Perforating Gunshock Loads: Simulation Capabilities and Applications
Authors: Brinsden Mark S , Boock Andrea , Baumann Carlos Erik 
Published: March 2015

Prediction of perforating gunshock loads and the associated risk of tool damage is very important because of the high cost of nonproductive time associated with fishing jobs, particularly for deep-water wells.

We have examined real examples of gunshock damage to determine how these incidents could have been prevented by using the most current (2014) capabilities in simulation software to predict gunshock loads. Our goal was to evaluate the latest software advances for predicting perforating wellbore dynamics and the associated gunshock loads and gauge the usefulness of the simulations in common perforating operations.

Both low- and high-pressure wells are susceptible to gunshock damage when they are perforated with inappropriate gun systems and/or under adverse conditions. Examples of tool damage due to gunshock include bent tubing and unset or otherwise damaged packers and wireline weak-point pull-offs. Using gunshock simulation software, we can identify perforating jobs with significant risk of gunshock damage, and then we can make changes to the perforating equipment or job execution parameters to reduce gunshock loads to safe levels, thus reducing the risk of equipment damage and nonproductive time. Using the latest gunshock software, engineers can also evaluate the sensitivity of gunshock loads to changes in perforating equipment, such as gun type, charge type, shot density, tubing size and length, cable size, rathole length, and placement/setting of packers and shock absorbers.

We analyzed two examples of gunshock damage: a tubing-conveyed perforation (TCP) job with 7-in. guns that produced a bent firing head fill-sub and tubing joints and a deep-water wireline job that broke the cable weak point. For both cases, we first simulated and analyzed the jobs as run to understand the observed damage and then we developed solutions to reduce the gunshock loads to a safe level.

Introduction

The objective of well perforating is to connect the reservoir rock to the wellbore for hydrocarbons to be easily produced or for fluids to be easily injected. Perforating with hollow carrier guns begins with the detonation of shaped charges contained inside thick-walled tubes called gun carriers. Shaped charges create high-velocity jets (~ 25,000 ft/sec) that produce tunnels in the reservoir rock. Shaped charges are selected based on the target completion type, either to penetrate deeply into the reservoir or, sacrificing penetration, open up an enlarged area for flow. When the shaped charges detonate, the hollow carriers deform due to internal gas pressure and debris impacting the inner side of the carrier. At the same time, the perforating jets puncture the hollow carrier wall, and the detonation gas inside the gun interacts with the wellbore fluid. All of these events combined lead to the onset of wellbore hydrodynamics, which includes large-amplitude pressure waves that produce very large loads on the equipment. The origin of gunshock loads generated by wellbore pressure waves will be explained in the gunshock studies presented in the following sections.