An oil base fluid can be defined as a drilling fluid which has oil as its continuous or external phase and the water, if present, is the dispersed or internal phase. The solids in an oil base fluid are oil wet, all additives are oil dispersible and the filtrate of the mud is oil. The water, if present, is emulsified in the oil phase.
Oil Mud Brief
There are two basic classifications of oil-based fluids; invert emulsions and all-oil muds. The amount of water present will describe the type of oil base fluid. The oil used in these types of oil base fluids can range from crude oil, refined oils such as diesel or mineral oils, or the non-petroleum organic fluids that are currently available. The latter type fluids – variously called inert fluids, pseudo oils, nonaqueous fluids and synthetic fluids – are now considered more environmentally acceptable than diesel or mineral oils.
The oil industry needs oil-based mud; it is the most efficient drilling fluid in some applications prone to serious shale sloughing. But the potential environmental impact of invert mud drilling waste is a serious concern. Several other properties of oil muds make them a good choice for many difficult drilling problems.
INVERT MUDS HELP REDUCE SLOUGHING SHALE PROBLEMS
Conventional all-oil muds have oil as the external phase but they are designed to be free of water when formulated or in use. Since water is not present, asphaltic type materials are required to control the fluid loss and viscosity. Since there is no water added to this system during the formulation and water additions are avoided if possible while drilling, there is only a minimum requirement for emulsifiers.
All-oil muds can withstand small quantities of water; however, if the water becomes a contaminating effect, the mud should be converted to an invert emulsion. If the water is not quickly emulsified, the solids in the mud can become water wet and will cause stability problems. The water wet solids will blind the shaker screens and loss of whole mud will occur.
Invert emulsions are oil muds that are formulated to contain moderate to high concentrations of water. Water is an integral part of the invert emulsion and can contain a salt such as calcium or sodium chloride.
An invert emulsion can contain as much as 60% of the liquid phase as water. Special emulsifiers are added to tightly emulsify the water as the internal phase and prevent the water from breaking out and coalescing into larger water droplets. These water droplets, if not tightly emulsified, can water wet the already oil wet solids and seriously affect the emulsion stability.
Special lignite derivatives or asphaltites are used as the fluid loss control agents, and bentonite derivatives are used to increase the viscosity and suspension properties of the system. Invert emulsions are usually tightly emulsified, low fluid loss oil muds.
An improvement in drilling rates has been seen when the fluid loss control of the system is relaxed, thus the name “relaxed” invert emulsion. Also, the relaxed invert emulsions fluids do not use as much emulsifier as the regular invert emulsion systems.
Oil Mud Troubleshooting
Add water and emulsifier, add gellant. If high temperature add polymeric viscosifier. All of these affect the low-shear viscosity, gel strength and yield point more than the plastic viscosity.
Remove low gravity solids with solids control equipment and/or dilution. Increase o/w ratio if water content is too high. Add oil wetting agent to reduce viscosity.
Remove water wet solids and add oil wetting agent and oil. Ensure that there is no insoluble calcium chloride in the mud. Water wet solids will blind screens and give low E.S. readings. Suspected water wet solids added to water will disperse easily.
Water wet solids, undissolved solids, inadequate concentration of emulsifiers, inadequate concentration of lime for emulsifiers, and some weighting agents (such as hematite) generate low electrical stability readings. All except hematite require chemical treatment. Most muds made with mineral oil will have lower electrical stability than those made with diesel. Low viscosity muds usually have low electrical stability readings.
Mud viscosity will increase and electrical stability readings will decrease even though emulsifier concentration is adequate. Improve solids removal efficiency. Use dual centrifuge to remove drill solids while recovering the barite and oil phase.
Add additional emulsifier if water appears in filtrate. Organolignite will also emulsify water and lower filtrate. Ensure mud has excess lime. Newly formulated mud may have high HPHT until properly sheared. Sometimes small amounts of water will lower HPHT in high O/W ratio muds. Organolignites are not effective when bottom hole temperature is less than about 150°F.
Detected in mud by drop in alkalinity. If H2S is detected by the Garrett Gas Train, alkalinity has decreased so increase lime additions. Maintain lime additions and add sulfide scavenger such as Zinc Oxide. If carbon dioxide is present, add lime.
If loss is not complete, use oil-wettable fibrous material or solid bridging material such as calcium carbonate. Use same technique for seepage losses to minimize thick filter cake and differential sticking. If losses are complete, consider organophilic clay squeeze, cement or displacing to water based mud until loss zone is cased off.
After periods of inactivity, free oil may cover the surface of the pits. Agitate the mud in the pits or add organophilic clay to increase viscosity.
Oil Mud Properties
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.
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 650 Fahrenheit. 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.