Study On Dynamic Distribution Law Of Weekly Stress In Tight Gas Well Drilling Well

Because gas drilling is no solid/liquid phase and negative pressure difference, compared to conventional drilling, there is great advantage in terms of timely detection and effective protection of tight gas reservoir. But due to the complex reservoir characteristics, such as high pore pressure and developed micro-fractures, and the huge negative pressure difference, there is strong fluid-structure interaction effect when tight gas reservoir is drilled by gas drilling. This will result in dramatic change of geostress and threaten borehole stability. It has severely-restricted the further application and promotion of gas drilling on tight gas reservoir. Based on nonlinear percolation theory and linear elastic theory, the dynamic coupling models of geostress are established and the dynamic variations of geostress near borehole are researched. In addition, multi-physics simulations about geostress distribution are accomplished. The specific tasks include:(1) Based on tight gas reservoir of DB region, the basic features of tight gas reservoir as well as the site working condition are analyzed. The results show:intense flow-structure coupling interaction cause dramatic changes of geostress, it is key factor caused the underground high risk when tight gas reservoir is drilled with gas drilling.(2) Carried out laboratory experiments under high pore pressure and high confining pressure. The results show:the permeability of tight sandstone has strong stress sensitivity in high-stress environment. The mechanics parameters of sandstone and mudstone decrease with increasing of effective confining pressure. The instant high production phenomenon is existence when tight gas reservoir is opened.(3) The transient distribution models of crustal stress were established when gas drilling opened matrix formation or fissured formation. And then combined with on-site parameters, the dynamic distribution rule of crustal stress of sandstone and mudstone was calculated. The results show:the near-wellbore formation exist double stress environment, including radial stretch and vertical/circumferential compression, geostress change has complexity.(4) The multi-physics simulations of crustal stress distribution were implemented on different working condition, and the relationship between crustal stress distribution and fracture location and dip was researched. The results show:pore pressure of near-wellbore formation occurs to dramatic changes at moment of formation being drilled. The influence of fracture to radial stress is max when fractures dip equal to45°The research results can provide basis for the analysis of borehole instability mechanism and the evaluation of security when tight gas reservoir is drilled by gas drilling technology.