Abstract

This paper presents a study on effects of perforation flow geometry on the evaluation of the flow performance of perforations. From data obtained in the laboratory serious deficiency and poor accuracy were identified in exploring the linear perforation targets recommended by API RP 43 Section II for the evaluation of the flow performance of perforations. This discredits the validity of the procedure and poses doubt to the correctness of applying API RP 43 results to the downhole conditions. The work suggested that perforations with radial flow configuration should be utilised in such an evaluation.

Introduction

Perforating is one of the most important elements in the well completion process. The ultimate test of perforation effectiveness has usually been the well productivity. As a result, much attention has been devoted to the laboratory test of perforating equipment and perforations so generated as a means of predicting and improving the well performance. Over the past 30 years, laboratory procedures for evaluating gun performance have been evolved.

The first comprehensive laboratory investigation on the flow properties of perforation was carried out in 1953 ! This study led to the introduction of the laboratory flow test procedure which was devised in 1956. In the procedure, perforation effectiveness was described in terms of a Flow Index (FI). The Fl compare, the flow rate through a perforated target after backflowing (qp) to a flow rate previously obtained through the same target with both the faces open to flow (qi) i.e.:

FI = qp/qi

Having recognised the importance of the flow properties of perforations, the industry adopted in 1962 a standard test procedure, namely API Recommended Practice No. 43 (API RP 43). The 1962 edition of the RP 43 describes perforation effectiveness in similar terms as defines for the Flow Index above. It employs the Well Flow Index (WFI) instead of the Flow index. The WFI's defined as the ratio of the apparent permeability of the perforated core (Kp) made from cylindrical Berea sandstone to the permeability of the unperforated core with both ends open (Ko) i.e.

WFI = Kp/Ko

A perforation made into a cylindrical core target influences the flow properties of the core target in such a way that the core target is effectively shortened. The apparent permeability, Kp, determined for the perforated core target, based on the original length and cross-sectional area, hence Kp will increase with core target perforation. Thus, the WFI can be misleading because the effects of perforation damage masked by the effects of perforation length. The term, Well Flow Index, is also a misnomer because it has no relation to the downhole productivity in a radial system.

In an effort to provide more meaningful data, the API procedure was revised in 1971 and once more revised in 1974. The new procedure, the apparent permeability measured in a core with a real damaged perforation (Kp) is compared with the permeability of a core with an ideal, clean perforation of the same perforation depth (Ki). This new measure is called Core Flow Efficiency (CFE). The CFE is defined as:

where Ki is the permeability of a perforated core with ends open. Thus the API RP 43 now provides a measure of perforation quality.

The API RP 43 test procedure includes two types of tests which are referred to as Section I - Surface Test and Section II - Laboratory Flow Tests. Section I defines multishot perforation penetration performance in a large concrete drum type target using actual field guns.

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