pH is a value representing the hydrogen ion concentration in liquid and it is used to indicate acidity or alkalinity of drilling mud. The pH is presented in a numerical value (0-14), which means an inverse measurement of hydrogen concentration in the fluid.
The pH formula is listed below;
pH = -log10[H]
Where: H is the hydrogen ion concentration in mol.
According to the pH formula, the more hydrogen atoms present, the more acidity of substance is but the pH valve decreases. Generally speaking, a pH of 7 means neutral. Fluids with a pH above 7 are considered as being alkaline. On the other hand, the fluids with pH below 7 are defined as being acidic.
In the drilling mud, there are three main chemical components involved in Alkalinity of drilling fluid, which are bicarbonate ions (HCO3–), hydroxyl ions (OH–), and carbonate ions (CO3-2). The Alkalinity means ions that will reduce the acidity.
In order to get accurate measurements for the pH, using a pH meter instead of using a pH paper is recommended because it will give more accurate pH figures. Additionally, pH meters must be calibrated frequently.
Figure 1 – pH meter
Figure 2 – pH paper
Cause of a decrease in pH of drilling Mud
There are many factors that can cause reduction in pH of drilling mud as listed below;
Water contamination – The water contamination or water influx will decrease pH in the drilling fluid because water is neutral.
Carbonate or Bicarbonate – With these two chemical molecules, the pH of drilling mud will be reduced.
Acid gases as Carbon dioxide (CO2) and Sulphur Dioxide (H2S) – If there are acid gases mixing in the system, the pH will decrease. Additionally, acid gas has bad effect on mud properties. The rheology of the mud as PV, YP, and gel strength will increase, but Pm and Pf will decrease due to loss of pH.
Anhydrite – Chemical components of Anhydrite will neutralize hydrogen ions in the mud so pH of drilling mud will drop. It is important to increase mud pH while drilling Anhydrite formation.
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