Drilling Fluid Mud Rheology


  • Determining the density of mud drilling fluid,

  • Finding the specific gravity of water-based mud

  • Obtaining the hydrostatic pressure gradient for water based mud

The apparatus used in the experiment was OFITE mud balance.

Equipment and materials

  • Mud balance

  • A corrosion-resistant plastic case for holding the balance in a working position


Various types of clay can compose the drilling mud, all clays have a unique property and when they are prepared with water they portray different viscosity, gel strength and the rheological characteristic. It is quite clears that Bentonite and Barite exhibit characteristics that are different to the drilling mud (frank, 2001). Bentonite can be seen to exhibit a low yield of clay and it is not effective in salt water. Barite on the other hand has a high clay yield and it does not exhibit any signs that are significant to swelling. Bentonite undergoes enormous swelling when it is mixed with fresh water (Robello, 2010). This means that Barite is a much better choice when making drilling mud.

Drilling mud is very crucial in the petroleum industry. This is because oil bearing formations contain oil with a certain amount of pressure that depends on the depth and other properties (Joan, 2006). When drilling, the drilling fluid is circulated continuously so that it can provide hydrostatic pressure in the drilling hole column so as to control the pressure exerted by the oil bearing formations. This avoids blow ups for cases of high pressure reservoirs. Note that the hydrostatic pressure is kept sufficiently high than the oil bearing formation pressure to avoid fracture. Note that the hydrostatic pressure is dependent on the density of the drilling mud. This means that the process of determining mud density is very crucial (Elton, 2001).

The Gel strength can be determined at a low shear stress after it has been allowed to thicken for an amount of time typically about ten minutes. The strength of the formed mud cake not only helps to prevent water from going into the wellbore but also the drilling fluid circulating in the wellbore from leaking out into a fracture (Francis, 2002). Viscosity refers to the fluid’s resistance to flow, the mud viscosity determines the efficiency and the ability to lift cuttings out of the well bore. Adding different clay types of clay not only affects the viscosity but also the use of salt water as oppose to plain water.


The mud cup was cleaned and dried; the mud was then placed on the mud balance on a flat level surface. The temperature of the fluid was then measured and recorded on an appropriate mud record form. The lid was then removed from the mud cup and the clean dry cup was then filled to the top with the mud sample to be tested. The lid was the placed on the cup and it was then set in a gentle twisting motion until it was seated firmly. The hole in the lid was then covered with a finger and all mud on the outside cup and arm was washed and then thoroughly dried. The balance was then placed on the knife edge on the fulcrum rest. The rider was then moved along the outside of the arm until the cup and arm were balanced which was achieved when the spirit level bubble was under the center line. The mud weight was then read at the edge of the rider towards the mud cup. This procedure was then repeated for each of the three mud samples.





Mud density Ip/ft3

Mud gradient (psi/ft)

Fresh water





Mud sample-1-5%





Mud sample-2-10%





Mud sample-3-15%





Density =g/cm3 = (lb/gal)/8.345

1 lbs/gal = 0.119826427 g/cm3


For bentonite

Density for fresh water = 8.31×0.119826427 = 0.9958 g/cc

Density for mud sample 1 = 9.01 x 0.119826427 = 1.079 g/cc

Density for mud sample 2 = 9.63 X 0.119826427 = 1.078 g/cc

Density for mud sample 3 =10.46 x 0.119826427 = 1.253 g/cc


For barite

Density for fresh water = 8.31×0.119826427 = 0.9958 g/cc

Density for mud sample 1 = 1.07 x 0.119826427 = 0.1282 g/cc

Density for mud sample 2 = 1.153 X 0.119826427 = 0.1381 g/cc

Density for mud sample 3 =1.251 x 0.119826427 = 0.1499 g/cc


Mud gradient, psi/ft = (lb/gal)/19.24


The results above depict that Bentonite can work well in Fresh Water, although it does swell a few times its own size. This means that Bentonite cannot do well when used with Salty Water. The graph of barite can however be observed to be linear which means it is suitable for use in both fresh and salty water and it doesn’t swell much.


Note that drilling mud is a non-Newtonian fluid and therefore the determination of which model type the fluid follows cannot be done by eye judgment. There must be graph construction and after that linear or power law regression has to be done to determine the rheological character of the drilling mud. For instance a mixture of Bentonite with fresh water results in a Bingham Plastic fluid.


Frank, J. (2001). Spectrometry and Reservoir Characteristics of Marine oil fields. New York publishers.

Robello, S. (2010). Friction factors: torque, drag, vibration, bottom-hole assembly and transient surge/swab analysis. Oxford university press.

Joan, K. (2006). Impact of frictional pressure losses along the completion on well performance. Cambridge university press.

Park, L. (1997). Rheological properties of biopolymers drilling fluids. Paperback publishers.

Francis, M. (2002). Frictional Pressure Loss and Rheological characteristic of Drilling and Stimulation fluid. Oxford university press.

Newton, J. (1994). Effective high-density wellbore cleaning fluids. Paperback publishers.

Elton, H. (2001). Hydraulic optimization of foam drilling for maximum drilling rate in vertical wells. New York publishers.

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