Perforating the extremely large deepwater wells in the Santos Basin fields, offshore Brazil, is done in a single-trip, shoot-and-pull operation, using 7.0-in. guns loaded with ultra-deep penetrating charges, which produce 65-in. of penetration depth per API RP 19B. These wells have 9-5/8-in. and 9-7/8-in. production casing with gross perforated lengths sometimes exceeding 600-m, and bottom hole pressures larger than 8,000-psi, in some cases reaching 13,500-psi. Perforating these wells with 7.0-in guns is very challenging because of the large downhole loads acting on the tubing string and on the drillship. To evaluate gunshock overloading risks, we utilize a simulation model to predict gunshock loads. This simulation model helps to assess the maximum loads for different perforating scenarios, and helps to devise strategies to reduce the peak tension on the tubing string and drillship to safe levels.
Perforating shock loads are generated by the detonation of the guns and by the associated pressure waves in the completion fluid, such pressure waves act on the guns, tools, and tubing string. Shock loads can pose a serious risk of parting the tubing string and/or damaging the drillship's hoisting equipment. A fully coupled fluid-structure simulation model is used to predict perforating shock loads. Before every perforating job, the operator evaluates the peak transient loads on the tubing string and heave compensator, and decides on the best strategy to prevent gunshock-related damage.
Many drillship operators believe that large-size gunstrings can damage the heave compensation system. Often, afraid of damaging the heave compensators, drillship operators opt for disabling the heave compensation system when perforating, and this is what can create unfavorable conditions that can lead to extremely high loads on the tubing-string. Computer simulation of the perforating event with models having varying degrees of heave compensation show the need for heave compensation to reduce the peak tension load on the tubing-string. Actual drillship measurements and simulation results of transient hook-load are presented side-by-side, as well as sensitivity studies of the transient tubing-load dependence on the heave compensator's load-movement relationship. Actual hook-load measurements from one perforating job done with heave compensation and one without heave compensation show the need to use heave compensation to reduce the peak tension load on the tubing-string.
Gunshock loading simulations are described in detail, using actual jobs data to analyze the transient shock load on the tubing string and on the drillship. Detailed comparisons between simulated and measured peak drillship hook loads are presented, as well as the tubing axial load dependence on the heave compensator's load-movement relationship. This information will help operators to decide on the strategy to avoid having non-productive time because of shock related equipment damage.