0°
1.25°
2.50°
+/– 12.5° stability window
3.75° Cumulative error
50 gun sections @ 1.25° /gun section= 62.5° potential rotation from vertical
> Accumulation of alignment error. In previous perforating gun designs, a slight rotational error was introduced at each gun section as the right-hand threads at each connection tighten under increasing compressional load. Although each rotational error may be small, the cumulative alignment error becomes signi cant, placing an indeterminate number of the guns outside the perforation-tunnel stability window. Bending and rotational torque from wellbore trajectory changes often exacerbate the total orientation error. In this example, a 1.25° error per gun section translates into 62.5° rotation from vertical if accumulated over 50 gun sections.
strength between 5 and 20 MPa [725 to 2,900 psi]. Rock mechanical studies indicated that suffi cient perforation-tunnel strength could be maintained with a maximum 25° deviation from the vertical plane, the direction of the maximum stress.
Hydro planned perforation intervals of more
than 2,000 m [6,561 ft] across near-horizontal wellbore intervals. The combined allowable drift for gun alignment and orientation would be
±10° , an accuracy beyond the capability of ordinary perforating equipment. Engineers from Hydro
realized that a new perforating
system design could enhance productivity in the Visund fi eld. Hydro set performance targets for a new oriented perforating system. Gun tubes would be required to orient to within 20° of vertical through a 5° /100 ft [5° /30 m] bend, or dogleg. Charges would need to perform at 75% effi ciency relative to standard steel-cased charges. Criteria
were set for swivel performance under loads reaching 50,000 lbf [222 kN] in straight and devi- ated environments. Hydro also required a method of confirming perforation orientation with an accuracy of 2° . Lastly, postfi ring criteria were set, including a requirement limiting gun debris to 50 gm/m [0.5 oz/ft] of nonzinc material. Schlumberger engineers developed an advanced, tubing-conveyed perforating system capable of accurate perforation orientation
Spring 2004
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