Method and system for detecting changes in drilling fluid flow during drilling operations

A drilling system comprising:

• a drill string including a drill bit for drilling a wellbore;
• a drilling fluid pump for pumping drilling fluid down the wellbore proximate to the drill bit, wherein at least some of the drilling fluid in the form of return drilling fluid having dissolved formation gases, is returned back up to a wellhead of the wellbore;
• a tracer gas injector for injecting a tracer gas at a constant rate into the return drilling fluid;
• a flow line for receiving the return drilling fluid with injected tracer gas;
• a gas extractor for extracting a sample of gases from the return drilling fluid received from the flow line;
• and
• gas analysis equipment to determine the concentration of tracer gas in the sample of gases.

A method of detecting changes in return drilling fluid flow during drilling operations, the drilling operation including pumping drilling fluid to a drill bit in a wellbore, and receiving return drilling fluid at a wellhead of the wellbore, the method comprising the steps of:

• injecting at least one tracer gas for a first discrete time period into the return drilling fluid;
• detecting the tracer gas from the first discrete time period in the return drilling fluid at a location downstream of injection of the at least one tracer gas;
• measuring a first time delay between injection of the tracer gas at the first discrete time period and detection of the tracer gas from the first time period at the location;
• injecting tracer gas for a second discrete time period into the return drilling fluid;
• detecting the tracer gas from the second discrete time period in the return drilling fluid at the location;
• measuring a second time delay between injection of the tracer gas at the second time period and detection of the tracer gas from the second time period at the location;
• determining a change in measured time delay between the first time delay and second time delay;
• and
• using the change in measured time delay to infer a change in the flow rate of the return drilling fluid.