Abstract
To address the deformation and instability characteristics of a formation after an offshore shallow gas well blowout, a theoretical model of formation deformation caused by shallow gas blowouts was constructed, based on porous elastic medium theory and incorporating the sand-out erosion criterion. The spatiotemporal dynamics of formation subsidence were then investigated, and deformation patterns during a blowout were analyzed under various factors. The results indicate that, following a blowout, a shallow gas formation near a borehole experiences significant subsidence and uplift at the upper and lower ends, with the maximum subsidence values at 12 h, 24 h, 36 h, and 48 h post blowout being 0.072 m, 0.132 m, 0.164 m, and 0.193 m, respectively. The overlying rock layer forms a distinctive “funnel” shape, exhibiting maximum subsidence at the borehole, while more distant strata show uniform subsidence. The effective stress within the shallow gas stratum and surrounding rock layers increases gradually during the blowout, with lesser impact in distant areas. The ejection rate and sand blast volume demonstrate an exponential change pattern, with a rapid decline initially and later stabilization. Formation deformation correlates positively with factors like burial depth; shallow gas layer extent; pressure coefficient; sand blast volume; gas blowout rate; and bottomhole difference pressure. Formation pressure, ejection rate, and bottomhole difference pressure have the most significant impact, followed by sand blast volume and burial depth, while the extent of the shallow gas layer has a less pronounced effect. These simulation results offer valuable theoretical insights for assessing the destabilization of formations due to blowouts.