Properties of mud

Properties of mud

 Plastic Viscosity (PV)

•Control the magnitude of shear stress develops as one layer of fluid slides over another.

•Measure of friction between layers.

• Provides a scale of the fluid thickness.

• Decreases with increasing temperature; with liquids; the reverse with gasses .

•Effective viscosity depends on fluid velocity flow pattern, difficult to measure, but can be calculated.

• PV is the difference between readings at 600 rpm and that at 300 rpm

yield point

•A measure of the attractive forces between particles due to positive and negative charges

• Measure the forces causes mud to gel in case of motionless

•Shows a minimum level of stress must be provided before mud flows

•Expressed in lb/lOOft2

•A parameter of the Bingham plastic model. YP is the yield stress extrapolated to a shear rate of zero

•It can be increased by Bentonite .

 Gel strength

•Ability of mud to develop gel structure .

•Defines the ability of mud to held solids and measures thixotropy

• Is Determined using viscometer .

• The sample stirred at high speed and the allowed to rest for 10 sec or 10 m.

•The torque readings at 300 rpm is taken as gel strength at specified time.

• Expressed in lb/100 ft2 .

•Converted to metric by multiplying by 0.478 .

  Filtration and filter cake

• Filtrate is the liquid that passes through the porous and permeable rock medium, leaving the cake on the medium .

• A layer of solids deposited on the rock is described as filter cake

• Loss occur when mud pressure is higher than formation pressure

•Quantity of mud loss depends on volume of filtrate and thickness and strength of filter cake, and differential pressure .

•Ideal mud gives small filter loss and thin and tough mud cake

 

•Filtrate is a disadvantage due to formation damage & plugging pores by  shale swelling .

•Mud cake is an advantage due to stabilizing the well bore  but when increasing it decreases the hole diameter

•Polymers are used in mud cake .

•Drilling muds are tested to determine filtration rate and filter-cake properties.

• Cake properties such as cake thickness, toughness, slickness and permeability are important because the cake that forms on permeable zones in the wellbore can cause stuck pipe and other drilling problems.

•Reduced oil and gas production can result from reservoir

     damage when a poor filter cake allows deep filtrate invasion.

• Can be determined by filter press, called API filter press

  

pH of mud 

•Describe the acidity or alkalinity of mud

•Defined as the negative logarithm of the hydrogen ion (H)

•Measured by pH meter or strips or dyes

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Emulsion mud

Emulsion mud

•Water is a continuous (normal oil 5-10% by volume)

•Formulated by using sodium soap as emulsifier

•Oil is added to

 increase penetration rate:oil mud allows to drill faster than watermud

• reduce filter loss,

•improve lubricity,

• reduce lost circulation

•reduce torque and drag in directional well

•The stability of emulsion mud is very important to insure best performance.

 The degree of stability of a certain emulsion can be measured by several observations. The resistance of an emulsion to conduct electric current (or the breakdown voltage) is one of the important measurements which determine emulsion stability. Also filtration loss and mud viscosity are often considered measures of mud stability.

 

•The oil soluble polymers in a form of a gel, according to the instant invention, allow a good thermal stability and avoid any rheological contribution. They may be used at high temperature and high pressure conditions.

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oil base mud

oil base mud

•Water in oil emulsion

•Diesel or crude oil forms its continuous phase

•Sometimes called invert emulsion

•Water droplets are emulsified in oil

•Water is used for gel strength and barite content

•Soaps are used as emulsifiers

•Soups are made from monovalent ion (Na+) or divalent ion

•(Ca++)

•The soap molecule bridge together oil and water interfaces

•Agitation is required to break the water into small droplets

•Oil/water ratio determine the final properties

 

Oil-based muds generally use hydrocarbon oil as the main liquid component with other materials such as clays or colloidal asphalts added to provide the desired viscosity together with emulsifiers, polymers and other additives including weighting agents. Water may also be present, but in an amount not usually greater than 50 volume percent of the entire composition. If more than about 5% volume water is present, the mud is often referred to as an invert emulsion, i.e., water-in-oil emulsion

  

ADVANTAGES:

(1) performs all the functions of a water-base mud or permits an acceptable

substitute for these functions;

 (2) has most of the desirable properties of a water-base mud;

(3) does not hydrate and disperse cuttings;

(4) will not dissolve salt or other water-soluble formation deposits and can be

used to core such sections

(5)exhibits good penetration rates

 (6) protects productive horizons

 (7) does not increase the interstitial water of cores; and

 (8) is not affected by contaminants such as cement, anhydrite, salt or shale, and

can tolerate contamination with water and raw crude. 

 Higher oil water increases resistance to contamination and

temperature stability

Used to drill hole with severe stability

More stable at high temperature

An excellent to drill pay zone; reduce formation damage and

preserve original permeability

 Disadvantages:

 

• Contaminate the environment

• Flammability hazards

• Difficult removal of drilled solids due to high plastic PV 

• Difficult electric logging

        

      

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Water base mud

Water base mud

Consists of:

1-Liquid water, continuous phase

2-Reactive solids, for viscosity and yield point

3-Inert solids, for density

4- Chemical additives, to control properties

Hydration of clays

•Clays with high cation exchange capacity exchange large amount of water into the exchangeable layer and adsorb water onto the outer surface of plates

•This effect gives high viscosity and high yield point

•Adsorption of water causes a very sticking expansion of clay

•Overall hydration transform clay from dry power to plastic slurry

•Clay yield depends on: purity, nature of atoms in exchangeable layers and salinity of water

Bentonite and attaplugite

•Bentonite consists primarily of montmorillonite

•Came from French town Montmorillon, where first mined 1874

•Basic structure is close to pyrophllite.

•There are a small number of exchangeable ions, sodium calcium and magnesium

•Most common bentonite are those with sodium and calcium as exchangeable ions

•Attapulgite belongs to a different family of clay minerals

•Instead crystallizing as platy crystals, it forms needle like crystals

•Have excellent viscosity and yield strength when mixed with salt water

•Disadvantage is suffering high water loss and poor sealing properties

Dispersion, flocculation and defloccullation

•Agitating of clay suspension in water gives three modes: edge to edge; face to edge; and face to face

•Dispersion occurs with no face or edge association

•It results in increase in viscosity and gel strength

•Aggregate occurs with face to face association

•Aggregate results in a decrease in viscosity and gel strength

•Flocculation occurs with face to edge association

•It causes excessive gelation

•Flocculation can be broken by chemical thinners

•The resulting suspension is called deflocculated

Inert solids

•Include low gravity and high gravity

•Low gravity include sand and chert

•High gravity are added to increase mud weight or density

• Referred to as weighting materials

•Mud named as weighted mud, they are:

                                1)Barite (barium sulphate, BaSO4) sp.gr. 4.2

                                   Used to prepare mud in excess of 10 ppg

                                  Refened to as weighting agent for low cost and high purity

2)Lead sulphides (galena) sp.gr. 6.5-7.0

• Allowing mud weight up to 35 ppg

•Iron ores, sp. gr. 5+ More erosive

• Contain toxic materials

 

 

 

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Drilling Fluids Functions

–They  

 

1-Cool the drill bit and lubricate its teeth
2-Lubricate and cool drill string
3-Control formation the pressure
4-Carry cutting to the surface
5-Stabilize the well bore
6-Help in the well logs
7-limitting corrosion

1-Cool bit and lubricate its teeth

  

•Drilling action requires mechanical energy in form of weight on bit, rotation and hydraulic energy

•Large part of energy dissipated as heat

•A Heat must be removed to allow drilling

•Mud helps remove heat

•Mud helps remove cutting between the teeth and prevent bit balling

2-Cool and lubricate drill string

•Rotating drill string generates heat

•Mud help dissipate heat from hole

•Mud absorbs heat by convection and release it by radiation

•Mud lubricate drill string and reduce friction

 

 3-Control formation pressure

•For safe drilling, high formation pressures must be contained within the hole to 

prevent damage and injury

•This achieved by hydrostatic pressure of mud

•Mud pressure should be higher than formation pressure

•An overbalance of 100-200 psi is used

•Pressure overbalance id referred as chip hold down pressure

    (CHDP)

•Penetration rate decreases as CHDP increases

•For abnormal pressure CHDP becomes negative and kick will happen

 

4-Carry cutting to surface

 

•Cutting generated by the bit must be removed

•Drilling mud carries cutting form bottom to surface

•A Carrying capacity depend on annular velocity, plastic viscosity and yield 

point of mud and slip velocity of generated cutting For power law fluid

 

 5- Stabilize the wellbore  

•Good mud cake stabilize the hole

•Differential pressure between mud and formation keep hole stable

•Reduce drilling time also help keep hole stable

 

 6-Help in the evaluation 

 
and interpretation of the well logs

•During logging mud fill the hole

•Logs used to detect hydrocarbon, measure porosity, formation ,pressure

•Mud should posses properties that help evaluate these properties

•Water base -mud  is  better  for  electric  logging  

7-Limiting corrosion

•The drilling  fluid in most cases  will have water which contain dissolved salts

•This serves as a medium for corrosion

•Precautions should be taken  to prevent  damage  of the  equipment

•Mud containing oil as the continuous phase may decrease

   or  prevent corrosion  completely

 

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