The hydraulic delay firing (HDF) head is an absolute pressure operated firing head triggered by the sum of the hydrostatic pressure and the applied pressure. When the absolute pressure exceeds a predetermined level, the activating piston is forced up, breaking the shear pins.

The HDF head is immune to wellbore fluids with high solids content and retains all safety features built into every Schlumberger firing system. The HDF head incorporates a specially designed chamber impervious to debris. Strategic placement of the ball retainer ensures proper functioning regardless of the condition of the wellbore fluid. This innovative design isolates the shear pins from corrosive wellbore fluid through independent and sequentially operated shear pin and hydraulic sections. Hydraulic delay cannot affect the shear-out process. An adjustable hydraulic delay provides sufficient time for bleeding off pressure, even when nitrogen gas is used, and establishes underbalance before the guns are detonated.

The HDF head is used for multizone TCP to delay firing of the guns until all firing heads in the string are activated and the cushion pressure is established. For HPHT applications, the HDF-H head enables reliable and efficient perforating operations, even in extreme conditions. When no time delay is required, the direct firing head (DFH) can be used as an alternative to the HDF. The DFH has no delay time and is suitable for extreme overbalance fracture treatment at the time of perforating.