Linking Interdependent Technologies Hydro identifi ed several areas for improvement other than perforation orientation, including formation damage, depth of charge penetration, charge chemistry, detonation debris and fluids chemistry in the perforating environment. Near-wellbore formation damage, or skin, occurs for various reasons. During drilling, reser- voir rock is exposed to signifi cant environmental changes, including pressure, shock from the bit, and invasion of mud fi ltrate and solids. Depth of damage is generally a function of rock porosity, permeability, pressure differential, exposure time and the characteristics of the drilling or completion fl uid.
Long drilling times on the Visund wells resulted in deep mud-fi ltrate invasion. Engineers believed that standard deep-penetrating charges might not reach beyond the damaged zone. Even under ideal conditions, American Petroleum Institute (API) testing specifications suggest that the effective perforation-penetration depth under actual downhole conditions can be signifi - cantly less than predicted.8
Laboratory analysis
conducted on core sections from the Visund fi eld indicated that 40- to 50-cm [16- to 20-in.] effec- tive perforation-penetration depth would be required to bypass damaged reservoir rock. In early perforated completions, zinc-cased charges were thought to be the least damaging, because the zinc casing disintegrates into a fi ne powder during detonation. In theory, this should minimize the damage and debris that accompany standard steel-cased charges. As part of the
Visund evaluation, engineers found that zinc- cased charges did not penetrate as deeply as previously thought. Further, powderized zinc- detonation by-products have the potential to react with the wellbore fl uid and connate water to precipitate zinc oxide in near-wellbore reser- voir rock and the borehole. Also, studies conducted at SRC demonstrated that interac- tions between zinc and calcium bromide-base kill pills could cause failure of fl uid-loss control additives.9
The result is excessive invasion of
fi ltrate that leads to reduced productivity.10 In several early Visund wells perforated with zinc-cased shaped charges, zinc oxide precipitate
was observed during postperforation cleanup. Hard nuggets of zinc oxide measuring 5 to 20 mm [0.2 to 0.8 in.] in diameter circulated to surface (below left). These nuggets plugged subsea chokes and other control equipment, posing a dangerous and diffi cult situation to remediate. To address these problems, Schlumberger engineers developed the nonzinc PowerJet Plus low debris, deep penetrating shaped charge. While retaining the high performance of a deep-penetrating steel-cased charge, PowerJet Plus charges generate minimal debris, are weighted to aid in orientation, produce no resid- ual zinc and, based on API penetration testing,
0 0
1 cm 1 in.
> Zinc oxide precipitation. Zinc detonation by- products may react with wellbore and formation
uids to form zinc oxide precipitate. The material may precipitate within the formation, perforation tunnels and the wellbore, resulting in an increased reservoir-skin factor and plugged perforations. Large nuggets, such as those shown here, can plug downhole safety and well-control equipment, posing serious operational risk and remediation problems.
> Deep penetration with PowerJet deep penetrating shaped charges. These charges produce long, narrow perforation tunnels. The concrete block in the photograph is an API Section 1 test target. The block was shot with steel- cased PowerJet charges through 7-in., 32 lbm/ft, L-80 casing. The average penetration in this target was 54.1-in. [137.4 cm]. The perforation tunnel being measured reached 60 in. [152.4 cm] into the target. Similar performance is seen with the PowerJet Plus low debris, deep penetrating shaped charges.
34
Oilfi eld Review
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