Análise da propagação de pressão em fluidos de perfuração durante kick de gás

Abstract

An important task during well drilling in deep water is the control of the bottomhole pressure within a narrow range. Whenever the bottomhole pressure becomes smaller than the formation pressure there is a risk of formation fluid invasion (oil, natural gas and/or water) into the wellbore. The influx of the formation fluid to the wellbore, called kick, can escalate to a blowout if not controlled when the formation fluid reaches the surface. Therefore, a small inflow of gas should be detected as soon as possible. Nevertheless, the pressure is only measured while drilling and also a small influx of gas cannot change significantly the bottomhole pressure. Another indication of kick is the change of pressure at the wellhead which is only noticed when a large amount of gas has invaded the well. The current work presents a compressible transient flow model to predict pressure transmission within the wellbore when a gas influx occurs. The model comprises the conservation equations of mass and momentum which are solved by the method of characteristics. In this work, to the drilling fluids a thixotropic model is considered. The influx of gas is defined as a function of the rock permeability and the pressure difference between the reservoir and the well. Model results show that the pressure variation along the time depends on the pressure wave propagation and the thixotropic properties.