OIL WELL DESIGNINGCasing Design and Seat selection

Prerequisites to casing design Formation properties : pore pressure; formation fracture pressure; formation strength (borehole failure); temperature profile; location of squeezing salt and shale zones; location of permeable zones; chemical stability/ sensitive shales (mud type and exposure time);  lost-circulation zones, shallow gas; location of freshwater sands; and presence of H2S and/ or CO2. Directional data : surface location; geologic target(s); and well interference data. Minimum diameter requirements : minimum hole size required to meet drilling and production objectives; logging tool outside diameter (OD); tubing size(s); packer and related equipment requirements; subsurface safety valve OD (offshore well); and completion requirements. Production data : packer-fluid density; produced-fluid composition; and worst-case loads that might occur during completion, production, and workover operations. Other : available inventory; regulatory requirements; and rig equipment limitations.

Design Method

Preliminary design Typically the largest opportunities for saving MONEY  are

present while performing this task. This design phase includes:

Data gathering and interpretation Determination of casing shoe depths and number of strings Selection of hole and casing sizes Mud-weight design Directional design The quality of the gathered data will have a large impact

on the appropriate choice of casing sizes and shoe depths and whether the casing design objective is successfully met.

Detailed design The detailed design phase includes selection of pipe weights

and grades for each casing string. The selection process consists of comparing pipe ratings with design loads and applying minimum acceptable safety standards (i.e., design factors). A cost-effective design meets all the design criteria with the least expensive available pipe.

Classification of CSG1. Outside diameter of pipe (e.g. 9 5/8”)

2. Wall thickness (e.g. 1/2”)

3. Grade of material (e.g. N-80)

4. Type to threads and couplings (e.g. API LCSG)

5. Length of each joint (RANGE) (e.g. Range 3)

6. Nominal weight (Avg. wt/ft incl. Wt.

Coupling) (e.g. 47 lb/ft)

Eg : A typical piece of casing might be described as

9-5/8" 53.5# P-110 LT&C Rg 3

specifying OD, weight per foot (53.5 lbm/ft thus 0.545-inch wall

thickness and 8.535-inch inside diameter), steel strength (110,000

psi yield strength), end finish ("Long Threaded and Coupled"), and

approximate length ("Range 3" usually runs between 40 and 42

feet).

Standardization of Casing

se

The collapse load, Pc at any point along the casing can be calculated from:

The burst load, Pb at any point along the casing can be calculated from:

Axial Load

The axial load on the casing can be either tensile or compressive, depending on the operating conditions.

The axial load on the casing will vary along the length of the casing. The casing is subjected to a wide range of axial loads during installation and subsequent drilling and production.

The axial loads which will arise during any particular operation must be computed and added together to determine the total axial load on the casing.

The sources of axial loads on the casing are a function of a number of variables :

W the dry weight of the casing; φ the angle of the borehole; Ao the cross sectional area of the outside of the casing; Ai the cross sectional area of the inside of the casing; DLS the dogleg severity of the well at any point; Pi the surface pressure applied to the I.D. of the casing; As the cross sectional area of the pipe body; DT the change in temperature at any point in the well ; dPi and ddPe the change in internal and external pressure on the casing;

and n the poissons ratio for the steel.

8

STRESS

Tension

Burst

Collapse

Collapse

Tension

Depth

Burst

Collapse:

Tension:

Assume full reservoir pressure all along the wellbore.

Hydrostatic pressure increases with depth Tensile stress due to weight of string is highest at top

Safety FactorsCollapse 0.85 -1.125Burst 1.00 -1.10Tension 1.60 -1.80

Safety factors can be defined as the ratiobetween rated capacity of casing and the actual load.

API standards include three length ranges, which are :

• R-1: Joint length must be within the range of 16 to 25 feet, and 95% must have lengths greater than 18 feet.

• R-2: Joint length must be within the range of 25 to 34 feet, and 95% must have lengths greater than 28 feet.

• R-3: Joint length must be over 34 feet, and 95% must have lengths greater than 36 feet.

The API has specifications for four types of couplings.

Short round threads and couplings (CSG)

Long round threads and couplings (LCSG)

Buttress threads and couplings (BCSG)

Extremeline threads (XCSG)

• Nominal Weight: Based on the theoretical calculated weight per foot for a 20 ft length of threaded and coupled casing joint.

• Plain End Weight: The weight of the joint of casing without the threads and couplings.

• Threaded and Coupled Weight: The weight of a casing joint with threads on both ends and a coupling at one end.

The Plain End Weight, and the Threaded and Coupled Weight are calculated using API formulas. These can be found in API Bulletin 5C3.

4.3) Move across to Point C

which identifies the mud weight requirement

for that depth

4) To determine initial

estimates of casing setting depths –First:

Enter the mud weight curve at Point A (Total Depth(TD))

4.6) Point E is the normal pressure range and no further casing is

required to withstand the associated mud weight. However, a structural and conductor casing are

required, and the setting depth criteria for those strings are

discussed later

1) Draw the mean pore pressure gradient curve along with the

lithology, if available

2) Draw the mud weight curve. The mud weight curve should include a

200 to 400 psi trip margin

4.2) Move up to Point B which

determines the initial estimated setting depth for the intermediate casing (actually run it 300–400 feet deeper)

4.4) Move up to Point D which

determines the preferred setting

depth for the surface casing/ intermediate

string

3) Draw the predicted fracture

gradient curve

4.5) Move across to Point E to identify the mud weight required at that

depth

Design “Bottom -to- Top”

A design should be developed by well planning that provides for economic production from the pay zone consistent with safety requirements. The pay zone should be analysed for its flow potential and the drilling problems that will be encountered upon reaching it. The well should be designed from bottom-to-top. The opposite approach can result in a well that limits the production capacity of the pay zone.

Panipat Refinery

Atmospheric and Vaccum Distillation Unit (AVU)

Continuous Catalytic Reformer Unit (CCRU)

Vis-Breaking Unit (VBU)

Hydrogen Generation Unit (HGU)

Resid Fluidized Catalytic Cracking Unit (RFCCU)

Once Through Hydrocracker Unit (HCU)

Coking Unit (Delayed and Fluid Coking)