Limited entry perforations have been used as a completion technique in steam injection wells to improve profile control for stratified heavy oil reservoirs. This technique is a practical application of critical flow of steam through chokes. Under critical flow conditions, the injection rate through each perforation reaches a maximum value that is independent of the downstream conditions. Therefore, no single perforation or sand can act as a thief zone. However, a fieldwide analysis of limited entry performance revealed wide discrepancies between actual and theoretical design injection rates and steam injection profiles have not always been uniform.

This paper examines the theoretical framework for both critical and subcritical flow through chokes. The limited entry theory based on ideal gas in isentropic flow process agrees well with experimental results, indicating the theory is valid under test conditions. However, the flow of wet steam in limited entry perforated injection wells under reservoir conditions can be quite different from theoretical calculations assuming critical flow. The wide spread of field data can be attributed to two factors: variation between designed and actual wellbore mechanical conditions and existence of subcritical flow conditions due to high pressure downstream of the perforation.

A new surveillance method using steam step rate test has been developed to help diagnose the bottomhole injection conditions. In many cases the tests have been used successfully in the field to 1) identify critical and subcritical flow, 2) determine the downstream pressure, and 3) identify a perforation impairment problem without running a steam-injection survey.

A simple two-layer well model has also been developed to evaluate the impact of permeability contrast on profile control using limited entry perforations. The model demonstrated that limited entry perforations can not ensure successful elimination of poor injection profiles in multisand completions in highly heterogeneous reservoirs. When permeability contrast is too large and the downstream pressure is too high, application of limited entry perforating technique is limited.

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