Core and core handling: Part One

The choice of well location, and the decision to core at a certain depth, may well have been made by a geologist. But the job is then usually handed over to the driller. Although the wellsite geologist often will (and usually should) monitor drilling progress and advise the driller during the cutting of a first core, this is generally a rather slack time. If the geologist present has staked his reputation on what the core will reveal, it can also be an extremely anxious time! Then suddenly the core barrel emerges from the hole, and there is intense activity while it is retrieved from the barrel, laid out, cleaned, measured, labelled, logged, and packaged for transport to the laboratory. These can be very exciting moments but in that excitement it is vital that the greatest care is taken with the core: that it is not disordered, no pieces are lost, and it is handled carefully and correctly. This requires a methodical approach.

Precisely how any core is handled will depend on its nature, what it will be used for, how it will be transported, and the nature of the planned analytical programme. Some of the available procedures are specific to certain purposes, and some analytical techniques will require particular core handling methods. It is important to consider well in advance the purposes for which a particular core is being cut, and then plan a core-handling program accordingly.
Sometimes the core will reveal something unexpected, and require certain analyses that were not anticipated (it would not be the first time that a supposed water well has found oil, or an oil well has found coal). It is therefore best to cater for all reasonably plausible eventualities in the design of a core-handling program.
In general, the inspection and testing of core occurs in two main phases. First, it is retrieved at the wellsite. At this stage it is rapidly undergoing numerous physical and chemical changes as a result of its removal from the possibly high-pressure and fluid-saturated subsurface to the low-pressure, dry, surface environment.
Although these changes will not affect the basic lithology and some other rock properties, they will certainly affect other characteristics, such as the pore-fluid chemistry, and possibly the rock strength and clay mineral morphology. Thus it is often important either to test the cores at the wellsite, or to preserve them so as to arrest the process of alteration, prior to later testing. It is also worthwhile to make a core log at the wellsite, even if this is to be repeated in greater detail later. This may be of immediate value in deciding whether or not to take further core or continue drilling, especially if the decision is going to be made by, or in discussion with, others away from the rig-site to whom the log may be sent. It will also serve as a precaution lest the core is lost or damaged subsequently while in transit. Wellsite procedures for core handling tend to follow similar basic principles no matter what the purpose of coring.
However, it is often not possible to undertake all the necessary analyses on core at the wellsite, so it is moved to a laboratory for further work. The laboratory may be a trailer 50 metres away, or a large complex in company HQ on the other side of the world. In either case, it is a location provided with at least basic facilities where the core can be laid out and studied, away from the bustle and distraction of the drilling floor.

To read the second part of this series click here

* Cores and Core Logging for Geoscientists by Graham A. Blackbourn
Whittles Publishing 2009 - RRP$99.95. Available in Australia from Inbooks
Email: orders@inbooks.com.au



Fig. 1 - The (square-section) kelly bushing within the (circular) rotary table in the centre of the drill floor. The rotary table is here stationary, prior to a coring run. (Photo courtesy of Diamant Boart Stratabit)
 

Fig. 2 - These lengths of core have been unpacked and laid out at the core analysis laboratory. They are on heavy-duty trolleys, so that they may be viewed, measured and transported around the laboratory with the minimum of disturbance and effort. (Photo courtesy of Geochem Group)