Transient Numerical Model for Narrow MPD Window

Abstract

In some wells, especially those drilled through fractured formations, the difference between the fracture pressure and the reservoir pressure becomes narrow. In these wells, maintaining ECD within the limits of fracture and reservoir pressures is a very challenging task, as many parameters have to be adjusted simultanusly to maintain the ECD.

When changing the pump rate and the backpressure to maintain the ECD within the MPD window, drilling fluid mass will change and an extra amount of fluid will be accumulated or released from the system as a result of the transient flow.

A numerical model is developed to enable the computation of the system mass change at any time due to change of the drilling parameters, which has to match the difference between the inflow and outflow. Any deviation will be interpreted as an external mass source (formation influx) or leak (fluid losses) and this can be used to enables early detection of losses or of formation influxes.