Viscosity Standards for High-Pressure, High- Temperature, and High- Viscosity Conditions Kurt Schmidt Houston, Texas, USA January 22, 2010
Viscosity in Reservoir Applications Ultradeep GOM (Gulf of Mexico) offshore prospects are being found at higher pressures with heavier oils that are characterized by low gas/oil ratios (GORs). These prospects typically have low reservoir energy, low compressibilities, and/or no aquifers. Extraction frequently requires artificial lift (i.e., waterflooding, gas flooding) and the potential for success is significantly affected by viscosity. Higher temperatures are required to deal with the challenges of bitumen reserve extraction from oil sands (which is also viscosity-driven). Importance of viscosity is twofold: Determining productivity using Darcy s law (describing flow through a porous medium). Determine displacement (i.e.,mobility ratios) in waterflooding (recovery).
Production Rate Key factors fluid viscosity (µ) formation volume factor (β) rock permeability (k) Reservoir-Fluid Sampling and Characterization Key to Efficient Reservoir Management ; N.R. Nagarajan, M.M. Honarpour, K. Sampath, SPE 103501, (2007)
Fractional Flow Viscosity uncertainties have significant impact. Reservoir-Fluid Sampling and Characterization Key to Efficient Reservoir Management ; N.R. Nagarajan, M.M. Honarpour, K. Sampath, SPE 103501, (2007)
Viscosity Measurement Viscosity of reservoir fluids is routinely measured in commercial laboratories with electromagnetic (oscillating piston), capillary, and falling-body viscometers. The viscometers are typically calibrated at atmospheric conditions at specific temperatures with standard calibration fluids Extended to elevated pressure and temperature conditions with empirical correlations. can also be calibrated at limited temperature and pressure conditions with available reference fluids. Quality of viscosity data (reference model) and purity of solvent can be issues. The accuracy of viscosity measurements, however, cannot be verified by the equipment operators under the desired HTHP test conditions.
Case Study Study looked at viscosity up to 200 cp at 240 degc at low enough pressure to keep volatiles from coming out of solution (e.g., pressures < 2,000 psi). The viscometer was calibrated under ambient conditions and could only be validated for accuracy at ambient pressure and temperatures up to 150 degc using traceable fluids. It was found that, at best, expected readings of Cannon calibration fluids at other pressures and temperatures could only be extrapolated from the test certificate values. The uncertainty of data at these conditions, especially when they deviated from predicted values based on past models, hindered engineering decisions.
Pressure Corrections Accurate information is necessary to adjust readings for higher pressures.
Viscosity Results Measurements with the oscillating piston viscometer match literature values at four different temperatures.
Viscosity Calibration With capillary viscometers, the K factor can vary 20% or more with pressure.
HTHP Viscosity Standards With greater frequency, we are asked to supply high-temperature, high-pressure, and high-viscosity viscometers and experimental viscosity data at these conditions. There is interest in establishing a viscosity standard for the petroleum industry; at this time the group consists of Cambridge Viscosity and Schlumberger. We will be expanding this to major petroleum companies in the future. Our goal is the development of a reference fluid with viscosities close to that required in practical HTHP situations.
Viscosity Standard Requirements Presently the acceptable (petroleum industry) accuracy of live oil measurements is ~10%. An uncertainty of 2% in the viscosity standard would allow laboratories to experimentally determine the viscosity to within 5%. The long term target specifications on which we have been focusing are aggressive: 300 degc, 200 cp at 300 degc, 35,000 psi with an 1% accuracy rating. Near term, we believe a reasonable solution would be 200 degc, 200 cp at 200 degc, 25,000 psi with a 2% accuracy rating. This will yield significantly better information for oilfield decisions and performance.
Viscosity Standard Key Issues Fluid selection: Thermally stable Nonreactive, not sensitive to UV exposure, etc.. Easily available at specified purities Existing dataset Available throughout the world Quality, health, safety, and environment (QHSE) friendly One fluid, certain temperature and pressure ranges, initially (e.g., Squalane) Measurement: Round-robin effort with variety of methods and different systematic errors Molecular dynamics simulations to supplement experimental data set
Viscosity Standard Key Issues Timing and measurement regions: Dissemination: Expand to other temperature and pressure ranges with same fluid or second fluids Wells are rarely found with both high temperature and high pressures: staged experiments Timing: What is the timing to arrive at the needed standards? We may need to develop an interim solution while working toward the longer term solution. Provide a correlation of the measurements with an expanded uncertainty of <5% This fluid and correlation would then be used to calibrate viscometers used in the oil and gas business. Industrial group Industry consortium funding Industry perceives this to be important and funding should be available at some level.
Initial Thoughts About Fluid Selection Our specifications with regard to the fluid may not be achievable with one pure substance, e.g., Squalane. Base fluids (alpha or iso-olefin mixtures) that go into making drilling muds as candidates for viscosity standards? Their thermal degradability is well known and so are their rheological properties from the work done by the drilling fluids industry. The other side benefit is a better handle for clients on using the viscosity data for the equation-of-state and viscosity studies for contaminated fluids.
Next Steps Affirm National Institute of Standards and Technology and International Association for Transport Properties (IATP) interest. Determine pressure, temperature, and viscosity ranges of interest to industry (and academia). HTHP viscosity standard work similar to IATP s diisodecyl phthalate (DIDP) efforts. Obtain funding from: industry government other? Anticipate results will be of wide interest in industry/academic journals.