Underbalanced, through-tubing perforating operations represent a compromise between the pressure differential needed for effective perforation cleanup and the differential that causes sand production or tends to the gun and cable. This paper explores the dynamic variables involved in through-tubing perforating and describes a new model developed to evaluate the underbalance perforating conditions and predict the effects on the wireline gun system. Using this model, formations can be aggressively perforated with the optimum safe underbalance pressure, and perforated with the optimum safe underbalance pressure, and completion operations can also be combined with closed chamber test techniques to accurately evaluate the completion efficiency and initial reservoir parameters.
Utilizing new wireline technology to measure downhole pressure and temperature during perforating, operational data were obtained from numerous field tests to validate the dynamic model. The results indicate that the model reliably predicts the maximum underbalance to avoid sanding, the change in gas cushion pressure, the volume of reservoir fluid influx and the gun/cable movement Examples from Louisiana Gulf Coast wells are used to illustrate the application of the design model and the associated closed chamber test technique.
The transient pressure data from the closed chamber test are used to accurately evaluate reservoir parameters and completion efficiency. The data are analyzed by assuming the formation is subjected to an impulse rate created by briefly flowing the well. In addition, by measuring the surface and bottomhole pressure during the perforating operation, the approximate fillup rate can be determined as well as the cumulative volume of fluid entry. In the field examples presented, these data are utilized in other interpretation techniques, such as rate convolved pressure analysis, and the derived formation parameters are compared with conventional well test results.
Underbalance perforating operations ensure a surge of reservoir fluid such that gun debris and crushed rock in the perforation tunnel are swept info the borehole, providing better flow performance for subsequent production. For maximum cleaning effect, it is desirable to perforate with the maximum possible amount of underbalance pressure. The degree of perforation tunnel cleaning is directly proportional to the amount of underbalance and fluid influx. In general, the larger the underbalance and fluid influx, the better the cleaning effect. The pressure differential required for perforation cleanup ranges from 200 psi to more than 50(x) perforation cleanup ranges from 200 psi to more than 50(x) psi and has usually been established by trial and error in each psi and has usually been established by trial and error in each field.
However, practical experience dictates that problems (sand production, casing collapse, etc.) will develop if too high an underbalance pressure is used. In unconsolidated or poorly consolidated formations, the mechanical strength of poorly consolidated formations, the mechanical strength of the formation must be considered to avoid sand production and/or the movement of the fine particles that could cause plugging of the matrix. Excessive underbalance pressure plugging of the matrix. Excessive underbalance pressure during the perforating operation can lead to significant sand production and to the sticking of wireline tools being in the production and to the sticking of wireline tools being in the well.
In the case of through-tubing perforating, a gas/liquid cushion is commonly used to obtain the desired underbalance pressure. The drag force exerted on the gun-cable system when the fluid accelerates upward must also be considered when perforating underbalanced The pressure differential must be designed to avoid blowing the wireline gun and cable up the hole during the initial fluid surge. In general, the larger the underbalance and fluid influx, the larger the drag force exerted on the gun/cable system.