Formation and Radial Evolution of Jet-Induced Transient Pressure in a Confined Gun-Casing Fluid Annulus during Shaped-Charge Perforation

Summary Shaped-charge perforation generates high-amplitude transient loads when a metallic jet enters the fluid annulus between the perforating gun and casing. Existing estimates commonly relate total charge energy to peak pressure without resolving the local jet-fluid pressure source, post-peak unloading, or annular confinement. For this study, we developed a mechanism-based model for jet-induced annular pressure. The detached shock ahead of the quasisteady penetration interface is represented as a finite-area pressure source and related to the initial radial motion of water through radial momentum balance, while cavity expansion and rarefaction-wave release govern post-peak unloading and pulse broadening. These mechanisms yield power-law relations for peak pressure and pressure-decay time constant, together with an annular-confinement correction. We simulated three progressively constrained configurations using an arbitrary Lagrangian-Eulerian fluid-structure interaction formulation: Case 1 established the free-field reference through direct jet penetration into unbounded water; Case 2 retained the unbounded water domain but introduced gun-barrel penetration to examine whether the power-law relations persisted after the reduction in jet water-entry velocity; Case 3 further introduced the casing boundary to assess the power-law behavior and peak-pressure correction under annular confinement. We conducted an independent perforation experiment to evaluate the predicted peak-pressure attenuation. Arrival-time differences measured by adjacent strain-gauge pairs bonded to the casing inner surface yielded interval-averaged propagation speeds, from which equivalent peak pressures were reconstructed using the Hugoniot relation for water. Within the 40-mm shock-dominated near field, numerical regressions gave coefficients of determination of 0.983–0.991 for peak pressure and 0.962–0.983 for pressure-decay time constant. At the six experimental locations included in the regression, the matched Case 3 numerical and experimentally reconstructed peak pressures gave attenuation exponents of −1.363 and −1.315, with coefficients of determination of 0.992 and 0.985, respectively. The confined-to-unbounded peak-pressure ratio followed the proposed correction relation, with a coefficient of determination of 0.968. The resulting single-source model provides a physically defined pressure input for annular shock-load prediction and subsequent multisource wavefield analysis in sequential perforating operations.