查看更多>>摘要：The oil-bearing layer in the first member of the Upper Cretaceous Yaojia Formation (K2y~1) in the middle of the Changling Sag of the southern Songliao Basin, northeast China, has low-permeability and is deemed to be an unconventional reservoir. Therefore, better evaluation of reservoir quality and related controlling factors are crucial for the reduction in exploration risk and sustainable development. The current study integrates diagenesis. sequence stratigraphy, and sedimentary facies to decipher their influence on reservoir quality using core and thin section observations, X-ray diffraction, scanning electron microscopy, fluid inclusion, and carbon and oxygen isotope analyses. The K2y~1 is composed of shallow delta sediments of transgressive system (TST) and lowstand system (LST) tracts. The depositional microfacies of the sandstones include TST deltaic front underwater distributary channels and sheet sands, and LST deltaic plain distributary channels, distributary channel intersections, and crevasse splays. The sandstones are dominantly fine-grained moderately sorted feldspathic litharenites and lithic arkoses with silt. Early diagenetic events included compaction, development of smectite, and alteration of feldspar and kaolinite whereas mesogenetic alteration included feldspar dissolution, precipitation of quartz and carbonate cements, and clay mineral alteration. The reservoir quality of the K2y~1 sandstones is generally poor, exhibiting variations among different deltaic depositional facies within the LST and TST. The (underwater) distributary channel (intersection) sandstones, particularly those of the LST, have relatively higher reservoir quality owing to their larger grain size, better sorting, lower carbonate cement content, mixed-layer I/S and illite. and greater occurrence of chlorite.
查看更多>>摘要：Matrix acidizing is a common stimulation measure used for carbonate reservoirs in the petroleum industry. Carbonate rocks are characterized by rich fractures and vugs, however, limited work has been done to date to examine the effect of fractures and vugs on the carbonate acidizing process in three-dimensions (3D). In this paper, the temperature and vugs with high porosity are introduced into the two-scale continuum (TSC) model. The 3D radial fractured-vuggy carbonate acidizing process is simulated using the present hydro-thermal-chemical (H-T-C) coupled model. The sensitivity of temperature, natural fracture and vug parameters on the acidizing process is evaluated. The results show that a high-temperature significantly improves the optimal injection rate of the acid. The existence of fractures and vugs significantly reduces the amount of acid consumed as the acid breaks through the formation. For fractures of the same direction, with the increase of fracture number and fracture length, the normalized initial pore volume (PV_(BT)) is reduced first, and then stabilizes. For intersecting fractures, there is an optimal number and length of fractures with the minimum PV_(BT) value. Furthermore, an increase in the vug size reduces the PV_(BT) value. There is an optimal number of vugs with the minimum PV_(BT) value. Finally, some possible extensions of the current work are discussed.
查看更多>>摘要：Thin interbedded tight sandstone of Shanxi Formation in Ordos basin, China, contains mudstone interlayers and laminas universally, and exhibits ultra-low matrix permeability, low brittleness, and areal heterogeneity. The stimulation effectiveness of this formation depends largely on whether multiple hydraulic fractures (HFs) can initiate and penetrate into the multiple thin sandstone layers upward and downward. To clarify the multi-fracture growth behavior of such formation, multi-cluster fracturing experiment in the horizontal well was performed on specimens prepared from the sandstone-mudstone outcrops of Shanxi Formation based on a true triaxial fracturing simulation system. The influences of interlayer stress difference, mudstone thickness, fluid viscosity, pumping rate and the number of clusters were mainly analyzed through post-fracturing specimen splitting and pressure curve analysis. Results show that the difference of rock mechanical parameters between sandstone and mudstone is relatively large. When interlaver stress difference is less than 3 MPa, the mudstone layer cannot function as a barrier to restrain the vertical growth of HFs, and the essential mechanism causing HF height containment is mainly the opening of laminas or/and interfaces. By contrast, when the interlayer stress difference is equal to or greater than 3 MPa and the dimensionless thickness of mudstone is more than 0.4, the HF height tends to be restrained. Increasing the fluid viscosity or/and pumping rate can reduce filtration, and facilitates HFs penetrating through the laminas or interfaces and consequently propagating into adjacent layers. It has been observed that when fluid viscosity reaches 100 mPa s (cross-linked gel) and pumping rate is 100 mL/min, HFs can pass through the overlaying mudstones. However, uniform initiation of all clusters is difficult to achieve due to the low brittleness of sandstone and heterogeneity along the horizontal wellbore. The experimental results prove that temporary plugging within the wellbore using the plugging agent of appropriate particle size is necessary to create multiple HFs in such formation.
查看更多>>摘要：Shale gas reservoirs are uniquely characterized by extremely low values of permeability, porosity, and complex fracture network which adequately hinders gas transfer and production evaluation. At the reservoir scale, the analysis of complex gas flow and transfer mechanisms from one domain to another is still a challenge. In this regard, a fully coupled multi-scale quadruple-continuum model is proposed, where the gas transfer mechanisms across the four domains (Kerogen, inorganic matrix, natural fractures, and hydraulic fractures) are adopted and the flow terms are corrected to suit real gas and account for shale deformation considering dynamic effective stress. The complexity of modeling gas transfers from the kerogen to the inorganic matrix to fracture system (natural and hydraulic fractures) and the production well is overcome by using the Warren-Root and Vermeulen transfer terms, respectively. The current model matches field data from typical shale reservoirs and accurately predicts gas production. Numerical simulation results show that increasing the kerogen pore volume and stress sensitivity coefficient decreases cumulative gas production. Increasing domain permeability, porosity, number of natural fractures, and the width and length of natural fractures improve the flow of gas, leading to higher cumulative gas production. Moreover, crisscrossing natural fractures and larger stimulated reservoir domain (SRD) could extend the area of contact with the unstimulated reservoir domain (USRD), thus allowing more gas to flow to the production well. Overall, more gas is produced when the four domains are considered than when either of them is ignored. This study introduces a comprehensive understanding of the four domains' primary flow and transfer mechanisms. It provides a practical baseline for evaluating gas production in fractured shale reservoirs.
查看更多>>摘要：The evolution of fractures during shut-in after hydraulic fracturing is an important topic in petroleum engineering. Considering the closure and secondary expansion of fractures during shut-in can further improve the accuracy of predicting hydraulic fracture parameters. Based on linear elastic fracture mechanics, elastic mechanics, and the implicit level set algorithm, a numerical model of planar hydraulic fracture propagation was established in this study. In the proposed model, the reservoir was considered as a homogeneous elastic medium, and the fluid loss was characterized using Carter's filtration model. The secondary propagation and fracture closure behavior during shut-in could be simulated by solving the strongly nonlinear system of fluid and stress coupling. Based on this model, the evolution behavior of hydraulic fractures during injection and shut-in was analyzed. According to the variation characteristics of the fracture length, the evolution process could be divided into three stages: fracture propagation during injection, secondary growth, and fracture stopping (but not closing). According to the variation characteristics of the fracture opening and net pressure, the evolution process could be divided into three stages: nonlinear increase, linear decrease, and gradual decrease. For low-permeability reservoirs, fracture closure and secondary propagation may take several times the injection time after shut-in, and the secondary propagation length may reach tens of meters. The results show that the larger the filtration coefficient, the faster the fracture closure will be, and the shorter the secondary propagation distance. The higher the viscosity of the fracturing fluid, the slower the fracture closure will be, while the secondary propagation distance is almost unaffected. The fracture toughness has little effect on the fracture closure and secondary propagation. The crack will close first in the region with large interlayer stress during shut-in. The findings of this study can help for better understanding of the evolution mechanism of fractures during shut-in and closure.
查看更多>>摘要：Due to the constraint of fresh water, the reuse of flowback and produced water (FPW) with high TDS. is critical to minimize the environmental impact of oil/gas exploration. The underlying challenge is the negative effect of salts present in FPW on the chemistry of fracking. The solution is to develop excellent fracking chemistry with salt- and temperature-resistant. Herein, we have comprehensively discussed FPW characteristics, and the technical challenges of reusing FPW; we then focus on the design strategy of developing fracturing fluids that are effective under harsh conditions. The environmental benefits and economic implications of using EPW in fracking are highlighted. In summary, we proposed that a new type of fracturing fluid should be developed in future, which contains a new functional gelling agent prepared by grafting the synthetic polymer on the biopolymer, bio-based nano-crosslinkers, cost-effective nano-breakers, and FRs with high elasticity and salt-resistance, which will have great potential for the reuse of FPW and the development of unconventional formations. We envision that this review will provide insights to stimulate new thinking and innovation in oil/gas fracking that is not only green/sustainable, but also economic/practical, while meeting the demand for high performance under harsh environments with high salt and high temperature.
查看更多>>摘要：Hydraulic fracturing operations to support production from unconventional oil and gas reservoirs have been subject to public concerns and regulatory oversight regarding the generation of induced seismicity. These concerns have been shaped by experiences such as those for the Montney Formation in northeastern British Columbia that have seen an increase in seismic activity in response to hydraulic fracturing activities. Although the geological conditions present play a central role in the susceptibility of a well to induced seismicity, operational factors such as fluid injection volume and rate are of special interest as these can be engineered and controlled to mitigate the induced seismicity hazard. Results are presented in this paper from an empirical investigation analyzing data from the Montney that explore the influence of injection volumes and rates to the triggering of induced seismicity. The results from the empirical analysis indicate that injection volume correlates more strongly with induced seismicity for wells that target the Middle Montney formation, whereas injection rate was seen to be a more significant influencing factor for wells targeting the Upper Montney formation. Our hypothesis is that this difference is due to the presence of a more dense and interconnected natural fracture network in the Middle Montnev that influences the likelihood of fluid-flow diffusion and connectivity with a critically stressed fault. Results are provided from a series of advanced 3-D numerical models used to support this hypothesis and provide mechanistic understanding.
查看更多>>摘要：A new transparent resin material with a compression-tension strength ratio of up to 6.6 at -15～-10 °C has been developed. It possesses more brittle fracture properties than before and can competently simulate many kinds of engineering rocks. The fabrication demands strict temperature control and treatments. Specimens are inventively made with a hollow injecting inner pre-crack. Based on self-designed water injection devices and low-temperature loading equipment, uniaxial and biaxial hydraulic fracturing experiments are carried out under fixed water pressures. The specimens' uniaxial hydraulic failure process can be separated into four stages. At each stage, the variation of water injection is analysed. For all experiments, both crack initiation stress and peak strength have declined significantly, by more than 80% and 70% compared with dry specimens. The evolution of wTapping wing cracks and fin-like cracks in biaxial experiments has rarely been reported before. Moreover, numerical verification is conducted by FLAC~(3D). A novel fluid-solid coupled model has been developed that takes into account both the pre-peak damage and post-peak sharp degeneration. The principles are dependent on the element's failure type, i.e., tensile failure, shear failure, or a combination of the two. The mechanical parameters and permeability coefficient of damaged elements are redefined. The simulation results match well with experiments, demonstrating the feasibility of this method. Finally, the proposed model is applied to explore the hydraulic failure of the specimen under triaxial compression with fixed stress. The impact of injecting pre-crack on hydraulic fracture growth is illustrated.
查看更多>>摘要：Water hammer fracture diagnostics is a promising fracture diagnostic method, which is nonintrusive, economical, timely performed and easily operated. In present studies, the propagation behavior of the water hammer in the wellbore-fracture system remains unclear, which brings uncertainty to locating the hydraulic-stimulated fractures. In this paper, the transient flow model was used to describe the water hammer pressure fluctuation, which was generated after the pump shut down during the hydraulic fracturing treatment, and the wellbore-fracture system was built by introducing the branch pipe to the wellbore as the fracture. The method of characteristics (MOC) was used to solve this model, then was validated by the computational fluid dynamics software FLUENT. Combining water hammer signal analysis in time, frequency, and quefrency domain, the fracture effect on the water hammer response characteristics in the wellbore-fracture system was investigated in multiple dimensions. We then built eight cases to simulate the water hammer pressure fluctuation in different wellbore-fracture systems, including the existence, the varied location and the varied length of the fracture and conducted the multi-dimensional analysis. After that, results were discussed and the response characteristics of the water hammer in the wellbore-fracture system were summarized. From this work, a multi-dimensional analysis method in time, frequency and quefrency domain is proposed. Meanwhile, the water hammer response characteristics of the wellbore-fracture system are sufficiently presented for the first time to accurately investigate the water hammer pressure propagation in the wellbore-fracture system and provide useful insights for water hammer diagnostics in field application.
查看更多>>摘要：In a fractured-vuggy carbonate reservoir (FVCR) are distributed many irregular natural fractures and cavities. They are the main oil/gas storage bodies. To stimulate such a reservoir, it is desired to connect the hydraulic fractures (HF) to them. To study the interaction mechanism between HFs and cavities in this paper, the natural fracture is modeled by the element partition method and the cavity is described by an ellipse equation at first. The surrounding fractured zone of a cavity is considered. Then the local and wellbore-nearby HF-cavity interactions are studied. It shows that the magnitude of in-situ stress has significant impact on their interaction due to the stress concentration surrounding a cavity. The higher in-situ stress leads to a stronger repulsion of a cavity to HF. The pore pressure and the surrounding fractured zone of a cavity can relieve the stress concentration, which facilitates the HF-cavity connection. The distributed natural fractures have the leading effect on HF direction when the in-situ stress difference is small. Next, the interaction between HF and cavity cluster is studied. It suggests that the injection mode has significant impact on the generation of fracture network. If the injection rate is adjusted properly, a complex fracture network can be generated. These findings help to deeply understand the HF-cavity interaction and provide valuable references for the HF treatment design in FVCR.
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