STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

676 Drilling and Well Completions
only decreases the degree of seventy. If the water phase of the oil mud is a solution
of CaCl, (10 to 15 lb/bbl), dehydration of the wet (20 to 30% water) gumbo shale
occurs; the shale becomes harder and it acts like a common water sensitive shale.
The general practice is to deliver the oil-base mud ready mixed to the rig,
although some oil-base muds can be prepared at the rig. In the latter case, the
most important principles are (1) to ensure that ample energy in the form of
shear is applied to the fluid, and (2) to strictly follow a definite order of mixing.
The following mixing procedure is recommended:
a. Pump the required amount of oil into the tank.
b. Add the calculated amounts of emulsifiers and wetting agent, stir, agitate,
and shear these components until adequate dispersion is obtained.
c. Mix in all of the water, or the CaCl, water solution that has been premixed
in the other mud tank. This requires shear energy. Add water slowly
through the submerged guns; operation of a +-in. gun nozzle at 500 psi
is considered satisfactory. After emulsifying all the water into the mud, the
system should have a smooth, glossy, and shiny appearance. On close
examination, there should be no visible droplets of water.
d. Add all the other oil-base mud products specified.
e. Add the weighting material last; make sure that there are no water additions
while mixing in the weighting material.
When using an oil-base mud, certain rig equipment should be provided to control
drilled solids in the mud and to reduce the loss of mud at the surfaces, i.e.,
a. Kelly valve-a valve installed between the kelly and the drill pipe will save
about one barrel per connection.
b. Mud box-to prevent loss of mud while pulling wet string on trips and
connections; it should have a drain to the flow line.
c. Wiper rubber-to keep the surface of the pipe dry and save mud.
Oil-base mud maintenance involves close monitoring of the mud properties
along with the mud temperature, as well as the chemical treatment (in which
the order of additions must be strictly followed). The following general guidelines
should be considered:
a. The mud weight of an oil mud can be controlled within the interval from
7 lb/gal (aerated) to 22 lb/gal. A mud weight up to 10.5 lb/gal can be
achieved with sodium chloride or with calcium chloride. For densities above
10.5 lb/gal, barite or ground limestone can be used. Limestone can weigh
mud up to 14 lb/gal; it is used when an acid soluble solids fraction is
desired, such as in drill-in fluids or in completion/workover fluids. Also,
iron carbonate may be used to obtain weights up to 19.0 lb/gal when acid
solubility is necessary.
b. Mud rheology of oil-base mud is strongly affected by temperature. API
procedure recommends that the mud temperature be reported along with
the funnel viscosity. The general rule for maintenance of the rheological
properties of oil-base muds is that the API funnel viscosity, the plastic
viscosity, and the yield point should be maintained in a range similar to
that of comparable weight water muds. Estimated properties of two oil mud
systems are shown in Figure 4113 and Table 450. Excessive mud viscosity
can be reduced by dilution with a diesel oil-emulsifier mixture that has been