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Table of Contents• Synthetic Muds• Synthetic Mud Compositions• Types of Synthetic Mud• Properties of Synthetic Mud• Functions of Synthetic Mud• Aphron Drilling Fluids• Structure of Aphron• Composition• Working

Drilling FluidsAll the fluids used in a well-

bore during drilling operation

are called “Drilling Fluids

Classification of Drilling Fluids

A drilling fluid can be classified by the nature of its continuous fluid phase

There are four types of drilling fluids:

1. Water Based Muds2. Oil Based Muds3. Synthetic Muds 4. Gas Based Muds

Synthetic Muds • A material produced by the reaction of

a specific purified chemical feedstock, as opposed to the traditional base fluids such as diesel and mineral oil which are derived from crude oil solely through physical separation processe.

or• A drilling fluid where the base fluid

consists of non water soluble organic compounds and where neither the base fluid nor the additives are of petroleum origin

Synthetic Mud Compositions There composition is similar to the oil based mud:• the continuous phase is a water insoluble synthetic organic material.

• Lignite

• Lime

• Emulsifier

• Wetting agents

• Rheology modifiers

• Organophilic clay

• Barite

• Thinners

• Gelling agents

Emulsifiers, which often are metal soaps of fatty acids, are added to the SBF to aid in forming and maintaining the inverted emulsion.

Wetting agents are added to ensure the solids in the mud are SBF-wet, include polyamines, fatty acids, and oxidized tall oils.

Lime is added to make calcium soaps that aid in emulsification of water in the SBF.

Rheology modifiers and organophilic clays are added to aid in suspending drill cuttings in the mud.

Barite (barium sulfate) is used to increase the weight of the drilling mud, counteracting formation pressure and thus preventing a blowout.

Additives Concentration in Drilling MudPounds/Barrel

Rheological Modifier 9.0 - 14.0Fluid Loss Additive <1.0 - 2.0Lime 1.0 - 8.0Organophilic Clay 6.0 - 9.0Wetting Agent 5 – 8Emulsifier 0 - 1.0

Table : Typical concentration ranges of the major additives in SBFs. Modified from McKee et al. (1995).

Types of Synthetic MudSynthetic base fluids may be classified into four general categories:

1. Synthetic hydrocarbons 2. Ethers

3. Esters

4. Acetals

1. Synthetic HydrocarbonsPolymerized olefins are the most frequently used synthetic hydrocarbons in SBFs today.They include:

Linear alpha olefins (LAOs)

Poly alpha olefins (PAOs)

Internal olefins (Ios)

Linear Alpha Olefins• LAOs are produced by the polymerization of ethylene.• Ethylene (C2H4), the smallest unsaturated hydrocarbon, is oligomerized by heating in the presence of a catalyst and triethyl aluminum to produce LAOs with different hydrocarbon chain lengths.• LAOs have molecular weights ranging from 112 (C8H16) to 260 (C20H40).• The LAO mixture is distilled to produce different molecular weight blends.• The physical-chemical properties of the mixtures can be altered systematically by changing the chain lengths and branching of the LAO molecules. • Typical LAO mixtures used in SBFs are LAO C14C16 (a blend of C14H28 and C16H32 LAOs) and LAO C16C18.

Poly Alpha OlefinsPAOs are manufactured in a four- to five-step process: 1) polymerization of ethylene to form a series of linear alpha olefins;2) distillation to isolate LAOs of the desired chain length;3) oligomerization of the LAOs to produce PAOs;4) hydrogenation to saturate the PAOs;5) distillation to isolate PAOs with the desired physical-chemical properties PAOs may be hydrogenated, producing alkanes for some applications• LAOs used to manufacture PAOs include 1-octene (C8H16) and 1-decene (C10H20

• unsaturated PAOs are preferred to the saturated in applications where PAO cuttings may be discharged to the ocean.

• The average PAO is C20H42 (Eicosane) with a molecular weight of 282.6 and an aqueous solubility less than 1 µg/L.

Internal Olefins• IOs are formed by isomerization of

LAOs in the presence of heat and a suitable catalyst.

• Isomerization of a LAO decreases its pour point and flash point.

• Commercial IOs usually have a chain length of 16 (C16H32) or 18 (C18H36) carbons.

• The IO mixture may be hydrogenated to produce saturated hydrocarbons.

• In today’s market, LAOs and IOs usually are preferred over PAOs. LAOs and IOs often are used in blends that are designed to achieve a balance among the physical properties important to the drilling operation (e.g. viscosity, pour point, flash point, etc.).

2. Ethers• Alcohols with different chain lengths

are condensed and partially oxidized to produce mono- and di-ethers.

• Ethers are saturated hydrocarbons with an oxygen atom in the center.

• Hydrocarbon chain lengths and branching are selected to optimize drilling properties and minimize toxicity.

• Ethers are more stable both chemically and biologically than esters or acetals.

• Ether SBFs have a high hydrolytic stability.

3. Acetals• Acetals are dialkylethers that are closely related to ethers.• They are formed by the acid-catalyzed reaction of an aldehyde with an alcohol or carbonyl compound (One mole of aldehyde and two moles of alcohol).

• Acetals are relatively stable under neutral and basic conditions, but may revert back to the aldehyde and alcohol under acidic conditions.

• A typical acetal in a SBF has the formula, C20H42O2, and has a molecular weight of 314.3

4.Esters• Esters are formed by the reaction of a

carboxylic acid with an alcohol under acidic conditions.

• The ingredients of esters used in SBFs include fatty acids (carboxylic acids) and alcohols with different chain lengths.

• 2-Ethylhexanol is the alcohol used most frequently; however, glycerols may also be used.

• The fatty acids usually are derived from natural vegetable or fish oils.

• Esters are somewhat polar and more water- soluble.

• An example of ester used in SBFs is a mixture of C8 through C14 fatty acid esters of 2-ethylhexanol.e.g C26H52O2

• Esters also may be mixed with synthetic hydrocarbons (LAO, IO, or PAO) in an SBF to attain some particular drilling performance characteristic.

• Esters are relatively stable under neutral conditions, but may undergo hydrolysis and revert back to the acid and alcohol under basic or acidic conditions.

Properties of Synthetic Mud Mud Weight or Mud density(MW)

Plastic viscosity(PV)

Yield point(Yp)

Gel Strength

Filtration

Sand content

pH

Colorless

Odorless

Non-toxic

Functions of Synthetic Mud1) Containment of pore pressure2) Hole cleaning/ Transport cuttings3) Suspend cuttings4) Clean the bit and other in-hole

tools5) Lubricate the drill string & drilled

hole6) Assist in gathering of subsurface

geological data and formation evaluation

7) Prevent adverse effects of H2S and

CO2

8) Assist in cementation9) Deposit filter cake10) Stabilize weak or incompetent

zones11) Reduce filtrate invasion12) Protect formation productivity13) Powering down-hole tools14) Support weight of tubular

Advantages:-• High ROP• Long bit life• Excellent shale inhibition• Thermally stable• Low reservoir damage• High lubricity• Low torque• Low Corrosion• Low fluid loss• Reduced chance of pipe sticking• High solid tolerance

• Salt not dissolve• Natural gas is less soluble in SBFs than in most OBFs, making it easier to detect gas kick• SBFs released much smaller amounts of vapors than diesel or mineral oil based drilling fluids these vapor losses are of importance mainly because of their toxicity through inhalation to personnel working near the mud storage and treatment systems on the platform.• Particularly useful for deepwater and deviated hole drilling.• SBFs are designed to be less toxic and degrade faster than OBFs.• Synthetics have certain technical and human health advantages over most mineral oils and diesel fuel• They are less volatile than OBFs and their vapors are free of aromatic compounds • Thus, the use of SBFs can reduce vapor inhalation by workers in closed, poorly ventilated areas on the drilling platform.

Disadvantages:-• High cost• Electric log difficulties• Expensive lost circulation• Poor cement bond possible• Messy working environment• Difficulty detection crude oil• Fire hazardous• Logistics• Pollution

Aphron Drilling Fluids

• Used to minimize fluid invasion into low-pressure permeable or fractured formation by I. Forming soft internal sealII. Creating microenvironment

Structure of Aphron The aphron is composed of a core of

air (gas) that is stabilized by a

polymer/surfactant shell The air core of an aphron is enveloped

by a much more stable surfactant tri-

layer

• an inner surfactant film enveloped

by a viscous water shell

• an outer bi-layer of surfactants

Structure of Aphron

Stability of Aphron

• Stability of aphron depends on thickness and viscosity of the encapsulating shell

• Aphrons in circulation change their volume with pressure change according to Boyle’s law

• Shell must have a minimum viscosity to prevent phenomenon known as the “Marangoni effect”

Composition (water-based aphron)

• Fresh water/brine • Soda ash • Biopolymer blend • Polymer blend • pH buffer • Surfactant • Biocide

• The base fluid is highly shear-thinning and exhibits an extraordinarily high LSRV (Low-Shear-Rate Viscosity) with low thixotropy (flat gels).

Composition (oil-based aphron)

• oil or synthetic fluid • clay or Polymer Blend • Surfactant • Water • Emulsifiers

Working• When the drilling fluid enters a

formation, the aphrons expand to a small extent and, more importantly, move forward rapidly by means of “bubbly flow” to concentrate at the fluid front and create a “microenvironment” that separates the borehole from the formation pressures.

Mud Cake Formation