In cased completions, perforations provide the essential link between the wellbore and the reservoir. Productivity of the completion is promoted by optimizing perforation characteristics such as geometry, phasing and density, but unfortunately it is restricted by the perforation damage zone---a region of low permeability material surrounding the perforation tunnel, and created by the impact of the shaped charge jet on the rock fabric.

Perforating underbalanced has become the primary means of removing perforation damage and maximizing productivity, though the mechanism by which it does so is still not very well understood. Underbalance perforating also serves to remove some or all of the comminuted sand grains that fill the perforation tunnel immediately after penetration of the rock by the shaped charge jet. Predictions of the required underbalance to remove the damage zone or remove the comminuted fill are at best uncertain.

In this paper we describe the development of mathematical models that predict the pore pressure in the rock surrounding the perforation tunnel as the wellbore pressure drops during, for example, a dynamic underbalance operation. From this we calculate the magnitude and duration of the induced surge flow. The third stage in the analysis investigates the mechanisms through which the damage zone is removed.

As a result of these calculations we are able to predict the surge rate (and associated underbalance) required to remove the damage zone. Moreover we predict the perforation skin that results from incomplete removal of the damaged zone and for the first time we are able to determine how the skin depends on the degree and rate of underbalance.

1. Introduction

A perforation is created by the impact into rock of the high-velocity jet from a shaped-charge explosive. This jet punches a hole at speeds sufficiently rapidly that the displacement of the rock is achieved by the creation of a large number of microfractures extending through grains of sand (Pucknell & Behrmann[1]).

The radial displacement of the rock creates a residual elastic stress in the far-field undamaged porous medium (known as a " stress cage"). As the rock decompresses, this stress cage causes the failure of the most damaged rock adjacent to the perforation tunnel. The failed rock collapses into the tunnel, where it mixes with the remnants of the perforation jet to form a fill of loose debris.

Figure 1 shows a cartoon of a perforation immediately after it is created. The perforation tunnel is filled with a loosely packed debris of high permeability (1-10 Darcies). Immediately surrounding the perforation tunnel is a "damaged zone" of fractured rock grains, extending a distance of order 20mm from the perforation tunnel. For liquid-saturated rocks, the porosity (and density) of the damaged zone is close to that of undamaged rock; for gas-saturated rocks, the porosity of the damaged zone may be much reduced. In the damaged zone, the pores are much smaller than in the virgin rock: many sand grains have micro-fractures, and large pore throats are filled with small fragments (Figure 2). This decrease in pore size results in the damaged zone having a permeability much decreased from that of undamaged rock Pucknell & Behrmann[1]. Additionally, the damaged zone is partially deconsolidated: its strength is much less than undamaged rock.

The perforation tunnel creates a flow path between the reservoir (at pressure pr), and the wellbore (at pressure pw). The pressure difference pr-pw can drive a surge flow, either from the wellbore into the formation, or from the formation into the wellbore. Traditionally, wells are perforated with a " static underbalance", with the far-field wellbore pressure less than the reservoir pressure. Usually this generates an underbalance surge flow (of timescale around 1 second), though this depends on the details of the wellbore and perforating gun design. Novel perforating techniques[2,3]exploit knowledge of the transient wellbore dynamics to engineer a surge flow that is both more rapid (building up over a timescale potentially as rapid as 10ms), and of greater magnitude than traditional perforating. (Figures 2 and 3 in ref 2 illustrate these two cases with pressure profiles measured in laboratory tests.) This dynamic underbalance can produce much cleaner perforations than traditional underbalanced perforating, leading to significantly more productive wells.[3, 4]

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