Oil-base mud

Oil-Based Muds. Oil-based drilling fluids have diesel or mineral oil (11) as a continuous phase with both internal water and soHd phases. 

The specifications for drilling fluid hematite have been set by the API and are Hsted in Table 2 (24). Hematite is used most frequently in high density oil-based muds to minimise the total volume percent soflds (26). The abrasivity of hematite limits its utiUty in water-based muds. 

Testing of Drilling Fluids 652. Composition and Applications 664. Oil-Based Mud Systems 675. Environmental Aspects 682. Typical Calculations in Mud Engineering 687. Solids Control 691. Mud-Related Hole Problems 695. Completion and Workover Fluids 701. 

Oil-Base Muds. Oil-base muds contain oil as the continuous phase and water as the dispersed phase. Oil-base muds contain less than 5% (by volume) water, while oil-base emulsion muds (invert emulsions) have more than 5% water in mud. Oil-base muds are usually a mixture of diesel fuel and asphalt the filtrate is oil. 

Proper control of the properties of drilling mud is very important for their preparation and maintenance. Although oil-base muds are substantially different from water-base muds, several basic tests (such as specific weight, API funnel viscosity, API filtration, and retort analysis) are run in the same way. The test interpretations, however, are somewhat different. In addition, oil-base muds have several unique properties, such as temperature sensitivity, emulsion stability, aniline point, and oil coating-water wettability that require other tests. Therefore, testing of water and oil-base muds will be considered separately. 

Plastic Viscosity and Yield Point. Plastic viscosity and yield point measurements are obtained from a direct indicating viscometer. Due to the temperature effect on the flow properties of oil-base mud, the testing procedure is modified. The mud sample in the container is placed into a cup heater [23]. The heated viscometer cup provides flow property data under atmospheric pressure and bottomhole temperature. 

Gel Strength. The gel strength of oil-base muds is measured with a direct indicating viscometer exactly like that of water-base muds. 

Filtration. The API filtration test for oil-base muds usually gives an all-oil filtrate. The test may not indicate downhole filtration, especially in viscous oils. The alternative high-temperature-high-pressure (HT-HP) filtration test will generally indicate a pending mud problem by amount of fluid loss or water in the filtrate. 

To approximately 20 ml of a 1 1 mixture of toluene (xylene) isopropyl alcohol, add 1 ml of oil-base mud and 75 to 100 ml of distilled water. Add 8 to 10 drops of phenolphthalein indicator solution and stir vigorously with a stirring rod (the use of a Hamilton Beach mixer is suggested). Titrate slowly with H SO, (N/10) until red (or pink) color disappears permanently from the mixture. Report the alkalinity as the number of ml of H SO (N/10) per ml of mud. Lime content may be calculated as... 

Calcium Chloride [25]. Calcium chloride estimation is based on calcium titration. To 20 ml of 1 1 mixture of toluene (xylene) isopropyl alcohol, add a 1-ml (or 0.1-ml, if calcium is high) sample of oil-base mud, while stirring. Dilute the mixture with 75 to 100 ml of distilled water. Add 2 ml of hardness buffer solution and 10 to 15 drops of hardness indicator solution. Titrate mixture with standard versenate solution until the color changes from wine-red to blue. If common standard versenate solution (1 ml = 20 g calcium ions) is used, then... 

Total Salinity. The salinity control of oil-base mud is very important for stabilizing water-sensitive shales and clays. Depending upon the ionic concentration of the shale waters and of the mud water phase, an osmotic flow of pure water from the weaker salt concentration (in shale) to the stronger salt concentration (in mud) will occur. This may cause a dehydration of the shale and, consequently, affect its stabilization. 

A standard procedure for estimating the salt content of oil-base muds consists of the following steps [26] ... 

Example. Find the total salinity of the oil-base mud using the test data below and Figures 4-108 and 4-109. 

Water Wetting Solids. The water wetting solids test (oil-base mud coating test) indicates the severity of water wetting solids in oil-base mud [24]. The items needed are... 

Oil-base muds are composed of oil as the continuous phase, water as the dispersed phase, emulsifiers, wetting agents, and gellants. There are other chemicals used for oil-base mud treatment such as degellants, filtrate reducers, weighting agents, etc. 

The oil for an oil-base mud can be diesel oil, kerosene, fuel oil, selected crude oil, or mineral oil. There are several requirements for the oil (1) API gravity = 36° - 37°. (2) flash point = 180°F or above, (3) fire point = 200°F or above, and (4) aniline point = 140°F or above. Emulsifiers are more important in oil-base mud than in water-base mud because contamination on the drilling rig is very likely, and it is very detrimental to oil mud. Thinners, on the other hand, are far more important in water-base mud than in oil-base mud oil is dielectric, so there are no interparticle electric forces to be nullified. 

The water phase of oil-base mud can be freshwater, or various solutions of calcium chloride (CaCl ) or sodium chloride (NaCl). The concentration and composition of the water phase in oil-base mud determines its ability to solve the hydratable shale problem. Oil-base muds containing freshwater are very effective in most water-sensitive shales. The external phase of oil-base mud is oil and does not allow the water to contact the formation the shales are thereby prevented from becoming water wet and dispersing into the mud or caving into the hole. 

Oil-base muds are expensive and should be used when conditions justify their application. It is more economic to use oil base mud. 

The general practice is to deliver the oil-base mud ready mixed to the rig, although some oil-base muds can be prepared at the rig. In the latter case, the most important principles are (1) to ensure that ample energy in the form of shear is applied to the fluid, and (2) to strictly follow a definite order of mixing. The following mixing procedure is recommended ... 

When using an oil-base mud, certain rig equipment should be provided to control drilled solids in the mud and to reduce the loss of mud at the surfaces, i.e.,... 

Oil-base mud maintenance involves close monitoring of the mud properties along with the mud temperature, as well as the chemical treatment (in which the order of additions must be strictly followed). The following general guidelines should be considered ... 

Figure 4-113. Approximate rheology of two oil-based mud systems. (VERTOIL = invert emulsion of oil and CaClj brine OILFAZE = invert emulsion of oil and freshwater)...

There are several significant permit conditions. As with all other OCS permits, the discharge of oil-based muds is prohibited. Similarly, the permit does not unconditionally authorize the discharge of any of the eight generic muds. Their discharge is subject to limitations on additives. To monitor the use of mud additives, the permit requires the additive not to drop or to decrease the 96-hr median lethal concentration (LC ) test below 7,400 ppm on the basis of the suspended particulate phase or 740 ppm for the whole mud. This parameter is based on a test of Generic Mud 8, which is formulated with 5% mineral oil. 

The chemical treatment methods reduce dispersability property, of drilling fluids through the increase of size of cuttings which improves separation and prevents the buildup of colloidal solids in the mud. These methods include ionic inhibition, cuttings encapsulation, oil phase inhibition (with oil-base muds), and flocculation. The mechanical solids removal methods are based on the principles presented in Table 4-55. 

SANDS Low pressure. Water or mud blocking. Loss of crude or diesel oil used as completion fluid. Minimum filtration rate water-base muds. Minimum filtration rate water-base emulsions. Miminum filtration rate oU-base emulsions. Oil-base muds. Inhibited muds. Minimum weight muds. Crude oil or diesel oil. Add oil-soluble lost circulation material. 

Velocity and Attenuation of the Pressure Waves. The velocity and attenuation of the mud pulses or waves have been studied theoretically and experimentally. The velocity depends on the mud weight, mud compressibility, and on the drillpipe characteristics, and varies from 4920 ft/s for a light water-base mud to 3,940 ft/s for a heavy water-base mud. An oil-base mud velocity will vary from 3,940 ft/s for a light mud to 3,280 ft/s for a heavy mud. 

The attenuation of the pressure waves increases with depth and with the mud pressure wave velocity. More attenuation is observed with oil-base muds, which are mostly used in deep or very deep holes, and can be calculated with the mud and pipe characteristics [108] according to the equations... 

Figure 4-252. Wave amplitude variation as a function of distance in water-base mud and in oil-base mud (a) mud weight, 9 Ib/gal (b) mud weight, 17.9 Ib/galt. (Courtesy Petroleum Erigmeer International [108]. ...

We have built a fluidic puiser system that can generate approximately 100 psi peak to peak with 500 gal/min mud flowrate. It is to be used down to 15,000 ft. The surface detector needs a 5-psi peak to peak sine wave for proper phase detection. The following oil-base mud is used ... 

Note that from Table 4-128 the very large volumes that can dissolve in oil-base muds. For the water-base muds, 0.6 to 0.9% of gas will dissolve and not appreciably change the density or compressibility of the mud. It will be difficult to detect these low concentrations with downhole physical measurements. Free gas will be easily detected as shown hereafter. For the oil-base muds we will assume no free gas is present at bottomhole and the mud properties are changed only due to the dissolved gas. The detection will be more difficult than with free gas. 

For oil-base muds Equation 4-197 can be applied, but K and p must be calculated for an average natural gas using tables or the corresponding algorithms. 

Table 4-128 shows maximum dissolved gas concentrations in drilling muds at the bottom of the hole. Figure 4-264 shows the variation of the acoustic velocity for two water-base muds and two oil-base muds of 9 and 18 Ib/gal at pressures of 5,000 and 10,000 psi. 

The oil-base muds having no free gas behave differently and the 500-ft/s threshold is not reached before approximately 5% of gas is dissolved. Then the velocity decrease is almost as fast as with the water-base mud. 

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