| Multi-stage and multi-cluster fracturing is critical to develop tight sand gas.Multiple transverse fractures are expected to initiate,which increase the stimulated reservoir volume.However,the data shows some perforation clusters do not initiate and have no contribution to productions.This paper proposes two pressurization regimes,named abnormal high borehole pressure and abnormal low borehole pressure,according to the size relationship between borehole pressure and breakdown pressure.True triaxial fracturing simulation experiments are performed using the artificial rock specimen and natural tight sand stone.Rock type,wellbore conditions,borehole pressure and natural fracture heterogeneity are considered to analyse the influence of initiation.Analytical formula of stress intensity factor for natural fractures were established based on fracture mechanic.According to the criteria of fracture initiation in fracture mechanics,mathematical model for fracture initiation pressure under abnormal high regime is proposed.Calculate the stress at the tip of the natural fractures.Calculate the stone tensile stess based on static fatigue.Initiation pressure under the abnormal low regime based on the maximum stress criterion is establish.Calculate and analyse the initiation of multi-fractures with the formation parameters and experimental parameters.The main conclusion include: multi-fractures are hard to initiate in normal fracturing regime,while it can initiate when well with fractures in abnormal high fracturing regime.The higher borehole pressure the easier it initiate,the higher heterogeneity of fractures,the harder it achieve.And fractures tend to initiate along the bedding surface when stone with bedding architectures.Multi-fracture can not initiate in simulation experiment under low borehole pressure due to size limitation of specimen,but the initiation pressure decreased.Static fatigue damage has time cumulative effect on rock strength reduction.The closer the borehole pressure approaching to the initiation pressure,the shorter the time required for fracture initiation.And the stronger the brittleness of rock,the shorter the initiation time for static fatigue damage. |
