INTRODUCTION TO CASING AND CEMENTING

PROGRAM DESIGN

1- Types of Casings and Functions

There are five main types of casings

· The Conductor Casing or stove pipe

· The Surface Casing

· The Intermediate Casing

· The Production casing

· The Production Liner System – Liner set through the productive interval and landed inside the casing on a hanger or with tieback to the surface

The main function of these casings are:

1. To furnish a permanent and gauge wellbore of precise diameter through which subsequent drilling, completion and production operations can take place
2. To allow selective production from heterogeneous formations without interformational flow
3. To allow the a means of attaching the wellhead system and Xmas tree to control and handle the produced fluid.

More specifically, the conductor/surface easing is used to :

• Control caving and washing out of poorly consolidated surface formations
• Furnish the means of handling the return flow of drilling fluid
• Protect fresh water sands from possible contamination by drilling fluid, oil/gas or formation water.
• Allow the attachment of BOP systems.

The main function of the intermediate easing is to seal off troublesome zones, which contaminate the drilling fluid or jeopardise drilling progress with possible hole problems
The production string or liner provides the means of segregating the pay section from all other zones and allow for selective production.

2- Casing String Design Considerations

Casing Strings are designed to withstand four principal types of loading ;

1. Collapse stress : Stress due to unbalanced external pressure on the casing . Most conservative approach is to assume casing is evacuated with external hydrostatic pressure imposed on the casing.
2. Burst Loading : This is is the condition of unbalanced internal pressure imposed by formation pressure with no external pressure through the annulus.
3. Tensile Loading : Each string suspending the weight of subsequent strings below it. Active at JOINTS.
4. Compression Loading : Active at JOINTS of two casing

Best approach is to have a combination string design to minimise cost. That is sometimes impracticable. May be convenient to use a single string system.

Casing Design Criteria

Design of a casing program is based on :
· specification of surface and bottom hole well locations
· size of production casing
· number and sizes of tubing string
· type of artificial lift system

Design specification includes :
· Bit sizes
· Casing sizes. grades and setting depths
· Design based on multiple combination sizes, grades, wall thickness, coupling type

Determination of Casing Setting Depths

Knowledge of geological conditions in a given area can aid the determination of casing depths. There are basically four major types of casing but the number of each type of casing and the setting depths are influenced by the geological conditions of the area. The principle of selecting the casing shoe setting depths starts with the knowledge of formation and fracture gradient. A typical plot of a projected fracture and production setting depths. A typical well
configuration with casing is shown in fig

Steps

1. A line representing the planned mud density programme is plotted This is Pore pressure plus trip margin(200-500psi or .5ppg) – Safety over pore pressure or equivalent mud density reduction during trip
2. Select Point a at required depth(To prevent kick)
3. Plot the fracture gradient profile
4. Plot a secondary safe working fracture margin line – This is fracture gradient less kick or trip margin.
5. Extend point a vertical and intercept the safe fracture margin line at point
   b. This is the equivalent safe mud density needed to drill to point a
6. To drill to point b and set intermediate casing, drilling fluid at point c is needed and will require surface casing to be set at point d.
7. The conductor casing SD is based on the amount required to prevent washout, etc.

Selection of Casing Sizes

The size of the casing string is dictated by the ID of production string and number of intermediate string desired.
For a given Production string O.D, the bit size must be greater than O.D. of casing joint to provide sufficient clearance.
Table 1 shows commonly used bit sizes. In special circumstances, other bit sizes can be used.

Selection of Casing Weight, Grade and Coupling

Load Conditions are: Burst, Collapse, tension, Compression, bending or buckling, thermal effect, kick Consideration

Surface Casing

1. Burst Loading – Based on maximum internal pressure under kick control condition.
   Design pressure = Fracture pressure + SF.
   Design assumes casing is evacuated of mud with gas in the casing
2. Collapse Design – Based on the most severe lost circulation problem
   · Uses maximwn possible external pressure to cause casing collapse due to mud weight. Ignores effect of cement or mud degradation.
   · Also assumes casing is evacuated with mud hydrostatic pressure acting externally.
   · Correction is made for axial tension

0.052rmax(D_k, – D_m) = 0.052 rp D_k

rmax,= maximum mud density anticipated at lost circulation depth
rp = equivalent pore pressure density of the LC zone
D_m is depth to which mud level would fall.

3. Tension design – requires consideration of axial stress correction is also made for bending stresses in directional wells.

Intermediate Casing

Intermediate easing is similar to surface in that its function is to permit final depth objective to be reached.
To minimise cost of casing, the following anticipation in line with illustration below can be anticipated.

rmax + 0.052rm(D_k, – D_m) = pi
Dm = depth of mud-gas interface.
pi = injection pressure opposite lost circulation zone.

Production String

Special Considerations
· Gas production in well
· Casing to withstand tubing leak near surface
· Depleted reservoir condition
· Tension consideration same as for other casings.

Casing Policy

Different types of casing schemes are used by different companies depending on the type of well, type of formation, lithology and stratigraphy; well depth, etc.
An appropriate casing design exercise involves careful determination of factors that influence casing failure under various conditions and selecting the most suitable, safe and economical casing strings. Design is based mainly on size and grade, which take into account the possible loads to be encountered. These include yield, collapse and burst pressure considerations. In many cases the design result in combination string selection. Nevertheless, companies generally evolve their own policy and are guided by official regulating policies of host country in the choice of appropriate string to use.

The policies generally cover:

1. Types of casings to use
2. Choice of single or combination string design.
3. Size of casings in 1’me with overall well plan.
4. Grades of casings and setting depths(Grades according to API in Table1)
5. Completion strategy In terms of especially presence of multiple pay sections, etc
6. Production strategy

In all the overriding parameters or factors are:
· cost
· Completion strategy
· Downhole conditions
· Presence of problem zones
· Availability of casing grades.

Choices of casing setting depths are based mostly on the analysis of formation pressure and temperature gradient profiles.

3- Cementing Operation Design.

Key Functions of Cementing are:

1. To afford additional support for the casing either by physical brazing or seal off of formation
2. To reduce casing corrosion by minimising contact between casing and formation fluid
3. To reinforce the junction of multilateral wells
4. Repair job through squeeze cementing

Types Of Cements

Class A: Depth = 6000ft; No special properties
Class B: Depth = 6000ft; Moderate to high sulphate resistance
Class C Depth = 6000ft; High early strength requirement
Class D: Depth = 6000 -1000Oft; Moderately high temperature and pressure

Class E: Depth = 1000Oft to 1400Oft High temperature and high pressure
Class F: Depth = 1 000Oft to 1600Oft; Extremely high temperature and pressure
Class G/Class h: Basic Cement up to 800Oft

Cement Additives

Wide range of additives to provide acceptable slurry properties such as
· density control : bentonite for reduction, pozollan
· setting time control
· lost circulation control
· filtration control
· viscosity control
· temperature control

Normal Terms

1. Percent Mix = Water Weight/Cement Weight X 100
2. Yield of Cement = Slurry volume per sack of cement.
3. 1 Sack = 94Ibm.
4. WOC – Waiting on Cement = Setting time
5. Accelerators – Reduces setting time
6. Retarders – Prolong setting time

Cement Placement Techniques

1. Casing Cementing/Liner Cementing
2. Cement Plugs
3. Squeeze Cementing
   

Casing Cementing

Here a conventional wiper plug method is usually used. Placement involves the following :

1. Bottom casing must have a cementing head with float collar
2. Bottom plug is dropped in until it hits the float collar.
3. Cement is displaced until pressure build-up causes rupture of diaphragm in bottom plug.
4. Slurry then flows out into annulus
5. At appropriate time top plug is dropped in and surface pressure builds up to indicate cement fillup.
6. Sometimes no wiper plugs are used.
7. Cementing can equally be done with coiled tubing.
   To perform this job requires a knowledge of :
   · Slurry volume
   · Sacks of cement
   · Mix water requirements
   · Additives requirements
   Design takes into account the fact that some cement would remain in float collar to be drilled out.