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ρ = density in g/cm 3 t = quart funnel time in seconds For example, a mud of funnel time 40 seconds and density 1.1 g/cm 3 has an effective viscosity of about 16.5 cP. For the range of times of typical muds above, the shear rate in the Marsh funnel is about 2000 s −1. [4]
See mud log for an example of the corrected d-exponent plotted on a mud log. The parameter is an extension ("correction", hence the "c" notation) to the d-exponent method previously used for estimating formation pore pressures. The extension consists of a correction for the mud weight in use, compared to "standard" mud for the region.
In the oil industry, mud weight is the density of the drilling fluid and is normally measured in pounds per gallon (lb/gal) (ppg) or pound cubic feet (pcf) . [1] In the field it is measured using a mud scale or mud balance. Mud can weigh up to 22 or 23 ppg. A gallon of water typically weighs 8.33 pounds (or 7.48 ppg).
The mud in the wellbore must exert enough hydrostatic pressure to equal the formation pore pressure. If the fluid's hydrostatic pressure is less than formation pressure the well can flow. The most common reason for insufficient fluid density is drilling into unexpected abnormally pressured formations.
Density is read at the point where the slider-weight sits on the beam at level. Calibration is done using a liquid of known density (often fresh water) by adjusting the counter weight. Typical balances are not pressurized, but a pressurized mud balance operates in the same manner.
Wentworth grain size chart from United States Geological Survey Open-File Report 2006-1195: Note size typos; 33.1mm is 38.1 & .545mm is .594 Beach cobbles at Nash Point, South Wales Grain size (or particle size ) is the diameter of individual grains of sediment , or the lithified particles in clastic rocks .
Using the figures above, we can calculate the maximum pressure at various depths in an offshore oil well. Saltwater is 0.444 psi/ft (2.5% higher than fresh water but this not general and depends on salt concentration in water) Pore pressure in the rock could be as high as 1.0 psi/ft of depth (19.25 lb/gal)
which depends only on equilibrium state variables like temperature and density (equation of state). In general, the trace of the stress tensor is the sum of thermodynamic pressure contribution and another contribution which is proportional to the divergence of the velocity field. This coefficient of proportionality is called volume viscosity.