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Designed, Produced and Published by OPITO Ltd., Petroleum Open Learning, Minerva House, Bruntland Road, Portlethen, Aberdeen AB12 4QL

Printed by Paul Matthew Print & Design, 2 Coldside Road, Dundee DD3 8DF

© OPITO 1993 (rev.2002)

ISBN 1 872041 85 X

All rights reserved. No part of this publication may be reproduced, stored in a retrieval or information storage system, transmitted iii any form or by any means, mechanical, photocopying, recording or otherwise without the prior permission in writing of the publishers.

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Oilwell Drilling Technology

Unit 1 : Basic Concepts Contents

Page Visual Cues

*

Training Target

1.2

*

Introduction

1.3

*

Section 1 - Reservoirs and Reservoir Rocks

1.5

*

Section 2 - Exploration Techniques

1.15

*

Section 3 - Drilling Rig Types

1.21

*

Section 4 - Drilling Personnel

1.33

~~ training targets for you to achieve by the end of the unit

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Check Yourself - Answers

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test yourself questions to see how much you understand

check yourself answers to let you see if you have been thinking along the right lines activities for you to apply your new knowledge or find things out for yourself

[~ summaries for you to recap on the major steps In your progress

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Training Target

The aim of this unit is to give you an insight into the basic concepts and techniques used in the exploration for oil and gas. We will also look briefly at the drilling process, design features of drilling rigs and the personnel involved in the drilling operation. When you have completed the unit you will be able to: •

Name the two main types of sedimentary rock.

•

Define the rock properties of porosity and permeability.

*

Explain in broad terms the origin of petroleum.

*

Identify the types of rock structure which can form a petroleum reservoir.

•

Describe the large scale survey techniques used in petroleum exploration.

•

Explain the principles of three small scale survey techniques used in petroleum exploration.

•

Describe the basic concepts of the drilling process.

•

List the main types of drilling rig in use and the key features of each.

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* List the main personnel involved in the drilling operation and the functions of the drilling crew. Tick each box when you have met the target.

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Unit 1 : Basic Concepts

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Introduction Natural petroleum is contained in underground reservoirs. The aim is to get it from the reservoir to the surface in a safe and efficient manner. There are two main stages involved in this:

* finding the petroleum * transporting it to the surface and processing it for the next phase The first of these stages is called exploration, and the second production. It is worth noting here that natural petroleum is also referred to as:

* petroleum

Therefore, we can say that drilling a well is the last step in the exploration stage. Once we know that hydrocarbons are there, underground, it is necessary to decide whether the amount present is sufficient to justify the expense of installing production facilities - offshore production platforms for example. So, a programme of appraisal drilling is planned, which aims to define: * the size and shape of the reservoir * how much hydrocarbon is in place there

* how much of this oil and gas can actually be brought to the surface

*

oil and gas or

* hydrocarbons I will be using all of these expressions from time to time. We are never absolutely sure whether oil or gas is present in a reservoir until we have: drilled into that reservoir * obtained a sample of the reservoir fluids at the surface *

* what difficulties the operation is likely to encounter.

As the number of wells drilled into a reservoir increases, so does the cost but there are benefits:

* the overall rate of petroleum extraction can usually be increased * long term damage to the reservoir can be minimised by avoiding points of high production at a few isolated wells. Development or production drilling is now carried out, giving the ideal distribution of wells over the reservoir to achieve the best economic return for the whole operation. That is - the most hydrocarbon, at the least cost, in the shortest time. All these different types of drilling activity ­ exploration, appraisal, or development - will use a wide range of skills contained within the area we call Drilling Technology.

If, after carrying all this out, we still believe that it is worthwhile proceeding, we enter the production stage proper. Production facilities need to be designed and installed - for example, how many wells or production platforms are required? There are usually a number of wells drilled into one reservoir.

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Before we look into the detail of the drilling operation, this unit on Basic Concepts provides you with some background on four related topics:

* Section 1 talks about the structure of a reservoir and the types of rock which will be found there. * Section 2 examines the various exploration techniques which need to be carried through before the location of the first hole to be drilled is selected.

*

Section 3 looks at the different drilling rig types and indicates why a particular design is selected for a particular purpose. It also covers some basic drilling concepts.

* Section 4 considers the people who work on a drilling rig and how they relate to each other.

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Unit 1 : Basic Concepts

Section 1 - Reservoirs and Reservoir Rocks A reservoir is not a huge underground cavern filled with fluid, as many people still imagine. It is actually a rock system and within the pores, cracks and channels of this system the reservoir fluid - gas, oil, water or, in many cases, a mixture of all three, is stored. In this section you will find out about:

* the basic geology of reservoir rocks * the types of reservoir fluid and how they got there * the structure of reservoirs - why they act as reservoirs

Basic Geology of Reservoir Rocks Most reservoirs are made up of sedimentary rocks. There are two principle types of sedimentary rock in which hydrocarbons are commonly found. These are:

* clastic (or detrital) rocks * biochemical rocks Clastic rocks are formed by the settling out and accumulation of solid particles such as sand. These particles are formed by the weathering of larger rocks. They are carried (by rivers, etc.) to the point where they are deposited. Further layers of rock particles (many thousands of feet thick in some cases) may be laid down on top of this sediment layer which will eventually form the reservoir. The force exerted by these further layers (known as the overburden) together with other chemical and physical changes result in the formation of typical clastic rocks such as sandstones and shales. Biochemical rocks are formed by the accumulation of marine life remains - fragments of shells, coral, skeletons and so forth. Again, application of pressure and other changes result in the formation of typical biochemical rocks such as limestones, chalks and dolomites.

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Figure 1 shows photographs of a typical sandstone and dolomite, taken through an electron microscope and magnified many times. Details of individual rock particles and pores can clearly be seen.

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Figure 12 shows the status of the hole at this stage.

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In the swivel.

Choose the missing words from: under reamers drill string

top drive tri-cone roller bits kelly bushing

bail gooseneck drill collars

kelly bit sub stabilizers

You will find the answer in Check Yourself 6 on page 2.57.

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Summary

In this section we have been looking at the equipment used to actually drill a hole. This consisted of a cutting tool, equipment to apply weight to the tool and equipment to rotate the tool. You saw that the cutting tool, which is called the drill bit, must be capable of making hole in a variety of different rocks. To do this, several types of bit are available, from a simple drag bit to sophisticated polycrystalline diamond bits. We looked at the drill string next and I explained the function and construction of drill pipe and the components of the bottom hole assembly. Finally, we considered the rotating mechanism. I pointed out that there are a number of ways of turning a bit but the most common system utilised a swivel, kelly, bushing and rotary table. We looked at the way this system operates to transmit a rotary motion from the rotary table through to the drill bit. At the end of the section we had a brief look at a couple of alternative methods of rotating the bit. In the next section we will move on to the circulating system and you will see how drilling fluids are pumped and conditioned.

2.27

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Unit 2 : Drilling Systems and Equipment

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Section 3 - The Circulating System I have listed below the individual components of the circulating system:

On a number of occasions already, I have mentioned the term drilling fluid, without saying much more about it. This term in fact covers a range of liquids (and sometimes gases) which perform a number of functions during the drilling operation. Initially, the primary function of the drilling fluid was to clean, cool and lubricate the bit and to carry cuttings from the hole. Nowadays much more is expected of this fluid as you will see in Unit 4. The drilling fluid is more commonly called drilling mud or simply mud and I will use this term during the rest of the section. When drilling is in progress, mud is continuously pumped down through the drill string, and out of the jet nozzles in the bit. Since the diameter of the bit is larger than that of the drill string, an annular space is left around the drill string as drilling progresses. The mud returns to the surface through this annulus carrying with it the cuttings from the bottom of the hole.

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mud pits

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mud pumps

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standpipe and rotary hose

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swivel

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shale shaker

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mud conditioning equipment

Figure 20 on the next page shows the complete circulating system.

At the surface, the cuttings are sieved from the mud. The mud is further cleaned as necessary and then pumped back down the hole again. In this section we will look at the equipment used to pump the mud and condition it at the surface. In other words the circulating system:

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Mud Pits mud pumps

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These are simply a series of interconnected tanks in which the mud is initially prepared and stored, The end tank from which the pumps take their suction is known as the active pit. A mud mixing hopper is located by the active pit. This is used to add chemicals to the mud when its weight and consistency needs to be changed.

--.r-- suction line

At the other end of the line of tanks is the pit which receives the mud as it flows from the hole. This is known as the settling pit or sand trap. The underside of this tank is usually sloped. This means that any solid particles which settle to the bottom, gravitate towards valves. The valves are opened periodically to dump the accumulated solids. Between the active pit and the settling pit are other tanks in which mud is stored and conditioned. We will look at the conditioning equipment shortly.

Mud Pumps

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At the heart of the circulating system are the mud pumps. Their function is to circulate the mud under pressure from the active pit, through the drill string, to the bit, and return it up the annulus to the settling pit.

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action of a double acting pump discharge

discharge

There are usually two pumps on a drilling rig. They are always of the positive displacement type. In other words plunger pumps rather like a bicycle pump.

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* duplex, double-acting or * triplex, single-acting suction

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A duplex pump has two cylinders. Each cylinder has two suction and two discharge valves. As the piston moves through the cylinder it is discharging mud in front at the same time as mud is filling the cylinder behind.

action of a single acting pump discharge

A triplex pump has three cylinders with each cylinder having only one suction and one discharge valve. The cylinder is filled as the piston moves back and is discharged as the piston moves forward.

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For one complete cycle of each piston a triplex pump discharges one cylinder full of mud. In a duplex pump however, because it is double acting, two cylinder volumes are discharged for every cycle of each piston.

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Figure 21 shows the pump action for each type. Figure 21

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Standpipe and Rotary Hose After leaving the pumps the mud is piped to the gooseneck of the swivel. The connecting pipework consists of high pressure piping from the pump, the standpipe and the rotary hose. This pipework must be capable of handling large volumes of mud under high pressure but keep pressure losses to a minimum. The pumps discharge mud to a manifold, an assembly of pipes and valves which permits isolation of pumps for maintenance and repair. The extension of the piping in the derrick consists of a vertical pipe firmly clamped to the derrick. The pipe is known as the standpipe. From the top of the standpipe the mud is passed to the swivel. During normal drilling operations the swivel will be slowly moving down while the standpipe is of course stationary. This means that the connection between the two must be flexible. The rotary hose provides this flexible link. If you look back to figure 20, you can see the relationship between the pumps, standpipe and rotary hose in the circulating system.

In my list of the components of the circulating system the swivel came next. However, as I have already described the swivel as part of the rotating mechanism, we will move on to the following component, the shale shaker.

Shale Shaker Before looking at this item, think again about the path of the mud after the swivel. It is travelling down through the hollow kelly, the hollow drill pipe and collars and out through the jet nozzles in the bit. From there it is going to return to the surface via the annulus and flow into the settling pit. On its return journey from the bottom of the hole, the mud will be carrying rock particles cut by the bit. Before the mud can be pumped back down the hole, these cuttings must be removed by the shale shaker. Figure 22 The shale shaker which is shown in figure 22 is mounted above and at the rear end of the settling pit. It consists of a sloping, wire mesh screen which is made to vibrate. Mud returning from the hole flows through a pipe and passes over the screen. The liquid mud falls through the screen and into the settling tank. Larger particles are trapped on the screen from where they are shaken to the bottom edge to be collected for disposal.

Fine particles of sand and silt however will pass through the shale shaker. These must be removed in special desanders or desilters. We will look at some of this equipment now in the final part of this circulating system section.

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Mud Conditioning Equipment The properties of the mud must be very carefully controlled in order that it can do its job properly. Chemicals may be added as we have seen already. Unwanted substances such as sand and silt, or sometimes gas, may have to be removed. Hydrocyclones are used as desanders and desilters. Mud is pumped into the hydrocyclone via a tangentially fitted inlet. This causes the mud to whirl round the cone shaped vessel creating high centrifugal forces. The suspended solids are driven towards the wall of the hydrocyclone and downwards in an accelerating spiral. The liquid moves inwards and upwards as a spiralling vortex.

The solids, i.e. sand or silt are discharged from the variable opening at the bottom of the unit, whilst' the liquid overflows from the top. If you look at figure 23 you will see this action illustrated.

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Another type of separation unit used for mud conditioning is the centrifuge. This is used for salvaging materials which are to be kept in the mud system. It consists of a rotating cone shaped drum

which spins at a high speed. Inside is a screw conveyor which moves the coarse particles towards the discharge.

From time to time, high pressure low volume gas accumulations may be encountered whilst drilling. This gas can enter the mud causing it to become gas cut.

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Two types of degasser units are provided to separate gas from drilling mud.

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mud gas separators

Test Yourself 7

vacuum degassers Think of two possible problems which may result from the mud becoming gas cut.

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The first of these units usually consists of a vertical vessel through which the gas cut mud can be circulated.

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Gas is released from the mud in the separator and is led away to be disposed of through a flare.

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The gas free mud can then be returned to the pits. This type of unit is suitable for handling high pressure gas and mud which flows from a well when a kick takes place. i

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You will find the answer in Check Yourself 7 on page 2.58.

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You will find the answer in Check Yourself 13 on page 2.60.

2.52

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Summary

In this section we have looked at the equipment used to contain and bring under control a potential blowout. This equipment is called the Blowout Prevention System. You saw that the functions of the system can be listed as enabling the drilling crew to:

*

close the top of the hole

*

release any fluid under controlled conditions

* permit the pumping of new mud into the hole

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In order to be able to perform these functions the system consists of: a blowout preventer stack having a number of preventers, both ram type and annular preventers an operating system which enables the driller to close and open the preventers remotely

We will also return to the subject of blowout prevention equipment in Unit 7 when we will cover pressure control in more detail. You should now be able to go back to the Training Target set at the beginning of this unit and check that you can tick all of the boxes. If you have any problems, look at the appropriate section again or arrange a meeting with your tutor, who should be able to help you.

a choke and kill system, used to release fluids under controlled conditions and allow the pumping of new drilling fluid into the well when preventers are closed Throughout the section we have concentrated on conventional land or production platform type of equipment. In Unit 6 we will be looking at BOP equipment which is used in floating drilling applications.

2.53

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Unit 2 : Drilling Systems and Equipment

Check Yourself - Answers

~ Check Yourself 1

Check Yourself 2

1. Yes

9. Yes

1. hard formation

2. Yes

10. No. The collars are part of the drill string which itself is part of the rotating system.

2. soft to medium formation

3. Yes

3. soft formation 11. No. We will look at a shale shaker in Section 3. It is a component in the circulating system.

4. No. The rotary table is part of the rotating system which we will look at in Section 2.

12. Yes 5. Yes 6. No. This is also part of the rotating system. 7. Yes 8. Yes

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~ Check Yourself 3 To obtain 50 OOOlbs weight on bit with fluid buoyancy factor of 0.833 will require:

50000

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Therefore the total weight required is the weight of the drill collar + the excess weight =

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75 000 Ibs

60 024 Ibs actual collar weight

0.833 Let's say 60 000 Ibs If 25% excess weight is required, this will be: 60000 x 25 = 15000 Ibs 100

If each joint weighs 4 410 Ibs the number of joints required =

75 000 4410

=

17 joints

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Check Yourself 4 1. False

2.

A bit sub is used to connect the bit to the collars but it has box connections, not pin connections at each end.

True

3. False

4. False

swivel bail

+

tri-cone roller

+

drill collar

+

synthetic diamond insert

Drill collars allow just part of the weight of the collars themselves to be applied to the bit. This means that the drill pipe is held in tension.

•

drill string

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rotating mechanism

+

stabilizer •

Crossover subs are used to connect different sizes of drill pipe or collars.

drill bit

+--­

kelly bushing • gooseneck

+

master bushing •

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Check Yourself 6 The words missing from the sentences are shown in bold type below. A rotating system has three main subsystems, the drill bit, the drill string and the rotating mechanism. There are a number of different drill bit designs available such as drag bits, tri-cone roller bits, diamond bits and polycrystalline diamond bits. Under reamers and hole openers are sometimes placed above a bit to enlarge or maintain a hole size. In a drill string, drill collars are used to hold the string in tension and maintain weight on bit. In a conventional rotating mechanism the rotary table turns the kelly bushing, which transmits the rotary motion to the kelly and from there to the drill string and bit. Drilling fluid enters the drill string via the gooseneck in the swivel.

Of the remaining words, a top drive is one of the alternative rotating systems, a bit sub connects the bit to the collars, and stabilizers help to maintain a straight hole of full diameter. A bail is simply a handle by which the swivel is suspended from the drilling hook.

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Check Yourself 8

Check Yourself 9

If gas cut mud is recirculated a number of problems may arise. These will include a reduction in mud weight (or density), giving rise to pressure control problems, which you will see in Unit 7.

The correct sequence is:

I'm sure that you wrote the same components as me, i.e.

Stand pipe

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Mud pumps

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Mud pits

Also, the mud pumps will have difficulty in dealing with mud which is gas-cut.

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Annulus Shale shaker

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Mud conditioning equipment

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