Perforating myths and misconceptions

Depending on whom is asked, the seemingly straightforward task of perforating a well might be characterised as anything from insignificant to a black art. The point of view will probably depend on the type of well that is being completed, the formation being targeted and whether the perforating operation will be followed by production, injection, stimulation, or sand control installation. Across the range of service companies and operators that GEODynamics works with there are a few general areas of consensus, but there are certainly many ways to skin the perforating cat. The industry has developed a collection of myths and misconceptions that can lead to poor decision-making, ill-advised perforating system selections, and – critically – suboptimal well performance. This article highlights some of the most pervasive issues and offers tips on avoiding the completion consequences they can cause.

It is okay to pick a system from the catalogue: that is API-witnessed data

Probably the most common mistake is picking a perforating system based on the manufacturer’s catalogue. Many well completion programmes call for a minimum depth of penetration and a minimum entry hole diameter. This leaves the service company to select the lowest cost system that meets or exceeds the required performance criteria. Unfortunately this guarantees very little in terms of downhole performance or, critically, well performance.

Almost all data published by perforating system manufacturers are generated by one of three test methods:

• API Recommended Practice 19B, Section 1 (system performance test).

• API Recommended Practice 43, Section 1 (system performance test).

• Qualicontrol test.

The common attributes of these three test methods are that they use an unstressed, cement target. As a benchmark, the RP19B test is the most reliable since it is witnessed by the API and the target formulation is relatively tightly specified. API RP43 is obsolete, having been superseded by RP19B, and both RP43 and QC tests are unwitnessed. However, none of these tests is representative of the downhole environment. Systems that exhibit superior performance in unstressed cement targets frequently do not outperform their competition when shot into stressed rock. The presence of an API-accredited witness merely assures adherence to the recommended practice; it does not guarantee any value of information.

Superior test methods are available to manufacturers and consumers but require a greater investment of time and effort. Within API RP19B, Sections 2 and 4 describe tests that use natural rock targets under stress conditions representative of the reservoir. The Section 2 test involves a single shot into stressed Berea Sandstone to determine tunnel geometry (penetration and diameter), while the Section 4 test involves pre-characterisation of a rock core, followed by a single shot under representative stress conditions and a flow experiment to determine the efficacy of the perforation tunnel that has been created. These tests are clearly superior to the Section 1 tests generally used to benchmark perforating systems, but for reasons of cost and complexity they are seldom used in system selection.

The problem of system selection is further compounded by simulation software that predicts downhole penetration from a correlation between catalogue data (performance into unstressed cement targets) and target formation characteristics. While these correlations may have some validity for moderate strength sandstones, they do a poor job of predicting downhole performance in the multitude of other rock types commonly encountered in today’s wellbores.

Recommendation:

Take care when using catalogue data as reference criteria. The less the target rock looks like moderate strength sandstone, the greater the risk of suboptimal performance in situ and the more value that can be derived from dedicated lab tests to help identify the best system for your application.

When fracturing a well, a hole is just a hole

In today’s world of unconventional reservoirs, an increasing proportion of perforated completions are immediately subjected to hydraulic fracture stimulation. This begs the question: ‘who cares about the perforations when I’m going to frac through them at high rate with abrasive fluid?’ Surely they will be obliterated within minutes?

This is an example of ‘rear-view mirror’ logic. Provided the frac is successfully pumped as designed using minimum horsepower, the perforations do indeed appear to be irrelevant. However, inadequate perforations can cause various difficulties, such as:

• Being unable to break the rock down and having to re-perforate (at best).

• Needing to pump acid ahead of the treatment in order to break down at acceptable pressure.

• Failing to achieve sufficient treating rate to pump the treatment as designed.

• Having to curtail proppant due to rising treating pressure.

• Losing the ability to transport proppant into the fracture, resulting in screen-out.

• Paying too much for stimulation equipment.

Recommendation:

Many of these symptoms can be mitigated by selecting a superior system - in particular a Reactive^® perforating system that assures clean tunnels and assists in breaking down the formation. At a minimum, it is worth experimenting with different perforating systems while keeping stimulation designs and parameters as constant as possible to determine the best solution for a particular field.

Re-perforating an old well only requires cheap and cheerful technology

As a reservoir depletes the effective stress acting on the formation increases, because the overburden remains constant while supporting pore pressure drops. This makes the rock act like a harder target, requiring a superior perforating system compared to the initial completion.

Depleted reservoir pressure means little or no driving force is available to clean debris from newly created tunnels. The maximum underbalance that can be created to generate surge flow and expel debris is only as great as the remaining reservoir pressure (a vacuum cannot be pulled in the well!), so a different way is needed to deliver clean tunnels. Poor quality perforations can also lead to an elevated risk of precipitation, waxing or asphaltene drop out because of significant near-wellbore pressure drop.

Reactive perforating systems are a useful tool since they generate pressure within the tunnel that expels debris without requiring underbalance or surge flow. This results in a high percentage of clean, large-diameter perforations.

Recommendation:

Take the time to simulate re-perforating operations to identify the most suitable perforating system for each unique set of wellbore and reservoir conditions. Always favour a higher-performance system to help mitigate the effects of reservoir depletion.

Read part two Dispelling well perforation myths.

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/14082013/perforating_myths_and_misconceptions_part_1/