A Mechanical Study Of Horizontal Wellbore Stability In Brittle Shale Gas Reservoir

Exploration and development of shale gas in China is still in infancy and serious problems of borehole instability occur when drilling horizontal wells in shale gas reservoirs?This paper analyzed the structural characteristics and mechanical properties of chang 7 and chang 9 shale gas reservoirs through the cores experiments,measured the geostress using Kaiser-effect acoustic emission test,in addition established the stresses distribution models of horizontal wells and calculated the collapse and fracture pressures,and used the ANSYS finite element analysis software for simulation in the end.The rock mechanics experiments indicate that the shales of Yan Chang oilfiled have high young's modulus,low poisson's ratio,and high content of brittleness mineral.The average brittleness index is about 50%.The shale uniaxial compressive strength is between 50?60MPa,much lower than adjacent sandstone.The acoustic emission tests show that overburden pressure is the maximum principal geostress,and the values of two horizontal geostress change little.Applicatiing elastic mechanics theory,embarking from equilibrium differential equation,combined with geometric equations,physical equation and compatible equations,and considering geostress(overburden pressure maximum),deviation angle,azimuth angle,pore pressure and seepage effect,this paper established a stress distribution model of rocks surrounding wellbore.Vertical wellbore rock stress distribution changes with shaft rotation angle,and 8 times wellbore radius away,the stress almostly doesn't change.When azimuth remains unchanged,inclination angle increases from 0°to 90°,wellbore stability gradually becomes worse,showing that horizontal wellbores collapses much more easily than vertical wells.While inclination angle remains unchanged,wellbore azimuth(assuming the well along the maximum horizontal geostress direction)varying from 0°to 90°,the maximum shear stress on the wellbore decreases and the whole shear stresses change smaller,showing that horizontal wells along the minimum principal stress direction are the most satable.Wellbore surrounding rock suffered the stress more than its strength well produce shear failure.Brittle shales collapse with wellbore enlargement,while plastic formation wellbore becomes smaller.Based on Mohr-Coulomb criterion and the tensile fracture strength criterion,calculated the collapse and fracture pressure of brittle shales.The results indicate that the vertical wells collapse pressure is between 0.85?1.01g/cm~3 in main group of shale gas,and in different horizontal wells along the direction,collapse pressure is between 1.18?1.26g/cm~3.ANSYS finite element software used D-P model to simulate the horizontal well under1844.5m of chang 9 shale gas reservoir.The result shows that the 1.25g/cm~3 density of drilling fluid can effectively control the collapse.