An Investigation of the Damaged Zone Created by Perforating

Abstract

SPE Members Abstract Underbalance perforation flow experiments were performed on reservoir and outcrop sandstones to investigate the perforation damaged zone. Cores from several different formations were perforated under reservoir conditions. After perforating, the perforated under reservoir conditions. After perforating, the cores were examined using CAT scans (Computer Aided tomography), thin sections and mercury porosimetry. In conjunction with these measurements, permeabilities in the damaged zone were measured using a minipermeameter and radial flow permeameter or were estimated from pore size distribution. The density and porosity of the damaged zone (at least for saturated rocks) is essentially the same as that in the undamaged rock. The damaged zone is not compacted, contrary to suggestions made in earlier work. However, the creation of this zone involves the destruction of large pores. The volume lost from these pores is replaced by microfractures created when rock grains are fractured by penetration of the shaped charge jet. This reduction in the average pore size causes a reduction in the permeability within the damaged zone. Although direct measurement of this permeability was made difficult by naturally occurring permeability permeability was made difficult by naturally occurring permeability variations, unambiguous measurements were obtained. Based on this work an understanding of the factors which control the permeability in the perforation damaged zone is advanced. Introduction Most oil and gas wells are cased, cemented and then perforated. To be effective perforations should penetrate several perforated. To be effective perforations should penetrate several inches into the rock around the well. Perforating is known to damage the fabric of the rock around the well, causing a reduction in permeability in a region around the perforation tunnel known as the "crushed" or the "perforation damaged zone". The amount of permeability reduction and its extent has an important impact on permeability reduction and its extent has an important impact on the productivity of the well. For this reason R was made the subject of a detailed investigation. This paper deals with the mechanism by which the rock's permeability is affected, a companion paper describes how the permeability is affected, a companion paper describes how the permeability loss can be quantified. permeability loss can be quantified. CAT SCANS CAT scans have been performed on 59 perforated cores. These include samples from 6 different sandstones, 3 from producing fields and 3 outcrop sandstones (Berea, Gold and Castlegate sandstone). Further details are given in Table 1. For each core, several cross sectional scans were obtained. Cores were first vacuum saturated with brine and then displaced to kerosene. They were then perforated with 3.2, 6.5 or 22g charges under realistic reservoir pressures and overburden stress (reservoir temperature was also reproduced in some cases). Most perforations were shot underbalance but some on balance tests were included. All cores were flow tested after being shot. The confining stress was then removed and the cores were preserved ready for CAT scanning. P. 511