pH
Drilling muds are always treated to be alkaline (i.e., a pH > 7). The pH will affect viscosity, bentonite is least affected if the pH is in the range of 7 to 9.5. Above this, the viscosity will increase and may give viscosities that are out of proportion for good drilling properties. For minimizing shale problems, a pH of 8.5 to 9.5 appears to give the best hole stability and control over mud properties. A high pH (10+) appears to cause shale problems.
The corrosion of metal is increased if it comes into contact with an acidic fluid. From this point of view, the higher pH would be desirable to protect pipe and casing.
Carbon Dioxide corrosion can cause severe pitting and cracks in fatigue areas. If moisture is present, CO2 dissolves and forms carbonic acid.
CO2 + H2O = H2CO3
This causes a reduction in the pH, which makes the water more corrosive to steel.
Fe + H2CO3 = FeCO3 (iron carbonate scale)
If a high pH is maintained, the water will tend to be less corrosive.
Standard treatments for CO2 are:
1. Kill the source of CO2 (if it is a kick, then circulate out the gas through the degasser).
2. Re-establish proper alkalinity and pH by additions of lime and/ or caustic soda.
While a high pH will combat corrosion, it may be necessary to add chemicals to remove the scale as well.
H2S as a gas is not particularly corrosive, however if moisture is present it will become corrosive and in the presence of CO2 or O2, it becomes extremely corrosive. Since H2S is soluble in drilling muds, as the pH increases, the total amount of sulfides existing as H2S is reduced. The pH should be maintained above 10 if known H2S bearing formations are to be drilled. A scavenger should also be added to remove sulfides. The most common scavengers are zinc carbonate, zinc chromate, zinc oxide, ironite sponge (Fe304) and copper carbonate. The pH will have to be treated as scavengers are added.
pH is commonly measured with pHydrion paper. This paper is impregnated with dyes that render a color which is pH dependent. The paper is placed on the surface of the mud which wets the paper. When the color has stabilized, it is compared with a color chart. An electronic pH meter may also be used.
