A Gun Jump Model for Underbalance Perforating

Abstract

A new methodology for evaluating the risk of gun jump during post-perforating surge is proposed and tested against field data. This methodology is corroborated by an advanced mathematical model of transient flow into the wellbore triggered by the creation of perforation tunnels under static underbalance and fully coupled with evolving transient flow inside the wellbore containing the gun string. The model predicts the viscous drag on the tool string and the gun string dynamics, which may induce cable tangling or breaking owing to gun jump. This model is used to mitigate the risk of gun jump by varying perforating job design options such as shot density, thickness of the perforated interval, and height of the liquid cushion. Examples of model application to the analysis of field data from Indonesia are provided. Introduction There are two basic mechanisms of gun jump during perforating underbalanced.1 One is associated with the dynamic impact of the shock wave created after initiation of the shaped charges on the gun string inside the borehole. This shock wave provides the gun string with momentum that can force it to move. This mechanism of gun jump can be activated perforating underbalance or overbalance and may result in both upward and downward gun movement, which is usually mitigated to some extent by shock absorbers included in gun strings. This mechanism of gun jump is not considered in the current study. The other mechanism of gun jump is related to the postperforating surge from the reservoir into the wellbore through perforation tunnels. If the underbalance pressure is high, the transient reservoir response coupled with the dynamic level of liquid inside the wellbore may create fast wellbore fill-up accompanied by viscous drag on the gun string and cable, causing the gun string to move upward.2 The more powerful the surge, the higher the risk of gun jump owing to viscous drag on cable. The risk of gun jump can be mitigated by reducing the static pressure underbalance, shot density, and thickness of the perforated interval. All these measures, however, can compromise the quality of perforation cleanup and the targeted well productivity. A reliable and quantitative technique is therefore needed to optimize perforating design, in particular, to choose the optimum cushion level, which would mitigate the risk of gun jump while providing maximum cleanup efficiency. A procedure for gun jump risk assessment based on a novel gun jump model is presented here. The proposed technique estimates the viscous drag on the cable by simulating transient surge from the reservoir coupled with wellbore fill-up. The paper is organized as follows. The surge model coupled with transient flow inside the wellbore is considered first using the approach developed in slug test and drillstem test theory.3, 4 The coupling takes into account the acceleration of the liquid column inside the wellbore, frictional pressure losses, and the variation of drawdown pressure to accurately calculate the drag force on the tool string. The drag force varies with time during surge to represent the input for a simple gun jump model that treats a gun on a cable as a mass on an elastic spring experiencing forced oscillations caused by varying cable tension. This drag force is compared with the critical drag force on the cable obtained from the condition that the cable tension becomes equal to zero when the gun reaches its highest position inside the wellbore. Surge Flow Physics The reservoir surge starts immediately after perforating with static pressure underbalance and continues until the equalization of pressure between the wellbore and the reservoir is achieved. The transient reservoir response during surge is coupled with the borehole fluid dynamics and controlled by different physical mechanisms during its evolution in time. As soon as the perforation tunnels have been created as shown in Fig. 1, the reservoir is in hydraulic communication with the wellbore, which is filled by the completion fluid up to an appropriate level with an air cushion above it. The initial size of liquid column inside the wellbore 0 L, or the size of the air cushion 0 H, is the main parameter that controls the static pressure underbalance during underbalanced perforating.

Perforating on Wireline: Maximizing Productivity and Minimizing Gunshock

“…The former produces gunshock forces, essentially a water-hammer effect that acts on all surfaces exposed to the wellbore fluid, whereas the latter cleans up the tunnels after the damaged rock around the perforations has been broken up by the DUB, thus producing very conductive perforation tunnels with the lowest possible perforating skin. Of note, guns blown uphole is a risk in gas wells perforated with a large initial static underbalance; when the initial static underbalance is large enough to produce a large reservoir surge, the gas surge produces a large movement of wellbore liquid which, in turn, produces a large drag on the gunstring and cable [see Zazovsky et al (2007)]. For this reason, the DUB approach is ideal because it produces a large-amplitude short-duration DUB environment at the sandface, which is enough to clean up the tunnels, but is short in duration so that there is no risk of guns blown uphole.…”

Risk Evaluation Technique for Tubing-Conveyed Perforating

“…This simulation technique is robust and delivers accurate solutions. Gunshock simulation is described in papers by Schatz et al (2004), Zazovsky et al (2007), Baumann et al (2010a, 2010b, 2011a, 2011b, 2012), Canal et al (2010, Sanders et al (2011), andBurman et al (2011).…”

Risk Minimization when Perforating with Automatic Gun Release Systems

“…This simulation technique is robust and delivers accurate solutions. Gunshock simulation is described in papers by Zazovsky et al (2007), Baumann et al (2010a, 2010b, 2011a, 2011b, and 2012), Canal et al (2010, Sanders et al (2011), andBurman et al (2011).…”