Recent Advances in Understanding Perforator Penetration and Flow Performance
- P.M. Halleck (Pennsylvania State U.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1997
- Document Type
- Journal Paper
- 19 - 26
- 1997. Society of Petroleum Engineers
- 5.6.5 Tracers, 2.4.3 Sand/Solids Control, 5.3.4 Integration of geomechanics in models, 1.8 Formation Damage, 5.4.2 Gas Injection Methods, 2.2.2 Perforating, 1.6 Drilling Operations, 4.3.4 Scale, 2.4.5 Gravel pack design & evaluation, 1.2.3 Rock properties, 5.3.2 Multiphase Flow, 1.6.9 Coring, Fishing, 5.6.1 Open hole/cased hole log analysis
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Driven by the importance of perforation performance in both natural and stimulated completions, our understanding of the factors that control penetration depth and formation damage has improved significantly over the last 10 years. Improved test procedures and their tabulated results are now available to predict downhole penetration as a function of rock properties and overburden stress. Progress has also been made in understanding the nature of permeability damage around perforations and how to prevent, remove, or bypass it. In particular, the mechanisms behind underbalanced perforating are being clarified, leading to better engineering of perforating procedures and better prediction of their results.
Productivity of a cased-and-perforated well depends largely on perforation depth and flow performance. Penetration of shaped-charge perforators depends strongly on the nature of the target and the effective stress applied to it. Downhole penetration is typically much less than measured in concrete targets under surface conditions. Shaped charges are also known to cause formation damage, possibly involving mechanical shock damage, deep filtration of fines from the wellbore or the formation, presence of rock and charge debris in the perforation tunnel, and multiphase flow effects.
This paper summarizes recent progress in understanding and predicting both penetration depth and perforation damage. Following a brief discussion of penetration-depth prediction will be a review of experimental observations of formation damage to the rock surrounding the perforation. Methods of preventing, removing, or bypassing perforation damage are reviewed, followed by a discussion of how underbalanced perforating works. This is followed by efforts to model the mechanism of underbalance surge cleanup and description of laboratory experiments that reveal its time scale and the flow volume needed.
Penetration of shaped charges depends on the character of the target rock formation, the effective overburden stress1 and, in extreme cases, wellbore pressure.2 Uniaxial compressive strength has long been used3 to correlate ambient-stress penetration. This approach is complicated by the lack of a comprehensive penetration theory incorporating target strength, by increased rock strength with effective stress,4 and by active target effects in some rocks5 which are not explained by conventional penetration theory. In Berea Sandstone, penetration can be reduced by as much as 50% with the application of 20.7 MPa (3,000 psi) effective overburden stress6. Little additional reduction occurs for higher effective stresses. The nature of the rock affects both penetration and the extent of the stress effect. Higher strength leads to lower penetrations but the effect of stress is less in such rocks. Thus, penetration data obtained from a single target material under atmospheric conditions do not reflect actual downhole penetration.
Recently, an attempt has been made7 to use existing laboratory data to predict down-hole penetration from American Petroleum Inst. (API) RP 43, section 1 certification data. The nomogram presented provides a painless method for estimating downhole penetration in lieu of the costly API standard test8-10 in stressed Berea Sandstone. The method relies on a series of empirical correlations between compressive strengths, porosity, and penetration in rock and concrete targets at atmospheric pressure, combined with data in stressed rock targets. Active target effects, normalization of the data to 20.7 MPa (3,000 psi), and statistical scatter in each of the successive correlations used may lead to substantial errors,11,12 particularly at lower effective stresses.
A more promising future technique may provide penetration estimates from acoustic log data based on direct relations between penetration and dynamic elastic moduli in stressed rock. Originally proposed by Venghiattis13 more than 35 years ago, recent data14 largely support the idea that differences in acoustic velocity caused by rock composition and overburden stress mimic differences in shaped-charge penetration.
The Nature of Permeability Damage
Perforation damage may refer to debris in the perforation tunnel and/or to a zone of reduced permeability surrounding the tunnel. The tunnel debris consists of broken rock (either loose or competent), jet metal, explosive products (mostly carbon), and other charge debris. If sufficient surge and/or production flow occurs, this debris may be removed, leaving a clean tunnel.
In well-productivity models,15-18 the perforation is usually treated as an empty cylindrical tunnel surrounded by a zone of reduced permeability, variously called the crushed or compacted zone. This zone is usually assumed19 to be 1.0- to 1.25-cm (0.4- to 0.5-in.) thick, based on easily-visible lighter-colored material around the tunnel, with permeability reduced to about 20% that of the undamaged rock.
Pucknell and Behrmann20 have confirmed earlier observations with optical microscopy which reveal broken grains in this zone. Asadi and Preston21 have used scanning electron microscopy (SEM) to estimate permeability damage from the size distribution of the broken grains. Reduced porosity is also thought to play a role. Fig. 1 illustrates broken grains near a perforation tunnel compared with unbroken material further away.
To predict the extent of such damage, Yew and Zhang22 applied the method of characteristics to calculate the shock-wave amplitude from jet impact. They assumed cylindrical radial loading and included poroelastic effects. They find, as expected, that stress-wave amplitude falls off rapidly away from the perforation tunnel, although they do not attempt to specify the specific amplitudes. Papamichos et al.23 have used this solution to estimate the extent of grain breakage to be expected from such stress waves, taking into account stress amplification at the contacts between sand grains. They predict that grain breakage also decreases rapidly away from the perforation and again suggest that this can be theoretically related to permeability reduction.
Several lines of evidence suggest that this view of permeability damage may be too simple. Damage is not necessarily related only to the visible crushed zone adjacent to the tunnel wall. Further, the material immediately adjacent to an open perforation is not highly compacted, at least in liquid-saturated sandstones, and shows little consistent permeability reduction. Instead, reduced permeability is observed outside this crushed zone, extending 3 or more cm from the tunnel. Permeability damage also varies along the length of the perforation, being thicker and more severe near the entrance hole. Mechanical damage patterns also reflect this observation.
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