Controlling dynamic underbalanced pressure when perforating using the PURE method results in clean perforation tunnels for higher productivity and better injectivity.

Perforating using static underbalance is the most widely accepted technique for optimizing perforated completions. This method alone, however, still delivers many underperforming wells. The explosive charges used when perforating pulverize formation rock grains and create low permeability crushed zones in the formation surrounding the perforations and also leave loose crushed rock debris inside the perforation tunnels that can seriously impair injection. This perforation damage can be consistently minimized or eliminated by carefully designing the completion to benefit from the new Perforating for Ultimate Reservoir Exploitation (PURE) perforating method. This method creates and optimizes a dynamic underbalance (the transient underbalance established just after the creation of the perforation tunnel) when perforating a reservoir, resulting in clean perforations and thereby increasing the productivity or injectivity of a well.

Along with clean perforations, jobs designed using this method can also improve well-site efficiency and may replace the need for costly post-perforation cleanup operations like perforation acid wash or a near-wellbore skin frac.

Static underbalanced perforating is a method that establishes a static wellbore pressure before perforating that is less than the adjacent formation pressure. However, research performed at the Schlumberger Productivity Enhancement Research Facility (PERF) in Rosharon, Texas, indicates that static underbalance alone does not ensure clean perforations. The research further indicated that previously neglected fluctuations in wellbore pressure immediately after shaped charges detonate actually govern perforation cleanup, not only the initial pressure differential between well and reservoir.
This research and improved understanding of perforation cleanup resulted in a perforating job based upon downhole parameters such as reservoir characteristics, completion type, gun string and the conveyance method such as wireline, tubing conveyed or coiled tubing.

The process uses customized perforating designs, special shaped charges and fit-for-purpose gun configurations to generate a large dynamic underbalance in a modest static underbalanced, balanced or even overbalanced environment. The technique has been shown consistently to minimize or eliminate perforation damage and maximize productivity or injectivity of wells in the North Sea, Ecuador, Algeria, Canada, the United States, Middle East, Far East and many other areas around the world. The technique has been utilized in more than 150 wells globally, in hard and soft rock formations, high and low permeability reservoirs, sandstones and carbonates, oil and gas reservoirs, and producer and injector wells. To date about 90 wells have been completed using wireline perforating and 60 wells using tubing conveyed (TCP) or coiled tubing conveyed perforating techniques.

Design is key to successful perforating jobs

Proprietary design and modeling software helps design the job by taking into account reservoir properties, completion parameters and gun configurations to deliver clean perforations. A unique perforating/completion system is designed for each well to generate and control the optimum dynamic underbalance for a particular downhole environment. This dramatically reduces the margin for error and potential operational issues created by relying on the estimated downhole pressures and a large static underbalance in the well bore.

Research into transient pressures

During studies and testing conducted by Schlumberger, researchers collected microseconds-resolution (fast) and milliseconds-resolution (slow) pressure data under simulated downhole conditions in an effort to better understand the resulting pressure transients.

During the first series of tests, four standard Berea sandstone cores were perforated using identical shaped charges and an initial underbalance of 1,000 psi. Another series of tests included three Berea cores similar to the first four cores but perforated with a 500 psi overbalanced static pressure. The results confirmed that wellbore pressure varies significantly immediately after detonation and that perforation clean-up is highly dependent on these variations.

Wellbore pressure increased immediately in each test due to extremely rapid transients associated with shock wave propagation, and then fluctuated over a period of tenths of seconds due to the interaction between the pressures in the well bore and the reservoir. Those tests that were designed on PURE principals exhibited large, sharp drops in wellbore pressure that resulted in highly productive perforations. This sharp underbalance can be achieved whether the initial state was under- or overbalanced. Furthermore, the initial state on its own was no indicator of future productivity. For example, depending on the job design, an initial underbalance could result in poorly producing perforations, and an initial overbalance could result in highly productive perforations.

Schlumberger researchers believe that the amount of debris left in perforations is indicative of variable levels of surge flow immediately after perforating. Perforation damage cleanup appears to be directly related to both the maximum dynamic underbalance and the rate of instantaneous surge flow. This concept helps explain occasional poor results from underbalanced perforating and unexpected good results from balanced and overbalanced perforating.

Some of the method's benefits include eliminating the need for large static underbalance pressure differentials, making well preparations prior to perforating more straightforward. Controlling surge flow limits produced fluid volumes during the perforation cleanup, reducing the risk of sand influx that can result in stuck guns. Additionally, perforation acid wash jobs that are often required to remediate perforation damage may be unnecessary when clean perforations are created.

Dynamic underbalanced perforating increases the effective shot density or the number of open perforations, thus improving productivity and the effectiveness of larger acid and fracturing treatments. Additionally, a reduction in perforation shock, a by-product of the method, may minimize damage to the cement/sandface hydraulic bond close to the perforation interval and help ensure zonal isolation after perforating.

Many wells are PURE method candidates

Many wells, producers and injectors, could be considered candidates for the perforation method. Actual candidates are evaluated by examining rock type, fluid types and formation porosity and permeability in conjunction with performing simulations with PURE job design software.

Both pore pressure and permeability are also considered during candidate selection. Wells have been successfully perforated using the method in reservoirs with pressures as low as 1,000 psi and permeabilities as low as 0.1 mD. The lower permeability candidates tend to be higher pressure, while the lower pressure candidates tend to be higher permeability.

Most injection wells are candidates for the perforating method as clean perforations are essential for optimal injectivity. The method can assure sufficient surge flow to remove loose materials from the perforation tunnels prior to injection and prevent debris and fine formation particles from being injected and sealing off the formation pore throats. The PURE perforation method has also been particularly effective in low permeability formations that often require extremely high static underbalanced pressures for perforation cleanup.

Case histories

North Sea. CNR International performed two PURE perforating jobs in the Ninian North field in the UK North Sea in wells designated N-41 and N-42 during shoot and pull operations with a drillstem test (DST) assembly. Eight zones in the N-41 well were perforated, totaling 992 ft (302 m) of net pay across a 2,200 ft (671 m) gross interval. The tubing conveyed perforating (TCP) test string included 33/8-in. guns designed to generate a dynamic underbalance. The well produced at an initial rate of 9,500 b/d of oil, significantly higher than the original projection for conventional perforating.

United States. In 2002, Anadarko Petroleum applied dynamic underbalanced perforating in its Brady gas field in Wyoming. In addition to containing high concentrations of H2S, the formation comprises about 600 ft (183 m) of interbedded sand, shale and dolomite stringers. The permeability ranges from 0.5 to 1.5 mD with reservoir pressure less than 2,800 psi at 14,000 ft (4,268 m). The 18 wells that had previously been completed used wireline-conveyed guns and static overbalanced perforating techniques resulted in minimal flow before being treated with acid.

A design was chosen that required a 3,250 psi overbalance to recomplete the Brady 38W well in an upper section of the Weber formation. A pre NODAL production system analysis indicated that the well should produce about 3.85 MMcf/d of gas with zero perforation damage. The technique resulted in a sustained flow rate of 5.2 MMcf/d only hours after perforation with no acid clean-up required.
Later, in 2002, Anadarko drilled the 56W well, the first new well in the field in more than 17 years. The success of the Brady 38W completion convinced the operator to use the technique again. A NODAL analysis indicted that the 56W well should produce about 3 MMcf/d with zero perforation skin. The well actually flowed at a stabilized rate of 4.2 MMcf/d, again with no acid required.

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