Mud Weight: Density
||Hydrostatic pressure in the borehole and solids content of unweighted muds
||Drilling rate, hole stability, transportation and settling rate of cuttings Useless solids accumulation slows drilling rate, wastes fuel, causes equipment wear, loss of circulation, differential sticking, and damages the productive formation
||Below 9.0 lb./gal
||BAROID® weighting agent to increase weight Water dilution, shale shakers, desander/desilter cones, and good pit design decrease density
The most significant, yet simple measurement the driller can make is that of mud weight or density. No visual estimate can be made. Density must be measured by weighing a known volume. Density can be stated in any convenient units, such as lb./gal, lb./ft3, g/cm3.
To prevent the flow of formation fluids into the hole, the drilling mud must exert a greater pressure than that of the fluids in the porous rocks that are penetrated by the bit. The pressure exerted by the drilling mud at any depth is related directly to its density.
Hydrostatic pressure, psi = lb./gal x 0.052 x depth (ft)
Loss of circulation may result from excessive pressure due to mud that is too dense or heavy.
With simple water-based muds, density is a reliable measure of the amount of suspended solids.
Solids that do not contribute useful properties (i.e., most drilled solids) are objectionable. Abrasive solids, like sand, cause excessive wear on pumps, drill string and bit. The drilling rate is reduced; a thick filter cake is deposited on permeable formations, and the pump does unnecessary work recirculating solids that have been allowed to collect in the mud.
For the water well driller, a most objectionable effect of useless solids is the formation of a thick filter cake on the water-bearing section. The thick filter cake on the water-bearing formation may not be removed completely and consequently impairs the flow of water. By weighing the mud regularly, the solids content can be estimated so that corrective steps can be taken before damage is done.
Procedure for Using the Baroid Mud Balance
- Fill the cup to capacity with fresh, screened mud.
- Replace lid and rotate until firmly seated, making sure some mud is squeezed out the vent hole. Wipe or wash excess mud from the exterior of the balance, and dry. Then seat the balance with its knife edge on the stand and level it by adjusting the rider.
- Read mud density from the edge of the rider as indicated by marker on the rider. Use any of the four scales to express the mud density as required.
- Calibration can be checked by filling the cup with fresh water. It should read 8.34 lb./gal or 1.0 g/cm3.