CASING

The main functions of the casing in any well are: Maintain hole integrity Isolate abnormally pressured zones Protect shallow weak formations from heavier mud weights required in the deeper portions of the hole Prevent contamination of freshwater-bearing strata Support unconsolidated sediments Along with cement hydraulic seal

Common Casing Designs

Conductor Casing Seal off unconsolidated formations at shallow depths Stop washouts under the drilling rig. Protect fresh water sands. To give a base and support for the next string of casing Set before the rig is set up

SURFACE CASING Surface casing is the first string of casing used after the conductor pipe. It is required in some instances by law (to protect ground water) and is normally cemented full length. Surface casing supports the BOP stack and subsequent casing and tubing strings. It is normally the only string designed to carry compression loads.

SURFACE CASING Support the wellhead and BOP equipments Protect, water sands Support the wellhead and BOP equipments Case unconsolidated formations Support other casings Case off lost circulation zones

INTERMEDIATE CASING Intermediate casing is any string between the surface and production string. Intermediate casing may or may not be cemented full length. In many the cases intermediate casing is circulated by regulations

INTERMEDIATE CASING Intermediate casing may be used to: Seal off weaker zones Protect previous casing strings from higher burst pressure. Provide support for liner casing

PRODUCTION CASING The production casing is used to control the hydrocarbon bearing zones that will be produced. This string of pipe adds structural integrity to the wellbore in the producing zones. Production casing should be set before completing the well for production. It should be cemented in a manner necessary to cover or isolate all zones which contain hydrocarbons.

PRODUCTION CASING Every well does not have all the strings. Depending on depth and conditions. Surface and Production are in almost every well.

The PIPE The size of the casing refers to the outside diameter (O.D.) of the main body of the tubular (not the connector). Casing sizes vary from 4.5" to 36" diameter. Tubulars with an O.D. of less than 4.5” are called Tubing.

STANDARDS Length The API standards recognize three length ranges for casing: Range 1 (R-1): 16 – 25 ft Range 2 (R-2): 25 – 34 ft Range 3 (R-3): > 34 ft usually about 42 ft Casing is run most often in R-3 lengths to reduce the number of connections in the string. Since casing is made up in single joints, R-3 lengths can be handled easily by most rigs.

STANDARDS Weight It is conventional to specify casing dimensions by size and weight per foot. • Nominal Weight: Based on the theoretical calculated weight per foot for a 20 ft length of threaded and coupled casing joint. This is not a true weight per foot. • Plain End Weight: The weight per foot of the joint of casing without the threads and couplings (pipe body). • Average weight per foot: is the total weight of an average joint of threaded pipe, with a coupling attached power-tight at one end, divided by the total length of the average joint.

STANDARDS For example: there are four different nominal weights per foot of 9 5/8’’ casing.

STANDARDS Each grade is designated by a letter, and a number. The letter refers to the chemical composition of the material and the number refers to the minimum yield strength of the material. N-80 casing has a minimum yield strength of 80000 psi and K-55 has a minimum yield strength of 55000 psi. Hence the grade of the casing provides an indication of the strength of the casing. The higher the grade, the higher the strength of the casing. (Cost)

STANDARDS

STANDARDS Yield point Proportionality limit: Stress is proportional to strain (Hooke’s law). If stress is removed --> it returns to original length. s = Ee (E: elastic modulus or Young’s modulus. For steel E = 30,000 psi) Elastic limit (yield point): Beyond the elastic limit, permanent deformation will occur. The lowest stress at which permanent deformation can be measured is defined as elastic limit or yield point. Typical yield behavior for non-ferrous alloys. 1: True elastic limit 2: Proportionality limit 3: Elastic limit 4: Offset yield strength

STANDARDS Tensile Strength It is the ultimate strength of a given alloy or product that determines how much load it can withstand before breaking, or being pulled apart.

STANDARDS Connections Connection represent less than 3% of the pipe length. More than 90% of pipe failures occur in the connection. Connections represent 10% - 50% of the total tubular costs Individual joints of casing are connected together by a threaded connection. These connections are variously classified as: API; premium; gastight; and metal-to-metal seal. In the case of API connections, the casing joints are threaded externally at either end and each joint is connected to the next joint by a coupling which is threaded internally.

STANDARDS

STANDARDS Connections The standard types of API threaded and coupled connection are: • Short thread connection (STC): - 8 threads per inch and threads have rounded crests and roots • Long thread connection (LTC): - 8 threads per inch; threads have rounded crests and roots; thread section is longer so has better sealability and tensile strength than STC.

STANDARDS Connections Buttress thread connection (BTC): - 5 threads per inch and not symmetric for the load and stab flanks

STANDARDS Connections • Metal-to-metal seal: Thread and Coupled (MTC): - Generally have burst, collapse, and tension ratings equal to the pipe body.