With more complex wells being drilled nowadays, casing connections, alongside casing steel grades, are considered cornerstones of well integrity.
Casing connections are widely labeled as API and Premium. This last doesn't necessarily command superior characteristics but different features, and is usually associated with a premium price..
API Connections - Some History First
Before the development of API standards, casing manufacturers used a sharp V-Shape thread that lacked standardization among industry participants. The V-Shape was fit for simple wells, but as deeper wells were being drilled, the need for a more robust connection became pressing.
This is when API standardized casing connections, and offered specifications for four connections types.
Most lectures and publications list the four connections types in the below specific order, which we believe corresponds to the evolution of the API Connections:
- Short Round Threads and Couplings (API CSG)
- Long Round Threads and Couplings (API LCSG)
- Buttress Threads and Couplings (API BCSG)
- Extreme Line Threads (XCSG)
- API 8-Round (8-Rd)
The first two types (CSG and LCSG) are known to the industry as:
- API 8-Round Short Threaded Connection (STC), and
- API 8-Round Long Threaded Connection (LTC) respectively.
The name 8-Round comes from the 8 threads per inch machined on the casing joint and coupling. While both connections have the 8 threads per inch, the LCSG has a longer connection, offering:
- greater tensile strength,
- higher reliability, and
- higher joint efficiency.
However, The API 8-Round connections have two major issues:
- prone to joint-pull-out failure caused by thread tapering
- Joint efficiency below 100% which reduces the tensile strength of the casing string
Buttress Connections (BTC)
The BCSG (which is widely known to the industry with the acronym BTC) came to solve the issues of the 8-Round connections.
Two major modifications where performed:
- thread shape changed from a rounded peak to a squarer peak
- extended the pin-box contact length by making the connection longer
These modifications have resulted in:
- significantly reducing the joint-pull-out failure risk
- Brought up joint-efficiency to 100%.
As with the 8-Round connections, Buttress Connection still need a thread compound to provide the seal (by filling the voids between the thread root and the crest). This sealing mechanism limits their sealing capability with:
- gases at high pressure (+500 psi)
- solid free and low viscosity fluids
- Extreme Line Connections (XLC)
To resolve the sealing limitation of 8-Round and BTC connections, API developed specifications for the XCSG (known also XLC) which brought-in two distinctive features that previous connections didn't have:
- First, by being the only API connection with the box machined on the casing joint (integral joint), resulting in eliminating the coupling and cutting the number of threads to half, which reduces the risk of leak.
- Secondly, by being the only API connection with a Metal-to-Metal seal, thus improving its gas-tight sealing capability
However, with these features come two constraints:
- The Metal-to-Metal seal doesn't eliminate the need for casing dope as the connection requires lubrication for make-up.
- The integral joint requires a thicker pipe-wall at the tip of the connection, which results in slightly restricting the casing string ID.
Premium Connections - A Bit Of A Premium Now
API connections offers an affordable variety of casing connections. However; they are far less diverse than proprietary connections (or what is known as Premium Connections).
- Premium connections were (and still are) designed and developed by casing manufacturers to address industry needs that API couldn't meet such as:
- Enhanced stabbing
- Reducing cross threading
- Fast make-up
- Over torque protection
- Dopeless make-up
- Improve sealing capability
- Maximizing clearance (minimizing O.D.)
- Galling resistance
- Tension, compression, torque, burst, and collapse resistance enhancement
- Maximize bending capability
Operating as an OEM in oil & gas downhole logging tools, Vigor prioritizes Premium Connection integrity. Our extensive research into certified suppliers has yielded deep technical insights and a vetted supply chain. This enables data-driven optimization of connections for your specific downhole conditions. For engineered solutions backed by empirical validation, contact our technical team.
For more information, you can write to our mailbox info@vigorpetroleum.com & mail@vigorpetroleum.com
