Drilling Fluid Circulation Systems

Drilling Engineering 1 Course
3rd Ed. , 3rd Experience

1.
Drilling Fluid Circulation System
A.
Introduction
B.
Mud Pumps
a.
Duplex PDP & Triplex PDP
C.
Solids removal
D.
Solid Control Equipment
a.
Shale shakers
b.
Degasser
c.
Mud Cleaners
E.
Treatment and Mixing Equipment

drilling fluid roles

The drilling fluid plays several functions in the drilling process.

The most important are:

clean the rock fragments from beneath the bit and carry them to surface,

exert sufficient hydrostatic pressure against the formation to prevent formation fluids from flowing into the well,

maintain stability of the borehole walls,

cool and lubricate the drillstring and bit.
Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 4

Drilling fluid circulation

Drilling fluid is forced to circulate in the hole

at various pressures and

flow rates.

Drilling fluid is stored in steel tanks located beside the rig.

Powerful pumps force the drilling fluid through surface high pressure connections to a set of valves called pump manifold,

located at the derrick floor. Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 5

Drilling fluid circulation (Cont.)

From the manifold, the fluid goes up the rig within a pipe called standpipe to approximately 1/3 of the height of the mast.

From there the drilling fluid flows through a flexible high pressure hose to the top of the drillstring.

The flexible hose allows the fluid to flow continuously as the drillstring moves up and down during normal drilling operations.

swivel

The fluid enters in the drillstring through a special piece of equipment called swivel located at the top of the kelly.

The swivel permits rotating the drillstring while the fluid is pumped through the drillstring.
A swivel

drilling fluid in wellbore

In wellbore

The drilling fluid then flows down the rotating drillstring and jets out through nozzles in the drill bit at the bottom of the hole.

The drilling fluid picks the rock cuttings generated by the drill bit action on the formation.

The drilling fluid then flows up the borehole through the annular space between the rotating drillstring and borehole wall.

drilling fluid at surface

At surface

At the top of the well (and above the tank level), the drilling fluid flows through the flow line to a series of screens called the shale shaker.

The shale shaker is designed to separate the cuttings from the drilling mud.

Other devices are also used to clean the drilling fluid before it flows back into the drilling fluid pits.

Process of mud circulation

The principal components of the mud circulation system are:

pits or tanks,

pumps,

flow line,

solids and contaminants removal equipment,

treatment and mixing equipment,

surface piping and valves,

the drillstring.
Rig circulation system

The tanks

The tanks (3 or 4 –settling tank, mixing tank(s), suction tank) are made of steel sheet.

They contain a safe excess (neither to big nor to small) of the total volume of the borehole.

In the case of loss of circulation, this excess will provide the well with drilling fluid while the corrective measures are taken.

The number of active tanks depends on

the current depth of the hole (bypasses allow to isolate one or more tanks.)

The tanks will allow enough retaining time so that much of the solids brought from the hole can be removed from the fluid.

Mixing and suction tanks

MUD HANDLING EQUIPMENT

Rig sizing must incorporate mud handling equipment as these equipment determine the speed of drilling and the quality of hole drilled.

The equipment includes:

Shale Shakers

The type of mud (i.e. oil-based or water-based) determines the type of the shaker required and the motion of the shaker. Deep holes require more than the customary three shakers.

MUD HANDLING EQUIPMENT (Cont.)

Mud Pits

The number and size of pits is determined by the size and depth of hole.

Other factors include: size of rig and space available, especially on offshore rigs. The size of a mud pit is usually 8-12 ft wide, 20-40 ft long and 6-12 ft high.

Mud degasser

Centrifuges and mud cleaners

Desanders and desilters

reciprocating positive displacement pumps vs. centrifugal pumps

The great majority of the pumps used in drilling operations are

reciprocating positive displacement pumps (PDP).

Advantages of the reciprocating PDP when compared to centrifugal pumps are:

ability to pump fluids with high abrasive solids contents and with large solid particles,

easy to operate and maintain,

sturdy and reliable,

ability to operate in a wide range of pressure and flow rate.

positive displacement pumps compartments

PDP are composed of two major parts, namely:

Power end:

receives power from engines and transform the rotating movement into reciprocating movement.

The efficiency Em of the power end,

that is the efficiency with which rotating mechanical power is transformed in reciprocating mechanical power

is of the order of 90%.

Fluid end:

converts the reciprocating power into pressure and flow rate.

The efficiency Ev of the fluid end (also called volumetric efficiency),

that is, the efficiency that the reciprocating mechanical power is transformed into hydraulic power, can be as high as 100%.

Pump configurations

Rigs normally have two or three PDPs.

During drilling of shallow portions of the hole, when the diameter is large,

the two PDPs are connected in parallel

to provide the highest flow rate necessary to clean the borehole.

As the borehole deepens, less flow rate and higher pressure are required.

In this case, normally only one PDP is used while the other is in standby or in preventive maintenance. Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 18

Affecting parameters on flow rate

The great flexibility in the pressure and flow rate

is obtained with the possibility of changing the diameters of the pair piston–liner.

The flow rate depends on the following parameters:

stroke length LS (normally fixed),

liner diameter dL,

rod diameter dR (for duplex PDP only),

pump speed N (normally given in strokes/minute),

volumetric efficiency EV of the pump.

In addition, the pump factor Fp is defined as

the total volume displaced by the pump in one stroke.

Types of the positive displacement pumps

The heart of the circulating system is the mud pumps.

There are two types of PDP:

double-action duplex pump, and

single-action triplex pump.

Triplex PDPs, due to several advantages, (less bulky, less pressure fluctuation, cheaper to buy and to maintain, etc,) has taking place of the duplex PDPs in both onshore and offshore rigs.

CENTRIFUGAL PUMPS

This type uses an impeller

for the movement of fluid

rather than a piston reciprocating inside a cylinder.

Centrifugal pumps are used

to supercharge mud pumps and

providing fluid to solids control equipment and mud mixing equipment.

Duplex vs. Triplex pumps

A basic pump consists of a piston (the liner) reciprocating inside a cylinder.

A pump is described as single acting

if it pumps fluid on the forward stroke (Triplex pumps)

and double acting

if it pumps fluid on both the forward and backward stokes (Duplex).

Duplex pumps
Piston scheme (double action)
A duplex unit

Triplex pumps
Piston scheme (single action).
A Triplex unit

Pump liners

Pump liners fit inside the pump cavity.

These affect the pressure rating and flow rate from the pump.

For a given pump, a liner has the same OD but with different internal; diameters.

The smaller liner (small ID) is used in the deeper part of the well where low flow rate is required but at much higher operating pressure.

The size of the pump

is determined by the length of its stroke and the size of the liner.

the pump factor

The duplex mud pump consists of two double–action cylinders.

This means that drilling mud is pumped with the forward and backward movement of the barrel.

For a duplex pump (2 double–action cylinders) the pump factor is given by:

The triplex mud pump consists of three single–action cylinders.

This means that drilling mud is pumped only in the forward movement of the barrel.

For a triplex pump the pump factor is:

VOLUMETRIC EFFICIENCY

Drilling mud usually contain little air and is slightly compressible.

Hence the piston moves through a shorter stroke than theoretically possible before reaching discharge pressure.

As a result the volumetric efficiency is always less than one; typically 95% for triplex and 90% for duplex.

In addition due to power losses in drives,

the mechanical efficiency of most pumps is about 85%.

Pump Flow Rate

For both types of PDP, the flow rate is calculated from:

For N in strokes per minute (spm), dL, dR, and LS in inches, Fp in in3, and q in gallons per minute (gpm) we have:

Note that in this particular formulation, the volumetric efficiency of the pump is included in the pump factor.

Pump operating pressure

The horse power requirements of the pump depends on the flow rate and the pressure.

The operating pressure depends on

flow rate, depth and size of hole, size of drillpipe and drillcollars, mud properties and size of nozzles used.

A full hydraulics program needs to be calculated to determine the pressure requirement of the pump.

Pump Power

Pumps convert mechanical power into hydraulic power. From the definition of power P=Fv

In its motion,

the piston exerts a force [F] on the fluid that is equal to the pressure differential in the piston Δp times the area A of the piston, and

the velocity v is equal to the flow rate q divided by the area A, that is

For PH in hp, p in psi, and q in gal/min (gpm) we have:

pump factor & hydraulic power

Compute the pump factor in gallons per stroke and in barrels per stroke for a triplex pump having

5.5 in liners and

16 in stroke length,

with a volumetric efficiency of 90%.

At N = 76spm, the pressure differential between the input and the output of the pump is 2400 psi.

Calculate

the hydraulic power transferred to the fluid, and

the required mechanical power of the pump if Em is 78%.

pump factor & hydraulic power

The pump factor (triplex pump) in in3 per stroke is:

Converting to gallons per stroke and to barrels per stroke gives:

The flow rate at N = 76spm is:

The hydraulic power transferred to the fluid is:

To calculate the mechanical power required by the pump we must consider the efficiencies:

Surge Dampeners

Due to the reciprocating action of the PDPs, the output flow rate of the pump presents a “pulsation” (caused by the changing speed of the pistons as they move along the liners).

This pulsation is detrimental to the surface and downhole equipment (particularly with MWD pulse telemetry system).

To decrease the pulsation, surge dampeners are used at the output of each pump. Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 34

schematic of a typical surge dampener

A flexible diaphragm creates a chamber filled with nitrogen at high pressure.

The fluctuation of pressure is compensated by a change in the volume of the chamber.

A relief valve located in the pump discharge line prevents line rupture in case the pump is started against a closed valve.
Surge dampener

aim of the solids removal system

Fine particles of inactive solids are continuously added to the fluid during drilling.

These solids increase the density of the fluid and

also the friction pressure drop, but

do not contribute to the carrying capacity of the fluid.

The amount of inert solids must be kept as low as possible.

Recall mud is made up of

fluid (water, oil or gas) and solids (bentonite, barite etc).

The aim of any efficient solids removal system is

to retain the desirable components of the mud system

by separating out and discharging the unwanted drilled solids and contaminants.

Solids in drilling fluids classification: based on specific gravity, (or density)

Solids in drilling, classified by specific gravity, may be divided into two groups:

High Gravity Solids (H.G.S.) sg = 4.2

Low Gravity Solids (L.G.S.) sg = 1.6 to 2.9

The solids content of a drilling fluid will be made up of a mixture of high and low gravity solids.

High gravity solids (H.G.S) are added to fluids to increase the density,e.g. barytes,

whilst low gravity solids (L.G.S) enter the mud through drilled cuttings and should be removed by the solids control equipment.

Solids in drilling fluids classification: based on particle size

Mud solids are also classified according to their size in units called microns (μ).

A micron is 0.0000394 in or 0.001 mm.

Particle size is important in drilling muds for the following reasons:

The smaller the particle size, the more pronounced the affect on fluid properties.

The smaller the particle size, the more difficult it is to remove it or control its effects on the fluid.

particle size classification

The API classification of particle sizes is:

Particle Size (μ)Classification Sieve Size (mesh)

> 2000Coarse10

2000 -250Intermediate60

250 -74Medium200

74 –44Fine325

44 -2Ultra Fine -

2 -0Colloidal-

solids control equipment

Solids contaminants and gas entrapped in mud can be removed from mud in four stages:

Screen separation:

shale shakers, scalper screens and mud cleaner screens.

Settling separation in non-stirred compartments:

sand traps and settling pits.

Removal of gaseous contaminants

by vacuum degassers or similar equipment

Forced settling by the action of centrifugal devices

including hydrocyclones (desanders, desilters and micro-cones) Mud cleaners and centrifuges.

Complete mud removal system with mud cleaner and centrifuge

sketch of a typical solids control system

Figure shows a sketch of a typical solids control system (for unweighted fluid). Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 44

a typical two–screen shale shaker

The screens are vibrated by eccentric heavy cylinders connected to electric motors.

The vibration promotes an efficient separation without loss of fluid.
A two–screen shale shaker

Linear shale shaker

The figure shows

a layout for solids control equipment for a weighted mud system.

shale shaker mechanism

The shale shaker removes the coarse solids (cuttings) generated during drilling.

It is located at the end of the flow line.

It constitutes of one or more vibrating screens in the range of 10 to 150 mesh over which the mud passes before it is fed to the mud pits.
Shale shaker configurations

The procedure

Shale shakers and scalper screens (Gumbo shakers)

can effectively remove up to 80% of all solids from a drilling fluid,

if the correct type of shaker is used and run in an efficient manner.

Removal procedure:

Mud laden with solids passes over the vibrating shaker

where the liquid part of mud and small solids pass through the shaker screens and

drill cuttings collect at the bottom of the shaker to be discharged.

types of shaker operation

There are two types of shaker operation:

elliptical shakers and

Field experience indicate they work better with water based muds

linear motion shakers.

more suited to oil based muds.

An absolute minimum of three shale shakers is recommended and that

these shakers are fitted with retrofit kits to allow quick and simply replacements.

The shakers should also

be in a covered, enclosed housing with a means of ventilation and each shaker fitted with a smoke hood.

Sample of shale shakers

SETTLING SEPARATION IN NON-STIRRED COMPARTMENTS

The solids control pits work on an overflow principle.

The sand traps are the first of the solids control pits and are fed by the screened mud from the shale shakers.

There should be no agitation from suction discharge lines or paddles.

Any large heavy solids will settle out here and will not be carried on into the other pits. Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 52

Degassers

Gases that might enter the fluid must also be removed.

Even when the fluid is overbalanced, the gas contained in the rock cut by the bit will enter the fluid and must be removed.

The degasser removes gas from the gas cut fluid by creating a vacuum in a vacuum chamber.

The fluid flows down an inclined flat surface as a thin layer.

The vacuum enlarges and coalesce the bubbles.

Degassed fluid is draw from chamber by a fluid jet located at the discharge line.

Removal Of Gaseous Contaminants

Gas entrapped in mud must be removed

in order to maintain the mud weight to a level needed to control down hole formation pressures.

Gas is removed from mud using a vacuum degasser.

It is a simple equipment containing a vacuum pump and a float assembly.

The vacuum pump creates a low internal pressure which allows gas-cut mud to be drawn into the degasser vessel and it then flows in a thin layer over an internal baffle plate. Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 54

Vacuum degasser

The combination of low internal pressure and thin liquid film causes gas bubbles to

expand in size,

rise to the surface of the mud inside the vessel

and break from the mud.

As the gas moves toward the top of the degasser

it is removed by the vacuum pump.

The removed gas is routed away from the rig and is then either vented to atmosphere or flared.

A typical degasser diagram(A vacuum chamber degasser)

FORCED SETTLING BY CENTRIFUGAL DEVICES

Desanders and desilters are hydrocyclones and

work on the principle of separating solids from a liquid by creating centrifugal forces inside the hydrocyclone.

Hydrocyclones

are simple devices with no internal moving parts.

are classified according to the removed particle size as

desanders (cut point in the 40–45μm size range) or

desilters (cut point in the 10–20μm size range).

At the cut point of a hydrocyclone 50% of the particles of that size is discarded. Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 58

The process of the Hydrocyclones (Desanders and Desilters)

Mud is injected tangentially into the hydrocyclone

the resulting centrifugal forces drive the solids to the walls of the hydrocyclone and

finally discharges them from the apex with a small volume of mud.

The fluid portion of mud

leaves the top of the hydrocyclone as an overflow and

is then sent to the active pit to be pumped downhole again.

Desanders

The primary use of desanders is

in the top hole sections when drilling with water based mud to help maintain low mud weights.

Desanders

should be used if the sand content of the mud rises above 0.5% to prevent abrasion of pump liners.

should never be used with oil based muds,

because of its very wet solids discharge.

The desander

It is a set of two or three 8in or 10in hydrocyclones,

and are positioned after the shale shaker and the degasser (if used). Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 61

Desilters

The desilter

is a set of eight to twelve 4in or 5in hydrocyclones.

It removes particles that can not be removed by the desander.

Desilters, in conjunction with desanders, should be used to process low mud weights used to drill top hole sections.

If it is required to raise the mud weight this must be done with the additions of barytes, and not by allowing the build up of low gravity solids.

Desilters should never be used with oil based muds.

Solid control equipment

Typical throughput capacities are:

Desanders

12"cone500 gpm per cone.

6" cone125 gpm per cone.

Desilters

4"cone50 gpm per cone.

2" cone15 gpm per cone.
(b) Desilter

Particle size classification

A typical drilling solid particle distribution and particle size range classification are shown in the diagram.

The diagram includes the particle size distribution of typical industrial barite used in drilling fluids.

Decanting centrifuge

The centrifuge is a solids control equipment which separates particles even smaller,

which can not be removed by the hydrocyclones.

It consists of a rotating cone–shape drum, with a screw conveyor.

Drilling fluid is fed through the hollow conveyor.

The drum rotates at a high speed and creates a centrifugal force that causes the heavier solids to decant.

The screw rotates in the same direction of the drum but at a slight slower speed, pushing the solids toward the discharge line.

The colloidal suspension exits the drum through the overflow ports. Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 65

Internal view of a centrifuge

The drums are enclosed in an external,

non–rotating casing not shown in the figure.

Mud Cleaners

A mud cleaner

is a desilter unit in which the underflow is further processed by a fine vibrating screen, mounted directly under the cones.

The use of mud cleaners with oil based muds should be minimized since

experience has shown that mud losses of 3 to 5 bbls/hr being discharged are not uncommon,

coupled with the necessity to adhere to strict environmental pollution regulations.

mud cleaner

Inert solids in weighted fluid (drilling fluid with weight material like barite, iron oxide, etc) can not be treated with hydrocyclones alone

because the particle sizes of the weighting material are within the operational range of desanders and desilters.

Weighting material are relatively expensive additives, which must be saved.

mud cleaner schematic

The mud cleaner separates

the low density inert solids (undesirable)

from the high density weighting particles.
Unit of a mud cleaner

Hydrocyclones

Hydrocyclones discriminate light particles from heavy particles.

Bentonite are lighter than formation solids because they are of colloidal size (although of the same density).

Barite particles are smaller than formation solids because they are denser.

The desilter

removes the barite and the formation solids particles in the underflow,

leaving only a clean mud with bentonite particles in a colloidal suspension in the overflow. Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 71

Hydrocyclones (Cont.)

The thick slurry in the underflow

goes to the fine screen, which separate the large (low density) particles (formation solids) from the small (high density) barite particles, thus conserving weighting agent and the liquid phase but at the same time returning many fine solids to the active system.

The thick barite rich slurry is treated with dilution and mixed with the clean mud (colloidal bentonite).

The resulting mud is treated to the right density and viscosity and re–circulates in the hole. Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 72

Principle of the mud cleaner

Mud cleaners

are used mainly with oil–and synthetic–base fluids where the liquid discharge from the cone cannot be discharged, either for environmental or economic reasons.

may also be used with weighted water–base fluids

to conserve barite and the liquid phase.
A diagram of a mud cleaner

Drilling fluid components

Drilling fluid is usually a suspension of clay (sodium bentonite) in water.

Higher density fluids can be obtained

by adding finely granulated (fine sand to silt size) barite (BaSO4).

Various chemicals or additives are also used in different situations.

The drilling fluid continuous phase is usually water (freshwater or brine) called water–base fluids.

When the continuous phase is oil (emulsion of water in oil) it is called oil–base fluid. Fall 14 H. AlamiNia Drilling Engineering 1 Course (3rd Ed.) 75

Mixing Equipment

Water base fluids are normally made at the rig site

(oil base mud and synthetic fluids are normally manufactured in a drilling fluid plant).

Special treatment and mixing equipment exists for this purpose.

Tank agitators, mud guns, mixing hoppers, and other equipment are used for these purposes.

drilling fluids physical propertiesblenders

The basic drilling fluids physical properties are density, viscosity, and filtrate.

Fresh water density is 8.37 pounds per gallon (ppg).

Bentonite adds viscosity to the fluids and also increases the density to about 9 to 10 ppg.

Higher density (15 to 20 ppg) is obtained with barite, iron oxide, or any other dense fine ground material.

Tank agitators or blenders

are located in the mud tanks

to homogenize the fluid in the tank.

help to keep the various suspended material homogeneously distributed in the tank

by forcing toroidal and whirl motions of the fluid in the tank.

Mud agitator
Tank agitators or blenders
toroidal and whirl motions

Sample of Mud agitator

Mud guns

Mud guns

are mounted in gimbals at the side of the tanks,

allow aiming a mud jet to any point in the tank

help to homogenize the properties of two tanks, and spread liquid additives in a large area of the tank (from a pre-mixed tank).

Centrifugal pumps power the mud guns.
Mud gun

mixing hopper

The mixing hopper

allows adding powder substances and additives in the mud system.

The hopper is connected to a Venturi pipe.

Mud is circulated by centrifugal pumps and passes in the Venturi at high speed, sucking the substance into the system.

Mud hopper

1.(CDF) Jorge H.B. Sampaio Jr. “Drilling Engineering Fundamentals.” Master of Petroleum Engineering. Curtin University of Technology, 2007.
Chapter 2
2.(WEC) Rabia, Hussain. Well Engineering & Construction. Entrac Consulting Limited, 2002.
Chapter 16 and 7

Drilling Fluid Circulation Systems

Drilling Fluid Circulation Systems

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