Drilling Fluid Design Enlarges the Hydraulic Operating Windows of Managed Pressure Drilling Operations
Authors: Oakley Doug , Conn Lee 
Published: March 2011

Abstract
With the increasing use of Managed Pressure Drilling (MPD) to mechanically control bottomhole pressures for drilling narrow mud-weight windows, little consideration has been given until recently to integrating drilling fluid design and MPD operations for a specific application. This paper describes how two converging technologies have emerged in recent years that both converge on more effectively managing narrow hydraulic operating windows in extended reach, horizontal, and through tubing drilling operations.
The performance limitations of drilling fluids are discussed within the context of the drilling fluid requirements needed to drill these more critical well types and how attempts to manipulate the rheological profile have only been partially successful at balancing the need for sufficiently high enough rheology to adequately suspend dense weighting agents and the requirement for low-rheology fluids to manage downhole hydraulic needs (ECD). It is suggested that drilling-grade barite has major limitations when designing fluids for wells with critically narrow hydraulic operating windows, which are overcome by a novel process that produces micronized barite with an average particle size of less than 2 microns. By formulating treated micronized barite (TMB) in drilling fluids instead of API Barite, low-rheology drilling fluids may be formulated that enable downhole frictional pressures to be reduced by 0.5-lb/gal equivalent density with minimal risk of barite sag or settlement.
Managed Pressure Drilling (MPD) is a fast emerging technology that has the ability to reduce ECD's by applying annular backpressure. The authors suggest that drilling fluids that enlarge hydraulic operating windows should be considered as one of the ‘tools’ of Managed Pressure Drilling as defined by IADC. Case histories from the Gulf of Mexico and the North Sea describe how the combination of TMB drilling fluids and MPD operations provide a powerful synergistic combination of technologies that is capable of drilling the narrowest of hydraulic operating windows. Break-circulation pressures, swab-and-surge pressures are lower and pressure transmission from downhole pressure-while-drilling (PWD) tools are more instantaneous in low-rheology fluids. This allows better control of downhole pressures, further reducing the risk of wellbore instability and lost circulation events during MPD operations. Aspects of software design that predicts ‘in time’ wellbore hydraulics by coupling downhole drilling fluid rheology to annular pressures in the absence of PWD measurements while tripping and casing are also described that further improve the capability to manage bottomhole pressures and wellbore security in critical narrow hydraulic operating windows.
The authors conclude that a combination of MPD techniques and low-rheology TMB drilling fluids have the potential to drill wells that are currently considered ‘hydraulically undrillable’ by enhancing wellbore security and reducing drilling risk in extended reach, horizontal and through tubing drilling applications for development drilling and redevelopment of ‘brownfield’ reservoirs.