Abstract: A double acting jar enhancer tool is capable of storing energy to enhance both upward and downward impacts delivered by a drilling jar. The enhancer stores energy in a single energy storage stack, typically of either bellville washer springs or a coil spring, and stores the energy in the storage stack irrespective of which direction it is moved, obviating separate energy storage stacks for upward and downward blows.

Abstract: A jar mechanism comprises an outer body member an, inner body member which is movably mounted on the outer body member, and a fluid chamber which is defined by the inner and outer body members. A resistance mechanism is in fluid communication with the fluid chamber. The inner and outer body members are moveable relative to each other between a first configuration in which the resistance mechanism resists relative movement therebetween, and a second configuration in which the resistance mechanism substantially does not resist relative movement therebetween. The resistance mechanism comprises two valve devices, where each valve device resists movement of fluid within the fluid chamber in one direction and the valve devices are arranged to resist the movement of fluid in opposite directions. A releasable locking mechanism locks the inner body member with respect to the outer body member, where a piston section permits a load to be applied between the body members after the locking mechanism has released.