Research On Failure Mode Of Unconsolidated Sandstone Formation During High Pressure Packing Operations

Outer screen packing for unconsolidated sandstone mainly includes two different packing mode,which are plastic squeezing pack and frac-pack.Final packing mode selection for definite formation depends on both operating parameters and formation characteristics.However,the failure mechanism of unconsolidated sandstone is complex and the system for identify failure mode under high pressure injection conditions for unconsolidated sandstone isn't established yet.Thus caused the design of operation parameters is not suitable for different formations and holding up the development of integrated outer screen packing technology.Complicated formation deformation procedure for unconsolidated sandstone during high pressure packing is considered in this paper and a new porous media model is provided to calculate the stress around wellbore for both vertical and directional wells.Transient temperature effect is considered to check the transient thermal stress influences on failure mode.However,the simulation result shows that transient thermal stress have little influence on both wellbore stress and failure mode of unconsolidated sandstone.By inheriting pre-researchers' findings,unconsolidated sandstone is divided into three different types including brittle rock,ductile rock and compacting rock according to rock mechanics characters.By analyzing both formation properties and operation effects,we conclude that the failure process of brittle rock is controlled by tensile criterion,the failure of ductile rock is managed by shear criterion while the failure of compacting rock is dominated by compressive strength criterion.The detailed failure process of ductile rock is analyzed based on elasto-ideal plastic constitutive model.Formation configuration identify model,critical failure pressure model,critical fracture initiate pressure model and fracture propagation model are provided in this paper.Both simulation and field data showed the suitability of Mohr-Coulomb model for unconsolidated ductile rock.It also shows that the essential requirement to initiate effective man-made fracture in unconsolidated ductile rock is to make sure the radius effective stress at maximum stress direction become intermediate principal stress.Otherwise,shear failure can only lead shear failure zone formed but not fracture.On the base of analyzing the micro-deformation regulations of unconsolidated compacting rock,Modified Cam-Clay model is introduced to calculate the critical failure pressure and relevant compacting failure radius around the wellbore.Simulation results indicate vast anisotropy of both critical failure pressure and relevant compacting failure radius,which is mainly caused by stress anisotropy.In the procedure of high pressure packing,the rock at maximum principal stress compacted firstly.With the increase of injection pressure,compacting failure radius in the formation increased.The results concluded from this paper provide a basis for frac-pack feasibility analysis and operation parameters design for high pressure packing.It is a preparational and prospective basis for integrated high pressure packing technology.