In order to solve the casing deformation problem of shale gas wells in Sichuan Basin, the casing deformation of H19 platform is calculated by using the small bridge plug size and Multi-Finger Image Tool (MIT) data. The characteristics of the spatial and temporal distribution and magnitude of microseismic events at the casing deformation points are observed, and the fracture surface model is established. The quantitative relationship among casing deformation, fracture zone scale and microseismic moment is analyzed by using the model. There were 9 deformation values, among which the minimum value was 6.1 mm, the maximum value was 50.43 mm. The characteristics of the spatial and temporal distribution is: 1) the microseismic events are not symmetrical with the wellbore; 2) most of the microseismic events in different fracturing stages are overlapped, showing linear distribution; 3) there are many large magnitude events; 4) in temporal, the frequency of large magnitude events is relatively high in the middle and late part of fracturing. According to the fracture surface model, it is concluded that the fracture area causing casing deformation in Changning area is 40000–70000 m2, and the seismic moment of casing deformation is 2.57.109–7.57.1010 N·m, and the magnitude distribution is (−1.16,0.79). These results can be used for real-time monitoring and early warning of casing deformation and judging fault scale to prevent casing deformation.
During the development of shale gas, in Changning-Weiyuan shale gas demonstration area, the problem of casing deformation is prominent. As of October 2018, casing deformation occurred in 75 wells among 187 wells with the ratio is 40.11%. Casing deformation leads to using downhole tools difficulties, increasing operating cost, reducing fracturing stage number and well production, and influencing the overall economic benefits of shale gas development. Statistical data shows that 61.7% of the casing deformation in Changning-Weiyuan area are related to natural fractures or weak bedding planes, and show the characteristics of shear deformation. Some scholars have analyzed the mechanism of fault slip leading to casing shear deformation: in the process of hydraulic fracturing, a large amount of fracturing fluid can enter the fault through different ways, so that the pore pressure in the fault increases and the effective stress decreases, resulting in the activation of the fault, and then causes casing shear deformation[1-9].
