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Abstract Underbalanced perforating has become accepted as one of the best methods for creating open, undamaged perforations. However, the magnitude of the underbalanced perforations. However, the magnitude of the underbalanced pressure which can safely be used to complete the well is pressure which can safely be used to complete the well is a major uncertainty, especially in unconsolidated or poorly consolidated sands. This paper describes an integrated well data analysis procedure to estimate the safe underbalance perforating pressure and the well response to a short perforating pressure and the well response to a short 'cleanup' flow period. The design procedure utilizes open-hole logs, fluid data, and reservoir data to determine the formation mechanical properties and to build a reservoir fluid flow model. The design results can then be compared to the actual well performance. This is evaluated using transient pressure analysis of the data acquired in conjunction with pressure analysis of the data acquired in conjunction with the underbalance perforating. A field example is used to illustrate the design and evaluation of a tubing-conveyed perforating well completion. perforating well completion
Introduction In underbalanced perforating, the pressure differential creates a surge of fluid from the reservoir into the wellbore. This helps clean the perforations and remove the near wellbore permeability damage. The pressure differential required for perforation cleanup range from 200 psi to over 5000 psi and have usually been established by trial and error in each field.
In unconsolidated or poorly consolidated formations, the mechanical strength of the formation must be considered to avoid sand production and/or the movement of fine particles which could cause plugging of the matrix. In addition, variations in permeability and other physical properties within the perforated zone may create different properties within the perforated zone may create different degrees of formation damage. Thus, all perforations within the zone may not respond the same. Those in higher permeability zones will clean up more easily and will, permeability zones will clean up more easily and will, therefore, respond to a lower pressure differential than those in lower permeability zones. Unless the well pressure is adjusted to the cleanup requirements of all perforations, only the better zones will flow efficiently, resulting in lower effective shot density and reduced production ratio.
Therefore, the optimum perforating operation is a balance between the pressure differential level which will cause sand production into the wellbore and the level needed for effective cleanup of the perforations. Using the perforating design methods discussed in this paper, the perforating design methods discussed in this paper, the following parameters can be estimated:*maximum underbalance pressure limits for perforating
*transient well response to the perforation/test operation
*transient lithologic zone response and flow profile
*initial surge volume per perforation
*minimum underbalance pressure for cleanup
*transient pressure distribution in the reservoir
*log-derived permeability
*production drawdown pressure limits
Following these design calculations, the actual well performance can then be evaluated by combining performance can then be evaluated by combining underbalance perforating with an Impulse Test* or surge test. This yields the first direct evaluation of the well and reservoir parameters, and provides the information necessary for optimizing completion operations.
This combination of underbalanced perforating and Impulse Testing is increasingly used in the Gulf of Mexico. It identifies wells with wellbore damage problems restricting their production, and indicates whether a problem occurred before or after the well was gravel packed. This information is then used to consider stimulation (economics and design) and also to correct the process and and/or fluid at fault so that the problem does not recur on the next wells.
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