Tight multi-medium oil reservoirs are the main source of hydrocarbon resources around the world. Acid fracturing is the most effective technology to improve productivity in such reservoirs. As carbonates are primarily composed of dolomite and calcite, which are easily dissolved by hydrochloric acid, high-permeability region will be formed near the well along with the main artificial fracture when acid fracturing is implemented in tight multi-medium oil reservoirs. In this study, a comprehensive composite linear flow model was developed to simulate the transient pressure behavior of an acid fracturing vertical well in a naturally fractured vuggy carbonate reservoir. By utilizing Pedrosa’s substitution, perturbation, Laplace transformation and Stehfest numerical inversion technology, the pressure behavior results were obtained in real time domain. Furthermore, the result of this model was validated by comparing with those of previous literature. Additionally, the influences of some prevailing parameters on the type curves were analyzed. Moreover, the proposed model was applied to an acid fracturing well to evaluate the effectiveness of acid fracturing measures, to demonstrate the practicability of the proposed model.
To investigate the paleoenvironmental controls on organic matter accumulation of Upper Paleozoic shales in the eastern Ordos Basin, China, 26 shale samples were collected from two wells drilled into the Shanxi and Taiyuan Formations. The total organic carbon (TOC) content, mineral compositions and elemental geochemistry of the samples were analyzed. Quartz (35.42%) and clay minerals (48.34%) are the dominant minerals and trace elements (Li, Cs, Cu, V, Co, and Cr) are commonly enriched in the shale samples compared to the Upper Continental Crust. C-values (ranging from 0.2 to 4.5), chemical indices of weathering (CIW) values (48.82 to 99.11), and Sr/Cu ratios (1.00 to 11.05) suggest that the paleoclimate was humid in the study area during the Late Paleozoic. Elemental redox indices (e.g., Al2O3/(Al2O3+Fe2O3), V/Cr, Ni/Co, V/(V+Ni) and U/Th) indicate a dysoxic to oxic paleoenvironment characterized by transitional sedimentary deposits in a continental margin setting. In addition, chemical index of alteration (CIA, ranging from 77.92% to 98.36%) and CIW (89.19% to 99.11%) values suggest that there was intense chemical weathering in the study area, while the Al2O3-CaO*+Na2O-K2O (A-CN-K) ternary diagram demonstrates that the shales were not subjected to potassium metasomatism during diagenesis. Al2O3/TiO2 and TiO2/Zr ratios, as well as REE characteristics suggest a felsic source rocks and discount seawater as an REE source. Ce anomalies indicate an oxic environment with terrigenous input during black shale deposition, and LREE enrichment with negative Eu anomalies suggests that both shale formations were affected by detrital input rather than hydrothermal fluids. The correlation of TOC (ranging from 1.10% to 6.39%, with an average of 2.77%) with trace elemental redox indices (Sr/Cu, Sr/Ba, V/Cr, and U/Th) indicates that a warm-humid, dysoxic to oxic environment preserved much of the organic matter.
The Lower Member of the Longmaxi Formation is generally dominated by siliceous shale, but recently we found some siltstone-mudstone rhythm sections developed in the Lower Member of the Longmaxi Formation. The study of formation mechanism of siltstone-mudstone rhythmic sedimentary sections may provide new insights into the shale sedimentary environment. Therefore, we studied the characteristics and formation mechanism of siltstone-mudstone rhythmic sedimentary sections in the Lower Member of the Longmaxi Formation in the Changning area based on core observation, thin section identification, major elements and trace elements analysis. The results show the following: 1) Two siltstone-mudstone rhythmic sedimentary sections are characterized by frequent interbed between black or gray-black shale and light gray siltstone, abundant argillaceous laminas and silty laminas, with obvious lithological boundaries having developed. Horizontal laminas and rhythmic laminas are well-developed in the shale layer, while the wavy laminas are well-developed in the siltstone layer. 2) The major compositional elements are SiO2, Al2O3 and CaO, followed by Fe2O3, MgO, K2O and Na2O. 3) Compared with the world average shale, these siltstone-mudstone rhythmic sedimentary sections are rich in Mo, U and Ba, but less in V, Co, Ni, Cu. Compared with the shale layer, the siltstone layer has lower contents of V, Co and Ni. 4). The geochemical redox indices, Mo-U and CIA values suggest the formation of the siltstone-mudstone rhythmic sedimentary sections are related to influences from bottom currents in an oxic condition with a warm and humid paleoclimate.
Pore pressure is an important parameter in coalbed methane (CBM) exploration and development; however, the distribution pattern and mechanism for pore pressure differences in the Upper Permian CBM reservoirs are poorly understood in the western Guizhou region of South China. In this study, lateral and vertical variations and mechanisms for pore pressure differences are analyzed based on 126 injection-falloff and in-situ stress well test data measured in Permian coal reservoirs. Generally, based on the pore pressure gradient and coefficient in coal reservoirs of the western Guizhou region, five zones can be delineated laterally: the mining areas of Zhina, northern Liupanshui, northern Guizhou, northwestern Guizhou and southern Liupanshui. Vertically, there are two main typical patterns: i) the pore pressure gradient (or coefficient) is nearly unchanged in different coal reservoirs, and ii) the pore pressure gradient (or coefficient) has cyclic variations in a borehole profile with multiple coal seams, which suggests the existence of a “superimposed CBM system”. The mechanism analysis indicates that coal permeability, thermal evolution stage and hydrocarbon generation contribute little to pore pressure differences in coal reservoirs in the western Guizhou region. The present-day in-situ stress field, basement structure and tectonic activity may be the dominant factors affecting lateral pore pressure differences. The sealing capacity of caprocks and the present-day in-situ stress field are significant parameters causing vertical pore pressure differences in coal reservoirs. These results are expected to provide new geological references for CBM exploration and development in the western Guizhou region.
Regarding CO2 enhanced shale gas recovery, this work focuses on changes in the multiphase (free/adsorbed) CH4 in the process of CO2 enhanced shale gas recovery, by utilizing a rigorous numerical model with real geological parameters. This work studies nine injection well (IW) and CH4 production well (PW) combinations of CO2 to determine the influence of IW and PW locations on the dynamic interaction of multiphase CH4 during 10000 d of CO2 injection. The results indicate that the content of both the adsorbed CH4 and free CH4 is strongly variable before (and during) the CO2-CH4 displacement. In addition, during the simulation process, the proportion of the adsorbed CH4 among all extracted CH4 phases dynamically increases first and then tends to stabilize at 70%–80%. Moreover, the IW-PWs combinations significantly affect the outcomes of CO2 enhanced shale gas recovery – for both the proportion of adsorbed/free CH4 and the recovery efficiency. A longer IW-PW distance enables more adsorbed CH4 to be recovered but results in a lower efficiency of shale gas recovery. Basically, a shorter IW-PWs distance helps recover CH4 via CO2 injection if the IW targets the bottom layer of the Wufeng-Longmaxi shale formation. This numerical work expands the knowledge of CO2 enhanced gas recovery from depleted shale reservoirs.
A validated particle flow code (PFC2D)-based model was developed to investigate the indirect tensile mechanical behavior of shale containing two central parallel cracks under Brazilian splitting test conditions. The results show that preexisting cracks have a significant and insignificant influence on the tensile strength of shale under LPL and LVL conditions, respectively. When L≥10 mm, changing the L and H values has little effect on the tensile strength of shale. However, the inclusion of preexisting cracks have a positive effect on reducing the anisotropy of the shale specimens, and in the case of an L/D ratio of 0.3, the shale anisotropy is the lowest. Four failure modes were formed at different β and θ values under LPL conditions. In the case of β≥60°, the failure mode is mainly affected by β, and when β≤45°, the failure mode is more complicated than in the case of β≥60°. Only three major failure modes were observed under LVL conditions; in the case of 45°≤β≤75° and θ≤30°, the most complex failure mode occurred.
Understanding the damage behavior and cracking mechanism of brittle shale is crucial for hydraulic fracturing design. In this research, uniaxial compression tests are conducted on shale samples with different bedding plane orientations, and acoustic emission monitoring is implemented synchronously. The results indicate that the apparent elastic modulus increases with increasing bedding orientation. For the bedding orientations of 45° and 90°, the lateral deformation is anisotropic due to the bedding structure, revealing the anisotropic Poisson effect. A shear failure surface and tensile failure surfaces form parallel to the bedding plane for bedding orientations of 45° and 90°, respectively. For the bedding orientation of 0°, shear failure mainly occurs through the bedding planes. Additionally, the damage mechanism of shale is investigated by crack classification based on AE parameters. It is found that crack initiation is induced by the generation of shear cracks for the bedding orientation of 45°, whereas by the generation of tensile cracks for other bedding orientations. According to damage attributable to different type cracks, shear cracks dominate the damage behavior for bedding orientations of 0° and 45°, whereas tensile cracks dominate the damage behavior for bedding orientation of 90°. Finally, the information entropy is calculated by AE energy. A low value of information entropy, approximately 0.36, predicts failure with a low degree of instability for the bedding orientation of 0°, whereas a high value of information entropy, more than 1.5, predicts failure with a high degree of instability for other bedding orientations. This finding indicates that the failure behavior is gradual progressive damage for bedding orientation of 0°, whereas sudden damage dominates failure behavior for other bedding orientations.
Special deposition environment makes organic-rich shales in Ningwu Basin have type III kerogen and high kaolinite content, which are also famous as the kaolinite ore. The specific composition of shale in Ningwu Basin can change the pore structure and thus, influence the shale gas storage and transport. This study focuses on the pore structure and its evolution in shales with type III kerogen and high kaolinite content. In this study, 14 Upper Paleozoic shale samples, whose total organic matter contents (TOC) range from 0.39% to 5.91% and maturities (represented by vitrinite reflectance) range from 1.22% to 2.06%, were collected. Scanning electron microscopy (SEM), high-pressure mercury injection, and low-temperature N2 adsorption experiments were used to analyze pore structure of these shale samples. Results show that when the TOC content is smaller than 1.4%, the kaolinite content decreases linearly and quartz content increases linearly with increasing the TOC content. In contrast, when TOC content is>1.4%, the clay content tends to increase with increasing TOC. Based on the SEM images, organic pores and clay pores were identified in shale samples with type III kerogen and high kaolinite content. During the maturation process, the kaolinite content decreases and illite content increases with increasing the vitrinite reflectance. At the same time, the pore volume and pore surface area both increase with increasing the vitrinite reflectance, and it may be because more organic pores and clay pores in the illite were generated during the maturation process. This study can provide further understandings of shale gas accumulation in shale with type III kerogen and high kaolinite content.
Prediction of shale gas production is a challenging task because of the complex fracture-pore networks and gas flow mechanisms in shale reservoirs. Empirical methods, which are used in the industry to forecast the future production of shale gas, have not been assessed sufficiently to warrant high confidence in their results. Methane carbon isotopic signals have been used for producing gas wells, and are controlled by physical properties and physics-controlling production; they serve as a unique indicator of the gas production status. Here, a workable process, which is combined with a gas isotope interpretation tool (also known as a numerical simulator), has been implemented in Longmaxi shale gas wells to predict the production decline curves. The numerical simulator, which takes into account a convection-diffusion-adsorption model for the matrix and a convection model for fractures in 13CH4 and 12CH4 isotopologues, was used to stabilize the carbon isotope variation in the produced gas to elucidate gas recovery. Combined with the production rates of the four developing wells, the total reserves ranged from 1.72 × 108 to 2.02 × 108 m3, which were used to constrain the trend of two-segment production decline curves that exhibited a transition from a hyperbolic equation to an exponential one within 0.82–0.89 year. Two-segment production decline curves were used to forecast future production and estimate ultimate recovery.
The pore structure of continuous unconventional reservoirs (CURs) in coal measures was investigated using different technologies for 29 samples (9 coal samples, 9 shale samples, and 11 sandstone samples) from Qinshui Basin, China. Results show that coals have relatively high porosities and permeabilities ranging from 4.02% to 5.19% and 0.001 to 0.042 mD, respectively. Micropores (<2 nm) are well-developed in coals and contribute to the majority of pore volume (PV) and specific surface area (SSA). The porosities and permeabilities are between 1.19%–4.11%, and 0.0001–0.004 mD of sandstones with a predominance of macropores (>50 nm). However, shales are characterized by poorly petrophysical properties with low porosity and permeability. Macropores and mesopores (2–50 nm) are well-developed in shales compared with micropores. For coals, abundant organic matters are expected to promote the development of micropores, and clay minerals significantly control the performance of mesopores. For shales and sandstones, micropores are mainly observed in organic matters, whereas clay minerals are the important contributor to mesopores. Moreover, micropore SSA significantly determines the adsorption capacity of CURs and sandstones have the best pore connectivity. The permeability of CURs is positively associated with the macropore PV since macropores serve as the main flow paths for gas seepage. Additionally, we also proposed that effective porosity has a significant effect on the permeability of CURs. The findings of this study could enhance the understanding of the multiscale pore structure of CURs and provide insights into the mechanisms that control gas storage, transport, and subsequent co-production for continuous unconventional natural gas (CUNG) in the Qinshui Basin and other coal-bearing basins worldwide.
Although various coaly source rocks widely developed in the Carboniferous–Permian (C–P) of the Bohai Bay Basin, their geochemical characteristics and hydrocarbon generation potential are poorly understood. This study aims to discriminate the contribution of hydrocarbon generation from different C–P coaly source rocks and clarify the differences within generated oils using organic geochemistry, organic petrology, and thermal simulation experiments. The coaly source rocks containt coal clarain and durain, carbonaceous shale, and shale deposited in deltaic and lagoonal environment. The results indicated that clarain, durain, and carbonaceous shale exhibited higher hydrogen index and liquid–gas hydrocarbon yields than lagoonal and deltaic shales, which was mainly associated with the concentrations of sporinite, cutinite, and hydrogen-rich collodetrinite. Aliphatic hydrocarbons originated from coal and carbonaceous shale presented lower Ts/(Ts+Tm), Ga/17α(H)21β(H)-C30 hopane, 18α(H)-oleanane/17α(H)21β(H)-C30 hopane ratios, and higher 17β(H)21α(H)-C30 Morane/17α(H)21β(H)-C30 hopane than deltaic lagoonal shales. Parameters of aromatic hydrocarbons generated from five lithologies of coaly source rocks trended as clear group distribution, e.g., clarain and durain showing lower MNR, DBT/Fluorene (F) ratios and higher DBF/F ratio than coaly shales. The distinct descending trend of hydrocarbon potential is obtained from clarain, durain, carbonaceous shale to lagoonal and deltaic shales, implying dominated the petroleum and natural gas supplement from coal and carbonaceous shale. The difference between aliphatic and aromatic hydrocarbons provides a significant contribution to analyze the generic relationship between coaly source rock and lacustrine shale. Our results illustrate the importance of coaly source rocks for the in-depth oil-gas exploration of the Bohai Bay Basin and understanding hydrocarbon generation potential of source rocks in coal bearing strata.
The sandstone of the third member of the Funing Formation (E1f3) in the northern slope zone of the Gaoyou Sag has the typical characteristics of high porosity and ultralow permeability, which makes it difficult for oil to flow. In this study, the lithological characteristics, sedimentary facies, diagenetic characteristics, pore structure, and seepage ability of this sandstone are characterized in detail. Correlation analysis is used to reveal the reason for the sandstone high porosity-low permeability phenomenon in the study area. The results indicate that this phenomenon is controlled mainly by the following three factors: 1) the sedimentary environment is the initial affecting factor, whereby the deposition of a large number of fine-grained materials reduces the primary pores of sandstone. 2) The Funing Formation has undergone strong compaction and cementation, which have led to the removal of most of the primary pores and a reduction in size of the throat channels. 3) Owing to fluid activity during the later stage of diagenesis, sandstone underwent intense dissolution and a large number of particles (feldspar and lithic debris) formed many dissolution pores (accounting for nearly 60% of the total pore space). Among these factors, dissolution has contributed the most to the development of high porosity-low permeability phenomenon. This is mainly attributed to the inhomogeneous dissolution process, whereby the degree of particle dissolution (e.g. feldspar) exceeds that of cementing minerals (clay and carbonate minerals). The secondary dissolution pores have increased the porosity of sandstone in the study area; however, the pore connectivity (permeability) has not been significantly improved, thus resulting in the special high porosity-low permeability characteristics of this sandstone.
This paper uses pollen climate analysis and coexistence analysis to systematically analyze the climatic evolution of the Shahejie Formation in the Chezhen Depression, Bohai Bay Basin, eastern China and discusses the relationship between palaeoclimatic evolution and lake level rise. The results show that the sedimentary period of the Shahejie Formation in the Chezhen Depression had an overall temperature change trend from hot to cold and simultaneously experienced a dry and wet balance–wet–dry and wet balance–wet transition process. The climatic parameters of the Shahejie Formation in the Chezhen Depression include a mean annual temperature of 8.1°C−15.1°C, a mean coldest monthly temperature of −0.1°C−2°C, a mean warmest monthly temperature of 18.6°C−28°C, a mean annual precipitation of 389−1164 mm, a wettest monthly precipitation amount of 215−262 mm, and a driest monthly precipitation amount of 8−48 mm. Climate change is believed to affect the rise and fall of lake levels to some extent. The quantitative reconstruction of these climatic parameters allows researchers to more intuitively understand the geological background of the Chezhen Depression and guide the exploration and development of oil and gas resources.
Retrospectively evaluating the efficacy of revegetation practices is helpful in planning and implementing future ecosystem restoration programs (ERP). Having a good understanding of how human activities can affect vegetation cover, both before and after ERP, is particularly important in sandstorm hotspot areas. The Beijing–Tianjin Sandstorm Source Region (BTSSR) is one such area. We conducted an investigation into vegetation dynamics within the BTSSR. This was done using remote sensing data in conjunction with climate data sets and land use data spanning the 1982–2014 period. The relationships between climatic factors (such as precipitation and temperature), and vegetative change were modeled using a neural network method. By a process of residual analysis, the proportions of human-induced vegetative change both before and after the ERP were established. Our results show that: 1) before the ERP (1982–2000), 40.96% of the study area exhibited significantly progressive vegetation changes (p<0.05). This proportion decreased to encompass only 20.23% of the study area in the period following the ERP (2001–2014). 2) 89.55% of the study area showed signs of human-induced vegetation degradation before the ERP. Between 2001 and 2014 however, following ERP, this figure fell to only 27.78%. 3) ERP implementation led to visible improvements in vegetative conditions within the BTSSR, especially in areas where ecological restoration measures were directly and anthropogenically applied. These results highlight the benefits that positive human action (i.e., revegetation initiatives implemented under the framework of an ERP) have brought to the BTSSR.
Evaluation of the river ecological environment can provide a basis for river management and ecological restoration. To conduct a comprehensive health assessment of the rivers in Tianjin, their biological, physical, and chemical indicators are investigated on the basis of 32 river monitoring sites from August to September 2018. The comprehensive pollution and ecological integrity indexes of the rivers are analyzed. Results of the two evaluations, compared to achieve the river ecological environment evaluation, are as follows. 1) Index of Ecological Integrity evaluation shows that among the sampling points, 18.8% are “healthy”, 28.1% are “sub-healthy”, 40.6% are “fair”, 6.3% are “poor”, and 6.3% are “very poor”. 2) The comprehensive evaluation of the chemical properties of the 32 river ecosystems in Tianjin shows severe overall river pollution and low standard water function area. Of the total sampling sites, 16 (50%) are heavily contaminated and 10 (31.3%) are moderately contaminated. Excessive chemical oxygen demand and ammonia nitrogen are the main causes of water pollution. 3) The Index of Ecological Integrity (IEI) has high correspondence with environmental factors. Pearson correlation analysis results show that the IEI index is significantly correlated with permanganate index (R= - 0.453; P = 0.023<0.05). Analysis results using BEST show that ammonia nitrogen is the best environmental parameter to explain the changes in IEI (Rho= 0.154; P = 0.02<0.05) and those using RELATE show significant correlation between the biotic index and the environmental parameter matrices (Rho= 0.154; P = 0.034<0.05).
The terminal settling velocity (TSV) calculation of drops and other spherical objects in fluid medium is a classical problem, which has important application values in many fields such as the study of cloud and precipitation processes, the evaluation of soil erosion, and the determination of fluid viscosity coefficient etc. In this paper, a new explicit approximation model of TSV is established, which combines the theoretical solution of N-S equation about fluid motion around spherical objects and the statistical regression of solution dimensionless coefficients with measurement data. This new model can adapt to different values of drop parameters and medium parameters in a large range of Re. By this model, the relative and absolute calculation errors of TSV are in range of −3.42%–+4.34% and −0.271 m/s–+0.128 m/s respectively for drop radius 0.005–2.9 mm. Their corresponding root mean square values are 1.77% and 0.084 m/s respectively, which are much smaller than that of past theoretical and empirical models.
