Ever since the late 80’s and 90’s there has been extensive research by most of the major operators such as, Exxon, Shell, BP, Total and others, to try and understand the cuttings transportation mechanism taking place in highly inclined and horizontal wells. It was seen, through practical experimentation, that cuttings beds were forming and taking up to 50% of the annular space when “conventional” drilling parameters were used. Hole cleaning effectiveness is a function of; Hole angle, Mud rheology, Hole size, Drillpipe size, drillstring RPM, Mud density, Mud annular velocity, Annular eccentricity, Cuttings size and ROP.
This is a very extensive and variable set of parameters, however, there are only a few that have the most pronounced effect and need to be optimised to ensure effective hole cleaning. These are Mud Annular Velocity and Mud Density. Mud weight has a high influence as gravity is the major reason for cuttings to settle and reducing the difference between the cuttings density and fluid density can reduce, the speed at which cuttings settle, can be reduced. Drillpipe eccentricity in the wellbore and speed of rotation are also very important factors in keeping cuttings in the mud flow.
However, it is Annular Velocity that plays the key role as it is responsible for keeping the cuttings moving and controlling concentration levels. The higher the AV, the more cuttings are removed from the wellbore. The chart below demonstrates how Mud Weight and Annular Velocity are key to cuttings removal.
RPM of the drillstring is essential to lift the cuttings out of the cuttings bed but it is the mud weight that will “float” the cuttings and Annular velocity that will dictate the efficiency of removal from the wellbore.