A New Extreme HP/HT Viscometer for New Drilling Fluid Challenges

Abstract

Growing demand for natural gas in North America is driving the E&P industry to look for new resources in previously unexplored areas and the deep Gulf of Mexico (GOM) Continental Shelf is currently attracting a lot of attention. Several of the current deep shelf HP/HT wells have anticipated bottom-hole temperatures that significantly exceed the limits of existing MWD/LWD tools; therefore, down-hole annular pressure measurements will not be available for pressure management. This leaves temperature/hydraulic models as our best, if not our only source of down-hole pressure information. These models depend on accurate surface inputs and laboratory measured fluid properties under down-hole conditions. Unfortunately, these anticipated temperatures and pressures also exceed the operating limits of conventional HP/HT viscometers. This lack of measured fluid properties under these extreme conditions will severely limit the ability of hydraulic models to predict down-hole pressures. A new extreme HP/HT (XHP/HT) concentric cylinder viscometer was designed and built to fill this important technology gap for the GOM deep shelf HP/HT wells. The instrument is capable of measuring typical drilling fluid viscosities up to 600°F (316°C) and 40,000 psig (276.0 MPa) and is capable of accurate measurements of drilling fluid properties containing ferromagnetic materials. Subsequent verification and validation testing proved the new viscometer compares favorably to commercially available field viscometers and more sophisticated laboratory rheometers and therefore lends itself towards widespread industry use. This paper reviews the development of the instrument, automated control system and HS&E issues related to testing drilling fluids at these extreme conditions. The paper will also present results of the verification and validation testing on invert-emulsion drilling fluids. Introduction Developing deep shelf gas requires overcoming some formidable drilling, as well as drilling fluid challenges. Rigs capable of drilling to these depths are larger, more robust and more expensive. Penetration rates tend to be low, extending time on location and adding to drilling costs. The extreme pressures, temperatures and acid gas levels limit down-hole tool, material and fluid selection. During the planning stage for several potential record depth deep gas wells, a technology gap was recognized for the measurement of fluid viscosity at the expected down-hole temperatures and pressures. HP/HT viscometer technology at the time was limited to measurements at =500°F (260°C), =20,000 psig (138.0 MPa). Some of the deep shelf HP/HT wells had anticipated bottom-hole conditions approaching 600°F (316°C) and 40,000 psig (276.0 MPa). Mathematical extrapolations of fluid properties could result in significant inaccuracies in hydraulics models, as fluid behavior has never been evaluated at these extreme conditions. Since present MWD/LWD tools are unusable at these extreme conditions, measurement of valid fluid properties for input into hydraulic models is critical for determination of the best available prediction of down-hole annular pressures. Based on these limitations, it was apparent that a new high temperature, high pressure viscometer would have to be developed for the industry. Oilfield Couette Viscometers Specialized concentric cylinder couette viscometers are used throughout the oilfield industry to determine the rheological properties of drilling fluids, cement slurries and fracturing fluids. ISO/API standards[1,2,3] exist to define and recommend test conditions, methods, bob and rotor geometries and shear rates used to determine the fluid characteristics. From these results, apparent viscosity of the sample is calculated at each shear rate and test condition. The data modeling methods differ with the nature of fluid being tested, as most of these fluids do not exhibit Newtonian behavior. The term "Couette Flow" originated from Maurice Frédéric Alfred Couette, Professor of Physics at the French provincial University of Angers during the 19th Century[4]. He described laminar flow of a liquid in the space between coaxial cylinders, now known as "Couette Flow" in his honor. Equations used to calculate values for shear stress, shear rate and viscosity for "Couette Flow" are included in the Appendix.