Drilling Fluid Selection

Over the years a considerable number of drilling fluid formulations have been developed to suit various subsurface conditions. Selection of the best fluid to meet anticipated conditions will minimize well costs and reduce the risk of catastrophes such as stuck drill pipe, loss of circulation, and gas kicks. Consideration must also be given to obtaining adequate formation evaluation and maximum productivity.

The different types of drilling fluids that are available have been summed up by Hutchison and Anderson17 (see Table 1-3) and discussed in detail by Kelly.18 Considerations that affect the choice of muds to meet specific conditions are given below.


In remote locations, the availability of supplies must be considered; for example, in an offshore well there is an obvious advantage in selecting a mud that can tolerate sea water for its aqueous phase.

Government regulations designed to protect the environment restrict the choice of muds in some locations.19 ;u-2122 Such regulations have made the handling of oil base muds difficult and expensive, particularly in offshore wells. But the substitution of low toxicity mineral oils204 25,26 for diesel oil has greatly alleviated this problem, and the regulations of the various governments concerned are being reviewed accordingly.

Methods and equipment for preventing environmental damage by drilling fluids and drill cuttings have been described by several authors.27-28 29 29a Tests for evaluating the toxicity and other relevant properties of mineral oils have been described by Burton and Ford.30

Thawing of the permafrost by the drilling mud is a problem in the Arctic.11 The hole may enlarge up to 8 feet, and special foams are necessary to clean the hole.31 a

Mud-Making Shales

Thick shale sections containing dispersible clays, such as montmorillonite, cause rapid rises in viscosity as drilled solids become incorporated in the mud. High viscosities are no problem with unweighted muds because they can easily be reduced by dilution and light chemical treatment; but with weighted muds dilution is costly because of the barite and chemicals required to restore the mud properties. The drilled solids can, of course, be removed mechanically, but the presence of barite greatly complicates the process. Therefore a mud which inhibits the dispersion of clays, such as a lime, gyp, or CL-CLS (see T&ble 1-3) mud. should be used when drilling thick sections of mud-making shales. The CL-CLS is the most popular of the group because it is easy to maintain and is the most stable at high temperatures, but it is also the most expensive because of the high concentration of CLS required. Also, it tends to destabilize shale formations.

Modified lime muds, in which potassium hydroxide is used instead of sodium hydroxide, and polymer thinners instead of lignosulfonates, have recently been introduced.32 u 14 Laboratory and field tests indicate much less dispersion of clays with these muds compared to conventional inhibitive muds.

Table 1-3 Selection of the Drilling Fluid


GAS: Dry Air



Stable Foam




Low Solids Muds*


Principal Ingredients


Dry Air

Fast drilling in dry, hard rock

No water influx


Air, water or mud Wet formations but little water influx

High annular velocity

Air, water, foaming Stable rock agent

Moderate water flow tolerated

Air, water containing All "reduced-pressure" conditions:

polymers and/or Large volumes of water, big cut-

bentonite; foaming tings removed at low annular veloc-

agent ity

Select polymer and foaming agent to afford hole stability and tolerate salts

Foam can be formed at surface

Fresh Water

Sea Water

Fresh water, polymer, bentonite

Fast drilling in stable formations

Need large settling area, floc-culants, or ample water supply and easy disposal

Brines for density increase and lower freezing point

Limited to low-permeability rocks

Fast drilling in competent rocks

Mechanical solids removal equipment needed

Contaminated by ccment. soluble salts

Detergents, lubricants, and/or corrosion inhibitors may be added to any water composition.

* When barite is added to raise the density of these muds, they are called "non-dispersed" muds.

(Table 1-3 continues on page 22)

(1) Density of oil muds can be raised by addition of calcium carbonate or barite. (21 Calcium chloride is added to the emulsified water phase to increase shale stability.

Geopressured Formations

Shallow formations are normally pressured and can be drilled with unweighted muds. When geopressured formations are encountered, the density of the mud must be increased so that the pressure of the mud column exceeds the formation pore (fluid) pressure by a safe margin. Exactly what is a safe margin is a matter of opinion, but remember that excessive density adds greatly to drilling costs and increases the risk of stuck pipe and loss of circulation. Lower margins can be carried if the viscosity and gel strengths are kept to a minimum so as to avoid swabbing the well when pulling pipe, and to facilitate the removal of entrained gas.

Because of the presence of barite the initial solids content of a weighted mud is high, and the incorporation of drilled solids soon increases it to the point where the viscosity rises rapidly. Therefore the solids content should be reduced to a minimum before adding the barite, or the old mud should be discarded and a fresh mud prepared containing only barite and just enough bentonite to suspend it.

When densities over 14 lb/gal (1.68 SG) are required, a mud that tolerates a high solids content, such as one of the inhibitive muds discussed above, or an oil base mud, should be used.

High Temperature

The constituents of drilling muds degrade with time at elevated temperatures, the higher the temperature the greater the rate of degradation. Both temperature and rate of degradation at that temperature must be taken into account when specifying the temperature stability of a mud or mud product. The critical temperature is that at which the cost of replacing the degraded material becomes uneconomical, which is generally established by experience, but may be calculated.- The critical temperature is about 225°F (107°C) for starch and 275°F (135°C) for cellulosic polymers. CL-CLS muds can be used at temperatures up to 350°F (177°C), and invert emulsions up to about 400°F (204°C). Asphaltic oil muds have been used in wells with bottom hole temperatures up to 550°F (287°C). Polyacrylates and copolymers with polyacylamides are stable up to 4(X)°F (204°C). Certain other acrylic copolymers are stable to over 500CF (260°C) (see Chapter 9).

Hole Instability

The two basic forms of hole instability are:

Hole Contraction. If the lateral earth stresses bearing on the walls of the hole exceed the yield strength of the formation, the hole slowly contracts. In soft plastic formations, such as rock salt, gumbo shales, and geopressured shales, large quantities of plastic spallings are generated, but the pipe becomes stuck only in severe cases. Hard shales and rocks are generally strong enough to withstand the earth stresses, unless there are stress concentrations at specific points on the circumference of the hole (e.g., at keyholes) in which case spalling occurs at the point of maximum stress. Although spalling and tight hole can be alleviated by shale stabilizing muds, they can only be prevented by raising the density of the mud high enough to balance the earth stresses

Hole Enlargement. This problem occurs in hard shales that can withstand the earth stresses unless destabilized by interaction with the mud filtrate, in which case the contaminated zone caves in hard fragments and the hole gradually enlarges. Such shales are called watersensitive. Hole enlargement can be prevented only by the use of shale stabilizing muds. Invert oil emulsion muds are best for shale stabilization, provided the salinity of the aqueous phase is high enough to balance the swelling pressure of the shale. Potassium chloride polymer muds are the best water base muds for stabilizing hard shales. If necessary to clean out an enlarged hole, exceptionally high YP/PV ratios can be obtained with them. Potassium chloride muds are less suitable for stabilizing soft, dispersible shales because high concentrations are required and initial and maintenance costs arc high. Gyp, lime, or modifications thereof33 34 are suitable alternatives.

Both oil base and, to a lesser extent, potassium polymer muds are expensive, but their cost can easily be justified if they prevent caving and hole enlargement, which greatly increase drilling time and costs. Besides the obvious costs of cleaning out cavings and bridges, freeing stuck pipe, bad cement jobs, and poor formation evaluation, there are the less obvious costs resulting from the necessity of carrying high viscosities and gel strengths, which reduce rate of penetration, cause swabbing and surge pressures, gas cutting, etc. Reduction of rate of penetration is especially damaging because caving is time dependent; the longer the shale is exposed to the mud, the greater the hole enlargement.

Fast Drilling Fluids

The characteristics of fast drilling fluids are low density, low viscosity, and low solids content. Air is by far the fastest fluid, but can only be used in stable, non-permeable formations that do not permit any significant inflow of water.36 Clear brines can be used to drill hard rocks, but only if a gauge or near-gauge hole can be maintained, because of their poor cutting carrying capacity.37 Low solid (non-dispersed) muds and potassium muds using soluble salts as weighting agents drill fast in hard rocks and non-dispersible shales, but the solids content must be kept below 10% by mechanical means. Rates approaching clear brines can be obtained if the solids can be kept below about 4%.

The density range of drilling fluids is shown in Figure 1-6.

Because of their high viscosity, standard oil base muds do not permit fast drilling rates. But a special low viscosity invert emulsion, which has proved particularly successful when drilling with polycrystalline bits, is available.^ This mud is less stable and has somewhat poorer filtration characteristics than standard oil base muds, and therefore should not be used when conditions are severe, as with highly deviated holes (because of the danger of sticking the pipe), deep hot holes.™ etc.

Rock Salt

To prevent the salt from dissolving and consequently enlarging the hole, either an oil base mud or a saturated brine must be used. The chemical composition of the brine should be approximately the same as that of the salt bed. A slight tendency for the salt to creep into the hole may be offset by maintaining the salinity slightly below saturation. As previously mentioned, high densities are essential for deep salt beds.

High Angle Holes

In highly deviated holes, such as are drilled from offshore platforms, torque and drag are a problem because the pipe lies against the low side of the hole, and

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