7,500 6,500 5,500 4,500 3,500 2,500
Reservoir pressure Wellbore pressure
-1 012 36 Time, s
45
> Achieving dynamic underbalance on the Visund A-21 well. Downhole pressure sensors recorded pressure changes throughout the perforating process. Changing wellbore pressure (black) is plotted against the previously measured reservoir pressure (red). The Visund A-21 well was overbalanced by 35 bar [508 psi] prior to perforating. The PURE perforating technique produced a 290-bar [4,206-psi] wellbore pressure drop within 1 second after perforating-gun detonation. A brief underbalanced condition just after ring perforating guns minimized formation damage and assisted in clearing perforation tunnels.
The Visund perforation program was designed with 35-bar [508-psi] static overbalance and PURE techniques, achieving a dynamic under- balance just after perforating. The dynamic underbalance removes the crushed-zone debris from the perforating tunnels. To prevent the wellbore from returning to an overbalanced state before the wellbore and pore pressure equalized, the design suggested a fl ow-restriction device to seal the top of the 7-in. liner hanger. The flow-restriction device would also serve as a depth-correlation tool. For these first applica- tions, engineers installed downhole pressure gauges just above the firing head to quantify the magnitude of the dynamic underbalance and confirm that the job was executed as designed (left).
Recent work conducted at SRC has indicated that maximum dynamic under- balance— not the initial static underbalance— determines perforation cleanup. The degree of underbalance required depends on several factors, including perforation-tunnel diameter, permeability, porosity and rock strength. How- ever, in the Visund reservoir, it is generally accepted that a minimum underbalance of 50 to 100 bar [725 to 1,450 psi] is needed to minimize skin effects and remove perforation debris. Achieving this level of static underbalance may or may not be practical on a given well. In some cases, wellbore conditions, method of conveyance and gun-string length may prohibit perforating with a static underbalance. The PURE Perforating for Ultimate Reservoir Exploitation system optimizes this critical step in connecting the well to the reservoir by maximiz- ing the dynamic underbalance, or transient underbalance, established just after the creation
are capable of producing deep and narrow perfo- ration tunnels (previous page, right).11 These long perforation tunnels can be diffi- cult to clean. Underbalanced perforating is a common technique used to clear the perforation tunnel of debris and minimize formation damage.12
of the perforation cavity. The PURE system cre- ates an instantaneous decompression of reservoir fluids around a perforation immediately after perforating, assisting in removal of the crushed material from the perforation tunnel while under static overbalanced conditions. The result is cleaning of perforation tunnels with minimal production impairment.13
In most cases, the PURE technique will produce less skin effect than conventional static underbalanced perforating. Laboratory and fi eld evaluations have shown that PURE system design concepts will often yield a crushed-zone perme- ability to reservoir-permeability ratio (kc/k) of 1, indicating no permeability impairment. Less advanced underbalanced perforating techniques typically yield 0.1 to 0.3 kc/k.
On the Visund completions, mechanical
issues precluded use of conventional under- balanced perforating. Project engineers determined that perforating damage could be removed with an underbalance of approximately 120 bar [1,740 psi]. Laboratory tests suggested that with a correctly designed perforating string and application of PURE dynamic underbalance concepts, sufficient underbalance to clear the perforation tunnel and minimize formation damage was achievable.
With the perforating environment defined, engineers at Schlumberger used a computer- based orientation simulator to design OrientXact perforating strings for six wells in the Visund field. The longest would encompass a 2,049-m [6,722-ft] gross perforation interval with 1,705 m [5,594 ft] of net perforations across a horizontal wellbore with high dogleg severity. Engineers chose a series of swivels and orienting weights sufficient to ensure accurate orientation. The length of individual gun sections between swivels ranged from 167 to 400 m [548 to 1,312 ft]. The OrientXact perforating system, inte- grated with PowerJet Plus charges, PURE underbalanced techniques and OCD perforation confi rmation, has now been used on eight wells in the Visund field— seven producers and one injector. Net production intervals ranging from 150 to 1,705 m [492 to 5,594 ft] have been perfo- rated successfully. When compared with previous designs, the PowerJet Plus charges produced only a fraction of the debris. Perforations were confi rmed by the OCD detectors with a variance of ±5° , half of the maximum allowable deviation
8. Stenhaug et al, reference 5.
9. In this context, kill pill refers to a kill-weight uid having a density suf cient to produce hydrostatic pressure greater than reservoir pressure, thereby shutting off ow of formation uids into the wellbore.
10. Chang FF, Kageson-Loe NM, Walton IC, Mathisen AM and Svanes GS: “ Perforating in Overbalance— Is It Really Sinful?,” paper SPE 82203, presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, May 13– 14, 2003.
11. American Petroleum Institute: publication API RP 19B (formerly RP 43):
http://api-ep.api.org (accessed March 15, 2004).
12. For more on underbalanced perforating: Bakker E, Veeken K, Behrmann L, Milton P, Stirton G, Salsman A, Walton I, Stutz L and Underdown D: “ The New Dynamics of Underbalanced Perforating,” Oil eld Rev iew 15, no. 4 (Winter 2003/2004): 54– 67.
13. Bakker et al, reference 12.
Spring 2004
35
Pressure, psi
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