The mud system used most frequently throughout the industry is the water-based system. Water is the continuous phase, but it may contain oil (i.e., emulsion muds) (check also: Oil Based Mud) or air (i.e., aerated mud) as the discontinuous phase. The oil must remain as segregated droplets and not combine in a distinct phase termed “discontinuous” (Fig. 1).
Clear, freshwater is one of the oldest drilling muds used in the industry. It is generally used with no special additives except perhaps a corrosion inhibitor. The freshwater may hydrate formation clays and convert itself into a “native” mud. In fact, the term “mud” is believed to have been first used when fresh water and surface soils were mixed to develop a viscous fluid that could increase the hole cleaning characteristics of the fluid.
Water Based Drilling Mud Chemicals
Freshwater is often the base fluid when adding many drilling chemicals such as clays, polymers, weight materials, and additives to control various properties.
- The clays include sodium and calcium montmorillonite, attapulgite, and subgroups of montmorillonite.
- Polymers cover a broad range of products such as CMC (carboxymethylcellulose) and HEC (hydroxyethylcellulose).
- Weight materials include barite, galena, iron oxides, and hematite.
- Special additives may be used for controlling mud properties such as fluid loss, viscosity, gel strength, and pH. Inhibited Water-Based Fluids.
Water Based Drilling Mud Types
A Inhibited Water Based Drilling Mud
An inhibited water-based mud is often used to minimize hole sloughing problems. “Inhibition” refers to retarding the rate at which formation clays hydrate. Hydration reduces the structural stability of the borehole, allowing it to fall, or slough, into the wellbore. Common inhibited muds such as lime muds use calcium to retard hydration, while others use high concentrations of various salts.
Four common mud systems that can be classified as inhibited muds are
- Gyp muds.
- Lime muds.
- Seawater muds.
- Saturated saltwater muds.
Lime Water Based Drilling Mud
Lime muds have been widely used for many years as inhibited fluids. The hydrated lime, which is calcium hydroxide, reduces the amount of water attached to the clay structure. High lime-content water based drilling mud should contain a Pf of 5-9 and a Pm of 2~O to maintain an excess of lime. The amount of undissolved lime can be estimated with Eq. 1:
Lime = (Pm – Pf )/ 4 Eq.1
- The excess lime is measured in Ib/bbl
- Pm is the phenolphthalein end point of mud
- Pf is the phenolphthalein alkalinity endpoint of filtrate.
Low-lime mud has been used successfully during recent years in high-temperature wells. High lime concentrations tend to cause clay flocculation in the upper-temperature ranges. Low-lime water based drilling mud should contain a Pf of 2-5 and a Pm of 12-18. This mud system must be monitored closely to maintain undissolved lime in the system.
Gyp Water Based Drilling Mud
Gypsum (calcium sulfate) water based drilling mud are used commonly in gyp and anhydrite formations. Gyp muds are similar to lime muds since they derive their inhibition properties from soluble calcium and require a chemical thinner for viscosity control. These muds function at lower alkalinity ranges than lime muds and contain more soluble calcium.
This type of water based drilling mud commonly used in offshore drilling are more economical than freshwater systems in some cases. The convenient source of salt water and the elimination of extra storage space on offshore rigs have made this type of mud very popular. Since the seawater contains salt, NaCl, it inhibits the hydration and dispersion of clays. Alkalinities are extremely important and must be monitored closely for the dispersants and clays to react properly.
Saturated salt muds
Saturated salt mud contain a large amount of dissolved salt and therefore can be classified as inhibited muds. However, saturated salt muds are used usually to drill salt domes or thick stringers of salt. Salt contamination and elimination of the development of large cavities in the salt formation are accomplished by saturating a water-based mud with salt.
B Dispersed Drilling Mud
A dispersed water based drilling mud usually uses drilling chemicals that cause the clay platelets to “disperse,” or separate, within the liquid phase (Fig.2). These dispersed muds tend to have better viscosity control, higher solids tolerance, and better filtration control than nondispersed systems. Dispersed systems are used frequently in drilling young, high-activity clays such as those found in the Gulf of Mexico and Nigeria. A common dispersed system is the lignosulfonate mud.
C Non-dispersed water based drilling Mud.
Nondispersed muds are often associated with low solids concentrations and relative low-density weights. These muds do not contain chemical dispersants and are formulated normally with a minimum amount of bentonite. The systems use a polymer(s) that extends the effects of small amounts of bentonite and selectively flocculates undesirable drilled solids. Proper solids control equipment, alkalinities, and bentonite concentrations must be monitored closely for a low-solids nondispersed mud to function effectively.
D Flocculated Mud Systems.
Flocculated water based drilling mud cause the clay platelets to arrange themselves in an edge-to-face orientation. This arrangement often occurs with the intrusion of some contaminant. Filtration, viscosity, and gel strengths will usually increase dramatically once a mud becomes flocculated. Chemical agents are available that decrease the sensitivity of the edge-to-face attraction of clay particles. These deflocculants restore the clay particles to a dispersed or aggregated state.
Clear water and some brine fluids have been used in drilling operations not requiring high densities and/or high viscosities. This tupe of water based drilling mud have been used extensively in completion and workover operations where very low solids and a clean environment are critical. Brines may range from 8.33 to 19.2 Ib/gal. These brines may be single-, two-, or even three-salt compounds such as the following:
- Potassium chloride
- Sodium chloride
- Calcium chloride/calcium bromide/zinc bromide
Brine makeup water should be mechanically filtered to eliminate undesirable solids contamination. Brine densities are usually measured at 60°F and, if possible, the density should be calculated from specific gravity as measured with a hydrometer. Tables and charts for brine formulations are included in the Appendix.
Single-salt brines are made from freshwater with the addition of a single salt to achieve the desired density. Density is controlled by adding salt or freshwater. Table.1 shows some of the more commonly used salt systems.
Potassium chloride brines
It can be used with densities to 9.7 Ib/gal. Potassium chloride brines are excellent workover fluids for water-sensitive formation The potassium ion is particularly effective at plugging the clay lattice to minimize hydration of formation clays, as shown in fig. 3 & 4.
A sodium chloride system (NaCI)
NaCl system is probably the most common brine water used in drilling, completion, and workover operations due to its availability. The maximum density for a sodium chloride brine is 10.0 Ib/gal at 60 °F. Note that a density of 9.7 Ib/gal is relatively easy to achieve. The range from 9.7 to 10.0 Ib/gal requires special attention.
The Calcium Chloride mixtures are used commonly when densities from 9.7 to 11.6 Ib/gal are required. The CaCl2 fluid is used in many workover and com- pletion operations. The freezing (crystallization) point of an 11.6-lb/gal CaCl2 brine is 44°F, which may cause operating difficulties at low surface temperatures.
Brine systems requiring densities to 15.1 Ib/gal use a two-salt mixture of calcium chloride and calcium bromide (CaCI2and CaBr2). The basic ingredient of this brine is a calcium bromide solution with a density of 14.1 to 14.3 Ib/gal. The density can be increased to 15.1 Ib/gal by adding calcium chloride flakes or pellets. Care must be taken for cold weather formulation by mixing proper amounts of calcium chloride and calcium bromide to maintain the solution above its freezing point during cool weather.
High-pressure drilling, workovers, and completion operations occasionally necessitate using densities to 19.2 Ib/gal. Adding calcium chloride and calcium bromide to a zinc bromide solution can achieve a density of 19.2Ib/gal. Various concentrations of the three salts, in addition to other special products, can be used to formulate summer and winter blends.
Solids Systems (Solids Tolerance).
The solids concentration in the water based drilling mud may be used to describe the system. A clear water mud has virtually no detectable solids. A low-solids system has some solids, although efforts are usually made to minimize solids concentration. A high-solids system is normally used when “mud-making” formations are penetrated or high mud weights requiring barite are used. The solids content of a high-weight mud must be controlled within acceptable ranges.
The degree to which formation solids contaminate a water based drilling mud depends on the characteristics of the continuous phase and the type and amount of clay in the rock cuttings. Excess solids may be controlled by surface mechanical equipment and/or water dilution. Water dilution is the most expensive process for correcting drilled solids accumulation. Fig.5 shows acceptable ranges for mud solids as developed by several mud companies.