Drilling Fluids – 2 – Mud compositions and additives

Base fluid: drilling mud has two main base fluids; aqueous and non-aqueous. Aqueous means fluids that their liquid phase is water, and it is referred to water-based mud WBM; non-aqueous phase refer to oil-based mud OBM, oil can be diesel or synthetic oil.

Commonly WBM used in onshore operations where fresh water is adequately available. However, hardness of fresh water has to be reduced to the optimum percentage in order for mud components to react appropriately. Soda ash and caustic soda used to condition hard water and to increase pH. Almost all WBM fluids require higher pH for chemical reaction to occur save Guar Gum Mud and PHPA Mud.

OBM has distinct advantages over WBM such as stability in sever downhole conditions HPHT, sufficient lubricity required to drill a highly deviated and horizontal wells, and also used to treat water-sensitive shale formations. OMB mainly used in offshore rigs with bearing into consideration that when only base fluid is environment friendly such as synthetic oil. Diesel never used as base fluids for OBM in offshore operations and that is why it is used in land rigs.

?Fluid loss reducers: filtrated fluids normally tend to migration into formation pores and void spaces, which can direct impact permeability and cause formation damage. Thus, fluid loss reducers role is to control this flirtation rate by plugging pores of filter cake in front of porous or permeable formations. Filtration rate depends on solids content and excreted pressure on formation face to form filter cake. There is inverse relationship between migration velocity and thickness of mud filter cake.

For WBM Bentonite can be used as fluid loss reducer escpically in spud mud. In WBM polymers are act as fluid loss reducer except in high mud weight in polymer never used because higher solids, the polymers will increase thickness of mud and make it immobile; thus, asphaltic materials are used instead. For OBM fluid loss reducers are special sort of asphaltic materials that are soluble in oil.

Viscosifier:??Polymers have attraction forces between mixture components to form hydrogen bond with fluids that leads to increase viscosity of mud. That is why polymers are used as visocsifiers in WBM. Furthermore, Bentonite swells in fluids and increases viscosity; it is used as viscosifier for WBM.?For OBM Bentonite treated with amine is the viscosifier.?Amine is an organic material that makes Bentonite soluble in oil.

Weighing agent: to increase weight of mud, materials with high solids content are added to mixtures; the materials should be inert and have no influence on reaction of polymers and/or other additives. Gelna, Barite, Hematite, and others non-acid soluble materials are used as weighting agent. However, acid-soluble materials such as calcium carbonates are used to avoid formation damage. In completion, salt is used as weighting agent to increase fluid density while the solids should remain free.

Alkalinity and PH control Materials: in WBM, pH virtually controls all things; if it is high polymers will scorch and if it is less polymers and other chemical additives will not?react properly. pH is the degree of hydroxide; thus, alkalinity and acidity of drilling fluids can be controlled by controlling hydroxide content in fluids. Additives contain hydroxide are lime and caustic soda, so they can be utilized to increase fluids pH; while acid such as citric acid and sodium bicarbonate are usually used to lower pH.

Bactericides: natural materials such as polymers are normally affected by Bacteria, also Bacteria is less active in higher pH environments. Thus, anything can increase PH and reduce Bacteria effects such as caustic soda, lime and B-54 is considered as Bactericide.?

Bridging Materials: rate of filtration is controlled by applying particle size distribution, hence, asphaltic material and acid soluble calcium carbonate with difference sizes are used as bridging agent.

LCM any inert solids used to plug opening of thief formations is LCM. There are two types of LCM, conventional materials such as mica, nut plug, wall nut, cotton seeds, etc that never used in pay zone; and acid soluble materials such as calcium carbonates typically used to cure losses in zones of interest.

Calcium reducer:?calcium causes polymers in WBM to flocculate polymers.?Therefore, calcium needs to be reduced to avoid fluids settling. Most common calcium reducer is soda ash.

Corrosion inhibitor: ?typically carbone steel is eroded in humidity and/or when it contacts water, and the main cause of corrosion is effect of oxygen and hydrogen sulfide. Hence, it is recommended to rise pH in order to counteract sulfide in WBM. OBM has corrosion inhibition properties. However, common types of corrosion inhibitors used in WBM are: oxygen scavenger, Zinc Oxide ZNO, and H2S scavenger.

Shale Stabilizers: water-sensitive clay/shale formations tend to interact and swell or disperse in fluids, which causes many wellbore stability problems. Thus, to control hydration, caving, and disintegration of shale zones, chemical additives should be used to inhibit clay interaction with WBM. The Asphaltic materials are used in all sections except in pay zones such as Gilsonite, ID cap, Soltex, Asphasol; Polyamine materials are used in pay zones such as KLA-stop and KLA Guard. Common types of shale stabilizers are: gypsum, sodium silicate, and calcium lignosulfonates.

Other chemical additives: salts, chemical interaction, and viscosity might create foams in mud, which can make it gelatinous and poor hydrated; thus, antifoamer and defoamers are used to kill or hinder foams. Emulsifier used to help hydrophilic and hydrophobic fluids such as oil and water to get mixed together in order to achieve lower density mud. Sometimes pipe sticks due to thicker filter cake, so a pipe freeing agent used in fluids in areas of stuck pipe to damage filter cake which help in freeing it. Types of pipe freeing agent: oil, soaps, detergent, and surfactant. The most common toxic agent used in deeper areas where no fresh water: Pipe lax, EZ-spot, and Glycol; in zones of fresh water coastal spot is used.