Perforation Strategy
The primary objective of perforating a cased and cemented well is to establish communication between the wellbore and the reservoir while controlling the allocation of fracturing fluid and proppant into multiple intervals during hydraulic fracturing. Treating multiple zones simultaneously reduces the number of stages required and improves operational efficiency. In horizontal wells, perforating is commonly performed using plug-and-perf operations, where perforations are created in stages along the lateral using a wireline or coiled-tubing conveyed perforating gun.
Experimental observations show that fractures do not initiate from the perforation tunnel itself. Instead, fractures grow from the surrounding rock, typically from a plane aligned with the least principal stress and located at the base of the perforation. Mineback experiments indicate that hydraulic fractures tend to avoid the high-stress regions created around perforation tunnels by shaped charges. As a result, the perforation tunnel within the reservoir rock is generally not a controlling factor in fracture initiation.
Several design parameters influence perforation effectiveness. The orientation and phasing of perforations relative to the wellbore affect the connection between the well and the fracture. Perforations aligned with the plane normal to the least stress provide a more direct flow path and promote fracture initiation at lower breakdown pressures. The diameter of perforations must be large enough to prevent proppant bridging, typically requiring the perforation diameter to be several times larger than the proppant size. Variability in perforation diameter due to gun clearance or operational factors should be minimized to ensure consistent performance.
The length and location of perforation clusters also play an important role. Shorter perforation intervals promote the development of a single dominant fracture within each cluster, whereas longer intervals may lead to multiple competing fractures and reduced fracture width. Perforations should be placed in mechanically stable rock intervals to avoid chaotic fracture initiation and excessive near-wellbore pressure losses.
Cluster density and spacing significantly influence treatment distribution. Increasing the number of perforation clusters can improve reservoir contact, but it also increases the risk that some clusters will not receive sufficient fluid. Closely spaced clusters can experience stress shadowing, in which fractures interact and inhibit each other’s growth. This effect can limit fracture width and alter propagation behavior, especially when fractures are bounded by neighboring fractures on both sides.
Perforation location should also take wellbore conditions into account. Clusters should be positioned to avoid external casing irregularities and regions where the cement sheath increases resistance to deformation, as these factors can restrict fracture growth near the wellbore.
Overall, the perforation strategy is critical for controlling how fluid enters the formation, influencing fracture initiation, propagation, and the effectiveness of the hydraulic fracturing treatment.
Limited-Entry Design
Limited-entry design is used to improve the distribution of fluid and proppant among multiple perforation intervals. This is achieved by reducing the number of perforations or decreasing their diameter, thereby creating a significant pressure drop across the perforations during injection. The resulting perforation friction generates backpressure in the wellbore that helps balance flow between intervals with different fracture-propagation pressures.
Variations in fracture propagation pressure between intervals can arise from differences in rock properties, stress conditions, or proximity to geologic boundaries. Without sufficient perforation friction, fluid tends to preferentially enter intervals with lower propagation pressure. Limited-entry design compensates for these differences by forcing a more uniform distribution of treatment volume across all intervals.
The pressure drop across perforations can be described using an orifice-flow relationship based on Bernoulli’s principle. This relationship shows that the pressure drop increases with injection rate and fluid density, and decreases with increasing perforation diameter and number of perforations. Therefore, careful selection of perforation size and density is required to achieve the desired pressure distribution.
Perforation Erosion
Perforation erosion occurs during hydraulic fracturing as proppant-laden fluid flows through the perforations. Field and laboratory studies show that erosion is a two-stage process. Initially, the proppant impacts the perforation inlet, rounding and enlarging the opening, thereby increasing the discharge coefficient by reducing the vena contracta effect.
As erosion progresses, the perforation diameter increases steadily, leading to a significant increase in flow capacity. This reduces perforation friction pressure unless compensated by increasing the injection rate. A decrease in perforation friction can negatively affect treatment distribution by allowing more fluid to flow into certain intervals.
Erosion tends to be more pronounced in intervals with lower fracture propagation pressure because these intervals receive higher flow rates. This can lead to a progressive increase in fluid and proppant allocation to those intervals, further accelerating erosion and reducing control over the distribution of the treatment.
