There are different methods employed in oil and gas well perforation. Well perforations create a channel that connects the reservoir to the wellbore. Most completion strategies require some means of creating these perforations. The choice of perforation type will depend on several factors including cost, efficiency, future stimulation plans and required perforation channel dimensions. Poor perforations can be the source of formation damage and low productivity. (Related reading: Understanding Reservoir Drive Mechanisms.)
Types of Well Perforation
There are four types of well perforation that we'll examine below.
In bullet perforation, the gun barrel is lowered into the wellbore and positioned adjacent to the interval to be perforated. When the guns are fired, they have to pass through the casing, the cement sheath and travel some distance into the formation.
Bullet guns are really effective when used on steel alloys with low strength. The stronger the steel casing, the less efficient this perforation choice will be. Bullet guns create perforations by shattering everything in its path. This shattering can be an advantage as it can improve the permeability of the producing formation. (Learn more about permeability in the article Grasping the Concept of Rock Permeability.) However, bullet guns are prone to depositing spent gun debris in the producing formation that may plug channels and lower permeability if it is not properly cleaned afterwards.
Even though bullet guns produce wide perforation tunnels, these tunnels are usually shallow. If you’re trying to bypass damaged zones in the near wellbore region, deeper tunnels are often desired and it may be necessary to consider other perforation options.
Perforation through Water Jets
This kind of perforation relies on water pressure to create tunnels in rocks that will connect the wellbore to the reservoir. Water is pumped in through the jetting tool and exits the tool through carefully designed nozzles. The shape of the nozzle increases the fluid velocity and pressure creating large impacts on the target. However, water jets have their limits. It may take a long time to create desired perforation dimensions in deeper wells. The more consolidated the formation is the longer it will take. As a matter of fact, water jetting perforation is adapted for open hole completions where there is an absence of casing and cement sheath. It is not a common choice of creating perforation tunnels.
This is a special kind of water jetting. It relies on abrasive jetting to create holes through the casing, any cement sheath between the casing and target formation and into the formation.
Abrasive perforations make use of a slurry containing abrasive solids like sand. The slurry is pumped down the jetting tool and concentrated on the target formation. If this goes on long enough, perforation intervals are created.
Obviously, apart from normal water jetting, abrasive perforation takes more time to create perforation channels when compared with other perforation methods. It also produces shallow perforations since the fluid being pumped in gets blocked by fluids leaving the channel created. This limits how deep the perforation can be. But the abrasive perforation method poses very little damage issues to the reservoir with few clean up requirements.
This method is well adapted for shallow reservoirs but will not give desirable results with deeper wells. With deeper wells, more back pressure has to be applied from the surface to prevent well control issues. This back pressure reduces the pressure at which the abrasive jets strikes the target and ultimately reduces the rate of perforation.
Shaped Charge Perforation
This is one of the most efficient perforation methods. Shaped charge perforation utilizes highly explosive powders that have the ability to pierce through the casing, the cement sheath and into the formation. It is also known as jet perforation.
Unlike the bullet perforation that shatters, shaped charges pierce through obstacles in its path. Thus, they can penetrate deeper into the formation.
There is a sequence of operation for shaped charges. The charge is initiated in the detonation cap or initiator. It then moves through the detonation cord and releases the charges. If all goes right, this produces excellent perforation intervals, but if any of the shots are fired with low energy as a result of any electrical or mechanical fault, then other shots will be fired with low energy.
Care must be taken and procedures strictly adhered to when using shaped charges. Shaped charges cause some damage in the near wellbore region. When the charges pierce through the target formation, they squeeze through the formation and compact rock grains in the process. This compaction limits the flow of hydrocarbons into the newly created tunnels. Sometimes, the well is stimulated afterwards to improve permeability and productivity.
Well perforation is a critical aspect of well completions. After the exploration and drilling phase, if hydrocarbons are present in commercial quantities, then the well may have to be completed.
Well perforation provides a path of communication between the reservoir and the wellbore as the trapped hydrocarbons will have to flow out through the perforations. If perforation is done correctly, production can be tremendously increased; the opposite goes for poor perforation methods.
Formation damage is common near wellbore regions, and the perforation tunnels created can help bypass any formation damaged during the drilling phase or could make matters worse.