In a cased-hole well, the reservoir is sealed off from the wellbore by steel casing and a sheath of cement. Before the well can produce, those barriers must be opened across the producing zone. Perforating is how that is done: explosive charges punch a pattern of holes through the casing and cement and into the rock, creating the flow channels that connect the reservoir to the inside of the well.
How perforating works
A perforating gun — a steel carrier loaded with shaped explosive charges — is lowered into the well, usually on wireline. When the gun reaches the target depth, the charges are fired. Each shaped charge collapses into a high-velocity jet that pierces the casing, penetrates the surrounding cement, and drives a perforation tunnel into the reservoir rock. After firing, those tunnels provide the path for oil and gas to flow from the formation into the wellbore.
An explosive charge geometrically shaped so that, when detonated, it focuses its energy into a high-velocity jet capable of punching a clean tunnel through casing, cement and rock.
Perforating guns are run on wireline and fire shaped charges that punch holes through the casing and cement into the reservoir, creating the flow paths that let fluids enter the well.
Perforating and hydraulic fracturing
Perforating is also the entry point for stimulation. In plug-and-perf completions, each frac stage begins by perforating the next interval so that frac fluid pumped at high pressure can reach the rock and crack it. The perforations are literally the doorways through which the treatment enters the formation, which is why perforating and fracturing are tightly coordinated in modern multi-stage completions.
The design of a perforating job — the number of holes, their spacing (shot density), their angular arrangement (phasing) and depth of penetration — is engineered to suit the reservoir and the planned stimulation. Good perforation design helps fluids flow efficiently into the well and ensures a frac treatment is distributed as intended across the stage.
Why it matters
Perforating is a small operation with an outsized effect on well performance. Too few or poorly placed perforations restrict flow and can cause a frac treatment to concentrate in the wrong spots; a well-designed perforation pattern lets the reservoir drain efficiently and the stimulation work as planned. It is the deliberate, controlled breaching of the very barriers — casing and cement — that were carefully installed to keep the well sealed.
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Frequently asked
A perforating gun is a steel carrier loaded with shaped explosive charges that is run into the well, usually on wireline. When fired, the charges punch holes through the casing and cement into the reservoir.
Casing and cement seal the reservoir off from the wellbore. Perforating punches holes through those barriers across the producing zone so oil and gas can flow into the well, and so stimulation fluid can reach the rock.
In plug-and-perf completions, each frac stage starts by perforating the interval. The perforations are the entry points through which high-pressure frac fluid reaches the formation to crack it.