slickwater increases, which shows that the flowback pressure gradient needs to be higher than 10 MPa to achieve economic output. With the proportion of CO2 injection increased from 20% to 58%, the CO2 flowback efficiency increased from 37.5% to 44.1%, and the oil recovery factor increased from 11.2% to 14.7%. With the increase of soaking time, slickwater imbibed into matrix pore throat according to the capillary force, meanwhile, CO2 diffused into deep formation. The degree of slickwater and CO2 backflow decreased accordingly. Considering the pressure change, stimulation effect and flowback efficiency during the soaking period, the optimal soaking time is 6 h in experiment and 15 d in field production. This study is expected to provide theoretical guidance for CO2 pre-fracturing in tight oil reservoirs.

Calibration of orthotropic plasticity- and damage models for micro-sandwich materials

Sandwich structures are commonly used to increase bending-stiffness without significantly increasing weight. In particular, micro-sandwich materials have been developed with the automotive industry in mind, being thin and formable. In the present work, it is investigated if micro-sandwich materials may be modeled using commercially available material models, accounting for both elasto-plasticity and fracture. A methodology for calibration of both the constitutive- and the damage model of micro-sandwich materials is presented. To validate the models, an experimental T-peel test is performed on the micro-sandwich material and compared with the numerical models. The models are found to be in agreement with the experimental data, being able to recreate the force response as well as the fracture of the micro-sandwich core.

Fracture Characterization of Lower Cambrian Niutitang Shale in Cen’gong Block, Southern China

The marine shale of southern China is characterized by old sedimentary formations, multiple tectonic activities, and poor preservation conditions. The fracture system in this shale reservoir is extraordinarily complex, greatly adding to difficulties for shale gas exploration and development. Based on field surveys, drilling cores, seismic data interpretation, and experimental tests, we try to characterize the developmental characteristics of shale fractures at different scales and delineate the stages of fractures in the Cen’gong block. The results show that the Cen’gong block is a saddle-shaped structure formed by the northeast-oriented Banxi anticline and the southwest-oriented Lannigan anticline. There are four types of core fractures developed in the study area, namely, pyrite-filled fractures, fibrous veins, subvertical fractures, and slip fractures, and the forming time of these fractures are later in turn based on the intersecting relationships and analysis of filling minerals. The fracture rose diagram and the paleomagnetic orientation experiments indicate that the fracture direction is mainly NNE and partly NW, consistent with the direction of faults identified in the seismic data. Quantitative statistics have been conducted for fracture aperture, length, and density distribution. The fracture abundance has a close relationship with buried depth and regional faults in the study area.

Tensile fracture characteristics of carbon fibers collected from the outer and inner parts of a CFRP laminate by thermal decomposition

Thermal decomposition treatment (TDT) is a typical recycling method for CFRP. However, the recovered CFs (rCFs) tend to show a decrease in performance owing to the thermo-oxidation during this process. In particular, the thermo-oxidation rates of the outer and inner parts of CFRP are different during TDT; thus, the amount of damage is different between the rCFs collected from the outer (oCFs) and inner (iCFs) parts of CFRP. This study investigated the tensile fracture characteristics of the oCFs and iCFs of the CFRP laminates subjected to TDT, through the single-fiber tensile test and fracture surface observations. The tensile strength of the iCFs was higher and presented a narrower variation than that of the oCFs because the fracture of iCFs occurred from the largest pit formed on the skin whereas that of oCFs occurred from the largest defect in the core. Finally, the strength distribution of the rCFs collected randomly from the TDT treated CFRP was estimated using a mixture distribution model with the aid of the experimental data. The model fitted the strength distribution well. The results indicated that the strength of the rCFs varied more widely with an increase in the number of rCFs undergoing the core fracture.