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Abstract The total stress distribution surrounding a perforation tunnel considering the influence of the casing, perforation tunnel and fluid penetration is derived to develop an optimal oriented perforation technique (OPT) for the hydraulic fracturing of a deviated cased well. The fluid penetration effect, injection rate, viscosity and compressibility and the rock permeability are coupled by combining Darcy's law and the equation of mass conversation. Published models and field results are used to verify the rationality of the presented model. The investigation results reveal that the well trajectory and perforation orientation have a significant impact on the fracture initiation pressure (FIP) due to a complex in situ stress field. A field application shows that oriented perforations could reduce the fracture initiation pressure by 16.6 MPa from that using a spiral perforation. The presented model sensitivity analysis implies the following. The fracture initiates away from the intersection of the wellbore and perforation tunnel. The relative magnitudes of the vertical stress, maximum horizontal stress, and minimum horizontal stress have a significant influence on the FIP and only a slight impact on the optimal perforation orientation. Under the normal faulting stress state, the perforation orientation decreases and the FIP increases slightly with increasing inclination angle. However, both the perforation orientation and FIP are highly sensitive to the azimuth angle. The FIP decreases with increasing rock permeability and fluid injection rate, whereas the FIP increases with the fluid viscosity. Highlights • A initiation pressure model for oriented perforations cased deviated wellbore in permeable formation is proposed. • An optimized perforation orientation procedure is presented for fractured cased well. • Oriented perforation could reduce the fracture initiation pressure effectively compared to that of spiral perforation. [ABSTRACT FROM AUTHOR]

Sea level changes more frequently during the deposition of transitional shale than marine shale, resulting in the strong heterogeneity and instability of the sedimentary environment and making transitional shale gas evaluation difficult. The appropriate identification and classification of transitional shale lithofacies types can improve evaluation of shale reservoirs and can provide the geological basis for the evaluation and potential of shale gas reservoirs during exploration and development. The lower Permian shale in the northeastern Ordos Basin is selected as the focus of this research. Based on X-ray diffraction (XRD), total organic content (TOC), vitrinite reflectance (R o), nitrogen and carbon dioxide adsorption, and argon ion polishing scanning electron microscopy (AIP−SEM) data, the lower Permian transitional shale in the study area is divided into silty mudstone and muddy siltstone lithofacies. On this basis, the differences in the organic geochemical characteristics, pore development characteristics and influencing factors of the shale reservoir in the two lithofacies are compared. The results show that the mean values of w (TOC) and w (R o) in the muddy siltstone lithofacies are 1.9% and 1.0%, respectively. Siliceous minerals dominate the composition of the rock, and the main types of reservoir spaces are mineral matrix pores, including interparticle (interP) pores and intraparticle (intraP) pores related to inorganic minerals. Macropores and mesopores are the main components of the pore volume, and account for 40.8% and 49.8% of the total volume, respectively. The pore structure is mainly affected by the TOC and siliceous mineral contents. In the silty mudstone lithofacies, the average values of w (TOC) and w (R o) are 2.4% and 1.2%, respectively. The clay mineral content is relatively high. The main types of reservoir spaces are organic pores and interP pores. Additionally, the pore volumes of macropores, mesopores and micropores are almost the same, accounting for 24.9%, 44.6% and 30.4% of the total pore volume, respectively. The pore structure is mainly affected by the organic matter content and clay mineral content. • Lithofacies classification is an effective method to evaluate the reservoir development characteristics of the transitional shale. • The effect of organic carbon content and inorganic mineral composition on pore volume of different lithofacies shale is various. • Although the reservoir capabilities of transitional shale are considerable, the recoverable reserves are still lower than marine shale. [ABSTRACT FROM AUTHOR]