98 / 2023-06-15 10:51:19

Investigation on dynamic mechanism of fault slip and casing deformation during multi-fracturing in shale gas wells

casing deformation,well integrity,fault slip

Fracturing horizontal well casing deformation has become very prominent, particularly in tectonic stress-concentrated shale gas fields, limiting the efficient development progress of shale gas. The main failure mode of casing shearing deformation had been attributed to fault slipping caused by multi-fracturing. The current research did not provide a clear picture of the dynamic evolution relationship between hydraulic fracturing, fault slipping, and casing deformation. As a result, research into the dynamic mechanism of fault slipping and casing shear deformation during the hydraulic fracturing process is critical. The dynamic sliding model of fault slip induced by formation pressure change is established in this paper, incorporating the effects of stress drop, the physical change of friction, and casing and cement-sheath resistance loads. The displacement discontinuity and explicit/implicit coupling iteration methods are used to reveal the relationships between the effective normal stress, shear stress, friction coefficient, and sliding speed during the fault slipping process. Furthermore, the microscopic process of casing deformation sheared by fault slip is investigated using static equilibrium theory. The characterization method of casing deformation caused by real-scale fault slip is proposed. It shows that three stages exist in the process of casing deformation sheared by fault slipping. In the first stage, as the pore pressure increases, the effective normal stress of the fault plane decreases, the stress equilibrium on the fault plane is destroyed, and the fault is induced to slip, with no shearing stress loads on the casing. In the second stage, the fraction coefficient drops abruptly as the fault slips, causing the fault to slip with acceleration while plasticity deformation exists in the casing. In the final stage, the fraction coefficient increases, the sum of the friction force and casing resistance force exceeds the shearing force, and the speed of fault slipping gradually decreases to zero. Moreover, the effect factors of fault slipping are discussed, laying the theoretical groundwork for the casing deformation control method in shale gas wells.