FILTRATION, WALL BUILDING, AND RESISTIVITY OF DRLLING FLUID

FILTRATION:

Theory

The loss of liquid from a mud due to filtration is controlled by the filter cake formed of the solid constituents in the drilling fluid. The test in the laboratory consists of measuring the volume of liquid forced through the mud cake into the formation drilled in a 30 minute period under given pressure and temperature using a standard size cell.

It has been found in early work that the volume of fluid lost is roughly proportional to the square root of the time for filtration, i.e. “V ∝ √t”

The two commonly determined filtration rates are the low-pressure, lowtemperature and the high-pressure high-temperature.

Test Equipment

The low pressure test is made using standard cell under the API condition of 100 + 5 psi for 30 minutes at room temperature. Another special cell, will be used to measure filtration rate at elevated temperatures and pressure. Filter press used for filtration tests consists of four independent filter cells mounted on a common frame.

Each cell has its own valve such that any or all the cells could be operational at the same time. Toggle valve on the top of each cell could be operated independently for the supply of air for each individual cell. Special high pressure and high temperature filtration tests are run in the laboratory simulating formation temperature and formation back- pressure.

standard-API-Filter-press
Fig 1. standard API Filter press
High-temperature-High-pressure-filter-press
Fig.2 High temperature High pressure filter press

TEST PROCEDURE FOR FILTRATION RATE AT 100 PSI & ROOM TEMPERATURE

  • Detach the mud cell from filter press frame.
  • Remove bottom of filter cell, place right size filter paper in the bottom of
    the cell.
  • Introduce mud to be tested into cup assembly, putting filter paper and
    screen on top of mud tighten screw clamp.
  • With the air pressure valve closed, clamp the mud cup assembly to the
    frame while holding the filtrate outlet end finger tight.
  • Place a graduated cylinder underneath to collect filtrate.
  • Open air pressure valve and start timing at the same time.
  • Report cc of filtrate collected for specified intervals up to 30 minutes.
  • Tabulate the results in an appropriate table.

WALL BUILD:

MEASUREMENT PROCEDURE FOR MUD CAKE THICKNESS (32nd of an inch or cm)

It should be reported in thirty-second of an inch in whole number. Vernier
caliper could be used to measure the thickness, however, while measuring care should be taken not to press vernier jaw on mud cake to penetrate through.

Results on mud cake thickness should be reported in whole number.

Example: 0.75/32″ should be reported as 1/32.
                    i.e. closest to the whole number.
                    Likewise 1.75/32″ should be reported as 2/32″

MUD RESISTIVITY:

Theory

The resistivity (Wm) of a drilling mud is influenced by the dissolved salts (ppm) or (gpg, grain per gallon) in the water portion and the insoluble solid material contained in the water portion. The greater the concentration of dissolved salts, the lower resistivity of the solution. Unlike metals, the resistivity of a solution decreases as temperature increases. It is necessary to measure resistivity because the mud, mud cake, mud filtrate resistivity exert a strong effect on the electric logs taken in that mud. The mud resistivity varies greatly from the actual resistivity values due to the various factors encountered in the actual operation.

Test Equipment

Equipment used is the Baroid Resistivity Meter (Fig. 3).

Analog digital resistivity meter
Fig.3 Analog digital resistivity meter

1. MEASUREMENT PROCEDURE FOR MUD CAKE RESISTIVITY

  1. Remove excess water from filter cake.
  2. Fill the slot on top of the cell with mud cake.
  3. Press black button and adjust for full scale.
  4. While keeping the black button depressed, press red button.
  5. Reading on the meter is the resistivity of mud cake.
  6. Record the resistivity of the mud cake and the temperature of the
    cell.
  7. Remove the cell and clean thoroughly with distilled water.

2. MEASUREMENT PROCEDURE FOR MUD FILTRATE RESISTIVITY

  1. Fill cell with mud and remove excess mud from the top of the cell
    surfaces).
  2. Connect cell to the resistivity meter.
  3. Press black button and adjust for full scale reading on the meter.
  4. While keeping the black button depressed, press the red button.
  5. The reading on the meter is the resistivity of the mud (when testing
    mud it may be necessary to allow 3-5 minutes after putting mud in
    the cell before taking a reading to allow cell and the mud filtrate to
    reach temperature equilibrium).
  6. Record the resistivity reading and the temperature of the cell.
  7. Remove the cell and clean with distilled water. (Pipe cleaner can
    be used to remove particles of mud filtrate that cannot be flushed
    out with water).

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