Horizontal Directional Drilling permits the creation of tunnels which pass beneath rivers and canals to allow the passing of services from one side to the other. The final stage of this process involves lining the tunnel with a plastic or steel pipe. The pipe is pulled inside into the borehole using the same drilling rig that was used to bore the tunnel. For the case of a plastic High Density Polyethylene (HDPE) pipe, issues of buoyancy may arise during this pullback process since the tunnel is pre-filled with drilling mud which now primarily acts as a lubricant. This presents a problem since with this buoyancy, the pipe is lifted up to brush against the tunnel wall, creating issues with the pullback process because of the added pulling resistance. There is the possibility of cutting slots in the pipe wall at the front end of the pipe in order to allow in drilling mud so as to ballast and offset this buoyancy. The drilling fluid is a suspension of bentonite in water and is characterised as a non-Newtonian shear-thinning Herschel-Bulkley fluid, which possesses a finite yield stress. This study aimed to find whether the current slot proportions used for a given borehole diameter, pipe diameter and pipe thickness are sufficient for allowing in drilling mud to ballast the pipe. This was a case where the multiphase flow given by the interaction of air and drilling mud can be simulated using CFD. OpenFOAM is used for this purpose to first simulate the current practice. The multiphase solver interFoam together with the non-Newtonian Herschel-Bulkley and air model was validated for a series of cases before the main simulations were run. These validations included the Marsh Funnel test and the Slump test. These are two kinds of workability tests used for cement pastes and drilling muds. Cement pastes and drilling muds are characterized as three-parameter Herschel-Bulkley fluids and the physical setups of the Marsh Funnel and Slump tests were replicated in computational space. The flow time of theMarsh funnel test and the slump diameter from the CFD simulations were compared with experimental data from literature thus validating the model in OpenFOAM. The main simulation setup recreates the situation of an HDPE pipe concentric with the borehole, with the axis of the domain at an angle to the horizontal. There is drilling mud above the slot at time t = 0s. The drilling mud comes into the domain from the annulus from below the slot. The simulations showed that the flow into the slot initially came in from both the drilling mud above the slot and from the inlet. After the volume of drilling mud above the slot is almost drained completely through into the inner pipe, the mud level inside the pipe starts to become comparable that outside the pipe and both interfaces rise upwards at approximately the same pace. Subsequently, certain parameters were changed from the first benchmark case in order to see what is the effect of these individual variables. The effects of a lower drilling mud yield stress, a longer pipe slot, lower drilling mud density, a different slot aperture shape, increased flow and a steeper angle of pipe penetration were all tested in the simulation campaign. The idea is that with better and faster filling, less pipe buoyancy results. An extension of this idea is that the difference between mud levels inside and outside the pipe should be kept to a minimum. That is why the aim is to increase the flow rate through the slot in the pipe. The results show that increasing the slot length by 30% from the current practice increases the throughput of drilling mud by 10%. The results also show that an elliptical slot profile has a neutral effect and decreasing the yield stress of the drilling mud has a slightly beneficial effect.