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The Luzhou Block in southern Sichuan Basin is an important shale gas production area in China, where the WufengLongmaxi Formation shale gas resources are abundant, indicating a high exploration and development potential. The present-day in-situ stress has important applications in the analysis of shale gas migration and enrichment, drilling and completion engineering, reservoir fracturing and well pattern deployment, etc. The present-day in-situ stress state of Wufeng-Longmaxi Formation deep shale gas reservoirs in the Luzhou block was analyzed, and natural fracture activity and hydraulic fracture propagation under the effect of presentday in-situ stresses were discussed. The research results show that the dominant present-day in-situ stress orientation in the WufengLongmaxi Formation shale gas reservoir is WNW-ESE-trending. The magnitude of horizontal maximum principal stress is the largest, magnitude of horizontal minimum principal stress is the lowest, indicating a strike-slip stress regime. The stress difference in the Wufeng Formation and S1 l 1 - 1 - 1 layer is 11. 90 ~ 15. 76 MPa and 11. 80 ~ 16. 75 MPa, respectively, which is beneficial for generating complex fracture networks. Under the present-day in-situ stress conditions, natural fracture activity in the Wufeng-Longmaxi Formation shale gas reservoirs of Luzhou Block is relatively weak. With fluid injection during shale gas development, natural fractures are gradually activated, and their critical pressure increase is 15 ~ 23 MPa. Hydraulic fractures vertically propagate under the strike-slip stress regime. The results can provide present-day in-situ stress analysis basis for further beneficial development of deep shale gas in the Luzhou Block of southern Sichuan Basin. [ABSTRACT FROM AUTHOR]

Based on structural distribution and fault characteristics of the Luzhou block, southern Sichuan Basin, as well as microseismic, well logging and in-situ stress data, the casing deformation behaviors of deep shale gas wells are summarized, and the casing deformation mechanism and influencing factors are identified. Then, the risk assessment chart of casing deformation is plotted, and the measures for preventing and controlling casing deformation are proposed. Fracturing-activated fault slip is a main factor causing the casing deformation in deep shale gas wells in the Luzhou block. In the working area, the approximate fracture angle is primarily 10°–50°, accounting for 65.34%, and the critical pore pressure increment for fault-activation is 6.05–9.71 MPa. The casing deformation caused by geological factors can be prevented/controlled by avoiding the faults at risk and deploying wells in areas with low value of stress factor. The casing deformation caused by engineering factors can be prevented/controlled by: (1) keeping wells avoid faults with risks of activation and slippage, or deploying wells in areas far from the faulting center if such avoidance is impossible; (2) optimizing the wellbore parameters, for example, adjusting the wellbore orientation to reduce the shear force on casing to a certain extent and thus mitigate the casing deformation; (3) optimizing the casing program to ensure that the curvature radius of the curved section of horizontal well is greater than 200 m while the drilling rate of high-quality reservoirs is not impaired; (4) optimizing the fracturing parameters, for example, increasing the evasive distance, lowering the single-operation pressure, and increasing the stage length, which can help effectively reduce the risk of casing deformation.

In recent years, significant efforts have been directed towards extracting deep shale gas (SG) in Luzhou. Understanding the factors influencing the accumulation and preservation of SG in the section extending from the Upper Ordovician Wufeng (WF) Formation (Fm) through the Lower Silurian Longmaxi (LMX) Fm is crucial for its large‐scale, profitable development. The shale layers in the LMX Fm located in the Luzhou‐Zigong area in the southern part of the Sichuan Basin have undergone unique geological processes, including deep deposition, intensive uplift, erosion and deformation. The Luzhou block, situated in the low‐fold zone in southern Sichuan between the southern slope of the paleo‐uplift in central Sichuan and the down‐warping fold zone in south‐eastern Sichuan, was studied using seismic, well‐log, drilling and core analysis and test data to understand its tectonic, sedimentary, reservoir and fracture characteristics and identify the principal factors controlling gas accumulation in this area. The study found that SG in the Luzhou block is primarily distributed in the WF Fm through the Longyi1 sub‐member. Class I continuous reservoirs, 2.6–20.3 m in thickness, are mainly found in the Longyi11–13 sublayers and those with large continuous thickness produced high yields during tests. Gentle anticlinal zones and wide, gentle tectonic structures with relatively high‐pressure coefficients have formed in this area. High‐angle fractures (HACs) in this area tend to accumulate free gas and, therefore, facilitate the accumulation and production of SG. The study concludes that the continuous thickness of Class I reservoirs and natural HACs controlled by the centre of the deep‐water continental shelf are the primary factors controlling SG accumulation in the Luzhou block. [ABSTRACT FROM AUTHOR]

In the Luzhou Block of the southern Sichuan Basin, the deep Longmaxi shales have become important exploration targets in recent years. However, the water-bearing properties of these shales are still unclear, which significantly limits evaluations of reservoir pore structures and gas-in-place (GIP) contents. In this study, twelve fresh shale core samples were collected at the well site, and the pore water (CPW) and equilibrium water (CEW) contents, as well as the pore structures of the shales, were analyzed under both as-received and dried conditions. The results indicate that the deep shales have low water-bearing extents with a pore water content (CPW) of 3.82–16.67 mg/g, and that both the organic matter (OM) and inorganic matter (IM) pores can be used for pore water storage. The extent of influence of pore water on nonmicropores and IM pore structures is more significant than that on micropores and OM pore structures. Meanwhile, the pore water obviously reduces the retention effects of nanopores and may block nanopores with pore widths < 0.5 nm. An average of 40% of pore spaces were taken up by pore water in the studied deep shales in the Luzhou Block, and the residual pore surface area and pore volume of the shales were mainly contributed from micropores and nonmicropores, respectively. [ABSTRACT FROM AUTHOR]

In the Luzhou Block of the southern Sichuan Basin, the deep Longmaxi shales have become important exploration targets in recent years. However, the water-bearing properties of these shales are still unclear, which significantly limits evaluations of reservoir pore structures and gas-in-place (GIP) contents. In this study, twelve fresh shale core samples were collected at the well site, and the pore water (CPW) and equilibrium water (CEW) contents, as well as the pore structures of the shales, were analyzed under both as-received and dried conditions. The results indicate that the deep shales have low water-bearing extents with a pore water content (CPW) of 3.82–16.67 mg/g, and that both the organic matter (OM) and inorganic matter (IM) pores can be used for pore water storage. The extent of influence of pore water on nonmicropores and IM pore structures is more significant than that on micropores and OM pore structures. Meanwhile, the pore water obviously reduces the retention effects of nanopores and may block nanopores with pore widths < 0.5 nm. An average of 40% of pore spaces were taken up by pore water in the studied deep shales in the Luzhou Block, and the residual pore surface area and pore volume of the shales were mainly contributed from micropores and nonmicropores, respectively.

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