Introduction to Drilling Fluids Additives

The drilling fluids have an important role in oil and gas drilling process. They have many functions such as hole cleaning, bit lubrication, pressure control, wellbore stability and many other functions. The cost of the drilling fluids in the overall cost of drilling project can reach 15% of the budget. Controlling the drilling fluids properties is the key in keeping drilling fluids efficient for the project targets. There are many additives can be used in the drilling industry to control the drilling fluids properties. Most common additives categories will be discussed below.

Weighting Materials

Weighting materials or density control additives are used in the drilling fluids system in order to increase the mud weight to the required level needed for safe operations. There are some features of the weighting materials which should be taken in consideration:

- Chemical inertness

- Specific gravity

- Availability in large quantities

- Easy to be grounded to the required particle size

- Limited abrasiveness

- Safe for drilling crew and equipment

Related the factors mentioned above, the barite is the most recommended weighting materiel used in the drilling industry. It has a specific gravity at 4.5, it is virtually insoluble in water and it does not react with other mud components. There are other good candidates for weighting materials such as the calcium carbonates , hematite and ilmenite.

Fig 1 - Barite Sacks

Viscosifiers

This type of additives defines the level of the viscosity of the drilling fluids. The suspension ability of the drill cuttings and weighting materials is related to the viscosity of mud. Missing the appropriate viscosity can lead to solids settlement when the circulation is ceased. The increasing pressure losses in the circulating system can be a straightforward indication of an increased viscosity. Clays can be used as viscosifier due to their feature of developing plasticity, the most used clay is bentonite.

Bentonite is classified as sodium bentonite or calcium bentonite depending on the dominant exchangeable cation and in terms of performance it is classified as high yield and low yield bentonite. Bentonite can be added to the fresh water for many reasons such as:

- Increase hole cleaning capability

- Reduce water seepage

- Form thin filter cake of low permeability

- Avoid loss of circulation

Attapulgite is another viscosifier which is also used in drilling fluids due to its ability to retain excellent viscosity and yield strength when mixed with salt water. The suspending property of attapulgite is not affected by the dissolved salt. In fact higher viscosity can be produced in saturated sodium chloride solution. Attapulgite has no filtration control and this can be positive or negative feature depending on the application. It can be a drawback when drilling through porous and permeable formations which can shows high water loss, but this feature can be an advantage when using the attapulgite in high filtration slurries to cure loss of circulation.

Organic polymers have many functions such as reducing filtration, stabilizing shales, increasing drill cuttings carrying capacity and flocculating drilled solids and they can be also used as viscosifiers. Polymers are chemicals composed of chains made up of many similar small units or groups of atoms called monomers consisting mainly of carbon compounds. The organic polymers have a high level of water affinity and they develop high swollen gels in low concentration.

Organic polymers which are used in drilling fluids can be roughly classified according to their origin and composition. Some are naturel and ready to be used after simple processing, others require more complex process and can semi-synthetic or purely synthetic. There are many factors which can affect the choice of polymers for instance: effect of shear conditions, temperature, dissolved salt, time of use of polymer, ease of preparation, environmental effects and polymer cost.

Filtration Control and Mud Conditioning Additives

These additives can be also called thinners and they are added to drilling fluids system to reduce flow resistance and gel development. According to experience, it has been proven that some of these thinners deliver other important functions in addition to improving the flow properties of drilling mud. They can be used to control the filtration, reduce the filter cake thickness, counteract the effect off salt, minimize the water effect on the formation, emulsify oil in water and stabilize mud properties at elevated temperature.

Filtration control compounds are additives used to reduce the amount of fluids that can flow from the drilling fluids into the formation due to the differential pressure between the exerted hydrostatic pressure and formation pressure.

Lost Circulation Material

Loss circulation is one of the drilling problems which can lead to very complicated situation such as well control situation. Curing the losses depends on the severity of the mud losses; they can be cured by reducing the flow rate to minimize the ECD effect for the very low rate losses or seepage till performing a cement job for high level of losses. One of the techniques to deal with the mud losses is pumping lost circulation materials LCM which can be used to plug the region from where the drilling fluids are lost.

The most common LCM can be classified as flaky (ex: cellophane & Mica), granular (ex: nut shells & calcium carbonate) or fibrous (ex: leather); related to the size they can be classified as fine, medium and coarse LCM. The LCM can be pumped down hole as LCM plugs with reduced flow rate to minimize the mud losses till getting the LCM at the thief zone or they can be diluted in the active system.

Fig 2 - Loss Circulation Material ( Courtesy of Prince International Corporation )

Surfactants

Surface active agent or surfactant is a substance which can build an interface between two dissimilar phases and diminishes the surface tension between them. The surfactant can be defined as an organic compound which its molecular structure is made of two dissimilar groups having opposing solubility tendencies.

Surfactants are used in drilling fluids systems for many reasons. They can be used as emulsifiers in oil based muds. For water based muds, the surfactants can serve many applications such as: shale swelling inhibitors, prevent cuttings sticking, wetting agents and corrosion inhibitors.

PH Control Additives

The PH level can affect many drilling fluids properties such as the solubility of many thinners. PH control is required for prevention of corrosion and hydrogen embrittlement. The desired level of PH in drilling fluids lies between 9.5 to 10.5. NaOH and KOH are some additives which can be part of the drilling system to control the alkalinity and PH.