Borehole Dynamics During Underbalanced Perforating
Authors: Behrmann L.A. , Li J.L. , Venkitaraman A. , Li H. 
Published: December 2007

Abstract
A knowledge of the shocks and resultant fluid dynamics in the wellbore from a perforating gun would be desirable in the design of downhole tools and in understanding the physics of perforation damage and cleanup. However, fast transient downhole data has not been published due to the cost and difficulty of obtaining these measurements.
To help close the 'no field-data' gap, underbalanced single shot perforation and flow experiments were conducted on outcrop and reservoir core samples under simulated downhole pressure conditions. Transient pressure and flow measurements were conducted in these laboratory experiments to provide a better understanding of the wellbore and reservoir fluid dynamics. Specific measurements included wellbore pressure, reservoir-wellbore pressure differential and the transient underbalance surge flow rate at millisecond resolutions. Fast quartz gauges were used to record the early gun-shock data at microsecond resolutions. Fast Fourier transform (FFT) analyses were carried out on the acquired data. Computer simulations were conducted for theoretical analysis of the gun-shock process.
Experimental gun-shock data and computer simulations showed the characteristic frequencies of the shock waves to be dependent on the size and geometry of the wellbore and independent of the charge and rock parameters. The effect of charge and sample sizes was evident during the first millisecond on the shock wave amplitudes. A short period of wellbore fluid injection into the perforation was detected prior to the underbalance surge flow. The magnitude of the pressure differential driving this wellbore fluid injection was dependent on the charge size, underbalance and the permeability of the rock sample. Slow data acquired provided information on the events in the wellbore-perforation vicinity prior to underbalance surge flow.

Introduction
Underbalanced perforating is the most widely accepted method for minimizing perforation damage in oil well completions. Several aspects of perforating have been studied previously. These studies have shown the importance of utilizing the underbalance surge flow to remove the 'crushed zone' responsible for perforation damage. Since then underbalanced perforating has evolved with recent studies focused on predicting underbalance pressure criteria for obtaining minimum perforation damage. Considering the impact perforation performance can have on well productivity and the role played by underbalanced perforating in particular, it is essential that we understand the physics and flow dynamics behind this method.
Conventional wisdom suggests that for an oil well, the wellbore pressure increases for a very short time period from the perforating gun-shock. Shortly thereafter, it (1) decreases from the filling of the gun with wellbore fluid followed by (2) an increase in wellbore pressure due to inflow of reservoir fluid. The differential pressure changes from an underbalance to an overbalance and then to an increase in underbalance (due to gun filling). These phenomena were studied experimentally in detail by collecting fast (microsecond resolution) and slow (millisecond resolution) data at the time of perforation. The current study reports the results from these single shot perforation and flow experiments conducted under downhole pressure conditions. Some of the parameters varied in the study included the charge size, underbalance pressure and the permeability of the rock sample tested. Theoretical simulation of the early gun-shock process was carried out using a commercially available numerical program.