3. well engineering, well design and drilling fluids

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MSc. Oil and Gas Enterprise Management Well Engineering Module

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Page 1: 3. Well Engineering, Well Design and Drilling Fluids

MSc. Oil and Gas Enterprise

Management

Well Engineering Module

Page 2: 3. Well Engineering, Well Design and Drilling Fluids

Well Engineering – Course Outline

• Drilling process, rig types and rig equipment

• Well design & well planning

• Drilling fluids & mud conditioning equipment

• Drillpipe & drillstring design

• Drilling Bits• Directional Drilling

• Casing design• Cementing• Hole problems & stuck

pipe• Evaluation• Well control & BOP’s• Completions• Complex wells• Risk management• Enabling technologies

Page 3: 3. Well Engineering, Well Design and Drilling Fluids

Engineering a Well

• Fully integrated team approach

• All disciplines represented from beginning

• All have an important part to play

• Iterative Process

Page 4: 3. Well Engineering, Well Design and Drilling Fluids

Well Planning

• Optimum Design a Fit-for-Purpose Well:– a well which contributes , over its life-cycle,

maximum monetary value, without compromising safety and environmental standards

Page 5: 3. Well Engineering, Well Design and Drilling Fluids

– Geological Column

• Formations to be drilled

• Rock types • Age of formations• Drillability

– Reactive or not?– Mobile?– Geopressured?– Unconsolidated?– Fractured?– Faulted?

Geological Prognosis

Well Design – Reaching the Target

Page 6: 3. Well Engineering, Well Design and Drilling Fluids

Well Design

• Formation & Fracture Gradients – Formation Pressure is the pressure exerted by the

formation fluids within the pore spaces of the rocks. – Formation pressures can be normal, over normal or

sub normal.• Normal pressure occurs when there is full pressure

communication between the pores of the rock throughout the geological column.

• Over pressure occurs when impermeable boundaries prevent fluid communication and trapped fluids support a larger proportion of the overburden.

• Sub-normal pressure occurs when a full column of interconnected pore fluid does not reach the surface.

Page 7: 3. Well Engineering, Well Design and Drilling Fluids

Well Design

• Fracture Gradient• Rock strength: tensile,

compressive, shear, or impact.

• Tensile strength important for fracture gradient. Rocks are more liable to fail in tension than compression.

• Fracture pressure is that pressure required to overcome rock strength and fracture the formation.

• Determination of Fracture Gradient– Direct: Leak-off test.– Indirect: Mathematical

modelling.

Leak-Off Tests (LOT) and Formation Integrity Tests (FIT)

Page 8: 3. Well Engineering, Well Design and Drilling Fluids

Well DesignHydrostatic Pressure

– The pressure created by a column of fluid due to its density and vertical height.

– Hydrostatic pressure (Phyd) = ρ.g.h in consistent units.

In API Units

(Phyd) = mud weight (ppg) x true vertical depth (TVD ft) x .052

e.g. Static bottom hole hydrostatic pressure at 13,000ft md (8,000ft tvd) with 12.5 ppg mud = 12.5 x 8000 x 0.052

= 5,200 psiCirculating Pressure

– When fluid is circulated in a well, additional pressure must be applied to overcome fluid friction.

Page 9: 3. Well Engineering, Well Design and Drilling Fluids

Well Design

BHCP = Phyd + APL

Where: BHCP = bottom hole circulating pressureAPL = annular pressure loss

• APL is the only frictional pressure loss in the circulating system that acts directly on the well bore.

• APL is the differential pressure over the length of the annulus required to move fluid up the annulus.

BHCP

APL

Circulating Pressure– A pressure must be applied to circulate the fluid in the well.– This additional pressure is provided by the mud pumps.– Pressure is lost as energy is consumed circulating fluid around the well.

Page 10: 3. Well Engineering, Well Design and Drilling Fluids

Downhole Pressures• The pore spaces of subsurface formations are filled with fluids.• The most common fluid is water but oil or gas may be present.

• In normally pressured formations, the pressure at any given depth is the hydrostatic pressure of the fluids filling the interconnected pore spaces of the subsurface formations above that depth.

• Pore pressure (also called formation pressure) varies with fluid density, vertical depth and geological history.

• Abnormally pressured formations have higher or lower pressures than normally pressured formations.

• To keep primary control of the well, the hydrostatic pressure of the drilling mud must exceed formation pressure.

Page 11: 3. Well Engineering, Well Design and Drilling Fluids

Circulating System

Ppump = Sum of well system pressure losses

= losses in surface lines, drill pipe, BHA, bit and annulus

Surface lines

Mud Pump

The only system pressure loss that acts on the bottom of the hole is the annular pressure loss (APL).

BHCP = Phyd + APL

Drill pipe

Drill collars

Drill bit

Page 12: 3. Well Engineering, Well Design and Drilling Fluids

Drilling and Completion Fluids

Functions of Drilling Fluid

October 2008

Page 13: 3. Well Engineering, Well Design and Drilling Fluids

Mud Conditioning Equipment

The solids control system shown is for a land well but the same type of equipment is used offshore.

Page 14: 3. Well Engineering, Well Design and Drilling Fluids

Functions of Drilling Fluids

1. Primary Functionsa. Drill cuttings removal and hole cleaning.b. Suspend cuttings when circulation is stopped.c. Control downhole formation pressures.d. Release the cuttings at surface.e. Seal off permeable formations with a thin

elastic wall cake.f. Stabilise the wall of the wellbore.g. Transmit hydraulic energy to the bit.h. Lubricate, cool and support the bit and drill

string.

Page 15: 3. Well Engineering, Well Design and Drilling Fluids

Lubricate, Cool and Support the Drill String• During the drilling operation considerable heat is generated by:

– work done at the bit– overcoming frictional forces between the drill string and hole wall– hydraulic energy expended at the bit.

• Circulation of the drilling fluid:– keeps the bit and drill string cool.– reduces friction between hole wall and the rotating drill string.

• In highly deviated wells, a drill fluid with good lubricity is essential.

• Oil and synthetic-base muds lubricate better than most water base muds but they are less efficient at cooling the bit and drill string.

• Lubricating properties depend on solids content - high solids content reduces lubricity.

• Additives are sometimes used to improve the lubricity of WBM. • The drilling fluid supports the drill string by the action of

buoyancy and by damping out drill string vibrations.

Page 16: 3. Well Engineering, Well Design and Drilling Fluids

Functions of Drilling Fluids

2. Secondary Functionsa. Prevent drillstring, casing and wellhead

corrosionb. Maintain stable properties under all downhole

conditionsc. Allow effective formation evaluationd. Avoid environmental pollutione. Minimise wear on circulation system

componentsf. Minimise the risk of sticking the drill string.

Page 17: 3. Well Engineering, Well Design and Drilling Fluids

Drilling Fluids (or Drilling Mud)

• Properties of Drilling Fluids– Density

• Weighting agents – barite, calcium carbonate– Viscosity or resistance to flow– Fluid loss control – filtrate, filter cake– Formation protection

• Zone dependant– Temperature Tolerance

• Additives & system stability at downhole temperature

Page 18: 3. Well Engineering, Well Design and Drilling Fluids

Drilling Fluids (or Drilling Mud)

• WBM– Most basic– More environmentally friendly– Fresh water, salt water, salt

saturated– Clays – viscosity and yield e.g.

bentonite– Polymers – filtration control,

viscosity, thinners

• OBM– Emulsion of oil and water– Inhibition of water sensitive

formations- shales– Lubricity– Temperature stability– Formation damage– Environmentally unfriendly –

spillage, drill cuttings disposal etc

Composition of Drilling FluidsRequired functions & properties achieved with range of fluids with differing compositions. They are composed of various combinations of solids, liquids, gases and classified according to continuous phase

Page 19: 3. Well Engineering, Well Design and Drilling Fluids

Hole Cleaning1. Cuttings should be removed

quickly from the bottom of the hole. If not the cutting action of the bit will be less efficient as old cuttings are reworked while new formation is being drilled.

2. Cuttings should be removed from the hole at the same rate as they are created. Failure to do this will result in cuttings build up, which in turn may cause hole problems.

3. Cuttings removal depends upon the annular velocity of the drill fluid, drill string rotation, rate of cuttings generation (ROP) and cuttings slip velocity.

Page 20: 3. Well Engineering, Well Design and Drilling Fluids

Minimising Formation Damage• Excessive filtrate and fine mud solids

invade the formation when we have:– High overbalance in the well– a poor quality filter cake.

• Invasion can seriously impair the porosity and permeability of the formation near to the wall of the hole in two ways:

– The mud filtrate can interact chemically with the formation causing clays and shales to swell

– The mud solids can block up the pores of the rock.

• Invasion damage (also called skin damage) can seriously reduce the productive capacity of the well. (Less oil produced per psi of drawdown.)

Page 21: 3. Well Engineering, Well Design and Drilling Fluids

Longitudinal stress acts vertically

σz = σ2

Maintaining Hole Stability

• When a hole is drilled, the equilibrium state of stress near the hole wall is disturbed.

• Stresses once carried by the rock removed by drilling are transferred to the rock remaining beyond the bore of the hole.

• As a result, circumferential stresses (sometimes called hoop stresses) are created around the wellbore.

• The drilling mud in the hole provides support to the wellbore by creating a radial stress.

Circumferential stress acts around the bore of the hole

σθ= σ1

Radial stresses acts towards the hole wall

σr = σ3

Usually in the UKCS,

σ1> σ2 > σ3

Page 22: 3. Well Engineering, Well Design and Drilling Fluids

Longitudinal stress acts vertically

σz = σ2

Maintaining Hole Stability

• Radial stresses act normal to the wall of the hole and reduce the circumferential stress.

• Borehole collapse occurs if the difference between the circumferential stress and the radial stress exceeds the shear strength of the rock forming the hole wall.

• Hole stability also depends on the degree of chemical interactionbetween the mud and the formation.

Circumferential stress acts around bore of the hole

σθ= σ1

Radial stresses acts towards the hole wall

σr = σ3

Usually in the UKCS,

σ1> σ2 > σ3

Page 23: 3. Well Engineering, Well Design and Drilling Fluids

Mud Solids Control

• Mud returned from the bit is loaded with drill cuttings and occasionally cavings caused by hole instability.

• Effective removal of these solids at the surface is essential before the mud is pumped back down the well.

• Poor solids control leads to a build up of fine solids in the mud system which adversely affects mud rheology, wear on equipment and rates of penetration.

• The alternative to good solids control is continuous dilution of the mud, which is very expensive in terms of mud chemical usage.

Page 24: 3. Well Engineering, Well Design and Drilling Fluids

Drill Cuttings

• Drilling cuttings are created at the bottom hole by the bit.• Hole cavings can be created by hole instability and/or the

movement of the drill string against the wall of the hole.• Drill cutting density affects how easily they may be

removed from the hole.• Rock density depends on the mineral or minerals making

up the rock and the porosity of the rock.• Most sedimentary rock densities range from 2.6 - 2.8 S.G.• Mud weighting materials have higher densities, e.g. barite

at around 4.2 – 4.3 S.G. • The shape of drill cuttings may give a clue to downhole

formation pressures. Acicular shaped shale cuttings are often associated with over pressured shales.

Page 25: 3. Well Engineering, Well Design and Drilling Fluids

Suspension and Release of Cuttings1. When circulation is stopped, drill cuttings must be held in

suspension in the mud and not allowed to fall down hole at their slip velocities.

2. Drilling fluids are thixotropic, i.e. they gel up when they stop moving. Gelling is usually sufficient to hold the cuttings in suspension i.e. slip velocity tends to zero.

3. When circulation is resumed, gels are broken as the mud “shear thins” and cuttings resume their upward path.

4. When the cuttings reach the surface, they are separated from the mud at the shale shakers. Finer solids are then separated from the mud using desanders, desilters and/or mud cleaners.

5. Highly viscous mud makes it difficult for solids control equipment to separate the cuttings from the drilling fluid.

Page 26: 3. Well Engineering, Well Design and Drilling Fluids

Annular Velocity and Slip Velocity• High annular velocities (Va) improve

cuttings removal but….• High annular velocities may cause

unwanted side effects, e.g. wash outs and high ECDs.

• The rate at which cuttings settle in a drilling fluid is called the slip velocity.

• Slip velocity (Vs) depends upon: Cuttings shape, size and densityDrill fluid density and viscosityAnnulus flow regime

Provided that Va>Vs, the cuttings will be lifted to the surface. This is called the transport velocity (Vt).Vt = Va – Vs Stoke’s Law can be used to calculate Vs.

• Va = Q/(π ( Dh2 – Dp)2/ K)

Where:Va = annulus velocity

(ft/min)Q = pump rate (bbl/min)Dh = hole diameter (in)Dp = pipe diameter (in)K = 4 x 144 x 5.61 = 3,231

Page 27: 3. Well Engineering, Well Design and Drilling Fluids

Mud Solids Control Equipment

Linear motion shale shaker.

•Shale shakers remove drill cuttings from the return mud flow.

•Modern offshore rigs may have up to four shale shakers operating in parallel.

•When fitted with fine mesh screens (200/250 gauge) they are able to remove particles down to sand size.

•Two electric motors rotating in opposite directions vibrate the shaker screens on an inclined plane at several hundred c/min.

Mud cleaner.

Desander

Used to remove course, sand sized particles from the return mud flow.

Page 28: 3. Well Engineering, Well Design and Drilling Fluids

Linear Motion Shale Shaker

• Two eccentric shafts are rotated in opposite directions ‘in phase’.

• Because counterweights rotate in opposite directions, net force on the shaker is zero except along the line of motion passing through the C of G of the rotating shafts.

Counterweights

Page 29: 3. Well Engineering, Well Design and Drilling Fluids

Hydrocyclones

• Induced vortex throws solids to the outer wall by centrifugal force. • These solids spiral down under gravity and exit at the cone apex. • Another spiral causes the mud in the centre of the cone to spiral upwards

and exit through the vortex finder.• A mud cleaner is a combination hydrocyclone and shaker screen.

Page 30: 3. Well Engineering, Well Design and Drilling Fluids

Principle of the Decanting Centrifuge

Page 31: 3. Well Engineering, Well Design and Drilling Fluids

Twin Centrifuges for Barite Recovery

• Mud in the holding tank is then fed to a High Speed Centrifuge via a 2nd positive displacement pump.

• Low gravity solids removed from this mud are discharged overboard.

• Processed drilling fluid from this centrifuge is then returned back to the active system.

• Process rate approximately 2 bpm.

• Drilling fluid from active system is fed to a Low Speed Centrifuge via a positive displacement pump.

• The barite removed from the mud is returned via gravity from the solids discharge end of Centrifuge No. 1 back to the active system.

• The mud from Centrifuge No. 1 is gravity fed to a holding tank.

Page 32: 3. Well Engineering, Well Design and Drilling Fluids

Mud Mixing Hopper

• The jet mixing hopper is simple to operate and very reliable.

• However, it injects a large amount of air into the mud system that encourages corrosion and other problems.

Jet Nozzle

Page 33: 3. Well Engineering, Well Design and Drilling Fluids

Paddle Mud Agitators

• Used to prevent suspended mud solids from settling out of the mud.• Position in the mud tank is important for good mud agitation. • 10 HP motor drives the impeller through reduction gearing.• Mud guns may supplement the paddle agitators.• Mud guns are supplied from ring main drawing from the active tank.

Page 34: 3. Well Engineering, Well Design and Drilling Fluids

Vacuum Degasser

• A typical vacuum degasser consists of a cylindrical chamber taking gas cut mud from the sand trap, degassing it and then discharging the degassed mud back into downstream active mud tanks.

• The chamber above the fluid level is kept at a partial vacuum by a compressor.• Gas from the compressor outlet is routed to a flare line which offshore is routed to the

top of the derrick or mast.