Gas kick detection and pressure transmission in thixotropic, compressible drilling fluids

Abstract

During well drilling operations, unexpected gas influx may take place when the bottom hole pressure falls below the pore pressure. The conventional procedure to control the influx consists of shutting-in the well and waiting for pressure stabilization. The pressures measured at the surface after the well closure and the pit gain are used to estimate the pore pressure, the gas density, and the kick magnitude. However, the drilling fluid compressibility and rheology can undermine pore pressure transmission to the surface, which can affect the pit gain and the well closing pressure. The current work presents a transient compressible isothermal mathematical model to predict pit gain and pressure transmission along the well during gas kicks. The model is based on the balance equations of mass and momentum that are solved by the method of characteristics. Three types of drilling fluids are analyzed: Newtonian, Bingham Plastic and thixotropic. The Darcy's law is used to represent the gas influx through the rock formation. The results indicate that the usual pressure balance used to estimate the pore pressure may lead to mistaken values and also that the drilling fluid compressibility and rheology have considerable effect on the pit gain and on the gas volume that invades the well. In all cases investigated, the gas volume within the well is at least 50% higher than the pit gain. Therefore, drilling fluid compressibility cannot be neglected on the estimation of the formation fluid volume within the well.