Abstract

Underbalanced perforating is widely used in well completions and is considered by many operators to be an effective method of obtaining improved well productivity. productivity. A measurement of downhole pressure before, during and after perforating can be made by installing a pressure gauge on the gun-string. pressure gauge on the gun-string. By using a wireline, the added capability of real-time read-out on surface allows the entire operation to be monitored 'live'. Correct underbalance can be accurately established prior to shooting, there is an unambiguous shot indication, and a pressure transient analysis can be made during the pressure transient analysis can be made during the initial flow or fill-up period. From this we can obtain an estimate of permeability, skin damage and, possibly, static reservoir pressure, which is a useful supplement to the shut-in buildup analysis which usually follows if flow reaches surface. Any subsequent conventional well-test can of course be planned without the need to retrieve or run in planned without the need to retrieve or run in additional equipment since the pressure gauge is already in place.

The advantages of perforating and testing in one run have proven particularly attractive in low energy or low transmissivity oil-fields. Here, testing of most wells by conventional methods is difficult, time consuming, or impossible because flow either does not reach surface or takes a very long time to do so.

The measurement of pressure on a perforating run is not a new idea, although real-time read-out of data is less common. The purpose of this paper is to describe the system used, and to present a method of interpreting the data which will have a wide range of applications, particularly if a simultaneous measurement of the downhole flow-rate is available. The examples demonstrate clearly the validity of this approach. Tubing-conveyed guns were used, but the technique is suited to any type of underbalanced perforating equipment.

Introduction

When a prospective pay-zone is perforated with pressure underbalanced, the sandface is suddenly pressure underbalanced, the sandface is suddenly exposed to the lower wellbore pressure and flow is initiated immediately. The correct underbalance should lead to effective clean-up of the formation near the wellbore, with minimal formation breakdown or sanding. The well will eventually either produce to surface, or die if there is insufficient produce to surface, or die if there is insufficient static reservoir pressure.

This initial flow provides an opportunity to conduct a transient pressure test based on the measurement of the downhole flowing pressure (backpressure) while the well is filling up. This is analogous to the "slug-test" of conventional drillstem testing, where flow is induced by the opening of a production valve in the DST string rather than by perforating.

For this to be possible, the sensors must be in place and recording when the perforators are fired. place and recording when the perforators are fired. Equipment is being developed for connection in-line with the normal types of wireline conveyed guns. The experimental system described in this paper was tested in conjunction with tubing conveyed perforators (TCP). perforators (TCP). In its final form, the system will serve the following functions:

  • depth control with gamma-ray and casing-collar locator;

  • checking correct underbalance before perforating;

  • detonation of the perforator/positive shot indication;

  • continuous monitoring on surface of down-hole pressure, flow-rate and, optionally, temperature and fluid density, during the operation;

  • interpretation of the data am the well-site.

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