Bottom hole firing head and method
View Patent ↗Detonation of a perforating gun is initiated by engagement of the lower distal end of the gun assembly against a bottom hole bore plug. A fluid pressure actuated firing head is initiated by well fluid that is pressurized by a free piston having an integral rod projecting from the distal end of the gun assembly. The piston is displaced against a closed fluid volume when the projecting rod engages the bore plug.
1. An explosive detonation sub for detonating explosive well tools, said sub comprising:
a cylindrical housing having an axially open fluid path between a detonator end and a piston end including a piston bore;
an axially translated piston within said piston bore that is resiliently biased toward a distal end of said piston bore;
a percussion detonator proximate of said detonator end;
a firing pin displaced by a fluid pressure pulse in said fluid path for detonating said detonator; and
a piston rod projected from a piston end of said housing for displacing said piston to generate a fluid pressure pulse in said fluid path; and,
a first aperture through a housing wall into said piston bore to vent a predetermined volume of fluid upon displacement of said piston prior to generation of said pressure pulse.
2. An explosive detonation sub for detonating explosive well tools as described by claim 1 comprising a second aperture through said housing wall into said fluid path.
3. An explosive detonation sub for detonating explosive well tools as described by claim 2 having a sliding valve sleeve for closing said second aperture.
4. An explosive detonation sub for detonating explosive well tools as described by claim 3 having fluid pressure responsive retainer means for securing said valve sleeve at an open second aperture position.
5. A method of detonating an explosive down hole well tool
comprising the steps of:
coupling a firing head to a down hole end of an explosive well tool;
providing said firing head with a percussion detonator and a fluid pressure driven firing pin for striking said detonator;
coupling a cylinder sub to a down hole end of said firing head;
providing a piston in said cylinder sub to displace fluid into a fluid link with said firing pin;
providing a resilient bias against said piston;
providing a projection from said piston to project past a down hole distal end of said cylinder sub;
providing a first vent from said fluid link to discharge a predetermined volume of fluid displaced by an initial translation of said piston against said resilient bias due to engagement of said projection with a well bore obstacle; and,
generating firing pin driving pressure in said fluid link after discharging said predetermined volume of fluid.
6. A method of detonating a down hole well tool as describe by claim 5 providing a second vent from said fluid link to discharge additional volume of fluid displaced by additional translation of said piston.
7. A method of detonating a down hole well tool as describe by claim 6 providing a second vent closure element responsive to a predetermined well environment pressure to close said second vent.