What immediate action should be taken in the event of lost circulation?

In the event of lost circulation, the immediate action taken should involve filling the annulus with water or the lightest mud available. This approach helps to stabilize the wellbore and prevent the further flow of drilling fluid into the formation, which could lead to additional complications, including well control issues or damage to the formation. By using water or a lighter mud, the hydrostatic pressure in the wellbore can be maintained or adjusted appropriately, allowing for better control over the situation.

Utilizing this method also aids in creating a barrier against any influx of formation fluids, which is crucial in maintaining well integrity. The formulation of appropriate mud solutions can assist in mitigating the losses experienced and promote a safer drilling environment.

While other actions such as increasing the pump rate, shutting down drilling operations, or activating the diverter system may seem relevant under certain conditions, they could inadvertently worsen the situation if not handled correctly or at the right time. For example, increasing the pump rate might exacerbate lost circulation by further pushing mud into the fractures, while shutting down operations may not address the immediate circulation loss that needs to be controlled.