Wellbore Stability Of Fragmented Coal Evaluation Under The Couple Effect Of Seepage And Stress

Micro-fractures can be commonly founded in fragmented coal layer.The invasion of drilling fluid during drilling will reduce the effective stress of the area surrounded the wellbore,which makes it easier to be damaged.Hence,the wellbore enlargement appears result from the collapse.Relevant laboratory tests are carried out,and numerical model is built based on the principles of solid-liquid coupling effect and damage mechanics,the fluid-structure couple computation to discuss the influence of the invasion on the wellbore stability in fragmented coalbed while drilling.The main task is introduced as bellow:According to the structural characteristics of the fragmented coalbed,the analytical method of wellbore stability is reasonably simplified,and the heterogeneous numerical model is built.Meanwhile,relevant physical input parameters of the model are presented combining the laboratory tests of rock mechanics and in-situ data.Besides,the permeability evolution experiment of coal is conducted,and the segmented coal permeability coefficient evolution equation is established in accordance with the damage mechanics and reasonable assumption.FORTRAN language is compiled to plug the evolution equation into the model,and variation of wellbore seepage field and damage field with open time are depicted.The results indicate that when the borehole is permeable,the variation of wellbore damage degree is not obvious with the enhancement of the mud density.Yet,after the drilling fluid is blocked off,the damage degree reduces significantly with the increase of the mud density.In addition,the lowest mud density is given to avoid collapse in example.