oil casing design.pdf

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Oil & Gas Well Casing :

By the time the crew drills the well to depth, it usually has several strings of casing in it. These strings

are called conductor casing, surface casing, intermediate casing and production casing.

Notice that cased well looks something like a telescope pulled out of full-length but it is as the crew drills

the well deeper, the size of the whole and the size of the casing gets smaller in diameter. Almost always,the drilling contractor cannot begin drilling at the surface and go all the way to total depth in one step.

For one thing, formations near the surface tend to crumble and cave-in easily so conductor casing

prevents cave-ins. For another thing, formations near the surface may also hold freshwater that the well

cannot contaminate. So surface casing prevents freshwater zones. For still another thing, deep

formations are sometimes so-called troublesome formations. That is, they can be drilled by adjusting

the properties of the drilling mud but once drilled, need to be sealed off to prevent problems in drilling

the deeper portions of the well. So, intermediate casing seals of troublesome zones. Sometimes, deep

wells required more than one intermediate casing string. Finally, once the producing zone is drilled, it

needs to be protected and sealed; so production casing isolates the producing zone. The first string of

casing is the conductor casing.

The hole drill first is pretty big; often as much as 36 inches or more as, almost a meter in diameter. The

conductor hold has to start out with a big because as drilling goes on, the hole’s diameter decreases.

In some cases, the rig will hammer the conductor casing in place if the ground near the surface is really

soft. If the conductor hole is drilled, the casing is cemented in it. Using a bit whose diameter is small

enough to easily go inside the conductor casing, the rig drills the cold below the conductor to a

prescribed depth.

The diameter of the surface hole can still be relatively large; say 17 inches, over 400 mm or even more.

The surface hole’s depth is usually set by regulatory agencies. They require that the surface hold a

drilled through all freshwater zones and that surface casing be set and cemented to protect the zones

from damage by additional drilling operations. This depth could be from hundreds to thousands of feet

or meters. Normally, crew-members nipple up or connect the BOP’s to the surface casing at the well

head. So this casing must be strong enough to support the BOP stack. In addition, it has to withstandthe gas or fluid pressures the well may encounter. Surface casing, also has to be strong enough to

support the addition of casing strings hung inside of it.

To drill the intermediate hole, the operator chooses are still smaller in diameter bit which easily fits inside

the surface casing. A bit of about 12 inches or 300 mm in diameter, is one example of the size.

Intermediate casing is also cemented into place to seal off troublesome formations like moss circulation

zones or abnormally pressured zones. It is often the longest section of casing in the well; Also the crew

connects or nipples of the BOP’s to the top of the intermediate casing by using an adapter and casing

head or a drilling spool which is stacked on or connected to the top of the surface casing wellhead. It

therefore anchors the BOP’s for the drilling that comes later. Remember, that the crew has to nipple up



a stack of BOP’s to each string of casing that is run into the well; first they nipple up on the surface

casing then on the intermediate casing and finally on the production casing.

To drill to final depth below the intermediate casing, the rig owner selects a bit whose diameter is small

enough to fit inside the intermediate casing; say from 8 to 10 inches or 200 to 215 mm. This part of the

hole penetrates the producing zone. When cemented in place, production casing seals off the producing

zone and readies it for production. Production casing also houses and protects the tubing and other

equipment used to produce the well. The operator usually perforates, puts holes in this casing when the

well is completed or ready for work to begin.

Well completion is the term describing the activities and methods of preparing the well for production of

oil and gas. Oil and gas flow into the well through the perforations. Sometimes well owners run liners

instead of casing into the well. A liner, is a shortened the string of casing used the case the smaller

open hole section below an existing casing string in the hole. It’s just like casing, except that a liner

does not run all the way to the surface. Instead, the casing crew hangs it from the bottom of the

previously run casing or liner string using a special piece of equipment called liner hanger. In this case

there is an intermediate liner and a production liner. Using liners saves money since they do not extend

to the surface.

Casing Size Selection – How To Select Casing Size to Match the Drilling and

Completion Goal:

We’ve learnt several topics in regard to casing design and this article will demonstrate you how to

determine casing size in the well.

Casing size selection is determined from the inside outward and it starts from the bottom hole.

The sequence of design is based on the following steps;

1. Proper sizing of tubing is determined by inflow performance analysis.

2. Completion equipment is planned to install with tubing string. Determine which part has the biggest

OD. This will directly impact of production casing.

3. Bit size for drilling the production section.

4. Casing size must be smaller than bit size and its ID must be bigger than the biggest component in

completion string.

5. Once you get the last casing string, the upper string is selected by repeating bit selection and casing

selection similar to step #3 and #4.

You may need to use the following table (Table 1) to determine bit size and casing size. This is based

on API casing therefore if you use special casing, you may need to check with the casing company to

give you exact casing ID.

http://www.drillingformulas.com/category/casing-design/






Table 1 – Commonly Used Bit Sizes That Will Pass Through API Casing



Figure 1 – Casing Design Based On Pore Pressure and Fracture Gradient

The casing string should be set at 3000’ TVD, 6,000’ TVD and 12,000’ TVD.

The Rig needs to drill and set the conductor casing to 500’ TVD in order to rig up the well control

equipment.

Completion information

• Tubing string – 3-1/2” tubing

• Completion equipment – TRSV, Side Pocket Mandrel, Packer, etc.

• The biggest size of completion equipment is 5 inch.

According to the Table 1, the bit and casing plan for this well is described below.

Note: this plan is based on size selection only. The load cases are not accounted for in this example.

Production Section Bit size = 8-1/2”

Casing = 7”, 26 ppf, ID 6.276”

Intermediate Section

Bit size = 12-1/4”

Casing = 9-5/8”, 40 ppf, ID 8.835”

Surface Section

Bit size = 14-3/4”

Casing = 13-3/8”, 48 ppf, ID 12.715”

Conductor Casing

Bit size = 17-1/2”

Casing = 16”, 55 ppf, ID 15.375”



Figure 2 – Casing Details

In order to determine what casing grades to be used in the well, you need do the detailed calculations

based on several load cases as burst, collapse, tensile, etc. We will cover the details later on.

Casing Seat Selection – How To Select Casing Setting Depth:

From the previous article, you’ve learnt about overall of casing design process and in this article we are

going to discuss about how to select casing setting depth. The selection of casing string and setting

depth is based on formation pore pressure and fracture gradient of the well.

For the casing setting depth determination, pore pressure and fracture gradient are normally described

in PPG (Figure 1).

Figure 1 – Pore Pressure and Fracture Pressure Plot



The solid lines in the chart are not accounted for safety factor; therefore, for the first step of casing seat

design, safety margin must be applied. For this example, we will add 0.3 ppg for safety for both pore

pressure and fracture gradient (Figure 2). You need to add the safety factor into formation pressure and

subtract it from the fracture gradient. What’s more, the safety factor value may depends on where you

work and how much confident in your data.

Figure 2 – Pore Pressure and Fracture Pressure Plot with Safety Margin

The dashed lines are design range which will be used for the design. There are two ways to determine

casing setting depths which are bottom-up and top-down method.



Bottom Up Casing Design

This design will start from the bottom of the well up to surface and the setting depths are designed within

the safety factor limits (dotted lines). Starting at the bottom (formation pressure dashed line – Point A),

draw a vertical line upward to fracture pressure dashed line – Point B (Figure 3). Casing should be set

from 4,500’ TVD to 12,000’ TVD because you can reach TD (12,000’ TVD) with highest equivalent mud

weight and you will not break the formation at shallow depth (4,500 TVD). We will apply this same

concept to another string.

Figure 3 – Bottom Up Design Step#1

The next casing string is determined by drawing a horizontal line from Point B to intersect the pore

pressure dashed line at Point C. Then draw a vertical line from Point C to the fracture gradient dashed

line at Point D (Figure 4). The Casing must be set from 1,800’ TVD to 4,500’ TVD.




With the same idea, the next casing string is determined by drawing a horizontal line from Point D to

Point E and a vertical line from Point E to Point F (Figure 5). The Casing must be set from surface to

1,800’ TVD.


Based on the bottom up design concept, we will need to have 3 strings of casing set at 1800’ TVD,

4500’ TVD and 12,000 TVD (Figure 6).

Figure 6 – Bottom Up Design Final



From the example, you can see how the bottom up casing design process is done. If you have the

different pore pressure/fracture gradient, you can repeat the process for other casing strings until you

reach surface casing.

Top Down Casing Design

This design will start from the surface of the well down to the bottom and the setting depths are designed

within the safety factor limits (dotted lines). We start by drawing the vertical line from the facture gradient

dashed line (point A) down to pore pressure dashed line (point B). See Figure 7. The first casing should

be set from surface to 3,000’ TVD.

Figure 7 – Top Down Design Step#1

Next, draw the horizontal line from Point B to Point C located in the fracture dashed line curve. Then

draw the vertical line from Point C to intersect the formation pressure dashed line curve at Point D. This

is the section casing string which should be set from 3,000’ TVD to 6,000’ TVD (Figure 8).




Applying the same concept to the next string, draw the horizontal line from Point D to intersect the

fracture gradient with safety factor chart at Point E and draw the vertical line from Point E to the target

depth at Point F. The last casing string should be set from 6,000’ TVD to 12,000’ TVD (Figure 9).


Based on the tow down design concept, we will need to have 3 strings of casing set at 3,000’ TVD,

6,000’ TVD and 12,000 TVD (Figure 10).



Figure 10 – Top Down Design Final

From the example, you can see how the top down casing design process is done. You can repeat the

same process to determine the setting depth based on your fracture and pressure plot.

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