A New Extreme-HP/HT Viscometer for New Drilling-Fluid Challenges William Gusler, SPE, Marvin Pless, SPE, Jason Maxey, SPE, Patrick Grover, SPE, and Jose Perez, SPE, Baker Hughes Drilling Fluids, and Jeff Moon, SPE, and Todd Boaz, Ametek Chandler Engineering Summary Growing demand for natural gas in North America is driving the exploration and production industry to look for new resources in previously unexplored areas, and the deep Gulf of Mexico (GOM) continental shelf is currently attracting substantial attention. Sev- eral current deep-shelf high-pressure/high-temperature (HP/HT) wells have anticipated bottomhole temperatures that significantly exceed the operating limits of existing measuring-while-drilling and logging-while-drilling (MWD/LWD) tools; therefore, down- hole annular-pressure measurements will not be available for pres- sure management. This leaves temperature and hydraulic models as the best, if not the only, source of downhole-pressure informa- tion for these wells. These models depend on accurate surface inputs and laboratory-measured fluid properties under downhole conditions. Unfortunately, these anticipated temperatures and pres- sures also exceed the operating limits of conventional HP/HT vis- cometers. This lack of measured fluid properties under these ex- treme conditions will severely limit the ability of hydraulic models to predict downhole pressures. A new extreme-HP/HT (XHP/HT) concentric-cylinder viscom- eter was designed and built to fill this important technology gap for GOM deep-shelf HP/HT wells. The instrument is capable of mea- suring typical drilling-fluid viscosities up to 600°F (316°C) and 40,000 psig (276.0 MPa) and is capable of accurate property mea- surements for drilling fluids containing ferromagnetic materials. Subsequent verification and validation proved that the new vis- cometer compares favorably to commercially available field vis- cometers and more-sophisticated laboratory rheometers and there- fore lends itself to widespread industry use. This paper reviews the development of the instrument and associated automated control system and explores health, safety, and environment (HSE) issues related to testing drilling fluids at these extreme conditions. The paper also presents results of verification and validation testing on invert-emulsion drilling fluids. Introduction Developing deep-shelf gas requires overcoming some formidable drilling and drilling-fluid challenges. Rigs capable of drilling to these depths are larger, more robust, and more expensive than ordinary rigs. Penetration rates tend to be low, extending time on location and adding to drilling costs. The extreme pressures, tem- peratures, and acid-gas levels limit downhole tool, material, and fluid selection. During the planning stage for several potential record-depth deep-gas wells, a technology gap was recognized for the measure- ment of fluid viscosity at the expected downhole temperatures and pressures. HP/HT-viscometer technology at the time was limited to measurements at 500°F (260°C) and 20,000 psig (138.0 MPa). Some of the deep-shelf HP/HT wells had anticipated bottomhole conditions approaching 600°F (316°C) and 40,000 psig (276.0 MPa). Mathematical extrapolations of fluid properties could result in significant inaccuracies in hydraulic models because fluid be- havior has never been evaluated under these extreme conditions. Because current MWD/LWD tools are unusable under these ex- treme conditions, measurement of valid fluid properties for input into hydraulic models is critical for determination of the best avail- able predicted values of downhole annular pressures. Because of these limitations, it was apparent that a new HP/HT viscometer would have to be developed for the industry. Oilfield Couette Viscometers Specialized concentric-cylinder, or Couette, viscometers are used throughout the oilfield industry to determine the rheological prop- erties of drilling fluids, cement slurries, and fracturing fluids. In- ternational Standards Organization (ISO)/American Petroleum In- stitute (API) standards [ISO 10414-2:2002, ISO 10414-1:2002, API RP 13B-2 (2005)] exist that define and recommend test conditions, methods, bob and rotor geometries, and shear rates for determining fluid characteristics. From the results of these tests, the apparent viscosity of the sample is calculated at each shear rate and test condition. The data modeling methods differ with the fluid being tested, as most of these fluids do not exhibit New- tonian behavior. The term “Couette flow” originated from Maurice Frédéric Alfred Couette, professor of physics at the University of Angers in France during the 19th century (Couette 1890). He described lami- nar 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 Appendix A. A coaxial-cylinder, or Couette, viscometer consists of an outer cylinder that rotates around a stationary inner cylinder. The outer component is known as the “rotor,” and the inner cylinder is known as the “bob.” A shear gap exists in the annular space between the bob and the rotor. In the interest of industry standard- ization, the diameters and lengths of the bob and rotor are defined by applicable ISO/API recommended practices [ISO 10414- 2:2002, ISO 10414-1:2002, API RP 13B-2 (2005)]. The bob and rotor are immersed in the target fluid. As the rotor turns at standard speeds ranging from 1 to 600 RPM, creating a specific fluid shear rate in the annular gap at each speed, the torque induced on the stationary bob by the fluid is measured accurately. The torque transducer connected to the bob is calibrated to indicate shear stress using known viscosities of Newtonian oils over the desired range of shear rates. Viscosity at a given shear rate is determined as the ratio of shear stress to shear rate. Couette-Flow Assumptions. Several assumptions must be ad- hered to when using Couette (sleeve-and-bob) geometry to ensure accurate measurement of fluid rheological properties. These as- sumptions include (Macosko 1994) • Steady, laminar, isothermal flow • v  r only and v r v z 0 • Negligible gravity and end effects • Symmetry in , where v i the velocity component in the i-coordinate direction, cm/s; rradial distance from centerline, cm; and angular velocity, rad/sec These assumptions often are not completely satisfied when work- ing with solids-laden drilling fluids and torsion-measurement de- vices. For example, the requirement for a steady, laminar flow assumes that the motion of solid particles is not imparting impact energy to the bob, which in practice may result in unstable torque measurements. Additionally, it is assumed that no secondary flows Copyright © 2007 Society of Petroleum Engineers This paper (SPE 99009) was first presented at the 2006 IADC/SPE Drilling Conference, Miami, Florida, 21–23 February, and revised for publication. Original manuscript received for review 20 December 2005. Revised manuscript received 26 December 2006. Paper peer approved 2 January 2007. 81 June 2007 SPE Drilling & Completion