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D rilling E ngineering 2 Course ( 2 nd Ed.)

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1. Laminar Flow in Pipes and Annuli

2. Turbulent Flow in Pipes and Annuli

3. Pressure Drop Across Surface Connections

4. Pressure Drop Across Bit

5. Optimization of Bit Hydraulics

6. Particle Slip Velocity

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1. Casing A. Review

B. Setting Depths

C. Connections

D. API Casing Performance Properties

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Casing functions

Casing costs compromise one of the largest cost items of the drilling project. Therefore proper planning of

casing setting depths and casing selection is vital to realize a cost effective and safe well.

The casings themselves fulfill multiple functions that can be summarized as:Isolate porous formations with different fluid-pressure

regimes from contaminating the pay zone,Prevent near surface fresh water zones

from contamination with drilling mud,Protect the hole from caving in,Provide a connection and support of the wellhead equipment,Provide exact dimensions for running

testing, completion and production subsurface equipment.

Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 5

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Casing Types

According to the different functions, the total casing program consists of different casings strings.Conductor Casing

Surface Casing

Intermediate Casing

Production Casing

Liners

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Conductor Casing

The function of the conductor is to enable circulation of the drilling fluid to

the shale shakers without eroding the surface sediments directly below the rig foundation.

The conductor prevents the subsequent casings from corrosion and may partly support the wellhead weight.

Commonly a diverter is installed on top of the conductor casing to divert an unexpected inflow of formation fluids into the wellbore away from the rig-site and the personal.

Conductor setting depths are in the range of 150 to 600 [ft] (46-183 m), their seizes range from 36 to 20 [in] (0.91-0.5 m).

Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 7

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Surface Casing

The function of the surface casing is to prevent cave in of unconsolidated, weak near-surface formations

as well as protect the shallow, freshwater sands from contamination with drilling mud.

As the conductors, surface casing protects the subsequent casings from corrosion.

Before the surface casing is set, no blow out preventers (BOP) are installed.

After setting the surface casing and installing the wellhead, a BOP is available to handle kicks when drilling the intermediate hole section.

Surface casing setting depths are in the range from 300 to 5,000 [ft] (91-1524 m), their diameters range from 24 to 17-1/2 [in]. the surface casing setting depth is often determined by government

or company policy and not selected due to technical reasoning.

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Intermediate Casing

The intermediate casing string is a purely technical casing.

One or more may be necessary to handle abnormal formation pressures, unstable shale formations, lost circulation or cave-in zones.

An intermediate casing may also be necessary to realize the planned mud weight profile. When for example an abnormally pressured formation is

encountered, it may have to be protected by an intermediate casing so when formation pressure of the formations below is normal, a lower mud weight can be applied.

Intermediate casing diameters range from 17-1/2 to 9- 5/8 [in].

Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 9

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Production Casing

The production casing is set through the prospective production zone(s).

This casing string protects the environment in case of production tubing failure and permits the tubing string to be maintained or replaced during the production life.

Commonly production casing and production liners have gas-tight connections,

their diameters range from 9-5/8 to 5 [in]. A production casing diameter of 7 [in] is encountered often.

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Liners

To save cost, the casing installed sometimes doesn’t reach until the surface but finishes within the previous string. Such a casing configuration is called

liner.

A liner is mounted on a so called “liner hanger” to the previous casing string.

Commonly the liner head is several hundred feet into the previous casing to enable a good cement seal.

LinerSpring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 11

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Various typical casing programs

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develop a casing program

To develop a casing program, first the various casing setting depths have to be determined.

Since the primary reason to drill a well is to produce hydrocarbons out of a reservoir, the final casing inside diameters

have to be large enough to allow for the forecasted completion and production schemes. Factors like completion type

(open hole, cased hole, monobore production, etc.), expected amount of production (production tubing seize), expected production forecast (e.g. need of gas-lift, etc.) and size of evaluation tools to be run have to be considered.

In general, for production purposes the well diameters shall be as large as possible.

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Advantages of small hole sizes

On the other hand, as small as possible hole seizes reduces the total cost of the well since:drilling times are faster,less mud has to be used

(purchase and disposal of mud),smaller mud equipment can be used (cleaning, pumps, etc.),smaller casings can be used

(cheaper, higher strength at same grade),smaller rig can be applied

(lighter casings, smaller mud volume),rig site can be smaller (especially important offshore and

platform types like TLP where weights are limited).

Out of this reason a technology called slim-hole drilling was developed.

Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 14

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Casing setting depth and hole size

In general, the casing setting depths calculation starts at the bottom of the well

with the minimum required hole size (often provided by the production department).

After determination of the hole size to drill and applying the corresponding mud weight, a kick (normally gas kick, volume and pressure company

depended) is assumed and it is calculated where, when the kick is circulated out, the pressure of the kick would fracture the formation. This is the highest depth the previous casing

could be set to handle the assumed kick.

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Casing setting depth and hole size (Cont.)Having the setting depth of the previous casing,

the proper corresponding hole size is determined using next slide charts.

From here on, the same procedure is applied to find the next casing setting depth and so on until the depth of the surface casing is reached.

As mentioned before, the setting depth of the surface casing is normally determined by government or local regulations.

The procedure described above gives the cheapest, since shortest casing strings possible.

In practice it is often required to have the ability to drill deeper than the planned well depth. To provide this flexibility casing strings are run

deeper than calculated by the previously described procedure.

Spring14 H. AlamiNia Drilling Engineering 2 Course (2nd Ed.) 17

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Commonly Used Bit Sizes that will pass through API Casing

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Parameters influences casing setting depthUp to now, casing setting depths determination

was only based on the fracture gradients of the different formations and the mud weights of the different sections.

The formations to be drilled themselves are also influencing the casing setting depths determinations. It is often required

to seal off a porous formation before drilling deeper, orto isolate various sensitive formations like salt. As common practice a casing is normally set into

a competent (shale) formation.

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Effect of geological uncertainty on casing designWhen drilling within an area where the geology

and formation properties are well known, the casing setting depths and design can be optimized

taking the considerations discussed above into account.

At locations where the formations are not well known or overpressures can be expected, additional strings have to be planned for.

These additional strings may be necessary to seal off overpressured formations or

handle various unexpected situations.

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Example casing programs

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Tally and casing string design

When running the casings into the borehole, a so called casing tally is produced

that keeps track of all casings (types, details) and casing equipment (centralisers, scratchers, etc.) that is actually lowered into the well.

When the casing setting depths are finally established, the individual casing strings are to be designed.

As drillpipes, casings are manufactured in different length ranges:Range 1 16 to 25 [ft], Range 2 25 to 34 [ft],

Range 3 34 to 48 [ft],

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Casing connections

Casings used for oil and gas wells have to be equipped with connections that can be made up easily and

are leak proof.

The casing joints itself are manufactures in different types that fulfill different requirements.

Threaded connections are rated according to their “joint efficiency”. The joint efficiency is defined as

the ration between tensile strength of the joint and the tensile strength of the body.

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API casing joint types

Short-Round-Thread: as long-round thread,

offer no pressure seal at internal pressures, threaded surfaces get further separated.

Long-Round-Thread: greater strength than short-round threads, often applied since reliable, easy and cheap, joint efficiency greater than at short-round threads but

less than 100%.

Since round-threads have eight threads per [in], they are sometimes called API 8-Round threads as well.

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API casing joint types (Cont.)

Buttress-Thread: offers a nearly 100% joint efficiency, is not 100% leak proof.

Extreme-Line: design is an integral joint

(box is matched on the pipe wall), pipe wall must be thicker near the ends of the casing to

provide the necessary strength, OD of this connection is significantly lower than

of others and half as many threaded, connections exist, metal-to-metal seal, much more expensive.

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API Connectors

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special features of casing connections

Along with these API connection types, other ones offering premium features are applied in the industry. Some of these special features are:Flush joints for maximum clearance.

Smooth bores through connectors for reduced turbulence.

Threads designed for fast make-up with low tendency to cross-thread.

Multiple metal-to-metal seals for enhanced pressure integrity.

Multiple shoulders for enhanced torque strength.

High compressive strengths for special loading situations.

Resilient rings for secondary pressure seals and connector corrosion protection.

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API standards for oilfield tubulars

The American Petroleum Institute (API) has developed internationally accepted standards for oilfield tubulars and summarized them in bulletins that contain the minimum performance properties and

equations to compute these properties.

It should be understood that these properties are guidelines (minimum requirements),

for casing design calculations, the exact strength values provided by the individual manufacture shall be applied.

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Casing properties

A casing is defined by:Casing outside diameter

(OD),Weight per unit length

(this determines the wall thickness),

Type of coupling,Length of joint.Grade of steel,

To determine the strength of various casing materials, API has designated defined grades.

The grading code consists of a letter followed by a number where the number gives the minimum yield strength of the material in thousands of [psi], the letter is an arbitrary one (e.g. N-80).

For wells that require very high tensile strength, collapse resistance or better corrosion resistance, non-API casings are used by the industry as well.

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Calculation of some of the individual casing propertiesFollowing Tables list API

and some non-API ones as well as gives the minimum performance properties of them. To calculate the individual

properties following equations can be applied:Yield strength collapse

pressure:

Nominal casing weight:Casing types can be

normally identified by their nominal weight, which are

based on 20 [ft] length of threaded and coupled casing joint. They can are calculated as:

Plain end casing weight:The plain end casing weight

is defined as the weight of the casing joint without inclusion of threads and couplings.

do [in] tubular outside diameter,

t[in] tubular wall thickness,σy[psi] minimum yield

pressure of tubular

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Minimum performance properties of Casing

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Minimum performance properties of Casing (Cont.)

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1. Dipl.-Ing. Wolfgang F. Prassl. “Drilling Engineering.” Master of Petroleum Engineering. Curtin University of Technology, 2001. Chapter 8

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1. General Casing Design Criteria

2. Graphical Method for Casing Design

3. Other Casing Design considerations

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