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During the development of shale gas, one of the major challenges is the casing deformation (CD) in the horizontal section due to the geological activity. Recently, the casing deformation before hydraulic fracturing (CDBF) occurred in multiple shale gas wells in L block in China. In this paper, based on the theory of tubular mechanics, the relationship between casing buckling and CDBF caused by casing running in is analyzed qualitatively and quantitatively. It is found that the buckling deformation caused by running the casing string process is not sufficient to prevent the tool (Φ99 mm bridge plug). On the other hand, the mechanism of CDBF is systematically analyzed based on the actual field data and finite element (FE) method. In order to analyze the CDBF mechanism, the comparison between the actual casing trajectory and the reservoir rock (S#1, S#2, S#3 and M#1) in horizontal section in H2 platform is carried out, and the preliminary CDBF mechanism is proposed. Then, two groups of FE models are established to reduce the CD process to verify the preliminary mechanism. It is found that the numerical simulation results (high stress section length from FE model) are in good agreement with the actual CD logging results. Based on the analytical results, the corresponding mitigation measures are proposed based on the analysis of the CDBF mechanism. Our work could offer a detailed theoretical basis and reference of CDBF for shale gas well application.

In this paper, the mechanical behaviors of different lithological rocks of coal measure strata from Shanxi Formation in the eastern margin of the Ordos Basin, China, were investigated through uniaxial compression tests, and the deformation characteristics and failure modes of different lithological rocks were investigated. On this basis, the energy evolution of different lithology rocks was also discussed. The results show that there are obvious differences in the mechanical properties of different lithology rocks in coal measure strata, resulting in different wellbore instability prevention measures and fracturing measures in different lithology strata. Under the uniaxial compression condition, the peak strain of different lithological rocks is obviously different, and the denaturation characteristics are also obviously different, and the failure modes of rocks are mainly the tensile fracture mode, suggesting that the rock samples have strong brittle characteristics. With the increase of the strain, the total energy of different lithological rocks of the coal measure strata increases, and the elastic energy first increases and then decreases rapidly, whereas the dissipated energy first increases slowly and then increases rapidly. Each energy at the peak point is different, and the average total energy of shale, silty shale, siltstone, fine sandstone, and coal is 0.022 J/cm3, 0.045 J/cm3, 0.052 J/cm3, 0.042 J/cm3, and 0.003 J/cm3, respectively, indicating that there are obvious differences in the energy evolution laws of the different lithological rocks.

Hydraulic fractures are difficult to initiate simultaneously during multi-cluster fracturing owing to the strong heterogeneity of shale reservoir and the stress interference effect between adjacent hydraulic fractures. Some hydraulic fractures can initiate early and propagate rapidly, whereas others exhibit late initiation (or even fail to initiate) and propagate slowly, resulting in non-uniform propagation behavior of multiple fractures. This non-uniform propagation behavior can significantly limit hydraulic fracturing performance in shale gas reservoirs. Therefore, minimization of non-uniform propagation of multi-cluster fractures is important in improving shale gas development. Currently, diverting fracturing technology is implemented to restrain overextended fractures while promoting restricted fractures to facilitate uniform propagation. Pumping diversion balls to block the perforations of overlong fractures has become an important method to improve non-uniform fracture propagation. This method is, however, limited by lagging behind of theoretical simulation, and significant blindness in current implementation of diverting fracturing. A dynamic propagation model for multiple-cluster hydraulic fractures was established in the current study by considering the stress interference effect between adjacent fractures and the effect of flow rate dynamic adjustment by diversion balls. This model is effective for dynamic simulation of fractures propagation after pumping diversion balls. A fractured well in Changning block was used as an example for simulation of the dynamic fractures extension and the distribution of SRV before and after diversion. The findings showed that the temporary plugging ball significantly promoted the uniform extension. The number of temporary plugging balls, the number of diversions and the timing of diversion were then optimized. The simulation method developed in this study has important theoretical significance and field application value in guiding regulation of uniform expansion of fractures and improving optimization of diverting fracturing design.