Drilling fluids types

Drilling fluid type and properties (density, viscosity, fluid loss, etc., solids content, differential pressure, etc.)... 

C = constant dependent upon bit and formation type Af = formation abrasiveness parameter in hr K = formation drillability parameter in ft/hr S = bit bearing parameter in hr aj,aj = bit weight and rotary speed exponents H,Hj,Hj or (W/D ) = constants which depend upon bit type (see Table 4-138) b = constant depending upon bearing and drilling fluid type (see Table 4-139)... 

Solid-solid interactions Inhibition level Drilling fluid type... 

Siace 1980 over 1000 patents have been issued for drilling fluid systems and materials ia the United States alone. A 1994 listing of products from 117 supphers offers ca 3000 trade names (6). This array of trade name products actually represents less than 100 separate chemical types that may be purchased iadividuaHy or as a blend. Moreover, some of these materials are for completion and workover fluids. These differ from drilling fluids ia that completion fluids are used after the well has been drilled and prior to the initia tion of production whereas workover fluids are used duting remedial work on older wells. 

Water-Based Muds. About 85% of all drilling fluids are water-based systems. The types depend on the composition of the water phase (pH, ionic content, etc), viscosity builders (clays or polymers), and rheological control agents (deflocculants or dispersants (qv)). 

Surfactants. Surfactants (qv) perform a variety of functions in a drilling fluid. Depending on the type of fluid, a surfactant may be added to emulsify oil in water (o/w) or water in a nonaqueous Hquid (w/o), to water-wet mud soHds or to maintain the soHds in a nonwater-wet state, to defoam muds, or to act as a foaming agent. 

To overcome these difficulties, drilling fluids are treated with a variety of mud lubricants available from various suppHers. They are mostly general-purpose, low toxicity, nonfluorescent types that are blends of several anionic or nonionic surfactants and products such as glycols and glycerols, fatty acid esters, synthetic hydrocarbons, and vegetable oil derivatives. Extreme pressure lubricants containing sulfurized or sulfonated derivatives of natural fatty acid products or petroleum-base hydrocarbons can be quite toxic to marine life and are rarely used for environmental reasons. Diesel and mineral oils were once used as lubricants at levels of 3 to 10 vol % but this practice has been curtailed significantly for environmental reasons. 

Temperature logs can be taken as wells are drilled and the temperature gradient determined for the particular region. These temperature logs taken at depth are used to determine the types of drilling fluids used as drilling progresses. Also, the temperatures at depth will determine the cements u.sed in well completion operations. 

Emulsions. Emulsions are formed when one liquid is dispersed as small droplets in another liquid with which the dispersed liquid is immiscible. Mutually immiscible fluids, such as water and oil, can be emulsified by stirring. The suspending liquid is called the continuous phase, and the droplets are called the dispersed (or discontinuous) phase. There are two types of emulsions used in drilling fluids oil-in-water emulsions that have water as the continuous phase and oil as the dispersed phase, and water-in-oil emulsions that have oil as the continuous phase and water as the dispersed phase (invert emulsions). 

For pipe flow of Bingham plastic type drilling fluid, the following can be used ... 

Corresponding equation for a Power law type drilling fluid is... 

Air and natural gas are often used as a drilling fluid with no additives placed in the injected stream of compressed fluid. This type of drilling is also often referred to as dusting because great dust clouds are created around the drill rig when no formation water was present. However, modern air and gas drilling operations utilize a spray at the end of the blooey line to control the dust ejected from the well. Figure 4-185 shows a typical site plan for air drilling operations. 

In most air and gas drilling operations, open-hole well completions are common. This type of completion is consistent with low pore pressure and the desire to avoid formation damage. It is often used for gas wells where nitrogen foam fracturing stimulation is necessary to provide production. In oil wells drilled with natural gas as the drilling fluid, the well is often an open hole completed with a screen set on a liner hanger to control sand influx to the well. 

The positive displacement motor of the Moineau-type design can be operated with unstable foam (or mist) as the drilling fluid. Some liquid must be placed in the air or gas flow to lubricate the elastomer stator as the metal rotor rotates against the elastomer. Positive displacement motors have been operated quite... 

Upon shutting in the well, the pressure builds up both on the drillpipe and casing sides. The rate of pressure buildup and time required for stabilization depend upon formation fluid type, formation properties, initial differential pressure and drilling fluid properties. In Ref. [143] technique is provided for determining the shut-in pressures if the drillpipe pressure is recorded as a function of time. Here we assume that after a relatively short time the conditions are stabilized. At this time we record the shut-in drillpipe pressure (SIDPP) and the shut-in casing pressure (SICP). A small difference between their pressures indicates liquid kick (oil, saltwater) while a large difference is evidence of gas influx. This is true for the same kick size (pit gain). 

Application of Corrosion Inhibitors. There are basically two main techniques used to apply corrosion inhibitors in drilling operations. In the first method inhibitors are added to the drilling fluid system either by mixing the additives through the rig s chemical hopper or through additions into the mud pit. The treatment can be achieved in two ways, batch treatment or continuous treatment. In some cases it may be necessary to use both types of treatment simultaneously. The second technique of applying is directly coating the corrosion inhibitors on the drillpipe. 

Direct Treatment. Although batch and continuous treatment are both quite effective, there is a problem with inhibitor waste. When corrosion inhibitors contact the circulating drilling fluid, they are likely to coat the solids in the fluid system (cuttings or other solids). By applying the corrosion inhibitor directly to drillstem components before they are run in the hole, the corrosion inhibitor is the first thing that contacts the exposed metal surface. There are two methods for this type of corrosion inhibitor treatment. 

Clear freshwater is the best drilling fluid in terms of penetration rate. Therefore it is desirable to achieve a maximal drilling rate using a minimal amount of solid additives. Originally low-solids mud formulations were used in hard formations, but they now also tend to find use in other formations. Several types of flocculants are used to promote the settling of drilled solids by flocculation. 

For many years uintaite and other asphaltic-type products have been used in water-based drilling fluids as additives assisting in borehole stabilization. These additives can minimize hole collapse in formations containing water-sensitive, sloughing shales. 

Drilling fluids usually contain water as the primary component. However, oil-based muds may be used for high temperature operations and for drilling highly water-sensitive formations. Oil-based muds are of two types, oil-external emulsions containing as much as 50% water in the internal phase or an oil-based fluid containing little if any water. 

A great many additives can be used to impart desired properties to the drilling fluid. In general, the deeper and hotter the well, the more chemical additives are needed to obtain the desired fluid properties. These additives can be classified into different types by function. These include ... 

Brine composition and polymer type should be considered together, and could lead to improved control of the viscosity of polysaccharide based drilling fluids. 

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