Method of detecting fluid influxes

A method of detecting a fluid influx in a well being drilled with a drill string comprising a plurality of pipes joined end-to-end and having a drilling fluid circulated therethrough from a tank via a supply line system connected to said drill string, said method being used while connecting or disconnecting a pipe to or from said drill string and comprising the steps of:

• a) ceasing circulation of said drilling fluid;
• b) disconnecting said supply line system from said drill string so as to allow any drilling fluid in said system to flow back into said tank while connecting or disconnecting a pipe;
• c) reconnecting said supply line to said drill string and recommencing circulation of said fluid;
• d) while repeating a)-c) during the normal course of drilling the well, monitoring the change in level of fluid in said tank so as to determine a model of the change in level in the tank with respect to time during steps a)-3);
• and
• e) comparing the change in tank level observed when subsequently performing steps a)-c) with the change predicted by said model so as to detect a difference therebetween indicative of an influx.