productivity (see “ Practical Approaches to Sand Management,” page 10).
Although oriented perforating has been suc- cessful in many areas, perforating tool designs have proved inadequate in highly deviated well- bores. During placement, tools experience varying degrees of frictional, bending and torsional stress, causing drag and gun-tube misalignment. Perforating gun manufacturers typically isolate the gun string from these stresses and the rotational
force of the
conveyance string by placing a number of swivel subs above the gun tubes to promote gravita- tional self-orientation.
Manufacturers have employed various methods to generate gravity-driven perforation- tool alignment.4
A common method uses fi ns, or
kickover lugs, welded on the gun tube (right). Kickover lugs alter the gun-tube center of gravity and promote an eccentered orientation. The ori- entation accuracy using kickover lugs is generally better at low angles of wellbore deviation. In an effort to overcome inherent short- comings in this method, some manufacturers increased fi n, or lug, height, while others made them shorter. Taller fins provided a tighter fit, but increased the potential for the gun string to become stuck in a wellbore. Shorter fins gave more clearance, but allowed the gun assembly more freedom to rotate, thus introducing an orientation error.
Early Schlumberger designs used internally
weighted spacers to help orient the gun string. A solid, half-round steel bar occupied the lower half of a spacer, typically 20 ft [6 m] long. This design worked well for short intervals in straight wells, at moderate deviations and in situations requiring only minimal orientation accuracy. In longer extended horizontal wellbore sections, 250 or more individual gun tubes are sometimes run on one string. Each gun section must be closely aligned with the next to assure perforation alignment. However, compressionsal forces acting on the connectors squeeze any manufacturing clearance tolerances, slightly twisting the right-hand connector threads and generating a gradual clockwise orientation error. Although each twist is small, the cumulative
4. Benavides SP, Myers WD, Van Sickle EW and Vargervik K:
“ Advances in Horizontal Oriented Perforating,” paper SPE 81051, presented at the SPE Latin American and Caribbean Petroleum Engineering Conference, Port-of- Spain, Trinidad, West Indies, April 27– 30, 2003.
5. Stenhaug M, Erichsen L, Doornbosch FHC and Parrott RA:
“ A Step Change in Perforating Technology Improves Productivity of Horizontal Wells in the North Sea,” paper SPE 84910, presented at the SPE International Improved Oil Recovery Conference in Asia Paci c, Kuala Lumpur, Malaysia, October 20– 21, 2003.
6. Sulbará n et al, reference 3. Shoulder screw Upper alignment plate Casing Half-bar weight
Tool Pipe
Kickover lug
> Kickover-lug designs. Older designs incorporating kickover lugs may become stuck inside casing. Often, the kickover lug is con gured as large as possible relative to the inside diameter of the casing (top). With this close tolerance, sand and debris accumulating in and around the gun tube after ring may cause the gun to stick, or hang up, in the hole. Smaller lugs allow the gun assembly to fall to one side resulting in perforation misalignment. During conveyance in highly deviated boreholes, the eccentric shape of the gun-tube assembly can promote rotational torque and friction resulting in perforation misalignment. Other designs use weighted spacers (bottom) to generate rotational torque. If insuf cient torque is generated to overcome frictional resistance, the result is perforation misalignment.
orientation error can be significant across perforation intervals with many connections (next page).5
Although the allowable deviation from optimal perforation angle varies from reser- voir to reservoir, a variance of more than 25° from maximum horizontal stress may promote sand production.6 Until recently, service providers tried to minimize the accumulation of alignment and subsequent orientation error by keeping gun-section length between swivels short. How- ever, short sections often generate insufficient gravity-derived torque to orient guns in more deviated wellbores.
Intervention in the North Sea
Hydro found that attempts to orient perforations in highly directional wellbores in the North Sea often resulted in as much as 45° perforation
misalignment. In 2001, lower than expected per- formance from early wells in the Visund fi eld led Hydro to assess the effects of perforating tech- niques on sand production and well productivity. The Visund field, operated by Statoil since 2003, lies about 150 km [90 miles] off the coast of Norway, northwest of Bergen. Production began in 1999 while the field was operated by Hydro. The first two wells in the Visund field were completed using sand screens. Over time, Hydro engineers found that zonal isolation was required to properly manage production from the field. Production liners were placed across the productive intervals and cemented in place; the wells were completed using a standard ori- ented perforating system with zinc-cased charges. The reservoir in the Visund field is geologi- cally complex and comprises weak sandstones with permeabilities ranging from 300 to 3,000 mD and an unconfined compressive
30
Oilfi eld Review
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