The presence of more than one pay sand in a well can be a blessing and a curse. The blessing is the presence of additional potential reserves. The curse is that it is often difficult to justify selective fracture treatments for each sand. Operators often feel compelled to group more than one sand in a single frac stage to reduce costs. When multiple zones or long intervals are perforated in a single frac stage multiple fractures of varying lengths may be created. Verifying individual zone frac lengths in multiple zone completions is difficult with traditional post frac analysis techniques due to the multiple reservoir layers involved. When production logs are run there is generally only one flow rate at one drawdown pressure for each zone. With this alone it cannot be determined if low production is a function of poor kh or a poor stimulation. Thus, even with PLT data it is often not clear whether the target frac length is being obtained in each sand in multiple perforated interval frac stages.
To address these issues the concept of “completion efficiency” has been proposed. The process involves estimating the productivity of a zone with a target frac length and comparing it to actual production. Key inputs include permeability correlated to well tests using logs, reservoir pressure from either wireline FT or fracture closure pressures, and PLT data. Subsequent to the initial work done in SPE 71651 (Barba and Allen 2001), this technique was applied on over 100 wells in four South Texas fields. From this analysis it is clear that shorter than optimum fracture lengths are being created when operators complete long perforated intervals or more than one perforated interval in a single frac stage. This determination could not have been made with PLT data alone or conventional post frac analytical techniques. While the statistical analysis was done in South Texas, the concept has been successfully applied in both West Texas and Mid-continent as well. Examples are presented from both hard rock areas to demonstrate where poor completion efficiencies have resulted when multiple zones or long perforated intervals are completed in the same frac stage.