MUD WEIGHT, MARSH FUNNEL VISCOSITY AND pH

MUD WEIGHT OR DENSITY TEST:

Theory

The density of the drilling fluid must be controlled to provide adequate hydrostatic head to prevent influx of formation fluids, but not so high as to cause loss of circulation or adversely affect the drilling rate and damaging the formation.
Normal pressure gradient by water is equal to (0.433 psi/ft) and equal to 433 psi/1000 ft.

Test Equipment

The Baroid Mud Balance as shown below is used to determine density of the drilling fluid. The instrument consists of a constant volume cup with a lever arm and rider calibrated to read directly the density of the fluid in ppg (water 8.33), pcf (water 62.4), specific gravity (water = 1.0) and pressure gradient in psi/1000 ft. (water 433 psi/1000 ft.)

typical mud balance
typical mud balance

Calibration

  • Remove the lid from the cup, and completely fill the cup with water.
  • Replace the lid and wipe dry.
  • Replace the balance arm on the base with knife-edge resting on the
    fulcrum.
  • The level vial should be centered when the rider is set on 8.33. If not,
    add to or remove shot from the well in the end of the bream.

Test Procedure

  • Remove the lid from the cup, and completely fill the cup with the mud to
    be tested.
  • Replace the lid and rotate until firmly seated, making sure some mud is
    expelled through the hole in the cup.
  • Wash or wipe the mud from the outside of the cup.
  • Place the balance arm on the base, with the knife-edge resting on the
    fulcrum.
  • Move the rider until the graduated arm is level, as indicated by the level
    vial on the beam
  • At the left-hand edge of the rider, read the density on either side of the
    lever in all desired units without disturbing the rider.
  • Note down mud temperature corresponding to density.

MUD VISCOSITY:

Theory

The viscosity of a fluid is defined as its resistance to flow. The desired viscosity for a particular drilling operation is influenced by several factors, including mud density, hole size, pumping rate, drilling rate, pressure system and requirements, and hold problems. The indicated viscosity as obtained by any instrument is valid only for that rate of shear and will differ to some degree when measured at a different rate of shear. For field measurements the marsh funnel has become the standard instrument. For laboratory, the Fann V-G meter, a direct indicating rotational multi-speed instrument, has become the standard, allowing measurements of plastic viscosity, yield point, gel strength to be made.The Stormer viscometer is still, however, used to some extent for single point (apparent) viscosity and 0-10 min. gel.

Test Equipment

Details of the Marsh Funnel and receiving cup are shown in Figures 1-2, and 1-3. The viscosity is reported in seconds allowed to flow out of the funnel. API specifications call for 1500 ml and one quart (946) ml out. For API water at 70 F + 0.5oF = 26 + 0.5 sec. The Marsh Funnel measures the apparent viscosity.

Marsh-Funnel-and-One-liter-Cup
Marsh-Funnel-and-One-liter-Cup

Calibration

Fill the funnel to the bottom of the screen (1500 ml) with water at 70 F (plus or minus 0.5 F) time of outflow of the quart (946 ml) should be 26 seconds plus or minus 1/2 second.

Test Procedure

  • With the funnel in an upright position, cover the orifice with a finger and pour the freshly collected mud sample through the screen into a clean, dry funnel until the fluid level reaches the bottom of the screen (1500 ml).
  • Immediately remove the finger from the outlet and measure the time required for the mud to fill the receiving vessel to the 1-quart (946 ml) level.
  • Report the result to the nearest second as Marsh Funnel Viscosity at the temperature of the measurement in degrees Fahrenheit or Centigrade.

HYDROGEN ION CONCENTRATION (pH):

Theory

The acidity and the alkalinity of the drilling fluid can be measured by the concentration of the (H+) ion in the fluid. As for instance, if H+ is large (1 x 10-1), then the (OH-) hydroxyl concentration is very low (1 x 10- 13), the solution is strongly acidic. If the (OH-) concentration is (1 x 10- 1) very high then (H+) concentration is very low then the solution is strongly alkaline. The pH of a solution is the logarithm of the reciprocal of the (H+) concentration in grams moles per liter, expresses as:

pH = log{1/H+}=-log[H+]

Example: If the solution is neutral then H+ and OHconcentrations are the same equal to 1 x 10-7.

pH = log{1/ (1×10-7)} = -log(1×10-7) =7.0

Therefore, if the pH of a mixture drops from 7.0 to 6.0, the number of (H+) increase ten times.

The pH of a mud seldom is below 7 and in most cases fall between 8 and 12.5 depending upon the type of mud. The pH is important because the pH affects the solubility of the organic thinners and the dispersion of clays presents in the mud.

Methods of measuring pH in the laboratory:

  1. The pH Paper: The pH paper strips have dyes absorbed into the paper display certain colors in certain pH ranges. It is useful, inexpensive method to determine pH in fresh water muds. The main disadvantage is that high concentrations of salts (10,000 ppm chloride) will alter the color change and cause inaccuracy.
  2. The pH Meter: The pH meter is an electric device utilizing glass electrodes to measure a potential difference and indicate directly by dial reading the pH of the sample. The pH meter is the most accurate method of measuring pH.

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