The Capillary Suction Testing (CST) equipment is used to measure
the propensity of a clay to swell once it is introduced to fresh
water. A slurry consisting of a portion of the core catcher with
distilled water is prepared. This slurry is placed in the small
stainless steel beaker seen in the picture at right. A piece of
blotting paper is located underneath the beaker and below the clear
plastic frame, which includes two electrodes. The slurry makes contact
with the blotting paper and a "liquid front" moves outward from the beaker. The liquid front passes the first electrode and starts a timer (the black box seen in the picture). The liquid front passes the second electrode and stops the timer. The recorded time is directly related to the sample's swelling potential -- the greater the time, the higher the swelling potential is. This time can be reduced by adding KCl to the slurry. In samples where the clays are predicted to swell, KCl may be added to the drilling fluid in a percentage determined by the CST to inhibit swelling. The Operations Superintendent may ask the logger to conduct a CST on several samples to determine the likelihood of encountering a swelling clay during logging.
The go-devil is attached to the bottom of each toolstring (except the GHMT, which acts as its own go-devil), and the LFV is opened and closed for each logging run. The disadvantage of this is that it sometimes takes a few attempts to get the go-devil through the LFV.
The go-devil is pumped down on its own before logging, and the LFV stays open for all logging runs until closed by the GHMT passing upwards though it (the WST would either have to be run before the GHMT, or after, with another go-devil attached to it). The disadvantage here is that the hole cannot be deepened, because the go-devil is sitting at the bottom of it.
