Abstract

The increasing demand to drill wells having narrow pore pressure and fracture gradient margins and the increasing concern that irreparable damage from extreme overbalanced drilling negatively impacts productivity and recoverable reserves, requires a new method of controlling bottomhole pressure while drilling, beyond the current capability of conventional drilling techniques. Where the dynamic pressure or equivalent circulating density can be supported, a solution has been developed to drill with a relatively constant bottomhole pressure equal to the dynamic conditions rather than static conditions, thereby reducing the maximum bottomhole pressure. The method also establishes a continuous method of well control that should reduce the risk of taking a kick.

Shell International Exploration & Production Research & Development has developed a robust solution using a flow model running in real time coupled to an automated choke control system to continuously maintain the bottomhole pressure at a relatively constant value. Several applications of this method have been implemented or proposed, including fractured reservoirs where it is difficult to control losses, HPHT wells subject to ballooning and wells with narrow pore pressure and fracture gradients margins subject to kicks and losses. Underbalanced drilling is another application where increased control of bottomhole pressure to limit inflow and decrease the potential for borehole instability can be realized. The development, testing, application and results of this automated system will be presented.

Introduction

With the advent of circulating drilling mud, bottomhole pressure has been subjected to two different pressure conditions; a static pressure state when the pumps are off and a higher dynamic pressure state caused by the addition of frictional pressure when the drilling pumps are on and drilling fluid is being circulated in the well.

The recently developed Dynamic Annular Pressure Control (DAPC) system now changes that by making it possible to drill with a relatively constant bottomhole pressure equivalent to the dynamic pressure at a given depth in the well by holding the necessary backpressure on the annulus as shown in figure 1. In essence, the pressure curve is reversed by having the increase in pressure at surface using backpressure control rather than at the bottom of the hole.

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