The presence of more than one pay sand in a well can be both a blessing and a curse. The blessing is the presence of additional net pay and potential additional reserves. The curse is that it is often difficult to economically justify selective fracture treatments for each individual sand. Operators often feel compelled to combine more than one sand in a single frac stage to reduce the total number of frac stages. 3D hydraulic fracture simulations suggest that when multiple zones are perforated in a single frac stage multiple fractures of varying lengths may be created, with the target propped frac length not being obtained in each sand. Obtaining verification of frac lengths in all zones completed in multiple zone completions is difficult with traditional post frac pressure transient analysis or rate-transient analyses 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, and not a time vs. pressure series of datapoints required to estimate fracture length, conductivity, and formation permeability. The PLT measurement by itself cannot determine whether low production is a function of low pressure, low kh, or an ineffective fracture treatment. As a result, even with PLT data it is often not clear whether each sand is being effectively stimulated 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 minimum acceptable frac geometry and comparing it to actual production. Key inputs to the model include permeability from pre-frac flow test data or correlated to flow tests using wireline logs, reservoir pressure from either a wireline formation test measurement or empirical correlation to fracture closure pressures, PLT rate, and PLT drawdown data. This technique was applied in four South Texas reservoir studies on 135 wells in 2002 and 2003. From these studies it was evident that when multiple perforated intervals were stimulated in the same frac stage that the highest flow rate sands had significantly higher completion efficiencies than the other sands in the same stage. In many cases the lesser performing sands in the stage received only skin removal or were not stimulated at all. In 40% of the stages the lower flow rate perforated intervals had higher kh values than the highest flow rate perforated interval. It was also clear that zones that were treated with a single perforated interval had significantly higher completion efficiencies than zones in multiple perforated interval frac stages, and that within that subset shorter perforated intervals had higher completion efficiencies. Another observation was that single perforated interval stages with less than 2 degrees wellbore deviation had higher completion efficiencies than zones with higher deviations. Zones with higher reservoir pressure and higher drawdowns had lower completion efficiencies than lower pressure and drawdown zones. Finally, higher performing zones recovered less load between multiple stages than lower performing zones.