What is a core barrel?
The core barrel is an essential part of each coring practice as it collects the core sample in its inner tube and simultaneously its outer tube acts as the BHA. Core barrels are not all the same, but rather differ and have different features and components depending on the coring method or system. This in turn affects their design and selection criteria (Ashena & Thonhauser, 2018).
What are the types of core barrels?
Different types of core barrels with various characteristics and components are employed, depending on the coring methods and systems to be used. These can be broadly divided into the following categories: invasion-mitigation, orientated, pressure/in situ, motor coring, conventional, wireline continuous, slim hole, high torque, jam indicating, and jam mitigation (i.e., antijamming and full-closure).
What are the parts of a core barrel?
First of all, a core barrel has a typical length of 1.5 to 3 m and is available with a blank reaming shell and thread protector (Selley & Sonnenberg, 2023). A core barrel typically consists of a double tube system consisting of an inner tube and outer tube. You should understand the role of each piece in the coring system which we will describe below (Fordia, n.d.):
Head assembly
This is the most complex part of the core barrel, and it serves several purposes including landing indication, providing circulation of drilling fluids to the core bit and latching the inner tube assembly in place. The latch head mates with the outer tube assembly, and it spins along with the rods. While rotating, the bearings in the head assembly allow the inner tube to stay stationary, to better accept the in-coming core sample. There are several options when choosing latching systems such as the traditional spring latch or the L-latch.
Inner tube
This is the part that receives the core and stores it until it is retrieved. It is available in 5 feet and 10 feet lengths, but extensions are also available.
Stop ring
The stop ring is one of three integral parts that accept, hold and break the core sample off from the host rock. The stop ring provides a hardened surface for the core lifter spring to bear against. It is locked into a machined groove in the core lifter case. When the core sample is being recovered, it keeps the core lifter in place, preventing it from being pushed up into the inner tube. In broken ground, it keeps core lifter spring square to the core.
Core Lifter Spring
The core lifter spring is one of the most important parts for core recovery. It allows the core sample to move into the inner tube as the bit advances in the formation. When advancing, the core lifter spring slides up against the stop ring. In this position the spring is open and allows the core sample to enter unobstructed. Once the tube is full, the driller will pull back on the rod string, this forces the tapered sections of the core lifter spring and core case together, gripping the core firmly.
The inside profile of the core lifter spring comes in two styles, slotted or broached, to ensure a good grip regardless of the ground conditions. A core lifter that is slotted is good for competent rock, whereas a fluted core lifter can provide good recovery in fractured ground.
Core lifter case
The core lifter case houses both the core lifter spring and the stop ring and is adjusted in relation to the bevel on the inside of the core bit. This adjustment will allow water to flow past the tube to flush the bit. The bevel on the inside of the bit matches the bevel on the core lifter case exactly. When the inner tube is full, the driller will pull back the drill string and this forces the core lifter spring to grip the core as it moves down the taper within the core case. The compression spring in the head assembly allows for the gap between the bit and core case to close, giving support to the inner tube to help break the core.
Bibliography
Ashena, R., & Thonhauser, G. (2018). Types and Components of Core Barrel Assemblies. In R. Ashena & G. Thonhauser (Eds.), Coring Methods and Systems (pp. 23–68). Springer International Publishing. https://doi.org/10.1007/978-3-319-77733-7_4
Fordia. (n.d.). Core Barrels: Do You Know Your Inner Tube? Fordia. Retrieved August 10, 2024, from https://www.fordia.com/en/resources/blog/2018/core-barrels-do-you-know-your-inner-tube
Selley, R. C., & Sonnenberg, S. A. (2023). 3—Methods of Exploration. In R. C. Selley & S. A. Sonnenberg (Eds.), Elements of Petroleum Geology (Fourth Edition) (pp. 43–166). Academic Press. https://doi.org/10.1016/B978-0-12-822316-1.00003-3
