In oil and gas industry, a specialized drilling fluid known as high-vis sweep is often used and its primary purpose is to optimize hole cleaning and remove cuttings from the wellbore. What distinguishes this fluid is its heightened viscosity, a quality achieved by incorporating polymers or other additives into the base drilling fluid. This augmented viscosity empowers the sweep fluid to transport cuttings up the annulus out of the well.
Water phase salinity is a factor showing the activity level of salt in oil based mud. In order to control the water phase salinity, salt is added into the drilling fluid. The salt added into the system will be dissolved by water in the mud; therefore, the chloride content will increase.
By increasing the chloride concentration (adding salt), the activity level in the mud will decrease. Salt is added in order to create an activity level which is equal to or less than formation water. Therefore, the water phase in the mud will not move into formation and cause a clay swelling issue. Practically, calcium chloride (CaCl2) or sodium chloride (NaCl) is the chemical to be used.
For most of oil based mud, lime (Ca(OH)2) is used in the system in order to perform a chemical reaction with fatty acid emulsifiers. Typically, 3 to 5 lb/bbl of lime is added in the drilling mud so that there is enough hydroxide (OH-1) ions to keep the emulsion stability in good shape.
Moreover, lime (Ca(OH)2) will control acid gases such as H2S and CO2. The following chemical equations demonstrate how lime reacts with H2S and CO2, respectively.
Ca(OH)2 + H2S -> CaS + 2(H2O)
Ca(OH)2 + CO2 -> CaCO3 + H2O
If drilling into zones where CO2 or H2S exists, the amount of excess lime should be increased to around 5 to 10 lb./bbl., because some lime is used for emulsion stability and others react with acid gases to maintain the mud’s properties.
What’s more, when the well is drilled into formations containing H2S, the excess lime must be kept constantly at 5 to 10 lb/bbl all the time. Do not try to reduce the amount of excess lime because the chemical reaction between H2S and lime is reversible. Therefore, if the level of excess lime is not maintained, the H2S gas can be released at the surface from the reversible chemical process.
Andy Philips, 2012. So You Want to be a Mud Engineer: An Introduction to Drilling Fluids Technology. Edition. CreateSpace Independent Publishing Platform.
Ryen Caenn, 2011. Composition and Properties of Drilling and Completion Fluids, Sixth Edition. 6 Edition. Gulf Professional Publishing.
The Electrical Stability (ES) is one of the vital properties for oil based mud. It shows the voltage of the current flowing in the mud. The ES number represents the mud emulsion stability. The more ES is; the more the emulsion stability is.
Oil based fluid is a non-conductive material. Therefore, the base fluid will not transfer any current. Only the water phase in the mud will conduct the electricity. If the mud has good emulsion, you will have high ES figures. On the other hand, if the emulsion of the mud is bad, you will have low ES value.
The Electrical Stability (ES) is obtained from an electrical stability tester kit (see the image below).
Electrical Stability Meter
“Total Hardness” or “Water Hardness” is a measurement of calcium (Ca2+) and magnesium (Mg 2+) ions in water base mud. The total of both soluble ions of calcium (Ca2+) and magnesium (Mg 2+) is given by titration with standard Vesenate solution.
What will be happening if you have a lot of total hardness in your drilling mud?
- Bad mud cake (thick and mushy)
- High fluid loss
- Flocculation of clay content
- Less polymer effectiveness
- Ineffective chemical treatment
For most of the water base mud, the acceptable value of total hardness must be below 300 mg/L. If the lime drilling mud is used, it is acceptable to have a higher valu,e but is should be kept below 400 mg/L. Continue reading