Field data and recently developed models provide some guidance for estimating the underbalance needed to obtain fully functional perforations, but there are little data available that relate flow efficiency to lower underbalances in different rock types. To improve understanding of the surge cleanup process, we have performed two series of perforation flow tests in Berea Sandstone and in Bedford Limestone cores at increasing levels of underbalance. Flow tests were performed according to modified API RP19B, section 4 test procedures. At the conclusion of the tests, the cores were analyzed using high-resolution X-ray CT techniques. The shape, dimensions and total volumes of both the open tunnel and the remaining embedded liner metal were extracted from the CT data and correlated with the underbalance and with the flow test results.
Open tunnel diameters and volumes are much lower in the limestone samples, and there is preliminary evidence of a compacted zone surrounding the tunnel that is not present in the Berea samples. While the amount of metal remaining in the tunnel and at the perforation tip decreases dramatically with underbalance in Berea Sandstone cores, the amount of metal is nearly constant in the limestone cores. Conversely, the tunnel volume increases with underbalance in the Sandstone cores but stays constant in the limestone. Preliminary core flow efficiency (CFE) results correlate well with these observations. There is a shape increase in CFE in the sandstone samples as the tunnel volumes increase and little change in CFE in the limestone samples corresponding to unchanging tunnel volume.
The tests also offer some evidence of the cleanup mechanism at the perforation tip, at least in the sandstone cores. Samples at intermediate underbalance levels show evidence of open tunnel space in an annulus surrounding the metal slug at the tip. This suggests that cleanup may proceed, at least partially, by axial flow through crushed rock surrounding the metal. As this material erodes away, the metal is loosened and is flushed from the tunnel. Existing models for cleanup are based primarily on radial flow.
Underbalanced perforating continues to be the technique most commonly used to prevent and remove permeability damage from perforation tunnels. While generally successful in improving the quality of perforated completions, much remains to be understood in predicting the underbalance pressure required in a specific situation and in predicting the flow performance of a given completion.
Significant progress has been made recently in developing models that can predict the extent of perforation cleanup for a given formation permeability and underbalance pressure.1,2,3 These models calculate transient radial flow into an empty perforation tunnel and are based on the principle that some minimum Darcy-flow velocity is needed to remove fines from the rock surrounding the tunnel. Experimental studies have been capable of mapping the permeability around the tunnel and correlating it with reduced particle-size distribution near the tunnel wall.4,5
However, little attention as been paid to the mechanism by which rock and charge debris are removed from the tunnel in order to achieve an open tunnel in the first place. This necessarily requires substantial flow along the tunnel in addition to the radial flow postulated by current models. Sufficient underbalance is known to remove even the metal slug at the perforation tip. It is difficult to imagine this being accomplished purely by radial flow.
Our objectives simply were to gain better insight into the mechanisms by which under-balance surge removes damaged rock and charge debris from a perforation and to better characterize the damaging materials left behind. Recognizing that dynamic failure occurs in different ways in different mineralogies, we also sought to distinguish between behavior in sandstones and limestones. To reach these objectives, we have performed a series of perforation flow experiments in two different rocks at successively higher underbalance conditions. Following conventional flow testing of the perforated samples, we applied high-resolution X-ray CT analysis to observe the extent, geometry and morphology of debris remaining in the perforation tunnel.