Surge and Swab Modeling for Dynamic Pressures and Safe Trip Velocities

Abstract

Surge and swab pressures, which could cause lost circulation or gas kicks and blowout, result from running or pulling a pipe in a borehole. Recent downhole measurements indicate that these pressures differ considerably from the pressures anticipated by traditional computational models which compute such pressures on the basis of steady state flow. In this paper, a new dynamic surge and swab model is presented that is based on the physics of the transient nature of the phenomenon. It treats friction as a distributed parameter and considers various parameters in hole geometry. hole expansion, varying trip velocity, return area for tricone and diamond drill bits, plugged jets, and the mud properties. The computer program for the model not only predicts the maximum surge and swab pressures or their variation with time at various points in the borehole, but also computes the safe maximum trip speed for a specified pressure margin. Computer studies for running a casing, liner, and drill string in several actual wells show the model to have fast computer run times. The results of some of these studies are reported in the paper. The program is both interactive and user-friendly, and is a significant improvement over the existing models.