Designing an Efficient Perforating Program

Abstract

King, George E., SPE, Amoco Production Co. When one thinks of perforating a well, it is often only the gun that receives attention. However, obtaining the most efficient response from perforating requires the design and application of an entire perforating program The first decision during perforating is what type of completion to use. A natural completion requires a different perforating technique than a sand-control completion or a completion of a well that will be hydraulically fractured. In a natural completion, the primary emphasis is on perforation length and shot density. Longer perforations are more important to productivity than such other effects as hole size. 1 The number of perforations per foot of pay zone depends on the production rate and type of production. In wells producing at very high rates, 6 or more shots (holes)/ft [20 or more shots/m] are useful, whereas in lower-productivity wells, 2 to 4 shots/ft [7 to 13 shots/m] are adequate. Higher-viscosity oil production requires more perforations to reduce friction losses than are needed in gas production. In sand-control operations, many large-diameter holes are needed to reduce the velocity and sandcarrying capacity of the produced fluid. I Perforation length is of secondary importance, especially in unconsolidated formations where the perforation tunnels may collapse. In wells that are to be hydraulically fractured, 6 to 8 small-diameter shots/ft [20 to 26 shots/m] or 4 large-diameter shots/ft 113 shots/m] are needed to reduce friction through the perforations during the high-rate pumping of large volumes of proppant and high-viscosity fluids. The length of the perforation beyond the cement is unimportant because the relative value of a 12- vs. an 18-in. [30- vs. 46-cm] perforation is of no consequence when a fracture will extend 200 ft [60 m] or more beyond the wellbore. Other parts of the stimulation may also depend on the number and type of perforations. The limitedentry concept for fluid diversion, in which only a few perforations per zone are used, is an example of the perforating design assisting in the stimulation. The limited-entry concept uses a few shots per zone to allow pumping friction at the perforations to distribute the fluid to zones of various permeability. After the stimulation or final completion in highly productive wells, the interval must be shot with the correct number of shots per foot to end the perforation friction for produced or injected fluids (Fig. 1). These new perforations must often be "broken down" with acid to form a connection with the fracture or the stimulated zone. The phasing (Fig. 2), or the angle between the vertical planes of the perforations, may also affect the production rate from a well. In an unfractured well. the fluid flow in the formation converges toward the wellbore in inward radial flow. In openhole completions, the fluids may flow into the wellbore at any point. In the perforated well, however, the fluids must flow through the perforations. -The 0deg. (0-rad) phasing common in most through-tubing guns alters this flow because only one side on the casing is perforated. Also, when a well is fractured, both wings of the fracture must wrap around the pipe, sometimes as much as 180deg. radi, before reaching the fracture orientation in the formation. Phasings of 120 and 90deg. [2 and 1.6 rad], which are possible with the larger guns, help resolve these problems. The purpose of perforating is to obtain open, clean holes. This can be done most effectively by perforating with a large, retrievable casing gun run on tubing and having the pressure differential toward the wellbore when the gun fires. If the formation has a higher pressure than the wellbore when the perforation is made, the fluids from the formation will surge through the perforation and clean out much of the debris left from the making of the perforation. P. 513^