Even though the effect of temperature on the viscosity of nanofluids at atmospheric pressure has been well studied, viscosity measurements of nanofluids at elevated pressures and temperatures have not yet been investigated.
Rheological characteristics of the nanofluids were measured using an automated high pressure high temperature (HPHT) viscometer (Model 7600, AMETEK Chandler engineering) The viscometer uses a rotor and bob geometry that is widely accepted in petroleum industrial applications. This equipment meets ISO and API standards for viscosity measurement of completion fluids under HPHT conditions up to 320 oC and 275 MPa. The assembly consists of three major parts: pressure vessel at the bottom, top plug in the middle, and top cap on the upper part. The bob and rotor cylinders are centered inside the pressure vessel where the fluid sample was gathered. The fluid sample occupies the spaces between, around, and inside the two concentric cylinders of the bob and the rotor. The rotational movement is transferred from a rotating outer drive magnet, placed around the top plug, to the rotor through the inner drive magnet, placed inside the top plug. The bob shaft extends through the whole length of the assembly and then is attached at the end to a spring assembly that is placed at the upper part of the top plug inside the top cap. The rotational movement of the rotor, at specified shear rates, causes the fluid in contact to move, which results in deflection of the bob cylinder. The angular deflection of the bob then causes a metal piece inside the spring assembly to deflect. This deflection is detected and recorded by an encoder that is placed above the top cap.
Study #1 Rheology of mineral oil-SiO2 nanofluids at HPHT
Here the rheological characteristics of mineral oil based nanofluids at high pressure and high temperature (HPHT). The nanofluids used in this work were prepared by mechanically dispersing commercially available SiO2 nanoparticles (w20 nm) in a highly refined paraffinic mineral oil (Therm Z-32, QALCO QATAR), which has wide applications in industrial heat exchangers. Mineral oil and nanofluids, with two volume concentrations of 1% and 2%, are studied in this work. The rheological characteristics of the basefluid and nanofluids are measured using an HPHT viscometer. During experimentation, viscosity values of the nanofluids are measured at pressures of 100 kPa and 42 MPa, with temperatures ranging from 25 _C to 140 _C, and at varying shear rates. The results show that the viscosity values of both nanofluids, as well as the basefluid, increased as the pressure increased. With the increase in temperature, viscosity values decreased following a power law for all the cases and, at temperatures below 100 oC, there was no substantial reduction in viscosity values of nanofluids.In addition, nanofluids exhibit non-Newtonian characteristics at elevated temperatures and pressures. Viscosities of nanofluids are unexpectedly altered at higher temperatures and pressure. The infrared spectroscopy analysis of the tested samples indicated a chemical alteration of the nanofluids when operated at high temperatures and pressure.
Figure: Comparison of viscosity values of nanofluids at near atmospheric pressures and at 42 MPa
Figure: FTIR spectrum of basefluid as well as nanofluids taken before and after HPHT experiments
Study #2 High-pressure rheology of alumina-silicon oil nanofluids
In this study, a standard calibrating silicone oil, which has no added chemical components, is used as the basefluid. The effect of nano-particle addition on suspension viscosity at elevated pressures is examined here. The rheological characteristics of silicone oil, as well as alumina-silicone oil nanofluids, are compared at pressures up to 100 MPa and at shear rates varying from 5 s−1 to 1000 s−1 while the temperature was maintained at ambient temperature. Silicone oil is widely used as a basefluid in the preparation of nanofluids as well as in specific drilling fluids. Both Newtonian and shear-thinning flow behaviours were observed for both the silicone oil basefluid and alumina-silicone oil nanofluids. The critical shear rate at which shear-thinning started to appear is observed to be a stronger function of the nanoparticle concentration than of the applied pressure. The viscosity increased with an increase in the particle concertation and was observed to be at several orders higher than that predicted by the effective medium theory, possibly due to the effect of particle aggregation.
Figure: Viscosity ratio variation with pressure for varying shear strees (8 wt% nanofluid)
Figure: Viscosity ratio variation with particle volume concentration at a given shear rate of 102 (1/s)
Study #3 Rheology study of colloidal suspension of carbon nanotube (CNT) particles in a water based drilling fluid
Water based muds are widely used as a drilling fluid in oil and gas industry while exploring the hydrocarbon reservoirs. These water-based muds are environmentally friendly and inexpensive. As deep-sea drilling attempts for extreme depths, high-temperature high-pressure (HTHP) condition which results in overheating of drilling equipment and malfunction due to lost circulation occurs, posing a severe limitation to the drilling fluid used. Several studies have been conducted to enhance the rheological and thermal properties of water-based drilling muds. Owing to the enhanced thermos-physical properties and stable nature, suspensions of nanoparticles have been suggested to be used along with drilling fluids. Here the rheology of water based drilling fluid with CNT nanoparticles are studied at high temperature and pressures. The effect of pressure and temperature are independently studied for the various particle loading of the nanoparticles. Viscosity values are measured at a maximum pressure of 170MPa with temperatures ranging from ambient to 180oC.