Taking the Proper Action to Gas Influx during Constant Bottom-Hole Pressure Technique of Managed Pressure Drilling
Authors: Yu Mengjiao , Karimi Vajargah Ali , Miska Stefan Z , Majidi Reza , Ozbayoglu Mehmet Evren 
Published: May 2013

Abstract
Managed Pressure Drilling (MPD) techniques rely on precisely controlling annular pressure profile and hence enabling us to overcome challenges due to narrow mud window in offshore and deep-water drilling. In a Constant Bottom-Hole Pressure (CBHP) MPD the bottom-hole pressure is kept relatively constant which allows circulating small influxes out of the well without shutting in. However, during circulation, annular pressure must not exceed the fracture pressure of the weak zones and maximum allowable surface pressure. Therefore, accurate prediction of the annular pressure profile is vital in the decision making process. It enables us to decide whether to continue drilling, shut in the well or take another action such as increasing the pump rate or mud weight.
Most of the annular pressure prediction methods during entrance of an influx rely on simplifying assumptions that can cause a significant error and a faulty well design. This issue becomes more important during drilling with Synthetic Based Mud (SBM) in which solubility of gas in the mud cannot be ignored.
We attempt to present a transient model for an accurate prediction of annular pressure profile during well control operations in CBHP technique of MPD. The presented model is developed based on the conservation equations and it takes the solubility of gas in synthetic based mud into account. The validity of the model has been tested against the available experimental data from the field scale low-pressure flow loop (8?? × 4.5??, 90' long) available at The University of Tulsa.
Annular pressure profile and maximum surface pressure (casing peak pressure) during circulation of a gas influx are calculated by the introduced transient two-phase model. A chart based methodology and a simple algorithm are developed by which, the proper action could be taken according to the simulation results. This provides a powerful decision making and design tool in MPD operations that ensures delivery of safer wells with lower Non Productive Time (NPT).

Introduction
Increasing demands for oil and gas and depleting conventional reservoirs during the past century have lead the petroleum industry to drill in harsh and hostile environments such as deep offshore that requires advanced technology. One of the most important issues in drilling such wells is the narrow mud window between fracture and formation pressures. Managed Pressure Drilling (MPD) techniques rely on precisely controlling annular pressure profile in the wellbore. The intention of MPD is to avoid continuous influx of formation fluids to the surface.
A permanent concern in any drilling operations is kick control and blow out prevention. A kick is defined as any influx that constitutes a well control emergency. In conventional drilling, this means using the blowout preventer (BOP) to shut in the well. It results in building up the casing pressure and increasing the bottom-hole pressure, which ceases entering additional influx to wellbore. Subsequently, kick will be removed by using Drillers Method or Wait and Weight Method. The key for a successful kick removal operation is to keep the bottom-hole pressure constant and prevent further entry of gas influx during circulation. The most important indicators for detecting an influx are pit gain and variations in pump pressure. In managed pressure drilling the well control emergency may not apply, as the system is already set up for this occurrence.