Perforating Underbalanced Evolving Techniques (includes associated papers 13966 and 14140 )
Authors: Bell W.T. 
Published: December 2007

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleum engineering.

Introduction
The usual objective in perforating a well for completion is to maximize well productivity, i.e. to minimize impairment to well flow (skin), whether due towellbore damage or to the perforating process itself. Criteria for minimizingskin vary with whether the specific completion problem is natural, sandcontrol, or hydraulically fractured. However, in perforating for enhanced productivity, two factors are significant in decreasing order of importance:establishing perforating pressure conditions that enhance system cleanup, andselecting the perforating technique and guns to achieve best flow performance.
Underbalanced perforating (in which guns are fired under reduced wellbore pressure conditions) addresses the critical factor in establishing perforating pressure conditions. In effect, the technique"surge flows" andcleans perforations as a system. When high-performance perforators are usedwith the technique, optimization of well response becomes possible. Underbalanced methods have been applied successfully since the early 1950s. However, resistance to their use has persisted because required through-tubingguns displayed lower performance capability and certain mechanical limitations, as well be discussed. With improved techniques and better guns for both wireline- and tubing-conveyed methods, more flexibility is now available todesign effective completions. Underbalance techniques are more widely applied-particularly in natural completion operations(to which this article is principally addressed).
Factors Affecting Perforated System Flow,: Natural Completions
Perforating Damage: Cleanup and Flow. Penetrating typical oilfieldformations with commonly used shaped charge perforators creates a "skin"that impedes flow into the perforation (Fig. 1). The jet punches its way into theformation at high velocity (Fig. 1B), radially displacing formation materialand creating a crushed or compacted reduced-permeability zone compared to undamaged rock. For example, immediately after perforating the API RP 43standard Berea sandstone target, the perforation is highly damaged (Fig. 1C)and must be flowed for cleanup (Fig. 1D). During flow of 10 to 50 liters, coreflow efficiency (CFE) increases from an initial 0.1 (10%) to the typicallyreported 0.7 to 0.8 (70 to 80%), after which there is no further improvement. The crushed zone remains, representing "permanent" damage-at least in terms ofinitial response to flow.
Sensitivity of Cleanup to Differential Pressure. In addition to flow, acertain "threshold" level of differential pressure (p) is required to effect cleanup. Again using the API RP 43 test as an example note that in Fig. 2 the typical 0.7–0.8 CFE is measured at the test standard, 200 psi [1378.8 kpa] p. If tests are made at lower levels (below the approximate 150-psi [1034.1-kPa] threshold for this sandstone) CFE decreases. As suggested by curve shape, testing at any p value above threshold yields no improvement in flow efficiency-i.e., permeability in the crushed zone is not further improved. Inactual operations, varying formation characteristics can alter theresponse-to-flow from that of the Berea sandstone example.
JPT
P. 1653^