Monday, December 11, 2017

The natural Completion

The natural completion is often designed as that in which little or no stimulation is required for production. This approach is usually chosen for reservoirs that are less prone to damage, have good transmissibility, and are mechanically stable.

Of primary importance in selecting the perforating gun are its depth of penetration and effective shot density. Depth is important because the deeper the perforation, the greater the effective wellbore radius; also flow is less likely to be influenced by formation damaged during drilling.

Shot density also ranks high because more holes mean more places for hydrocarbon to enter the wellbore and a greater likelihood that perforations will intersect productive intervals of an anisotropic reservoir. After shot density and depth of penetration,  most important is phasing because, when properly chosen, it provides hydrocarbon with with the most direct path to the wellbore. Under typical flow conditions, perforation diameter does not adeversely affect flow once it exceeds 0.25 in (6 mm), which today is provided by nearly all guns used in natural completions.

A key consideration in perforation design of natural completions is the selection of overbalance versus underbalance perforating. Overbalance means the pressure of wellbore fluids exceeds reservoir pressure at the time of perforating. Under this condition, wellbore fluids immediately invade the perforation. For this reason, clean fluids without solids are preferred to prevent plugging of perforations. Cleanup can occur only when production begins.

Incrasingly, wells that have sufficient reservoir pressure to flow to surface unasisted are completed in underbalance conditions. Underbalance is the trend because of wider recongnition that it provides cleaner perforations- therefore better production- and because of greater availability of gun systems that allow it. Underbalance perforating can provide large gains in reservoir productivity. The question is , how much underbalance is approriate? Excessive underbalance risks mechanical damage to the completion or test string by collapsed casing or a packer that becomes damaged, stuck or unseated. It can also encourage migration of fines within the reservoir, reducing its permeability. Insufficient underbalance, however, doesn't effectively clean the perforations. Production may therefore be hindered , mainly by lack of removal of the crushed zone and , secondarily, by lack of removal of debris. The crushed zone is the damaged rock in and around the perforation tunnel; debris is mainly the liner material of the spent shaped charge, plus fragments of cement and rock.

The optimal underbalance, which removes both debris and the crushed zone and does not damage the formation, accomplishes virtually all cleanup during the portion of inital production that is dominated by surge of reservoir fluids into the perforations. Cleanup after this point is negligible because hydrocarbon follows the already cleaned paths of least resistance. During production, pressure drops across damaged area is insuficient for further cleanup. Recent experiments have shown that if a suboptimal underbalance is used, some cleanup will take place during production, but productivity never reaches that achieved with optimal underbalance.

When well testing is planned, underbalance perforating has become the standard, particulary when a drillstem test (DST) is included. Underbalance perforating is ideally suited because a DST includes hardware that allows establishing underbalance and running high shot density guns. This setup provides excellent well control and often saves time because the perforating guns are run below the test string. Pressure measurements can be recorded either downhole or in real time at surface, and are available for decision-making during the test. The MSRT MultiSensor Recorder/Transmitter and LINC Latched Inductive Coupling equipment allow real-time measurement and surface readout of downhole pressure. The main advantage of this system is the added mechanical and safety realiability of measuring pressure below the DST shut-in valve. In addition, memorized data can be read out at surface when LINC equipment is run, eliminating the need for the cable in the test string while the well is flowing.

From an operations viewpoint, underbalance perforating by wireline-conveyed guns causes a surge that lifts cable and guns. The high flow rate or liquid slugs associated with this surge can blow the guns and cable up the well. A common limit on underbalance when perforating via wireline is 700 psi, although this is often higher in tight reservoirs, which are not capable of delivering a substantial surge.

The choice of underbalance may be based on data collected since the early 1980s from laboratory and field studies and from increasing use of underbalance completions (primarily tubing-conveyed perforating). More recently, computer programs have been developed. The IMPACT integrated mechanical properties analysis & characterization of near-wellbore heterogeneity interpretation program computes a value of safe underbalance based on the mechanical properties of the formation estimated from sonic and density logs. Local experience also helps guide the selection of optimal underbalance. 

Overbalance perforating still has a role, however. Often significant are its speed for short intervals and the availibility of larger, high shot density guns compared to those for through-tubing underbalance perforation. The selection of overbalance versus underbalance rest on weighting economic versus production variables.


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