Experimental tests were performed under simulated downhole condnitions to determine the minimum underbalance necessary to obtain zero-perforation damage skin in two sandstones of different permeabilities. The objective of this paper is to study the transient cleanup resulting from the dynamic surge of fluid through the perforation immediately after perforating, prior to any postshot flow.

Single-shot perforation flow tests were conducted in nominally 100-md Gold and 200-md Berea sandstone cores under various underbalances. Prior to shooting, the virgin whole core diametral and axial permeabilities were measured. Perforation damage skin was determined using analytical and finite element models.

For each of the two basic rock permeabilities,

  1. the minimum underbalance required to obtain zero-perforating skin without postshot flow as determined, and

  2. a relationship of skin to perforating underbalance was obtained.

The experimental results were used to establish a Reynold's number criterion for zero-perforation damage skin. Using the Reynold's number criterion, a simple equation can be employed to predict, as a function of formation and fluid properties, the minimum underbalance to achieve zero-perforating skin prior to any postshot flow.


The motivation to perforate underbalanced is to maximize the cleanup of the â??crushedâ?¿ or perforation damage zone. The mechanism and characteristics of the damaged zone are examined in two companion papers.1,2 This paper focuses on 1) the experimental investigation of the quantitative relationship of rock permeability and underbalance to perforation damage and 2) potential field applications.

The key factors in underbalanced perforating to minimize perforation damage are the level of underbalance, formation permeability, and fluid properties. Although perforation damage and cleanup are the subject of numerous investigations, the results presented in this paper represent the first extensive study to determine perforation damage skin versus rock permeability and underbalance under controlled simulated downhole conditions.

A radial, pseudosteady-state, turbulent flow model of the perforation damage zone has been proposed by Tariq3 to yield a Reynold's number requirement to obtain minimum perforating skin. Based on the results presented in this paper, a new Reynold's number criterion is obtained that can be used in the analytical equation to predict the minimum underbalance required to achieve 100% effective shot density as a function of the formation properties.

Experimental Procedure

Through-tubing, 1 11/16-in. (3.2-gram) perforating charges were shot in 4-in. diameter by 11-in. length sandstone cores that were cut perpendicular to the bedding plane. Prior to shooting, the cores were dried and vacuum saturated with 2% NaCl brine. For each core, the virgin whole core diametral and axial permeabilities were measured. Two types of sandstones were used in the experimental tests: the nominally 100-md diametral permeability Gold and theh 200-md Berea. The cores used in the investigation are thus chosen to have diametral permeabilities that are within a small deviation from either 100 md and 200 md.

All perforating tests were conducted with an effective stress of 3000 psi and a wellbore pressure of 1500 psi at ambient temperature. Pore pressures from 1500 to 4500 psi were simulated to achieve the various underbalances. A 5-gal accumulator was attached to the simulated wellbore to model a closed well with a gas cap. The test assembly is illustrated in Fig. 1. All cores were flow tested after shot. Postshot, combined radial and axial flow with clean kerosene did not exceed 20-psi pressure differential in order to minimize the secondary perforation washing.

An initial perforation geometry was estimated without disturbing the tunnel. The cores were then wrapped, waxed, and shipped to BP Research Centre, Sunbury, U.K., where computerized tomography (CT) scans of the perforations were taken. Details of the experimental procedure and the permeability measurements are given in Behrmann, et al.1

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