Effect of Perforation Damage on Well Productivity

Abstract

To maximize productivity, perforations must penetrate substantially beyond the zone of drilling damage, and they must be of the highest possible quality. In a well with drilling damage, a few deep possible quality. In a well with drilling damage, a few deep perforations are more effective than many shallow ones; but within perforations are more effective than many shallow ones; but within the limits of current technology and economics, severe perforation damage cannot be entirely overcome by increasing either shot density or penetration. Introduction Gun perforation has been used for more than 40 years for generating a controlled flow channel between oil and gas reservoirs and the bore of an injection or production well. The first well reported to be gun production well. The first well reported to be gun perforated, was a Union Oil Co. of California well in perforated, was a Union Oil Co. of California well in the Montebello field, Los Angeles County, Calif., in 1932. Since that time many types of special bullets and jets have been introduced to improve the perforating process, and we include both devices in perforating process, and we include both devices in the term "gun perforating." Although gun perforating became widely accepted as a practical completion method, engineers long suspected that well productivities should be better than observed. Experimental and field studies, ranging over more than a 20-year period, exposed deficiencies in perforator design and perforating procedures that accounted for the reduced productivities procedures that accounted for the reduced productivities or injectivities and lead to improved field results. Laboratory observations on linear, perforated cores demonstrated that crushing and compaction of rock during perforating substantially impaired the flow capacity of the hole. Although this experimental work provided a better understanding of the physical effects of perforating into rock, our ability to estimate how much oil, gas, or water should flow into a wellbore through perforations fanned under down-hole conditions has been limited by the simplifying assumptions required to handle this problem. Because of the difficulty of the problem, the early work on the productivity of perforated wells assumed flow through clean, undamaged perforations. Recently, Bell et al. in analytical and experimental studies related perforation efficiency for a single damaged perforation perforation efficiency for a single damaged perforation in a radial system to that observed in a linear core, but no attempt was made to estimate over-all well productivity. productivity. The purpose of our paper is to extend the earlier work on productivity of perforated completions by taking into account the depth and severity of permeability damage caused by both the drilling and permeability damage caused by both the drilling and the perforating processes. We are now able to relate flow efficiencies and permeability damage in a laboratory-perforated linear core system to that in a practical radial well system. To limit the length of the text and yet illustrate adequately the general combined effect of perforating and drilling damage, we have modeled a radial system with an optimum perforating condition of 4 holes/ft. At this short perforating condition of 4 holes/ft. At this short density and a penetration of 6 in., perforated well productivity approximates open-hole productivity. productivity approximates open-hole productivity. The actual numerical effects on well productivity will, of course, be different for other perforation patterns but the general effects of perforation patterns but the general effects of perforation damage on productivity will be similar. Thus, regardless of perforation pattern, the trends indicated in this work should be helpful in designing perforating jobs. perforating jobs. JPT P. 1303