Cleaning the Drilling Fluid

The purpose of a drilling fluid cleaning system is to remove the suspending solids (drill cuttings) entrained in the mud. High solids or sand content increases the fluid density, which leads to the following problems:

• High fluid density causes pressure in the formation of the borehole. This pressure drives the drilling fluid through the filter cake into the formation, leads to excessive drilling fluid loss to the formation, and extends well development time required to remove the mud from the formation.


• As the fluid density increases, the pressure required to move the fluid up the borehole also increases, leading to high mud pump pressure requirements.


• High solids or sand content also leads to significant abrasion in the drill tooling as the fine particles are re-circulating through the mud pump and drill string. Washed out drill strings and mud pump valves/seats, along with leaking swivel packing, are caused by the recirculation of sand through the system.


• If the gravel pack is emplaced in the annulus through drilling fluid with a high sand content, the fines will be entrained in the gravel pack leading to increased well development costs and reduced well yields.

Drilling fluid in a typical direct mud rotary drilling operation is directed through the following path:

1. Clean fluid is pumped from the mud pump into a flow line to the drill rig.


2. The drill mud travels down the inside of the drill pipe to the bit.


3. As the fluid exits the bit nozzles, heat and drill cuttings caused by friction, are carried away from the bit face.


4. The cutting’s laden fluid travels up the annulus between the drill pipe and the borehole wall.


5. The fluid is typically contained at the ground’s surface within an above ground pit at the drill rig.


6. A transfer pump moves the fluid to the cleaning unit.


7. The fluid enters the fluid cleaning system at the “possum belly” and flows across the first linear motion shaker called the scalping shaker. This “first cut” removes the large cuttings from the mud.


8. The fluid falls through the scalping shaker into a pit where some settling occurs.


9. Another pump drives the partially cleaned fluid through a set of hydro cyclones, which removes sand and silt particles.


10. The hydro cyclone discharge is directed onto a second linear motions shaker with small mesh size screens (140-200), where the sand size particles are removed from the drilling fluid.


11. The cleaned mud is then returned to the mud pump and the cycle is repeated

Linear motion shale shakers employ the latest in technology by allowing a finer screen on the shaker. This results in more solids removed from the mud and a drier solids discharge from the unit.