Unit 9 - Drilling Fluids (Drilling Mud) Flashcards
Primary functions of drilling fluid?
- remove drilling cuttings from wellbore
- prevent formation fluids from flowing into the wellbore
- maintain wellbore stability
- cool and lubricate the bit
- transmit hydraulic horespower to the bit
What happens if the rock cuttings generated by the drillbit are not continously removed from the wellbore?
- drilling efficiency will decrease
- drillstring might be stuck in wellbore
The carrying capacity of a mud depends on what?
- annular velocity
- density and viscosity of mud
The ability of a mud to suspend cuttings depends on what?
- gelling (thixotropic) properties of the mud
What is rheology?
- the study of the way in which materials deform or flow in response to applied forces or stresses
How can you add weight to a mud?
- by adding barite or sometimes haemitite because of their high density
How can mud and filtrate damage productive formations?
- a loss of mud can result in deposition of solid particles or hydration of clays in a pore space → this results in the reduction of permeability of the formation
- The filter cake can bekome so thick that it may cause a stuck pipe
→ the ideal filter cake is thin and impermeable
How can you overcome the hydration of clays?
- by using a non water-based mud or by treating mud with chemicals, which reduce the ability of the water in the mud to hydrate the clays in the formation → these muds are known as inhibited muds
How can the mud help to transmit hydraulic horsepower to the bit?
- as the fluid is circulated, the power ot the mud pumps will be expended in frictional pressure losses
→ efficiency of the drilling process can be improved if 65% of this power is expended at bit
- pressure losses in the system are a function of the geometry of the system and mud properties (viscosity, yield point, mud weight)
What are the basic types of drilling fluids?
- water based mud
- oil based mud
When can you use air as a drilling fluid?
- in areas, where formations are competent and impermeable
What are advantages of drilling with air?
- higher penetration rates
- better hole cleaning
- less formation damage
What are disadvantages of drilling with air as a fluid?
- air cannot support sides of borehole
- air cannot exert enough pressure to prevent the formation fluids entering the borehole
What are active solids?
- solids like clay, which react with water and chemicals in mud
How do you call solids, which do not react within the mud (e.g. Barite)?
- inactive or inert solids
What is the main disadvantage of water based muds?
- the water in the mud causes instability in shales
What are some of the advantages of Oil based muds?
- do not contain free water that can react with the clays in shale
- good lubrication
- temperature stability
- reduced risk of differential sticking
What is a big problem of oil based muds?
- the disposal of drilling cuttings contaminated by oil based mud can have a lasting environmental impact
How can you measure the mud density of a fluid?
- with a mud balance
- cup of balance is filled with a sample of your mud and a lid is placed on top
→ then a balance arm is leveled and you can read the density directly off a scale
What is the viscosity of a fluid?
- its resistance to flow
How can you measure the viscosity of a fluid?
- with a marsh funnel
- mud sample is poured into the funnel and the time to flow out into a measuring cup is recorded
- since most drilling fluids are non-newtonian fluids, (different viscosities at different flow rates) the flow rate of the mud will vary during the test and it only gives indication of changes in viscosity but cannot be used to quantify the rheological properties of mud such as Yield point or plastic viscosity
- funnel viscosity is only used to check for radical changes in the mud viscosity
Describe the function of a rotational viscometer?
- used to quantify rheological properties of drilling mud
- made by shearing a sample of mud at a series of prescribed rates and measuring the shear stress on the fluid at these different rates
How does a viscometer work?
- you have a plump bomb, which is suspended inside a cylinder and the whole is immersed in a sample of drilling mud → when the outer cylinder is rotated, the mud between the cylinder and plump bomb is sheared
- the deflection of the plump bomb is a measure of fluid viscosity at that particular shear rate → this test is conducted at a range of different speeds
- if the relationship between the shear stress and shear rate is linear, the plastic viscosity and yield poit of the fluid can be calculated from a graph
What is the gel strength?
- the strength of any internal structures, which are formed in the mud, when the mud is static
The gel strength of a mud gives you an indication of what ?
- of the pressure required to initiate a flow after mud has been static for some time
- also gives indication of suspension properties of mud and therefore its ability to suspend cuttings when mud is stationary
What is the Yiel Point (deutsch)?
- Fliessgrenze, Streckgrenze
For what do you use a Filter press ?
What exactly is measured with a filter press?
- to measure the filter cake building properties
following are measured during the test:
- rate at which fluid from a mud sample is forced through a filter under specified temperature and pressure → reflects the efficiency with which solids in mud are creating an impermeable filter cake
- thickness of solid residue deposited on filter paper caused by loss of fluids
→ reflects thickness of filter cake, that will be created in the wellbore
Why doesn’t the filter press reflect downhole conditions?
- because we measure only static filtration and in wellbore we have dynamic conditions
How do you create viscosity in water based muds?
- by adding clay or polymers (mostly used clay)
what are the two beneficial effects of clay in mud?
- viscosity increase → improves lifting capacity of mud to curry cuttings to surface
- building well cake in permeable zones → prevents fluid loss
clay minerals can be divided in which groups?
- expandable clays (hydrophyllic) → will absorb water (e.g. montmorillonite)
- non-expandable (hydrophobic) - will not absorb water (e.g. Illite)
What is the structure of clay minerals?
- sandwich structure → consisting of layers of silica and alumina
What do you call dispersion?
- when in fresh water, clay layers absorb water → chemical bonds holding them together are weakened and the stack of layer disintegrates → less face to face association → increases number of suspended particles → causes fluid to thicken or viscosity
How are the surfaces of clay particles charged when they are in suspension?
- flat surface → negatively charged
- edges → positively charged
What is flocculation?
- when clay plates make connections because of differently charged areas
→ edge to edge
→ or edge to face
What is aggredation?
- face to face
- natural state of clay minerals, small number of particles in suspension
→ plastic viscosity of mud is low
What is a problem with the salt concentration in water with clay?
- the hydration and therefore the suspension of clay are greatly reduced by the presence of Ca2+ and Mg2+ → it affects the yield of clay
How can you reduce the viscosity of a mud?
- lower the solids content
- reduce the number of particles per uni volume
- neutralize attractive forces between particles
What is the primary material used to increase the density in a mud/
- Barite (BaSO4) → Barium sulphate
The rate of fluid loss to a formation is a function of?
- differential pressure , thickness of the filter cake and the viscosity of the filtrate
If you have a thick wall cake, it can lead to what kind of problems?
- tight spots in the hole
- differential pipe sticking
- formation damage due to filtrate invasion
How can you control the fluid loss of the mud (filter cake)?
- by adding clay → because of its particle size and shape and because it hydrates and compresses und pressure
Why do you often have a better productivity from reservoirs with an oil based mud?
- because oil does not cause the clays in the reservoir to swell , or causes changes in the wettability of the formations
what are disadvantages of oil in drilling fluids?
- higher initial cost
- more stringent pollution controls required
- reduced effectiveness of some logging tools (resistivity logs)
- detection of kicks is more difficult due to gas solubility in base oil
What are the three types of oil-based muds?
- full oil (water content < 5%)
- invert oil emulsions ( water content 5-50%)
- synthetic or pseudo oil based mud
What means: invert emulsion?
the water is emulsified in oil
What is solids control?
- the control of the quantity and quality of suspended solids in drilling fluid so as to reduce the total well cost
What are inert solids?
- solids, which do not hydrate or react with other compounds within the mud (e.g. sand, silt, etc.)
Why are the solids like sand, silt, etc. undesirable?
- increase frictional resistance without improving lifting capacity
- cause damage to mud pumps → leading to higher maintenance costs
- filter cake formed from these solids tend to be thick and permeable → leads to problems like stuck pipe, increased drag and possible formation damage
Mud solids are divided into two groups according to their density!
- low gravity solids = 2.5 - 3.0 S.G.
- high gravity solids = 4.2 S.G (barite)
What are the three basic methods used to control solids content of a drilling fluid?
- Screening
- Settling
- dilution
Describe screening?
- shale shaker uses vibrating screen to separate solids according to size
→ material too large to pass through a given mesh size will be discarded while finer material will undergo further treatment
Describe settling
- natural settling rate is too slow → use of hydrocuclones and centrifuges to increase gravitational force on solid particles
Describe dilution!
after passing through screening and settling stages, very fine solids content remains in mud → can be discarded or diluted
→ because of limited capacity of the active system, some mud is usually discarded (together with desirable solids and other chemicals) before the remainder can be diluted and conditioned for re-circulating
Name some solids control equipment!
- vibrating screens
- Hydrocyclones
- Decanting Centrifuge
- mud cleaner
Describe vibrating screens !
- designed to remove particles, which will not pass through the mesh
→ screen is at the same time vibrated to prevent blinding or plugging
- screens can be arranged in series, so that finer mesh is put beneath coarser mesh
Describe the general function of Hydrocyclones!
- general term: includes desanters (6” diameter), desilters (4” diameter) and clay ejectors (2” diameter)
- designed to remove all sand particles and most of the silt particles from the mud retaining colloidal fraction
Describe the working principle of hydrocyclones in detail!
- pump feed mud tangentially at high speed into housing → creates a high centrifugal force → force multiplies settling rate so that heavy particles are thrown against the outer wall and descend towards an outlet (underflow)
- lighter particles move inwards and upwards as a spiralling vortex to the liquid discharge (overflow)
What is the cut point of a hydrocyclone ?
- the particle size at which 50% of the particles of that size will be discarded
Why can you not run hydrocyclones with weighted muds?
- because half of the barite would be removed due to its particle size
What is the difference from a clay ejector to a desander or desilter?
- the outlets are reserved, which means, that the underflow (which contains the barite) is returned to the active system, while the overflow is discarded
Are there moving parts in a hydrocyclone?
- no, the separation is due solely to the settling action of particles with different densities
Describe a centrifuge!
- consist of rotating cone shape bowl and a screw conveyer
→ as centrifuge rotates at a high speed, the heavier particles are thrown against the side of the bowl
→ screw conveyor moves these particles along the bowl and carries them towards the discharge port
- at opposite end is another port where the liquid containing the finer particles is discharged
so how would you do solid control with a weighted mud?
- underflow from shale shaker is led to centrifuge ( no hydrocyclones used) since the discharged solids contain valuable barite and are returned to the active system
Why are centrifuges more efficient than hydrocyclones for barite ?
- because they make a finer particle cut → 90-95% of barite can be reused
How would you use solid control with an unweighted mud?
- underflow from a desilter is led to the centrifuge → this time the liquid phase, which contains the fine material including bentonite, will be returned to the mud, while the solids will be discarded
When do you use a mud cleaner?
- to remove drill solids larger than barite - it consists of a desilter and a screen → removes the solids in two stages → used for a weighted mud to remove solids while retaining barite
