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Abstract
The invasion of pulverized rock formation grains and the resulting “low-permeability crushed zone” is the primary cause of wellbore damage in perforated completions, as established by Behrmann et. al. In order to minimize this damage during the perforating process, it is necessary to provide a dynamic underbalance in the well that will deliberately induce flow into the wellbore for tunnel cleanup. Traditional well fluids have a limited application in depleted reservoirs as the lowest achievable density is on the order of 6.6 ppg. In many depleted reservoirs this density can represent an overbalance. It is not always desirable or operationally practical to provide this underbalance with a gas cushion, and therefore in order to achieve underbalance, it is desirable to engineer a stable fluid with non-damaging chemical properties that would have a significantly lower density. This paper reports on the formulation of super light completion fluids consisting of Shell Sarapar 147 synthetic oil [Shell MDS (M)], 3M™ Glass Bubbles as a density reducing agent and an appropriate rheology control agent. Laboratory tests show that density values as low as 5.0 ppg could be achieved. Similar mixtures were prepared and used in perforation operations for Talisman’s Malaysia. A total of 72 barrels of lightweight completion fluids at about 5.5 ppg was pumped downhole and the perforation job completed successfully. Production history of the well shows a marked increase in production rate compared to neighboring wells, which produce from the same reservoir, but were perforated traditionally. This technology is not necessarily limited to depleted reservoirs. In normally pressured zones where permeability is extremely low, the fluid provides an opportunity to increase the available underbalance by an order of magnitude to assist cleanup.

Introduction
It is no secret that perforations conducted in overbalanced conditions can result in damage of the rock matrix. The damage zone usually extends about 1 centimeter into the rock with about 20 percent or more of permeability reduction9. Lower permeability rocks tend to exhibit a larger percentage of permeability reduction. The damage zone of the rock matrix occurs from the crushing of sand grains as the jet enters the rock. Figure 1 shows a typical perforation schematic of rock perforated in an overbalanced state. It indicates the presence of perforation debris and a low permeability zone of crushed and compacted material around the perforation tunnel. Perforating shock waves and high impact pressure shatter rock grains that break down inter-granular mineral cementation and de-bond clay particles, creating a low permeability crushed zone in the formation around perforation tunnels. It is essential to remove some or all of the perforation damage to ensure a successful perforation job9. A common practice is to conduct perforation cleanup through acidizing. This type of clean-up job imposes additional costs. Perforation cleanup or remedial perforation-wash acid jobs could be avoided if the perforation operation were conducted in an underbalanced state. Underbalance perforation is widely accepted as the most efficient method to obtain clean perforation. Optimal underbalance pressure criteria have increased substantially over the past decade as a result of hundreds of laboratory tests and field observations1,4. Field observations by King et. al were used to develop criteria based on the efficiency of sandstone acidizing. Behram correlated laboratory data with the viscous drag force to remove fine particles in perforation tunnels. Laboratory tests confirm that a higher degree of underbalance is indeed needed for clean perforation. Underbalanced perforation improves flow channels by effectively removing the crushed zone. This is achieved through an instantaneous surge of fluids from the reservoir into the wellbore when the jet penetrates the rock. Thus underbalance perforation aids in the removal of perforating debris, while minimizing or eliminating crushed-zone damage in and around the perforation tunnel.