The current research generally believes that fault slip in the hydraulic fracturing process is the main cause of casing deformation. Therefore, accurate risk assessment of fault slip is an important way to prevent casing deformation. However, there are few studies on the prediction of fault slip probability caused by hydraulic fracturing. The existing calculation formulas are considered in relatively simply factors. Based on the Mohr-Coulomb criterion, this paper further considered the influence of local in-situ stress changes and seepage pathways, and established a new slip risk assessment model suitable for arbitrarily distributed faults. The model could predict the accurate probability of fault slip through deterministic parameters, and calculate the probability cumulative curve of fault slip through random sampling by the Monte Carlo method. The verification results of an example showed that the model had certain feasibility and accuracy.
According to statistics, as of April 2021, there were more than 74 wells in the Weirong block of Sinopec, of which 32.4% had been deformed (24/74). As of December 2018, more than 377 shale gas wells were fractured in national shale gas demonstration bases such as Changning-Weiyuan, Zhaotong, and the casing deformation rate was 35.3% (133/377). We run multi-finger imaging caliper tool and lead mold to observe the casing deformation. The amount of deformation is relatively large, usually 1~3 cm, and the most serious one exceeds 5 cm. The lead impression is worn out on one side, which is in line with the characteristics of casing shear deformation.
At present, it is generally believed that the fault slip during hydraulic fracturing is the main cause of casing deformation, and a large number of studies have been carried out with the faults near the wellbore as the core. Chen et al. (2017) first argued that faults were the internal cause of casing deformation, and hydraulic fracturing was the external cause. To further verify this view, field tests, laboratory tests and finite element simulations have been carried out (Li, 2017; Liu, 2017; Jalali et al., 2016)., and it was confirmed that the shear deformation of casing downhole was closely related to fault slip. Theoretical research on fault slip has been adequate, but how to assess the risk of fault slip and prevent casing deformation in advance is the ultimate goal of all research. Regrettably, there are few studies in this area at present. And the existing software is only suitable for injection wells, which do not meet the calculation conditions of large displacement, high pump pressure and multi-stage fracturing of shale gas horizontal wells (Walsh et al., 2016).