Temperature affects mud density and viscosity

The graph shows the huge influence of temperature on the drilling fluid.  Mud is somewhat compressible, so at depth, the effect of pressure is to increase its density.  In addition, the following are other influencers that must be considered in order to accurately predict a dynamic mud density profile:

Highest influence:

• Full thermal history of formation and temperature of cement, casing and mud pits

• Mass flowrate has a large influence on temperatures, hence directly affecting the mud density

• Actual (not planned) hole sizes and wellpath

• Annular area, flowrate and friction that together act to increase the annular pressure – mostly in the deeper parts of the wellbore

• Thermal effects from offset wells

• Cuttings in suspension that can increase the apparent density of the mud

• Work done across the bit jets and other circulating outlets in the drillstring

Of lesser influence are:

• Additives to reduce friction, control viscosity

• Rugosity of the hole, filter cake

 As can be seen, the predictions of pressure and temperature profiles from simulations are not simple. Even with PWD, pressure and temperature profiles (PTP) between the tool and the surface are not linear.  Poor understanding of the PTP leads to inaccurate cuttings lift predictions, and potential profile pressures outside the limits of the drilling window (frac, pore pressure, hole stability, etc.).

 The best practice is to include as many influencers as practical; the process must be repeated as the hole depth advances.  Changes to the influencers (depth, hole temperature, offset wells, mud pit cooling, mud additions, penetration rate and cuttings ratio, and many more) must be tracked also.

 This requires not only premier up-to-date software, but serious experience level. There are few programs and practicing engineers with these skills and proficiency. This can be remedied through acquisitions and training, both of which are available from Altus Well Experts.