Mud Conditioning and Field Testing

I- Mud Conditioning Equipment

Drilling fluids carry from the borehole cuttings, mud solids and sometimes hydrocarbons which have to be removed from the mud before recirculate it into the well. Beside this issue, the mud has to be treated with chemicals to preserve its required properties.

Shale shakers

When fluids return from the well bore, they pass over shakers which contain vibrating screens. The mesh of the screen is designed to allow mud to return mud tanks. The cuttings are damped after moving to the edge of the screen.
Fig 01- Shale Shaker 

Sand Trap (Settling Pit)

In this tank, the particles which have passed through the shale shakers screens will segregate out and settle by gravity toward a clean-out valve. The pit is cleaned time to time by opening this valve in order to dump the solids.

Desander, desilter and centrifuge

The desander and desilter are used to separate solids in hydro-clone where the fluid rotates and solids are forced by the centrifugal force to be separated. The design of the hydro-clone causes a whirling motion to the mud. The separation of the solids is achieved after getting sufficient centrifugal force.

The centrifuge is more efficient than the hydro-clone system and it is used for preserving mud products and keep them in mud system. They are cone shaped and rotate at high rate of speed. 
Fig 02- Desander 

Degasser

Degassers are used to keep mud free of gas. The gas comes from the formation and can affect mud properties leading to difficult situation as well control situations. There are two types of degassers: 

- Mud gas separators: they are vertical vessel designed to free gas from the upper part and discharge gas-free mud from the bottom part. This degasser is used when shutting the well in and circulating drilling fluids through choke manifold.

- Vacuum degasser: it is mounted on a mud tank and it is used to separate entrained gas, which is determined as foam on surface of mud. The mud enters from the top of this horizontal barrel and flows on an inclined plane. The free gas mud flows toward a feed pipe to come back to the mud circuit and the gas is released when mud streams on the inclined plane and is removed from the degasser by a vacuum pump.
Fig 03- Vaccum Degasser

II- Drilling Fluid Testing:

The tests of drilling fluid are categorized into two categories: 

- Physical tests
- Chemical tests

Density (Mud Weight)

The drilling mud weight is measured with the mud balance. This balance is consisted of beam which has a mud cup on one end and a fixed counter weight on the other end. The beam is graduated scale. The mud balance is tested with water and has to give an indication of 8.34 ppg. During operations, the drilling fluid mud weight is checked by filling the mud cup and moving a rider until getting balanced beam. The mud weight is read on the position of rider.
Fig 04- Mud Balance 

The rheology

Knowing the rheological properties of the drilling fluid is very important to maintain optimum properties. The measurements also help to treat mud accurately to avoid drilling problems. 

There are four values to represent the rheology:
- The viscosity
- The plastic viscosity and the yield point
- Gel strength

Testing viscosity with the marsh funnel

There is a routine test which is performed on the field to measure drilling mud viscosity with a marsh funnel. The value obtained by this device is called the apparent viscosity. The flow through the funnel is timed and the calibration of the funnel is done by the fresh water which has to indicate 26 second with a tolerance of 1/2 a second. This test gives quantitative values of mud properties.

Fig 05- Marsh Funnel 

Testing viscosity with multi speed viscometer

The viscometer can quantify the rheological mud properties. The assessment is performed by shearing a mud sample at a series of speed rates and then measuring the shear stresses at these different rates.

The range of speeds is: 600 rpm, 300rpm, 200rpm, 100 rpm, 6rpm and 3rpm. The readings are taken for 600 and waiting till getting a steady reading then this value is recorded. The same procedure is applied for the rest of rates. These reading are used to determine the plastic viscosity and the yield point.

Plastic viscosity (PV) = R600-R300 (centipoise)

Yield Point (YP) = R300-PV (lb/100 ft²)
Fig 06- Viscometer 

Gel strength

The gel strength of drilling fluids gives two indications about mud property. It gives an indication about the required pressure to initiate flow after ceasing the circulation; and provides an indication about the ability of the mud to suspend cuttings and solids when drilling fluids are stationary.


The gel strength is measured using the viscometer. The measures are taken for 10 second and 10 minutes. The mud is remained static for 10 seconds, and then the rate is started to 3 rpm. The indication is called 10 second gel. The same procedure is performed for mud maintained 10 minutes to get the 10 minutes gel.

The filtration

The filter cake of drilling fluid can be measured with a filter press. On the filter press two indications can be got:

- The efficiency of solids to create an impermeable filter cake. 
- The thickness of the filter cake created in the hole.

The test is performed at room temperature and pressure of 100 psi but it can be also applied for 500 psi and 300 degree F. the pressure is applied for 30 minutes. The volume of filtrate is measured in cm3 and the thickness of the filter cake is evaluated in 1/32"s of inch.

It has to be mentioned that the test is performed in static condition which does not represent the down hole conditions where the filtration occurs in dynamic conditions. 
Fig 07- Filter Press 

Liquid and solid content

The stuck pipe can be avoided by minimizing the proportion of solids in mud. The volume of solids to liquids should not exceed 10%. The test is performed by a retort where the liquids (oil and water) are evaporated, and then the evaporated liquids are condensed in a measuring glass. The volume is quantified and rest of volume represents the solids volume.
Fig 08- Retort 

Hydrogen ion determination (PH)

The PH of drilling fluids should be well controlled because it can affect the chemical reactions in the mud. The test is based on measuring the hydrogen ions. The test is performed by chemically treated paper called the PHydrion paper. The colour appearing on the paper when immersing it in the mud indicates the PH.

The PH can be also measured by the PH meter. The probe of this instrument is placed in sample of mud; the reading is taken after the stabilization of the needle.

Chloride content

The amount of chloride content is an indication about salt contamination which comes from formation. The test is based on measuring the salt quantity in sample of mud. The phenolphthalein is added to the sample and titrated with acid till getting color change; 20 to 25 ml of distilled water and solution of potassium is then added. The solution is stirred while adding silver nitrate continuously drop by drop till reaching color changes. The chloride content is calculated as follows:

Cl content (ppm)= ml of silver nitrate x 1000/ ml of filtrate sample 

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