Abstract: A method of and product for use in cementing the annulus of a hydrocarbon well having a casing centered within a hole bored from a surface. The method comprises the steps of introducing an effective amount of colouring agent contained in a water-soluble bag into a portion of spacer or scavenging fluid, pumping the spacer or scavenger and a then cementing slurry down the casing to the bottom of the hole to displace drilling mud upwardly through the annulus to the surface, until the colouring agent becomes visible at the surface, displacing the casing volume and then stopping the pumping and allowing the pumped cement slurry to harden.