Swab and Surge Pressures with Reservoir Fluid Influx Condition during MPD

Abstract

Abstract With managed-pressure drilling (MPD), there is a need to have precise control on the profile of annular pressure while drilling and cementing. However, current methods using conventional calculations for controlling bottomhole pressures in MPD wells do not properly take into consideration the elasticity of drillpipes under reservoir fluid influx and, therefore, do not control the proper pressures. For these reasons, a method for more precisely controlling the annular pressure profile throughout the wellbore is required. The purpose of this study is to investigate the dynamic motion of the drillstring when there is an influx of formation fluid, with particular interest focused on the effect of bottomhole pressure. Increased underbalanced and managed-pressure drilling operations have necessitated predicting wellbore pressures more accurately, as they allow for reduced fluid loss and reservoir influx. While drilling through or reciprocating in a potential reservoir zone where the borehole and formation fluid pressures are in a narrow margin under a managed-pressure condition, the swab pressures may cause the wellbore to be underbalanced for a small period of time. At certain times, an incremental influx for a short duration may be potentially detrimental. Neither the steady-state model nor the transient model is comprehensive enough to predict the wellbore pressures correctly, as both models neglect the reservoir fluid influx or fluid loss under this condition. This paper presents a coupled swab/surge model, whereby the wellbore bottomhole pressure can be predicted more accurately with a reservoir fluid influx. A full balance of mass and momentum for pipe and annulus flow is solved. The model also includes the effects of fluid inertia and compressibility, wellbore elasticity, axial elasticity of the pipe, and temperature-dependent fluid properties. The transient-gas reservoir model allows for investigating the effect of produced gas when the formation pressure exceeds the bottomhole pressures. This model allows for measuring the influx as a function of pipe movement and predicting the amount of influx as a function of time. The practical usefulness of the theory, backed by the fundamental analysis, is demonstrated with numerical cases. The effects during underbalanced conditions have been analyzed, and several non-intuitive transient effects, such as the observation of reduction in surge effects while swabbing and vice versa, are highlighted.