Oil Drilling Overview

7/30/2019 Oil Drilling Overview

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ON OIL-DRILLING - A BRIEF OVERVIEW

A drill is a mining construction achieved through complex crust works , the primary elements

of which are: well spring, bottom hole and well wall, during the actual drilling phase

(dislocation and evacuation to the surface of the resulteddrill cuttings) followed, atcertain depths, by consolidation through casing and grouting operations. Dislocation at

the bottom hole is performed using drill bits or core cutter heads, and their connection

with the surface drilling unit is realised through the drilling column. Cleaning of the

bottom hole and drill cuttings evacuation is achieved through the circulation of the

drilling fluid (mud). The drilling rig allows, through the component subsets, a safe

achievement of the necessary operations for execution of a drill.

Rocks forming the earth's crust are natural mono or poliminerale aggregates

distinguished by chemical composition, mineralogy, structure and texture.

There are three main types of rocks in relation to their formation: igneous rocks (through

cooling of magma), sedimentary rocks - through transport and deposition of

detritus, derived from destruction of magmatic and metamorphic rocks; metamorphic

rocks result of the first two categories, under the action of temperature and / or high

pressure (simultaneous or separate action). Dislocation process (cutting, crushing,

erosion) of rocks (with drill bits or core cutter heads) and its efficiency is directly related

to the physico-mechanic features of the rocks which will determine the type of the rock

discolation tools, the system of drilling, and the necessary technical procedures for an

optimum drilling process. From the drilling point of view (dislocation efficiency), there

are only few physical features that are of interest.

Porosity describes the volume of spaces, in the interior of an unoccupied solid substance

rock. Porosity influences the mechanical features of the rock. Because of the prezence of

the pores, the resistance and toughness to the penetration are diminished. The elasticity of

the minerals and the presence of gas in the pores, offer the rock an increased

compressibility. Under the pressure of a solid body, until a certain value of the pressure

force, the rock will distort without achieving dislocation.

Permeability the capacity of a porous body of allowing, through its connective pores, the

circulation of a fluid. Because of the permeability of the rock, a drilling penetration

occurs at the bottomhole, or, especialy, a filtration on a certain depth. This causes a

decreasement, or even balancing, of the pressure difference between the region from

above and the regiom from underneath the bottomhole, creating favourable conditions for

dislocation of rocks by the active elements of the work instruments.

Heterogeneity is one of the specific features of the rocks. These are heterogeneous bodies,

from various points of view: mechanical constituion, porosity, permeability.
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The tension in the rock, when penetrated by a solid body, is extremely complex. It is

determined by the mechanical features of the rock and its composition. The tension state

is more uniform if the dimension of the component grains or cristals are much smaller

then the dimensions of the contact surfaces between the penetration elements and the

rock. As the pressure of the dislocation instrument increases, the tension keeps growing

in the rock and finaly achieves a limit-pressure. Further increasing of the stress is

followed (depending on the type of rock) by dislocation or plastic deformation. Because

of the complexity of the tension estate and the strong connection between the features,

aspect and intensity of the pressure, it is hard to make a clear separation between the

elastic deformation to plastic deformation and finally, to dislocation.

The penetration depth of the dislocation intrument does not depend on its pressure on the rock

only. A substantial influence is expressed by the period of time and the speed of the

application on the formation. At a certain pressure, and at a certain given striking speed,

achiving a maximum depth is only possible through if the critical value is passed. Thehigher the striking speed, the higher the value of the critical period. If the period passes

time passes the critic one, increasing the striking speed leads to increasing the penetration

depth.

The main factors in the drill that influence the mechanical features of the rocks and over the

efficiency of the dislocation are temperature, pressure and fluid.

During the dislocation, as a consequence of the circulation of the drilling fluid, the

temperatura of the rock at the bottomhole decreases, nevertheless keeping high values.

When the temperature increases, the domain of the plastic deformation also increases,which results in decreasing the rezistance. The influence of the temperature proved to be

very important in various cases, having a more pronounced effect of the rezistance

decreasement, then the resitance increasement produced by the exterior pressure.

Considering a certain depth in the rock, we may consider the following pressures: lateral

thrust, rock pressure, and formation pressure.

Rock pressure is given by the weight of the rocks from above

Lateral thrust is produced by the rock pressure

Formation pressure is the pressure of the drilling fluid in the rock pores


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The three-wing bits are used in harder work conditions with more pressure on the bit. It has

a good centering, therefore less possibilities of deviations, a better functioning and a good

evacuation of the drill cuttings, thanks to the three flushing holes positioned close to the

bottomhole.

The parts close of the two way leads and three wing bits that come in contact with the rock are

reinforced with tough material and by welding.

Diamond bit

Diamond bits are used for dislocation of a large range of rocks, starting with less harder rocks,

finishing with extra-hard ones. They are very efficient for abrasive rocks, especially in

depth, and characterized by great resistance to wear, so through large periods of time of

use. The active elements of those bits are industrial diamonds. The diamonds are

distributed on the contact surfaces of the bit with the rock so that when the bit rotates, it

will cover the bottomhole and calibrate the well wall. Depending on the nature of the

rocks, the diamonds have different sizes: big diamonds are used for subtirirocks and

small diamonds for tough rocks.

Rolling cutter bits

The active elements of the rolling cutter bit are generally called bit teeth, positioned on rolling

bodies, called cutter. Rolling of the cutters on the bottomhole is the result of the bit

rolling around its axis. The contact of the bit teeth with the rock is periodic and time

limited. Because of the pressure on the bit, the bit teeth strike and penetrate the rock atthe bottomhole. The penetration depth and the impression shape depend on a range of

factors: the nature of the rocks and the pressure estate at the bottomhole, bottomhole

cleaning degree, shape and dimension of the bit teeth, strike strength and speed, the

pressure on the bit teeth and movements performed by the bit teeth on the period of time

of contact with the rock. Simple penetration of the bit teeth determines what we call

crushing.

The dislocation capacity of a rolling cutter bit increases directly proportional with the increase

in the active elements slipping degree. But as the slipping degree increases, the friction

between the bit and the rock also amplifies. As a consequence, the wear speed increases.Trough construction, a well established report between the rolling and slipping

movements is established, for different types of rolling cutter bits, depending on the rock

nature.

Core barrel


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During drilling of a well, especially exploration and coring wells, for the study of the

penetrated rocks, (mineralogical and Petrographiccomposition, the nature of the useful

substances contained), and the conditions in which these rocks are (stratigraphic and

tectonic), samples are taken. These samples are called boring kernel, the operation is

called mechanical coring, and instruments are called core barrel. The boring kernels are

taken from the bottomhole or from the well wall, if the stratum has already been crossed.

Core barrels for bottomhole coring

Depending on their main construction, we have: single core barrel and double core barrel .

regradless of their type, core barrels have the actual rock dislocatin instrument at the

inferior position at their inferior part, called core cutter head.

Single core barrels are used only in case of consolidated rocks. They are formed by a coring

tube, which has, at its superior part, a fillet, for the drilling column, and at the inferior

part, the core cutter head. In the interior of the coring tube, there is a core breaker. The

circulation of the drilling fluid is performed through the annulus between the assay and

the coring tube, the boring kernel having smaller diameter then the interior diameter of

the tube.

Double core barre have the largest utilisation in the drilling industry of petroleum and gas.
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Drill column

The connection between the dislocation instrument and the surface installation is performed

through the drilling column. At the fluid circulation drilling, the component elements of

the drilling fluid are tubular-shaped, and the assembly is made through threads.

In different phases of work, the drilling column fulfills the following functions:

Transmits the rotation movement from the surface to the bit

Assures the axis thrust on the bit for dislocation of the rock, pressure given by a part of the

weight.

Builds up an interior drain and a second exterior drain (with the well wall) through which

clean drilling fluid passes along to the bottomhole and then, charged with detritus, to the

surface.

Represents the introduction and extraction assembly of the bits and other special instruments.

Assures the accomplishment of other auxiliary operations: mechanical coring, stratum

sampling.

The drilling column is formed by heavy-weight drill pipes, drilling rods, and griefstems,

connected by fillets and cross-overs.

Heavy-weight drill pipes are located at the inferior part of the drilling column, and thegriefstems, at the superior side. Between them, there are drilling rods, which represent the

longest part of the drilling column. At the inferior end of the heavy-weight drilling pipes,

the bit or the submersible engine is assembled and at the superior side of the griefstem,

the swivel casing.

Heavy weight drill pipe

Heavy-weight drill pipes have the role to perform, through their own weight, the pressure of

the axis thrust on the bit, and to mantain, through rigidity, the direction of the drillhole,

preventing deviation. Another element that contributes to mantaining the direction of thedrillholeis the fact that the game between the well wall and the exterior of the heavy-

weight drill pipes is reduced, compared to the one with the drilled rod.

In normal conditions, the thrust on the bit is realized with 70% - 80% of the weight of the

heavy-weight drill pipes but there are cases where the axis thrust is reduced (20% - 30%

of the weight) because of the crossing of geologic formations with strong deviation

tendencies.


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Drilling rods are 9 meters long pipes, which have a dowel at one end, and a sleeve at the

other, both threaded, in order to be connected in between. It is composed out of three

elements: the actual drilling rod (the body or the pipe of the rod) and two welded details

at its ends the dowel and the sleeve the tool joint.

Griefstem

The griefstem is the connection between the drilling column, the drilling rod and the swivel

casing. It receives the rotation movement from the turn table, through adapter pieces. To

be able to receive the movement, in longitudinal displacement condition, the griefstem

has shaped body to the exterior. Therefore, in cross section, the griefstem is square,

hexagon or octagon shaped. The most popular are the square and hexagon shaped

grieftems.

During the work process, the drilling column is exposed to various kids of stress. The nature

of the stress is determined by the drilling method, the conditions of the drill, and the

operation that is to be made. The conditions may be static or dynamic, may act separately

or simulaneous, and may act in dependency or in interdependence.

Manipulating the drilling column

During the manouvering operations ( introduction and extraction), specific tools are used at

the surface, which form the so called small equipment.

The links are used in pairs and connect the crane hook with the hoist. They are made up of

alloy steel cylinder bars, with end with slings.

The hoist sustains the drilling column. It is suspended in links.

The drill pipe slips assure the sustainment of the drilling column at the well spring, through

floats, that hold on to the exterior of the rods. For small weights, small drill pipe slips are

used, and for bigger weights, longer ones. Apart from the manually maneuvered drill pipe

slips, there are also mechanic ones, and pneumatically-actioned ones.

The rotary slide tongs are used for screwing and unscrewing the rods from the drill pipe

threads. The maneuvering is done manually but there are also mechanic ones, to which

shifting to and from the well spring are realized through electric engine or pneumatic.

The maneuvering operations of the drilling column requires increased carefulness of the

performer personnel, as well as taking appropriate measures for avoiding accidents:

wearing the protection helmet, using the special gloves. A continuous instruction of the

staff must be assured, and a careful supervision of the process, insisting the interdiction

of activity on the radius of action of moving equipments. Similar measures are imposed

when operations of introducing the casing column are performed.


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Drilling fluids

The system through which circulation of the drilling column is performed is divided in two

subsystems: exterior and interior. The exterior part of the circulation system contains the

elements located at the surface: ambar, pumps, pulsation dampeners, main piping

systems, stand pipe, boring hose, swivel casing, and cleaning equipment on the way out

of the wellbore. Then, the interior part of the system is formed out of the elements that

are in the drill: drilling column, bit and the annulus (between the drilling column and the

well wall).

The normal circuit of the drilling fluid is: ambar pump main piping system stand pipe

boring hose swivel casing drilling column bit annulus cleaning equipment

ambar. The circulation that is performed as presented above is called straight circulation.

Sometimes, in case of special operations, the circulation is performed in reverse; it is

pumped through the annulus and raises through the drilling column, realizing the socalled inversed circulation.

At the beginning, the role of the fluid in drilling was summed up to removing the drill cuttings

from the bottomhole, and evacuating it to the surface. Later, the role of the fluid

amplified and demands regarding its features increased.

During the drilling process, the drilling fluid fulfills the following:

Transports the drill cuttings to the surface, cleaning the bottomhole.

Maintains the drill cuttings in suspension when the circulation is stopped.

Creates counter-pressure on the well wall, which stops crumbling of the instable rocks or

deformation of the plastic ones, and does not allow in the wellbore fluids from the

crossed geologic formations

Clogs the well wall with a thin layer of solid particles, which isolate the drill from the crossed

rocks.

Cools and lubricates the bit and the drilling column

Contributes at the dislocation of the thin rocks

Reduces resistance of rocks, through moistening and absorption

Apart from these functions, the drilling fluids must also correspond to a range of

requirements:

It must not affect the affect the rocks that it crosses

It must be resistant to pressures and temperatures in the drill


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Must not cause erosion and corrosion of the drilling equipment

Must not be toxic or represent a element of fire risk

Must be easy to prepare and to maintain

Must be cheap and need preparation materials and substances that are easy to achieve

Not all drilling fluids fulfill the requirements presented above. Good quality drilling muds and

some special composition fluids have larger use, corresponding to most of the drilling

conditions imposed for a efficient drilling.

Water was the first drilling fluid. Having a low viscosity, and relatively reduced density, water

constitutes, when it can be used, a good drilling fluid. Thanks to certain advantages -

possibility of work with higher flow capacity, efficient cleaning of the bottomhole and the

affected parts of the bit, and the fast balancing of the differential pressures from thebottomhole a high-speed drilling is performed.

Still, usage of water is limited. The drilling fluids that are mostly used are drilling muds.

Depending on their composition, they can be divided in:

Natural or untreated muds

Treated muds, obtained by adding certain substances to natural ones, in order to decrease their

physic features.

Special muds, where the main role is no longer held by the mixture of water clay but by the

substances added in order to reduce viscosity and filtration.

Emulsion muds, obtained through natural mud, treated or special, mixed with petroleum , in

the presence of a emulsifier; the emulsion is oil mixed with water type.

Drilling mud is a mixture formed by two phases: liquid water, and solid clay. Actually,

drilling mud is not a simple mixture but a disperse, heterogeneous system. Its continuous

phase is water, where we can find dissolved reagents, for control of viscosity, and the

discontinuous phase is made out of clays, hardening materials, drill cuttings particles.Because the solid particles have very different sizes, the drilling muds are polydispers

systems.

Natural muds, built up during drilling, out of water and dislocated clay, or prepared at the

surface using moistening and dispersible clays, is the simplest of the drilling muds

Treated muds are obtained through adding relatively small quantities of reagents for

decreasing viscosity and filtration, as well as hardening materials, to the natural muds.

Because the base of the treated muds is clay, they remain sensitive to contaminate agents.


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Special muds are used in case of intense and long term contamination, where treated muds no

longer correspond. As it was mentioned, to these muds, the main role in maintaining

stability and control of the features is no longer held by the water clay system, but of

the fluidizing reagents

Emulsion muds are natural muds, treated or special, mixed with petroleum or gas. They are

relatively resistant to the action of temperature, and can stand strong loads of clay.

Because of the reduced filtration, they are recommended to crossing clay deposits, which

increase their volume to the contact with water, and productive firmations. Thanks to its

moistening features, emulsion muds facilitate rolling of the drilling column and of the bit,

and increase their durability.

Petroleum products based fluids

In the category of petroleum products based fluids, we may include two main types: oil-based

mud and inverse -emulsion fluids.

Oil based mud, or black fluid, are so called because of their dark brown colour. Some of

them do not contain water, others have a relatively small content of water. The continuous

phase of the fluid is petroleum, and the colloid, which gives it structural features and

clogging, is the naturally oxidated asphalt. If water is included in the composition,

emulsifiers are required.

Inverse emulsion fluids are disperse systems, to which continuous phase is petroleum, and the

dispersed phase is water. So, they are water in - oil type of emulsions. The structuralfeatures are given by a special clay, that disperse the petroleum product.

Petroleum products based fluids own specific features: reduced filtration, inertness to

contaminants, resistance to high temperatures, stability, and great moistening capacity.

Therefore, they are recommended for crossing clay deposits, productive formations,

preventing and solving technic accidents of gripping the drilling column. But, these fluids

also present disadvantages, that limit their usage: high costs, fire risk, difficult

maintenance, hard work conditions for the operating staff. It must be mentioned that lack

of filtration causes the differential pressure to un-balance at the bottomhole, which slows

down the speed of the drilling.

Liquid gas mixture drilling fluid

Some advantages presented by the gas fluids, in harsh drilling conditions, may be obtained by

using liquid gas mixture drilling fluids. Among these, we may mention the aerated

drilling fluid, which is obtained by injecting the drilling mud, through a mixing

installation, of a volume of compressed air. Trough settling the air-mud report, we have a

large range of densities for the drilling fluid. Forming of air-mud emulsions is not

wanted, the presence of the air in the pump making it worsens the functioning of the

pumps.


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A special interest for drilling are the stable foams, which are obtained through injecting of a

quantity of mud or water in the air course sent from the compressor to the drill., in the

presence of a certain foam agent. In case of injecting a small quantity of water with a

foam agent, we may obtain fog in the high speed air course.

Features of the drilling fluids

The quality of the drilling fluid, which makes it suitable for utilization, is given by certain

features. Because the fluids that are most used in drilling are muds, we will refer

especialy to the features of these fluids.

The density of the drilling fluid conditions the appearance of the counter-pressure over the

crossed geologic formations. The value of the density is determined by the pressure of the

fluids in the rocks and by the condition of the stability of the rocks in the well wall.

Rheology of mud characterizes its behavior when flowing. Viscosity and shear stress of the

drilling fluid contribute to evacuating the drill cuttings, preventing it from sedimentation

in the wellbore, and clogging the well wall. A too viscous fluid with high shear stress

does not assure a good cleaning of the bottomhole but it leads to difficulties in separation

of the drill cuttings and the gases and reduces the work capacity of the hydraulic

submersible engine. Generally, for drilling, fluids with very low viscosity and shear

pressure are recommended.

Thixotrophy is a specific feature of the drilling mud. Left in repose, it forms a structure, with

solid aspect. Through shaking, it becomes fluid again. The transformation phenomenasoil geland gel soil can be repeated on a infinity of times. As a consequence of

thixotrophy, when the circulation is interrupted, the drill cuttings are trapped in the

formed structure and they no longer deposit, avoiding disadvatages that may appear

because of the sedimentation.

Filtration and clogging capacity. Through filtration we understand the penetration of the

crossed rocks by a part of the liquid phase, usually water, contained by the drilling fluid,

because of a pressure difference between the drill and the crossed layers.

Simultaneously,, the deposit of particles in the superficial pores of the rocks and on the

well wall takes place, which is called clogging. On the well wall, a crust of solidparticles is formed, also called mud cake.

Cleaning the drilling fluids

The drilling fluid gets out of the drill loaded with rock fragments resulted from dislocation. At

the surface, it must be cleaned. Separation is usually made, mechanically, with special

equipment: oscillating screen, hydrocyclone, centrifuging.

Oscillating screens are the devices that are most used for cleaning of drilling fluids. The base

of the separation is a metallic sieve, that is in a continuous and complex oscillating move,


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Hydrocycloneare the most effective separation devices for solid particles, specially those

which are not normally separated with the oscillating screen. The introduction of the fluid

is done using a centrifugal pump. The dimensions of the separated solid particles and,

implicitly, the quantity of fluid evacuated, depend on a range of elements: the shape and

dimension of the hydrocyclon, the rheologic properties of the fluid, speed when entering

the hydrocyclon. When crossing geologic formation containing gases under pressure,

they penetrate the drilling fluid, affecting its density and rheologic properties. In order to

bring the fluid to its initial features, a degassing is necessary.

When preparing especially treating drilling fluids, high or lower-level toxicity substances are

used. In such cases, proper protection measures must be taken: work using special gloves,

glasses, wearing special costumes.

Casing and grouting of the drill

Opening up the crust causes changes in the balance of the crossed rocks. Depending on the

nature and state of the rocks, the nature of the fluid which is in the pores of the rocks and

the nature of the fluid in the drill, on the diameter and position of the wellbore and other

elements, after a certain time from the crossing, problems appear, especially rock

crumblings. Such problems cause great difficulties in normal development of the drilling

process. It can lead, in extreme cases, to impossibility of continuing the work, so to

abandoning the drill.

Supposing that there are no problems caused by rocks in the exploitation process, the

extraction process is not possible without difficulties. The drill has crossed formationsthat contain different fluids which penetrate the well and negatively influence the

extraction process. Also, there may be registered lose of exploitation fluids in layers with

high permeability and low pressure.

In order to assure a canal for the exploitation, without any unwanted penetration, the wellbore

is consolidated, and the crossed layers are isolated in between. Only the layers which will

be exploited may be related to the interior of the drill. Consolidation is performed

through casing, and isolation, through grouting.

Casing is performed through introducing, in the wellbore, a column made of steel pipes,which provides with an inside lining of the well wall. The casing column will prevent

rock crumblings and will assure a tight and durable canal, through which the extraction

process will develop. Simultaneously, the blow-out preventer control unit is assembled,

during the drilling process.

Grouting is performed through introducing, between the well wall and the casing column, a

fluid mixture which left in repose, becomes solid. The mixture we mentioned is called

slurry cement. After the slurry cement hardens, it prevents communication between layers

behind the casing column. Also, it does not allow contact between the drilling fluid and

the rocks in the well wall, which may be harmful to the stability of the rocks, and protects

the casing column from the corrosive action of fluids contained in layers.


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Construction of drills

A drill was defined as a mining construction. The programme which is respected when a drill

is realized is called construction programme. It only contains equipments that are used or

stay in the drill during the normal drilling process

The construction program includes

The casing columns number, introduction depth, types and diameters. This part of the

programme is called casing programme

The drill bits types and diameters

The drilling columns types and diameters of the component elements

Grouting of the columns intervals and applied methods

Construction of any drill through which fluids will be extracted must assure the following:

Safety and reliability, regarding the drill as a technical construction

Possibility of leading the drilling until it reaches the projected depth

Applying the designed exploitation methods and systems, as well as rational use of natural

energy for transportation of the extracted fluids to the surface

Possibility of passing to exploitation of new productive spheres when exploiting the initial

ones is no longer rational

Performing different operations in the drill during the drilling process, as well as during the

exploitation process , especially repairing operations.


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General casing scheme of drills

As part of the actual drilling method, casing has a discontinuous character. The well is drilled

at a certain interval, then a casing column is introduced, then it is grouted. Then the

drilling is continued, with a smaller diameter bit, then the second drilling column is

introduced and grouted, and so on. The number of the casing columns, the introduction

depth and their diameter are determined by the specific conditions of each drill.

The conductor pipe before starting the actual drilling, a square section is dug (manually or

mechanically)in the wellbore, with a transversal dimension of 0.8 1.0 meters and 3 6

meters depth. In this opening, a tube is introduced, with a 500 to 700 millimeters

diameter. The superior end of the tube is raised with 1.5 2.0 meters over the soil level.

On field, the tube is concreted. Laterally, on its superior part, there is the exit derivation

of the drilling fluid.

For the wells that are drilled in swampy locations or with unconsolidated superior

formations, instead of a simple tube, a conductor pipe is introduced, tens or hundred

meters deep. In such cases, digging the respective hole is done by drilling with the drill

installation.

The conductor pipe fulfills a complex role:

Assures the initial vertical direction of the wellbore

Assures raising of the drilling fluid to the penstock

Consolidates the superior part of the wellbore where weaker rocks are located, which

crumbling risk

Protects the superior aquifer layers, by preventing their contamination with drilling fluid

If the rocks from the surface are consolidated, and there is no risk of drilling fluid loss, the

conductor pipe is no longer introduced.

Surface casing - has the following functions:

Consolidates the wellbore from its surface to a small depth, where there are weak rocks

containing different fluids;

Protects industrial or clean water sources from contamination with drilling fluid or other

fluids resulted from crossed layers;

Represents the secure element to which the blow-out preventer unit is mounted

It is the support that holds the other columns and a part of the extraction equipment


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The introduction depth depends on the nature of the rocks from the surface, on the existence

of a high permeability layer of rocks where fluid loss may occur, on the nature and

pressure of the fluids from the crossed layers, on the final depth of the drill, and other

factors. In normal conditions, the introduction depth varies from 50 to 300 meters. There

are cases where surface casing is introduced to 900 1000 meters (cases of deep or very

deep drills).

The diameter of the surface casing depends on the depth of the drill, on the number of the

columns, and on the purpose of the drilling. This diameter usually varies from 273,05 to

425,45 millimeters.

Extracting column-fulfills the following:

Forms a safe circulating drain of the exploited fluids from the productive layer level to the

surface, and, in the same time, protects the drill equipment;

Allows a selective exploitation of the layers, the interior of the column being connected to the

layers that are of interest only, through perforations;

Assures realization of operations regarding improvement of the drilling process, such as

introduction of fluids in the layer in order to increase the final recovery factor,

interventions, and so on.

The introduction depth of the extracting column usually corresponds with the depth of the

drill. Its diameter is conditioned by flow capacity of the fluids which will be extracted

and on the extraction methods that are used. It is usually between 114,3 and 268,27

millimeters. In very rare cases, larger or diameter extracting columns the ones mentioned

are used.

Protective casing is a column that is introduced in the drill, between the surface casing and the

extracting column. It is not used at all drills but, in the same time, there are drills that

have two, three or even four protective casings.

In the majority of the cases, they are imposed by geologic conditions. The casing programme

stipulates that protective casings should only be used if, through technic measures, the

normal development of the drilling process is not possible. There are also situations that

impose using of protecting casing:

Existence of layers with high permeability, cracked rocks, which will cause drilling fluid loss

The presence of low stability rocks, which may crumble, or other rocks with strong plasticity,

which cause the reduction of the wellbore diameter , especially in contact with drilling

fluid

Existence of layers containing abnormal pressure fluids, especially in the case ofsuccession

of such layers
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The introduction depth of the protective casing depends on the causes that imposed their

usage in the programme. Usually, it has to cover the whole area that caused its utilisation.

For the majority of the cases, the diameter of the protective casing is 177,80 339,72

millimeters.

For all drills, the extracting column and the first protective casing are complete. Their

superior ends reach the surface.

For some drills, the extracting column or one of the protective casings only cover some parts

of the wellbore. The superior end of this kind of column is in the interior of the drill, 50

150 meters above the base of the previous column. Such column is called liner or lost

pipe. In some cases, the column that is previous to the lost pipe is not safe, weakened by

the mechanic action produced by the drilling column, or affected by the corrosive action

of the drilling fluid. In such situations, after introducing and grouting the lost pipe,

another pipe, with the same diameter or larger, is added. The inferior end is assembled to

the superior end of the liner.

The graphic representation of a drills columns containing the introduction depths and

diameters is called casing scheme.


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Establishment of the construction programme

A drill must correspond to the requirements of the drilling process, as well as requirements of

the following stage: production and exploitation. In order to establish a programme, as

rational as possible, a complex range of elements must be considered geological,

technical, and economic.

For establishment of construction programme for a drill, the following datas must be known:

the purpose of the drill, (geological search or exploitation), depth of the drilling, drilling

method which will be used, the geological formations which will be crossed and possible

difficulties created by these formations, as well as the conditions in which the extraction

of the fluids from the drill will be performed. For the beginning, the necessary columns

and their introduction depth must be established, mentioning the grouting interval for

each of these columns. The bit types are chosen, depending on the rocks which are to be

crossed, and if necessary, types of engines that will be used.

After these elements are established, we move on to determination of the diameters of the

columns, the diameter of the bits, the elements of the drilling column.

The diameters of the bit and columns are established in a unique process, called the down

up method. We start with the diameter of the extracting column and finish with the

diameter of the bits for drilling of the introduction interval of the surface casing.

The diameter of the extraction column depends on the specific conditions of the exploitation

(diameter of the extracting column conditions the diameter of the bits and other columns,so the cost of the drilling its advancing speed, the quantity of the drilling fluid, and so

on).

The extraction column must have the smallest diameter to which a normal drilling is

performed, and a future extraction with no difficulties, in whatever applied exploitation

method.

For the entire process of establishing bits and columns diameter, the values resulted in the

calculations must be the same with the existent values in standards.

After the establishment if the diameters, drilling columns must be chosen (diameters of the

component elements).

In order to assure the verticality of the wellbore, heavy weight drill pipes with the maximum

diameter are chosen. Still, it is necessary that a sufficient shell side is left, so that the

drilling fluid can circulate easily.

The type and diameter of the drilling rod are chosen on minimum energy consumption and

assuring fast maneuvering criteria.


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A casing column is made out of steel tubes, called drive pipes, assembled to the ends. The

exterior diameter of the drive pipes varies between 114.3milimeters and 508.0

millimeters. For each exterior diameter there one to eight thickness models of the walls;

the wall thickness varies between 5.21 and 16.13 millimeters.

Column accessories

In order to introduce the column without any difficulties and realize a good grouting, a casing

column is provided with certain accessories.

The foot stone is at the inferior end of the column and, through its round shape, it assures

passing of the end of the column without damaging the well wall

The buffer collar is located at 10 to 30 meters above the foot stone, in the interior of a

connection sleeve of the drive pipes. It is made out of an easy to mill material.

The role of the buffer collar is to stop the cementing plug from going above the foot stone , so

that, that part of the cement grout mixed with the drilling is retained. It is known that

cement grout mixed with drilling fluid does not assure a homogeneous and resistant

stone.

The centralisers are mounted on the exterior of the casing column, on its inferior part or

even on the entire zone that is to be grouted. The purpose of the centralisers is to maintain

the column concentric with the wellbore, assuring a uniform and continuous grouting ring

around the column.


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Execution of the casing operation

Before the introduction of a casing column, preparing operations are made, to the surface and

to the wellbore.

The drilling rig is prepared through: control of the derrick drilling and its substructure,

including remediation of eventual deficiencies, replacement of the steel line and

mounting it in the system which is correspondent to the work during casing, control and

deficiency remediation of the pumps from the installations and the air compressor,

providing the blow-out preventer unit with floats that correspond to the diameter of the

columns.

The tools used for the casing are brought to the drill and prepared drill pipe slips, hoists,

rotary slide tongs, and so on. Spare parts are assured.

The jacketed barrels are brought to the drill, and arranged on wall thickness and material

quality criteria, considering the order of their introduction in the drill, as well.

Testing of the drive pipes to interior pressure with water is recommended, using a specific

equipment. This operation is indicated for all extracting columns and the other columns

for very deep drills.

The column accessories (buffer collar, foot stone, centralizer) are controlled and assembled to

the respective drive pipes.

In order to assure an introduction of the column with no difficulties, and for obtaining an

efficient grouting, preparing works are made in the wellbore. As it follows, using a

corresponding diameter drill bit, the well bore is checked through its entire depth,

insistently correcting the areas which may cause problems. After the bit has reached the

bottomhole, a longer circulation of the fluid is performed; the entire volume of the fluid is

circulated, while appliance of the corresponding treatment is made.

For actual operation of casing, which consists in introducing the column in the drill, a set of

basic rules must be complied:

Respecting the order in which the drive pipes must be introduced

Control of the interior of each drive pipe using a casing tester

Careful maneuvering of the drive pipes

Realisation of welding of the casing joints, for the first drive pipes, in order to prevent

unscrewing caused by the turning of the drilling column, when the work continues; also,

for the casing joints, special blocking substances can be used as well.


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Careful realization of the screwing of each part of the drive pipe

Control of the temperature in the drill pipe thread area; abnormal increasing of the

temperature may be a sign of an non-corresponding unscrewing

Launch of the columns in the drill, with speed that will avoid producing of additional,dangerous pressure, which could cause layer cracks, followed by circulation loss.

Performing of circulations at certain introduction intervals, in order to evacuate the mud cake

from the drill, and the rock particles

Maneuvering of the casing column at the end of the operation, in order to remove the

clogging; simultaneously, the circulation of the drilling fluid is performed.


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Grouting equipment

For performing of the grouting operation, specific equipment is used. Generally, such

equipment is met at other methods of column grouting, as well

Squeeze aggregate. Pumping of the cement slurry and drilling fluid for repressing the cementslurry behind the casing column is performed using special equipment, called squeeze

aggregates. For grouting a casing column, one or more aggregates are used, depending on

given conditions. Placing of the aggregate must be realised in such a manner, that the

transport of the cement doesnt have to be done on distance, and to assure performing any

maneuver, without any difficulties, or any other specific operation that may intervene

during grouting.

The main characteristic of a squeeze aggregate is the maximum pressure of the main pump, at

its slowest, and to the smallest diameter of the pistons.

Plug container. It is mounted at the superior end of the column, it represents the connection

between the column and the slurry pipeline from the squeeze aggregates, allows

launching of the cementing plugs and assures closing of the column at the end of the

grouting operation.

Cementing plug. Separation of the cement slurry from the drilling fluid in the interior of the

column is assured by the cementing plugs, made out of tire or plastic material, and

provided with metallic reinforcement. After hardening of the grout, they are destroyed

by milling.

For a usual grouting, two cementing plus are used, in the majority of the cases, tire ones. The

first cementing plug is perforated, and provided with a separation membrane. When the

cementing plug reaches the buffer collar, because of the pressure increase, the membrane

is breaks and the cement slurry passes under it.


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Grouting methods

Grouting performed to the drills are organized in three categories: primary, secondary and

special. Primary grouting are characteristic to drilling, and the other two types of grouting

are met in following phases of the drilling. The secondary grouting completes the primary

grouting remedies inefficient ones, and special groutings assures blocking of some layers

through introducing cement slurry behind the column, or blocking of a part of a wellbore,

through formation of a cementing plug.

Primary grouting are casing columns groutings, which are made immediately after the casing

operation. They consist in placing the cement slurry in a certain area of the annulus,

between the column and the well wall.


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Necesary documentation for drilling

Conceiving the architecture of a drilling is performed under the responsibility of the Drilling

Department, in several phases, synthesized through preparation of a general

documentation, which consists in:

Drilling proposal (geological report)

Implantation report

Drilling and casing programme

The geological report (the main document that once prepared, offers the possibility of

drilling), defines: location of the drill, (emplacement of the drilling, coordinates, altitude,

or water depth for off-shore drilling), the objective (type, importance, estimated depth)

and contains the basic data: geological, geophysical, correlation drills, difficulties andimposed restrictions.

This document varies pending on the purpose of the drilling (exploration or developing

drilling) and must highlight the decisions taken during the drilling, whether it was taken

in consideration that certain operations take important periods of time (preparing the field

and choosing the drilling device). It also initiates economical studies for confirmation of

feasibility of the project and sends forth a pozitive or negative technical economical

decision regarding the execution of the drilling.

Implantation report is prepared after the drilling decision was taken. This is tasksnotebook type of document and sums up the set of operations to be executed and allows

the determination of the operating budget. It is the result and the establishment moment

of the colaboration profile of different departments taking part (exploration, deposits,

drilling, production).

The main chapters of the implementation report are:

Geographical situation

Drilling purpose and petroleun objectives (mentioning the basic datas or the objectives

defined in the drilling proposal, and have been modified or completed during the study)

Geological and geophysical framework

Drilling and casing programme

Mechanical coring and diferent drilling tests

Production tests programme and study of the samples taken
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All these report allows defining the necesary personnel, materials, services and consumables.

Drilling and casing programme, regarding the geological section given, is the essential

document that allows predictions about how and by which means will the drill be

realised. Predictions regarding the estimation of the drilling price are generally recorded

in additional programmes of the implantation report: the drill bits programme (the bores

defined in the drilling programme corelate with the type of bits through driling studies of

the formations which will be passed through; through the analise of the former

performances, the speed of the progress can be deduced, quality, and number of bits); the

drilling fluids programme defines the types and features of the fluids used on drilling

phase , their possible transformation. The fluid programme depends on geology,

architecture, objectives of the drilling, measurements and essential operations at the

productive formation level. The volume and flow capacity are determined, which will

guide the choosing of the drilling device and the calculation of the consums in products

(mud, water, petroleum). The ecological restriction becomes a determinant for choosingthe type of mud.

To sum up, it all starts with the drilling proposal. Once it is approved, a general sketch of the

drilling is made the implementation report for a general plan. Then, the drilling and

casing programme, which are all about planning the particular elements of the drilling.

Once these documents are drawn up, the actual drilling may begin. The drill is dug in, using

various dislocation instruments: drilling bits and drilling rods. For the dislocation of the rocks,

certain types of drilling bits are suitable for certain types of rocks, determined by theirfeatures; the bits are chosen based on the previously made studies (coring), using the core

barrel. The drill column makes the connection between the bits and the drilling rig, from

which the whole drilling is controlled. While the drill is dug in, remains of the dislocated

rocks (detritus) are evacuated to the surface through the circulation of the drilling fluids;

drilling fluids also have the important role of clogging the well wall, avoiding their

dismantling to the interior. Once the digging is over, in order to assure a canal necessary to

exploitation, without penetration of other fluids then the desired ones, the wellbore is cased,

isolating the crossed layers one from another. The interior of the drill is only connected to the

layers that will be exploited. Casing is performed by introducing steel pipes in the drill, calledcasing columns, which will line the drill in, avoiding crumbling of the well walls. After the

casing is over, the casing columns are grouted in place. The following part does not belong to

the drilling domain: after the grouting, the wellbore is perforated, allowing the petroleum to

get into the wellbore, from where it is extracted.

Drilling went from practice to science fast. For a long period of time, progress obtained in

drilling activity was based exclusively on the practic experience of the operators, on

assumptions more than certainty. The increasing demands for hydrocarbon caused a

considerable and fast growth of the drilling volume, as well as the depth of the drills, executed

in more and more complex conditions. Due to the importance of petroleum nowadays, drillingis and will be a very important branch of the industry.
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Documents

Oil Drilling Overview