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Tuesday, March 13, 2012

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
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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Monday, March 12, 2012

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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