Explosive perforating has been the dominant method of establishing communication between the reservoir and cased wellbore for more than 70 years. Effective perforations, which provide an unimpeded flowpath, are critical to deliver the well performance required to justify overall project investment. To reliably estimate or predict well flow performance, it is essential to have an accurate understanding of critical perforation parameters that exist downhole, including tunnel penetration depth into the formation, cleanness of the tunnels, and hole diameter through the casing.
Consequently, the industry has focused significant attention toward developing this understanding in recent decades. This is particularly true today, as downhole environments are becoming more extreme. Project investment decisions require increasingly accurate well performance estimates, both initially and over the life of a development.
This current state of affairs has motivated a recent and ongoing effort to better understand perforator performance at full downhole conditions, up to and exceeding 30,000 psi. A large program is underway to investigate the penetration and hole size performance of several charges across a range of rocks and pressure conditions. The goal of this program is to obtain fundamental insights into the effects of extreme values of certain downhole conditions on perforator performance. The current test program follows the recently revised API-RP 19B Section II protocol and includes high-pressure variations of the standard test configuration.
One area of key findings thus far is in the context of recent industry frameworks for analyzing laboratory penetration data, including ballistic stress and the ballistic indicator function. These are found to be useful tools that simplify analysis and provide insight and guidance. These frameworks make it possible to collapse multiple diverse penetration datasets, from across a range of test conditions, toward a single performance curve. This curve can be used to enable ballpark estimates of the performance of a given charge in a specific rock strength and stress regime. It provides the potential to identify a penetration asymptote (assumed to be a fundamental charge property that would be observed in very strong and/or highly-stressed rocks). It is also a useful framework to quickly visualize a vast spectrum of reservoir conditions, and to identify where a specific reservoir may fit in the broader context. Of particular interest to the perforating testing community is the relatively narrow range of values encompassed by the newly-revised API-RP 19B Section II standard test conditions.
To extend this framework to predictive models of charge penetration over a broad range of downhole conditions, however, study results indicate that more work is needed. It will be necessary, for example, to account for charge-dependent wellbore effects to move closer to a predictive capability that exhibits the level of quantitative accuracy required for many applications.
Other fundamental findings involve wellbore pressure influence on perforator performance. For one charge studied somewhat extensively, wellbore pressure was observed to exhibit an interesting non-monotonic influence on penetration. Moderate wellbore pressures increased penetration depth; higher wellbore pressures decreased penetration depth. Wellbore fluid pressure was also found to exhibit a charge-dependent influence on casing hole size performance; increasing wellbore pressure tended to reduce the hole size slightly for one charge tested, but had no effect for two other charges tested.