TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractPerforating underbalanced has become the primary means of removing perforation damage and maximizing productivity through a cased and perforated completion. The level of static underbalance (wellbore to pore pressure difference prior to perforating), considered the critical parameter in achieving acceptable productivity, has been investigated theoretically and experimentally through testing on outcrop and reservoir cores under laboratory conditions. We now report on a new series of laboratory experiments conducted on outcrop cores under more representative downhole conditions. Investigation of previously neglected parameters produced profound differences in perforation productivity. In some circumstances static overbalanced perforating proved to be as effective as static underbalanced perforating. The surprising results on underbalanced and overbalanced tests suggest a new approach to minimize perforating damage. Not only do these results impact our understanding of the mechanism of perforation damage removal, but they also have important implications for future design of perforating jobs.

For perforated natural completions, well productivity is dependent on the depth of the perforation tunnels extending beyond the drilling damage (all other things being equal). It is therefore important to accurately predict perforation depth at downhole conditions, in order to enable accurate prediction of well performance.Most industry penetration models in use today are based largely on laboratory experiments conducted during the 1960's through the 1990's. Over the past decade or so, it has been observed that the accuracy of these models has not kept up with the true downhole performance of modern shaped charge perforators. Furthermore, different models can give quite different predictions of penetration performance of the same perforating system, in the same downhole environment.To address this situation, the authors have recently conducted an extensive series of laboratory experiments, the results of which are enabling improved prediction of penetration performance in the field. Several hundred modern charges of different sizes have been shot into multiple rock types under simulated downhole stress conditions.The primary conclusions of this work include: (1) historical penetration models tend to overpredict penetration at downhole conditions; (2) some industry models overpredict to a greater extent than others; (3) the discrepancy is partly due to the industry's continued reliance on performance into surface concrete targets. In addition, we observe that historical models tend to treat all charges equally, imposing the assumption that increased performance in one target (i.e. surface concrete) always and everywhere ensures increased performance in all targets (i.e. various stressed rocks). While this intuitive assumption is proven true as the "rule" on average, we find some significant exceptions. Although these exceptions complicate predictive modeling efforts, they do suggest opportunities to optimize charges for certain targets preferentially over others.Our conclusions have led to ongoing work including (1) the development of improved penetration models, which rely exclusively on stressed rock, rather than unstressed concrete, performance; (2) the development of perforators optimized for downhole conditions, rather than for surface concrete.

For natural completions, well productivity is proportional to the depth of the perforation tunnels. Perforation depth, in turn, is generally inversely related to the formation effective stress. Accurate productivity modeling, therefore, requires accurate knowledge of the relationship between the downhole stress environment and perforation depth.A comprehensive experimental effort was recently conducted to evaluate the penetration performance of shaped charges into stressed Berea sandstone cores. Rock confining stress (σ c ) and pore fluid pressure (P p ) were varied from ambient to 10,000 psi, to simulate a range of downhole stress environments. This current work featured a broader and more systematic investigation of the influence of pore pressure than previous studies.Our experiments yielded the expected inverse correlation between penetration depth and effective stress (σ eff ). However, the data suggest a new definition of effective stress. Historically, the perforating community has defined σ eff = σ c -P p , but a new treatment (σ eff = σ c -aP p ; a=0.5) better fits present data. Furthermore, this new effective stress law better fits published historical penetration results. Pore pressure's influence on penetration depth is therefore weaker than previously thought; for a given confining stress, increasing pore pressure does increase penetration, but to a lesser extent than conventional models would indicate. The present work suggests that all shaped charges would be similarly affected.These findings are relevant to penetration modeling, and in turn to well productivity modeling and prediction. Further implications are to laboratory testing, regarding scaling of parameters to accurately simulate field conditions. This work culminates an initial application of combined penetration mechanics and geomechanics analyses to the investigation of shaped charge penetration into geologic materials. Future work will address different rock types, additional poroelastic quantities, and dynamic effects as they contribute to pressure-induced strengthening of reservoir rock.

Perforating laboratory experiments are being conducted more frequently in recent years. In some instances, the goal is to qualitatively compare multiple candidate perforating techniques. In others, the goal is to obtain quantitative insight into likely flow performance in the field. Although the laboratory will never perfectly replicate the downhole environment, it can yield useful results which – if properly interpreted – can enable informed prediction of downhole flow performance. A traditional flow lab experiment (along the lines of API RP-19B Section 4) yields many key results, four of which are required inputs to downhole inflow simulators. These are perforation tunnel length and diameter, and crushed zone thickness and permeability. In the case of natural completions, these parameters (in addition to other system and wellbore parameters), dictate the skin and ultimate flow performance of the completion. Crushed zone permeability is typically inferred from core flow efficiency (CFE) and an assumed crushed zone thickness. As traditionally applied, this technique can yield values which are not accurate, and more significantly can produce misleading predictions of downhole performance. To address this, we have developed new methods for both measuring and interpreting CFE. The new measurement technique yields CFE values which are more meaningful and relevant. The new interpretation technique provides a consistent method of translating CFE to crushed zone permeability, and is capable of accounting for the effect of partially plugged tunnels. This work clarifies and improves the link between lab and field performance of perforators, with the ultimate goal of increasing the value of downhole inflow performance predictions. While other work is ongoing to challenge the framework of the conventional skin models, the present paper accepts these models as a premise. This work simply presents a coherent methodology of interpreting laboratory data, with the intent of generating the required inputs for skin models as they currently are. Furthermore, it is recommended that this workflow be considered for inclusion in any revisions to Section 4 testing protocol.

We report on a series of laboratory flow experiments comparing the productivity of perforations created with reactive liner charges against those created with conventional liner charges. Three of the tests involved shots into an outcrop carbonate rock called Indiana Limestone. Three of the tests involved shots into an outcrop sandstone rock called Berea Sandstone. Four different charge types were tested, including one standard (conventional) charge and three different designs of reactive liner charges. Among all charges, the only difference of note was the design and composition of the liner. All other charge design parameters were kept constant. For both rock types, the reactive liner charges produced perforations with lower productivity than the baseline conventional charge. The reduction in the normalized Productivity Ratio (PRn) ranged from 29% to 66%. Furthermore, the reactive liner charges produced characteristic "dynamic overbalance" conditions in the wellbore, in a system configuration which produced dynamic underbalance for the conventional charge. We conclude that the reactive liner charges tested are detrimental to productivity in naturally perforated completions. Background The perforating of oil and gas wells with shaped charge perforators has been carried out by the industry since the 1940's. Throughout this period of time, charge performance has improved significantly, due to improvements in the case and liner geometries, explosive materials, liner composition, and manufacturing processes. All of these improvements have aimed to produce a better quality charge, more suitable for a given completion type. In many instances, particularly for natural completions, it is desirable to maximize penetration depth, while at the same time minimizing damage to the rock matrix caused by the violent jet penetration event. For maximized penetration depth, it is well known that increasing liner material density, optimizing the charge geometry to yield a jet of optimal velocity profile, and enhancing manufacturing precision are critical [1]. However, all jet perforators produce a region of significantly damaged formation material surrounding the perforation tunnel. This is due to the highly dynamic nature of the perforation event, involving impact pressures ranging from tens to hundreds of kilobars, applied in less than a millisecond. This damaged material is believed to contain a "crushed zone" of reduced permeability, which is detrimental to flow performance [2]. Therefore, it has been an ongoing goal of perforating charge / system design to either prevent this crushed zone from being created, or to remove it before the well is put on production (or injection, as the case may be).