The accumulation of methane in coal seams and surrounding geological structures as well as underground coal mines has been the major contribution to gas outbursts and mine explosions. Drainage of Coal Seam Gas (CSG) prior to mining using Surface to In-seam (SIS) and Underground In-seam (UIS) boreholes is crucial to reducing the potential risk to the safety and productivity of underground mining operations. Many researches have been carried out to identify the factors affecting the gas drainage performance such as coal properties, gas content and drainage borehole geometries. Two different flow conditions determine the gas drainage efficiency: borehole flow with injection from wall and reservoir flow in a porous medium. These two different types of flow have previously been studied separately. However simultaneous flow of gas through reservoir and borehole requires further investigation. In this research, a three dimensional model for simulation of integrated reservoir-borehole flow is developed to study the significant effect of borehole geometry on flow characteristics of coal seams. Computational Fluid Dynamics (CFD) simulations were carried out using finite volume based software ANSYS Fluent. Four different borehole diameters of 7.5, 10, 12.5 and 15 cm as well as three different lengths of 50, 100, and 150 m were chosen to accomplish the parametric study of borehole geometry. It is assumed that the boreholes are in a steady state condition for two different single phase scenarios of liquid flow (water) and gas flow (methane). The CFD simulations are validated with previous pressure drop models for internal single phase gas and liquid flow. The obtained results reveal that increasing the borehole diameter leads to reduction in fluid pressure throughout the coal seam. On the effect of borehole length it is seen that at a specific distance from borehole outlet, the pressure distribution is independent of the borehole length and upstream effects.