Valve seat

A downhole tool for attachment to a work string, the downhole tool having an inner wall disposed above a central longitudinal axis, the inner wall defining a central bore for passage of fluid from an upstream location to a downstream location;

• the downhole tool also having a valve seat, adapted for receiving a plug having a plugging dimension, for use in the central bore of the downhole tool, wherein the valve seat comprises:
• a substantially cylindrical body having a first bore therethrough defined by an inner surface of the body and the body being of a first volume;
• a first clearance through said body, defined by a portion of said inner surface, which is smaller than the plugging dimension;
• a seating surface located upon the inner surface facing upstream for sealingly engaging with said plug and substantially occluding passage of said fluid from said upstream location to said downstream location;
• wherein a pressure differential is developed across said valve seat when said plug sealingly engages said seating surface, applying force to said seating surface;
• said body being formed of an elastic material which compresses to a second volume, smaller than said first volume, by application of said force to provide a second clearance to said body which is greater than the plugging dimension, and thus allows passage of said plug downstream within said fluid conduit;
• and
• wherein after passage of said plug, said body returns to said first volume with substantially said first clearance;
• wherein the central bore of the downhole tool has an open upper end for receiving the plug, and
• wherein the tool is actuated and returned to its initial configuration by the passage of the plug through the valve seat.