Many operators are moving to high cluster-counts and extreme limited-entry treatments to improve diversion and stimulation coverage. When successful, extreme limited-entry treatments can allow completion of increased stage lengths, reducing the total number of pumping stages needed to complete a horizontal well, and reducing completion time and cost. There is a common perception that, with sufficient perforation restriction, a constant differential pressure and injection rate can be maintained through all the perforation clusters in a stage. There are several factors that can defeat this assumption, making it much more difficult, if not impossible, to maintain uniform injection throughout a treatment.
This paper discusses several mechanisms that affect the success of any limited-entry, multi-cluster, horizontal well stimulation design. Some of these mechanisms, such as perforation erosion, have been discussed before for simplified cases with single holes and constant backpressure (Long, Xu, 2019; Long, et al, 2018). The effects of variation in entry-hole diameter among multiple perforations in a stage or cluster have not previously been addressed. Other factors, including fracture stress interference and associated poro-elastic effects that change the backpressure outside the perforations, have largely been ignored. Understanding the interaction of all the mechanisms that control perforation breakdown and flow diversion will help to arrive at a practical design methodology for extending stage length and assuring effective stimulation.
Recently, the metrics used to judge a "successful" completion have changed. For many companies the goals have switched focus to cost reduction. Number of stages pumped per day, number of hours pumping per day, total number of treatment stages on a well, and total completion cycle time have become more important measures of success than long-term production or reserve recovery. To achieve these goals, operators are considering, or have switched to, extreme limited-entry perforating. In these jobs, the number of perforations (holes) per cluster are reduced to one or two, and the number of perforation clusters increased to extend the length of each stage. Pump rate per hole has also increased, sometimes to more than five barrels-per-minute (bpm)/hole. Under these conditions, the theoretical pressure drop across each perforation may exceed 4000 psi.