Well perforation myths - Part 3

Read part one Perforating myths and misconceptions and part two Dispelling well perforation myths

There is a specific shot phasing and gun orientation that makes my wells work

It is important to distinguish between shot phasing and gun orientation. Phasing refers to the fixed angular arrangement of charges within the gun body, which ensures that successive shots are distributed around the wellbore. This is advantageous for inflow performance, assuring some shots are close to a preferred fracture azimuth, and preventing the perforating gun or casing from failing because shots are too close together. Gun orientation refers to rotating the gun body within the wellbore to align the shots with a particular azimuth.

It is also important to consider how the gun, wellbore and formation are aligned at the depth of interest. In a vertical well, shot phasing and gun orientation can be used to align a vertical fracture with a known preferred fracture azimuth. In a horizontal well they can be used to align shots into a vertical or horizontal arrangement to encourage superior tunnel stability, but since the preferred fracture plane will in most cases still be vertical, the arrangement will not aid in that regard. Furthermore, neither the wellbore nor the formation is actually horizontal; most wells deviate up and down along the horizontal section, and the formation is typically folded and faulted. This means that the actual orientation of shots at any given depth is difficult to predict or control.

There is seldom one particular system design that truly out-performs all others in a particular field. Many completion engineers have their preference, but few rigorous field trials are conducted to technically validate a particular selection. Moreover, when a new engineer moves into the position they will often bring along a favourite system design, and well performance will be just as good, if not better, than before! One might conclude that simply paying more attention to the perforating operation is what leads to improved well performance, rather than the design itself.

Recommendation:

Do not assume that an established system design - i.e. one that has been replicated across many well programmes – is necessarily optimum, or that it was selected on some scientific basis. In the absence of any conclusive experimental evidence to the contrary, favour a system with moderate-to-high shot density (4 - 6 spf in most wells) and at least 90° phasing (60° is even better) to distribute shots around the wellbore.

When shooting highly permeable or poorly consolidated formations, always use big hole systems, as there will be no residual damage

While formations with permeability in excess of 500 mD ought not to suffer from perforation-related damage, several operators report positive skin values when completing such intervals. This may be because the permeability actually varies along the wellbore – perhaps 50 - 5000 mD – while being 500 mD on average, so that some intervals are readily completed, but lower quality intervals require the use of premium perforating systems.

Big hole charges only generate a few inches of penetration, which may not extend beyond the radius of drill-in fluid damage to connect with undamaged formation. Deep penetrating charges may not create a stable tunnel, but could disrupt the formation further from the wellbore to create an enhanced flow path. Some operators prefer a ‘salt and pepper’ approach, intermixing big hole and deep penetrating charges to benefit from their relative advantages.

If sand control is to be installed (e.g. stand-alone sand screen or gravel pack), then maximising the area open to flow becomes the overriding criterion to avoid hot-spotting – that is, excessive inflow through a limited flow area – that might lead to a premature sand screen failure.

If sand production is expected, but no sand control is to be installed (i.e. sand co-production), maximising the inflow area is also a good idea in order to keep the flux rate through each tunnel low, reducing drag and minimising sand production.

Recommendation:

If the formation is incompetent and sand production is expected, big hole systems are generally needed to create open flow area and minimise the risk of hot spotting. However, if minimal sand failure is predicted, experiment with high performance perforating systems to determine which will deliver the highest well performance.

For the latest and greatest technology, leave it to a major service company

While this statement may have held true for several decades, it is no longer valid to say that only major service companies have the best technology. The development of premium perforating systems – especially those that are reactive or customised for specific formations – requires special capabilities and equipment, such as a perforation flow lab. People and knowledge are also critical. After a period of general stagnation and under-investment across the sector, the past decade has seen companies starting to specialise and develop differentiated solutions. Several of these have been developed by independent manufacturers and are available through all service companies.

As mentioned above, the way most completion programmes are written provides no incentive for a service company to offer the latest and greatest technology. Instead they are incentivised to supply the system that is most profitable (to the service provider) while meeting the minimum criteria set by the operator. The completions engineer is responsible for the functional performance of the well and therefore for specifying what perforating technology will be deployed. This is a critical part of the job, and letting the service company pick the system is irresponsible.

Recommendation:

It is critical for operating company engineers to be aware of different perforating solutions available on the market. Evaluate the whole tool box and identify which systems might be most appropriate for the well or field in question. If necessary, conduct laboratory experiments under representative conditions to reduce the number of candidate systems. Next, conduct extensive field evaluations (at least five wells with each system) to determine which system is most effective in practice. Finally, explicitly specify the selected system on subsequent well programmes, naming the exact charge to be run and requiring that it be run in the same vendor’s gun system.

In conclusion

There is perhaps one more myth that should be discussed: that perforating is a trivial or meaningless activity that can be programmed without any real engineering effort. Hopefully the issues discussed in this article, which by no means represent an exhaustive list, will serve to illustrate some of the potential complexities and misunderstandings. They can lead to significant well under-performance if not given due attention.

We would love your feedback, share your thoughts with us on Facebook or Twitter

Tweet #geodynamics

Written by Matt Bell, GEODynamics, USA.

This is part one of a three part article that originally featured in the August 2013 issue of Oilfield Technology

Read the article online at: https://www.oilfieldtechnology.com/exploration/16082013/well_perforation_myths_part_3/