Abstract: A method is provided for reducing seepage through a leak (5) in a duct (1), the leak being defined in terms of its maximum dimension xL and its maximum width in an orthogonal direction yL and the effective size of the leak being given by formula (1): SL=(x2L+y2L)1/2. A plurality of sealing elements (3) are introduced into the duct, the sealing elements having dimensions a, b and c in mutually orthogonal directions and the effective size SE of the sealing element being given by formula (2): SE=(a2+b2+c2)1/2. The sealing elements have an effective size SE, which is less than the effective size of the leak SL, so that sealing elements (3A) are drawn to the leak locality and caused, by reason of a pressure differential at that locality and attributable to the leak, to move over the leak and reduce the seepage.