Numerical simulation on the effect of drilling engineering factors on coal fines output in coalbed methane horizontal wells: a case study of Liulin block

The coal fines output is the key factor restricting the development of coalbed methane.Factors affecting coal fines output include static factors and dynamic factors.Drilling engineering is one of the important dynamic factors affecting coal fines output.Taking Liulin coalbed methane block as an example, this study established 2D and 3D models by ANSYS software from the perspective of the wellbore stability, and simulated the force and deformation of the borehole wall under different drilling orientations, drilling fluid density, wellbore size, and wellbore inclination.The influence of factors in drilling engineering on the coal fines production was analyzed for the coalbed methane horizontal well.Results show that borehole size and inclination on coal fines production exert greater effect than borehole orientation and drilling fluid density.When the borehole orientation is along the direction of the maximum horizontal in-situ stress, it demonstrates the biggest difference in the equivalent stress around the borehole, and the most obvious stress concentration.Excessive density drilling fluid will lead to tensile failure of the wellbore.In the process of horizontal well drilling, the larger the radius of the wellbore and the larger the inclination of the well, the easier it is to generate coal fines.In order to reduce the generation of coal fines caused by drilling factors, it is suggested that when drilling horizontal wells in the study area, the borehole orientation should be along the direction of the minimum horizontal in-situ stress, the horizontal section should be drilled with clear water, and the inclination of the horizontal section should be consistent with the inclination of the coal seam.The research results provide a theoretical basis for the control measures of coal fines.