Hydraulically actuated connection for downhole tools

A downhole tool assembly comprising:

• a first downhole tool with a central axis, the first downhole tool configured to receive fluid pressure from the downhole tool assembly;
• a second downhole tool with a central axis joined to the second tool;
• and
• a releasable connection joining the first downhole tool and the second downhole tool, the releasable connection having a central axis;
• and
• a release plane normal to the central axis of the releasable connection, along which the first downhole and second downhole tool are joined by the releasable connection, the releasable connection being configured to be released by a shear force along the release plane;
• and
• wherein the releasable connection comprises,
• a shiftable pin that translates axially in alignment with the central axis of the releasable, the releasable connection having a relatively high shear strength when the shiftable pin is in a first position and relatively low shear strength when the shiftable pin is in a second position;
• and
• a fixed connector that extends between the first downhole tool and the second downhole tool across a release plane, the connector having relatively low shear strength where it intersects the release plane.

A hydraulic actuator for a downhole tool, the hydraulic actuator comprising:

• an actuator piston translatable in a cylinder along an axis, the piston having a first end and a second end opposite the first end;
• and
• a hydraulic circuit connected to the actuator piston and configured to activate the actuator piston in response to internal tool pressure increasing to an activation pressure beyond a predetermined threshold pressure and not to activate the hydraulic when the internal pressure is below the predetermined threshold pressure, the hydraulic circuit comprising a first path configured to communicate internal pressure to the first end of the piston and a second path configured to communicate through a flow restrictor the internal pressure in parallel to the second end of the piston and a rupture element, wherein
• the rupture element is configured to rupture when the internal pressure reaches the activation pressure;
• rupture of the rupture element connects the second path through the flow restrictor to the second end of the piston and external port in parallel;
• and
• the flow restrictor is configured to maintain differential pressure between the internal pressure and fluid pressure on the second end of the piston.

A downhole tool assembly for sidetracking in cased wellbore, the downhole tool assembly comprising:

• a mill with fluid at an internal pressure;
• a whipstock;
• a releasable connection joining the mill and whipstock, the releasable connection configured for release by a shear force along a release plane normal to the connection where the mill and the whipstock meet, the connection comprising:
• a hollow fastener connection mill and whipstock that is shearable by a shear force along the release plane;
• a shiftable pin extending from the mill along an axis normal to the release plane between the mill and whipstock, the releasable connection having a relatively high shear strength along the release plane when the shiftable pin is in a first position relative to the mill and relatively low shear strength along the release plane when the shiftable pin is in a second position relative to the mill;
• and
• a hydraulic actuator comprising an actuator piston connected to the shiftable pin and configured to receive internal pressure of the mill when the shiftable pin is in the first position and the second position and to generate retraction force on the shiftable pin when the internal pressure reaches an activation pressure above a predetermined threshold pressure.