API RP 19B Section 4 tests were established to evaluate perforator performance at field conditions. The question of how well such a small-scale laboratory test translates to downhole reality has long been raised. Furthermore, how accurately are downhole dynamics reproduced in single-shot, Section 4 tests, when in practice, an extensive formation is perforated with multiple shots?
To address this question, flow fields for typical axial and radial API Section 4 flow targets were calculated using a general analytical model based on potential theory and compared with the calculated results of various downhole configurations with different shot densities. When compared with calculations of downhole flow, we show that both radial and axial API targets can yield flow patterns that differ considerably from those downhole, which could lead to erroneous interpretations of results and dynamic effects, such as cleanup. As expected, results show that a radial flow target tends to overestimate the flow into a downhole perforation depending on the shot density and that an axial flow target tends to underestimate the flow depending on target length.
We discuss how to modify the API test core by attaching a low-permeability sleeve to create a more accurate downhole flow simulation. The result is a more representative test that better reproduces the initial static reservoir pressure and the post-shot flowing pressure.
To illustrate the concept, we made several tests using modified and unmodified axial flow targets to compare flow efficiency through perforations. Initial results indicate a difference in flow between the two sets of targets, with the modified targets having lower flow efficiency. Finally, we offer some possible physical reasons to explain the difference.