A clamp arrangement for concentric well casings comprising: a releasable clamp for clamping concentric well casings, and a smaller inner well casing (420, 52) having inner and outer walls and of a first outer diameter within a larger outer well casing (416, 50) having a central axis and inner and outer walls of a second, larger internal diameter, the releasable clamp comprising: a. a compression ring having an inner surface for peripherally surrounding and engaging the outer wall of the larger outer well casing (416, 50) and having an external surface which is conically tapered relative to the axis; b. a compression collar in communication with the compression ring for generating forces within the reaches of the compression ring, the smaller inner well casing (420, 52) being securable and engageable within the larger outer well casing (416, 50) by a reduction in the inner diameter of the larger outer well casing (416, 50) caused by said forces, and the smaller inner well casing (420, 52) being disengageable by a release of said forces, the compression collar having an internal tapered surface, the compression collar being positioned in axial alignment with the compression ring and in peripheral relationship therewith, whereby axial movement of the compression collar relative to the compression ring results in the application of a radial compression force on the compression ring for forcibly distorting the compression ring into the outer wall of the larger outer well casing (416, 50), forcing the inner wall of the larger outer well casing (416, 50) radially inward for providing a gripping force for clamping and securing the inner well casing (420, 52); characterised in that the releasable clamp further comprises a mechanical locking device positioned within the larger outer well casing (416, 50) which positively engages and locks the smaller inner well casing (420, 52) in position. a releasable clamp for clamping concentric well casings, and a smaller inner well casing (420, 52) having inner and outer walls and of a first outer diameter within a larger outer well casing (416, 50) having a central axis and inner and outer walls of a second, larger internal diameter, a. a compression ring having an inner surface for peripherally surrounding and engaging the outer wall of the larger outer well casing (416, 50) and having an external surface which is conically tapered relative to the axis; b. a compression collar in communication with the compression ring for generating forces within the reaches of the compression ring, the smaller inner well casing (420, 52) being securable and engageable within the larger outer well casing (416, 50) by a reduction in the inner diameter of the larger outer well casing (416, 50) caused by said forces, and the smaller inner well casing (420, 52) being disengageable by a release of said forces, the compression collar having an internal tapered surface, the compression collar being positioned in axial alignment with the compression ring and in peripheral relationship therewith, whereby axial movement of the compression collar relative to the compression ring results in the application of a radial compression force on the compression ring for forcibly distorting the compression ring into the outer wall of the larger outer well casing (416, 50), forcing the inner wall of the larger outer well casing (416, 50) radially inward for providing a gripping force for clamping and securing the inner well casing (420, 52);