Abstract: A method and apparatus for causing pressure variations in a wellbore. A valve mechanism and a sealing device are provided in the wellbore. The valve mechanism is part of a valve device which may be actuated between an open position and a closed position. The valve device is actuated to the closed position to allow an increase in pressure below the sealing device and is actuated to the open position to allow a fluid to pass through the valve device up the wellbore. The valve device is alternately actuated between the closed position and the open position to cause cyclical pressure variations in the wellbore below the sealing device. The valve device includes the valve mechanism, a fluid passage, a control mechanism for controlling the valve mechanism, and a delay mechanism for delaying the actuation of the valve device from the open position to the closed position.