Mud weight of oil muds ranges from 7.5 lb/gal to over 22.0 lb/gal. Downhole density is affected by temperature and pressure more than water base muds. Temperature will decrease the density of oil muds due to expansion and pressure will increase the density due to compression of the oil phase.
Viscosity is affected by temperature and pressure. As the temperature increases, viscosity decreases. Conversely, as the pressure increases, the viscosity increases. The funnel viscosity measurement of an oil mud is greatly affected by temperature. The funnel viscosity of an oil mud is usually used an indicator and is not normally used for treatment purposes. Rheological properties are usually made with a rotational viscometer. The plastic viscosity, yield point and gel strengths measurements (according to the Pseudoplastic Rheology Model) are made with the rheometer. More accurate descriptions of the rheology of the mud are made with the Yield-Power Law Model. Suspension of cuttings and weighting material is monitored with the gel strength (for static settling) and 3 or 6 rpm reading (for dynamic settling). Run the rheology of oil muds at the same temperature for each test. Plastic viscosity is greatly affected by temperature at which mud is normally tested. The higher the temperature the lower the plastic viscosity. Reduce plastic viscosity by solids control or dilute with base oil. Yield point is somewhat affected at temperatures where mud is normally tested but may be greatly affected by temperatures above 350°F. Increase yield point by additions of organophilic clay, oil polymers or water. Decrease with wetting agents or thinners or dilution with base oil. Gel strengths behaves similar to yield point. Increase with organophilic clay, water or rheological modifiers. Decrease with wetting agents or thinners or even dilution with base oil.
Electrical stability (E.S.) is the increase in voltage across a probe until the emulsion breaks and a current is established. The electrical stability will vary with the amount of water - the more water generally the lower the stability. Presence of conductive solids such as hematite and insoluble salt will result in low E.S. readings. New sine wave E.S. meters are more reproducible and reliable. The readings from these meters are about one-half the value of the previous meters. Falling E.S. readings and the presence of water in the filtrate indicate weakening of the emulsion. Emulsifiers and lime additions are usually required.
HPHT filtration is run at bottom hole temperatures under static conditions to determine condition of emulsion, the filtrate volume and filter cake quality. If water is present in the filtrate, this could indicate emulsion weakening. Thick filter cakes and high fluid loss indicate excessive drill solids content. Lower the filtration rate with filtration control agents, emulsifiers and lime. Increase the fluid loss by dilution with base oil.
Salinity determination of calcium and sodium chloride is done on the whole mud. A new method for this test is now established by the API to determine types of salts present and if any salt is insoluble in the mud. Insoluble calcium chloride can cause water wetting problems and should be reduced by adding water or oil mud with no salinity in the water phase. Insoluble sodium chloride can be reduced in the same manner, but it does not cause water wetting of solids.
Lime analysis determines the amount of excess lime in the oil mud. Lime is essential for the formation of the emulsion when using fatty acid type emulsifiers. Lime content should always be checked since emulsifier additions may not be required due to deficiency in the lime content. A decrease in lime content while drilling may indicate acid gases such as H2S or CO2 or high temperature deterioration of products.
Water Activity or relative humidity of the oil mud is determined with a hygrometer. The hygrometer does not determine if any insoluble salt is present.
Oil/Water/Solids ratio in the oil mud is determined with a retort, which is a still that operates at about 650oF. Results need to be accurate, especially for the salinity analysis. Small sources of error in water content can cause large differences in salinity analysis.
Sulfides in the oil mud are measured with the Garrett gas train. A sample of whole mud is used instead of filtrate. Zinc oxide is the preferred compound to treat for soluble sulfides. Increased lime additions are also necessary when H2S is present.
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