Abstract

Accurate control over bottomhole pressure during drilling is essential as the industry meets increasingly challenging drilling environments such narrow drilling margin formations, HPHT wells and fractured formations.

To increase the accuracy of the control over bottom hole pressures during drilling, a fully automated prototype system consisting of a hydraulics simulator, a computer controlled choke manifold and a pump as part of the mud return system has been developed and tested.

Rather than mud density alone, the system uses a reduced density mud in combination with a variable back-pressure at the annulus exit to achieve the required downhole pressure. The system is able to substantially compensate downhole pressure variations induced by the drilling operation by varying the surface back-pressure.

A number of possible advantages are associated with the use of the system:

  • Reduction of formation impairment

  • Reduction of mud losses

  • Reduction of formation fluid influx

  • Increased ROP

  • No flat time during weight-up/down

  • Potential to reduce number of casing strings

  • Automatic kick circulation

A successful experimental program has been conducted on a real size test well in preparation of a field test sequence.

The system was easily retrofitted to the existing test rig and normal drilling procedures were minimally impacted.

Introduction

Accurate control over bottomhole pressure during drilling is essential as the industry operates in increasingly challenging drilling environments, some of which are:

  1. Narrow margin between pore and fracture pressure where static and dynamic equivalent circulating density or surge and swab effects can result insignificant mud losses 1 or well control events.

  2. HPHT wells where formation pore and fracture pressure determination byadjusting the mud weight in small increments is time consuming.

  3. Fractured or highly permeable reservoirs where equivalent circulating density is above pore pressure and LCM is not effective resulting in early termination of the well due to losses or in the requirement to drill with a mudcap 2 or underbalanced.

  4. Transients during under balanced drilling (UBD) exceeding pore pressure resulting in lost benefits of UBD.

  5. Mechanical hole stability where weighting-up can result in significant flattime.

These difficult drilling conditions are now managed by incorporating bottomhole pressure control procedures primarily using mud density control and to a lesser extent control over pump rate. Manual back-pressure control methods using surface pressure and chokes have also been used3, however automation is essential in these more demanding environments to maintain a constant bottomhole pressure with a high degree of accuracy and dependability.

This paper will discuss the novel design and full scale testing of a fully automated system to maintain an essentially constant bottomhole pressure during drilling operations. The system consists of a computer controlled choke manifold and pump as part of the surface mud return system. The downhole pressure is then controlled by automatic adjustment of the choke manifold and pump based on various inputs and the calculation results of a hydraulics simulator. Possible operational methods and considerations are also discussed.

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