December 2010 SPE Drilling & Completion 577 Improved Kick Management During MPD by Real-Time Pore-Pressure Estimation Jan Einar Gravdal, SPE, International Research Institute of Stavanger; Michael Nikolaou, SPE, Øyvind Breyholtz, and Liv A. Carlsen, University of Houston Copyright © 2010 Society of Petroleum Engineers This paper (SPE 124054) was accepted for presentation at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, USA, 4–7 October 2009, and revised for publication. Original manuscript received for review 30 July 2009. Revised paper received for review 26 March 2010. Paper peer approved 29 March 2010. Summary Pressure maintenance within safe bounds and minimization of influx of fluids from the formation to the well during a kick are basic concerns of well control. Managed-pressure drilling (MPD) offers improved capabilities over conventional well control methods to address these concerns. In this work, we develop a methodology that capitalizes on the improved access to downhole measurements offered by wired-drillpipe telemetry to maintain pressure within desired bounds during kick management. The objective of this methodology is to improve MPD by reducing nonproductive time, reducing formation damage, and optimizing operational limits for the annular backpressure choke manifold. The proposed methodology estimates formation pore pressure automatically on the basis of real-time measurements when a gas kick is taken during MPD. The methodology relies on the char- acteristics of the pressure-buildup curve. Implementation of the methodology presumes the availability of standard MPD equip- ment for automatic annular backpressure control. A representative North Sea well is used as test-case geometry, and an advanced hydraulics model is used as a virtual well in com- puter simulations that provide the basis for the presented results. The proposed methodology is demonstrated to both maintain pressure within desirable bounds and reduce formation-fluid influx during a kick and thereby reduce the risk of hole-stability problems and the cost associated with nonproductive time. Introduction MPD is seen as a promising technology to meet the challenges of drilling wells with a narrow margin between pore pressure and fracture pressure. This is often the case in depleted reservoirs or deepwater wells. MPD is a general description of methods for well- bore pressure management and includes techniques and equipment developed to limit kicks, lost circulation, and differential sticking. The overall objective of MPD is to reduce the number of casing strings required to reach the target depth safely. The definition from IADC (2008) states that “managed pressure drilling (MPD) means an adaptive drilling process used to control precisely the annular pressure profile throughout the wellbore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly. MPD is intended to avoid continuous influx of formation fluids to the surface. Any flow incidental to the operation will be safely con- tained using an appropriate process.” So far, the main focus within the industry has been on develop- ment of automated chokes and on their control algorithm (Roes et al. 2006; Godhavn 2009), on improved flowmeters for influx and loss detection (Santos et al. 2005), and on alternative MPD con- cepts to actively control the pressure profile (Hinton 2009; Fossli and Hendriks 2008). New telemetry systems for real-time down- hole measurements (Hernandez et al. 2008), continuous-circulation devices (Jenner et al. 2005), and innovative downhole tools (Bansal et al. 2007) are valuable contributions to the MPD tool box. However, the issue of developing an appropriate process to con- tain any flow incidental to the operation has not yet been addressed thoroughly. Specifically, although MPD does not encourage influx into the wellbore, there is usually a higher chance of receiving formation fluids (a kick) in MPD compared to conventional drill- ing. This is because the wellbore-pressure profile is usually close to pore pressure somewhere in the openhole section. When a kick is detected, the well must be controlled properly to stop the influx, circulate out the formation fluid, and continue the drilling opera- tion. To control the well during these steps, it is advantageous to obtain an accurate estimation of the pore pressure at the influx zone as quickly as possible. In this work, we present a method for rapid estimation of pore pressure after a gas kick. The method relies on detecting the turning point in a pressure-buildup curve. The detection is based on stan- dard robust statistical inference. The rest of the paper is organized as follows. In the second section, we describe the real-time pore- pressure-estimation algorithm (RTPPEA). In the third section, we clarify the applicability of the proposed method. In the fourth sec- tion, results from a case study are presented. Conclusions are given in the fifth section, and Appendix A is devoted to discussion. The RTPPEA The MPD setup for this work is a traditional automatic annular back- pressure control system with backpressure pump. A rotating control device pressurizes the annulus, and the pressure is controlled actively by adjusting the opening of the surface choke valve. A surface back- pressure pump gives additional flow rate through the choke. Tradition- ally, this extra flow loop is used to maintain circulation through the choke during connection. A schematic overview of the setup is shown in Fig. 1. A downhole pressure sensor at the measurement-while-drill- ing (MWD) tool transmits data through a wired-drillpipe telemetry system and ensures bottomhole-pressure measurements even during times with low or no circulation. For the entire simulation, the choke valve is controlled automatically by a suitable control algorithm, described further in Appendix A. Reference pressure for the choke controller is either a bottomhole-pressure or surface-backpressure value. The hydraulics model representing the virtual well and the influx model are also described in Appendix A. To explain the proposed method, an example is used, as shown in Fig. 2. The figure shows pressure and flow-rate variations during the time from before the kick is detected until the kick is circulated out and drilling resumes. On the figure, some time points are highlighted and numbered from one to six. During the first five minutes of the simulation, an MPD scenario is in progress using automatic annular backpressure control. At Time Point 1, the influx initiates. In this example, a 10-bar (145-psi) underbalance from the “true” pore pressure bottomhole led to a 240-L/min (1.5-bbl/min) kick. After the kick has been detected at Time Point 2 and the influx size has been decided to fall within a certain volume range suitable for performing a shut-in, the main pump and annulus pump are shut down smoothly over a 2-minute period while keeping a constant bottomhole pressure based on the MWD pressure sensor. A harder (faster) shut-in may lead to pres- sure transients that can cause additional underbalance and thereby increase the influx. After the choke is completely closed (Time Point 3), the RTPPEA starts automatically. The pressure-buildup curve from the choke measurements is analyzed in real time by the algorithm, tracking the point where the curve stabilizes as lin- ear. This point is the signature of the equilibrated pressure in the