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Heavy Oil Viscosity Measurements – Best Practices and Guidelines Hongying Zhao, Afzal Memon, Jinglin Gao, Shawn David Taylor, Donald Sieben, John Ratulowski, Hussein Alboudwarej, James Pappas, and Jefferson L. Creek Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00300 • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 9, 2016
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Heavy Oil Viscosity Measurements – Best Practices and Guidelines Hongying Zhao*1, Afzal Memon1, Jinglin Gao1, Shawn D. Taylor1, Donald Sieben1, John Ratulowski1, Hussein Alboudwarej2, James Pappas3, and Jefferson Creek2 1
2
3
Schlumberger
Chevron Energy Technology Company
Research Partnership to Secure Energy for America (RPSEA)
KEYWORDS Heavy Oil, Viscosity Measurement, Viscometer
Abstract
Viscosity is an important parameter in reservoir development, especially in heavy oil production, processing, and transportation. Accurate measurement (±5%) of heavy oil viscosities can be affected by sample handling, storage, and cleaning procedures. In addition, the type of viscometers and the corresponding experimental procedures can impact the accuracy of viscosity measurements. The objectives of this paper are to present the results of a systematic evaluation
*Corresponding author:
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and comparison of different viscometers typically used in heavy oil viscosity measurements, provide references on the subject of viscometer selection, recommend developed measurement procedures for each viscometer, and generate a reliable viscosity database of dead and live heavy oils. The systematic study was performed using three viscometers typically used in heavy oil systems: a capillary viscometer (CV), an electromagnetic viscometer (EMV), and a rheometer (Rh). Viscosity measurements were performed over a range of temperature and pressure conditions varying from 293K to 422K (20°C to 149°C) and from atmospheric pressure to 31.0 MPa (4,500 psia). Three dead heavy oil samples ranging from 20° to 11° API gravity and three live heavy oil samples prepared to gas/oil ratios (GORs) of 44.5, 30.3, and 17.8 Sm3/Sm3 by the three dead oils and methane gas were used. The study results showed that within working limitations, each well-calibrated viscometer can reproduce reported values of viscosity standards with a relative error of less than 5%. The Rh with an open to atmospheric system provides the highest viscosity measurement scale but is limited to lower temperature tests in order to minimize light and/or intermediate component losses. The EMV and CV provide reasonably consistent viscosity values for both dead and live heavy oil samples as long as key conditions related to the experimental set-up, measurement procedures and sample preparation are met.
Introduction
Understanding the physical properties of viscous heavy oil systems under production and transportation conditions is a major factor in successful exploitation strategy and production design. However, heavy or viscous oils present challenges to obtaining accurate, consistent, and repeatable viscosity data. Obtaining a “clean” and representative heavy oil sample1,2 is the first and foremost critical step in producing reliable laboratory pressure-volume-temperature (PVT)
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data. In this instance, “clean” is defined as the sample with a Basic Sediment and Water (BS&W) content of less than 1 wt%, which is a typical export specification used in reservoir, flow line, and facilities design. This BS&W is also used as a criterion to evaluate the suitability of a fluid in fluid property measurements3,4. Heavy oil is generally produced with water and some solids5, which can affect viscosity measurements by altering the expected flow pattern resulting in unrealistic data. Therefore, it is important to remove solids and water from heavy oils samples prior to measurements, although it can be a challenge to be successfully removed from the produced water-in-oil emulsions without altering the oil composition. More often than not, the most difficult step is to reduce the residual water content from a few weight percent to be less than 1 wt% due to i) the very low density difference between heavy oil and water, and ii) the high viscosity of the heavy oil. An evaluation and recommendation summary of best practices in heavy oil sample dewatering and handling were discussed previously6. Accuracy and repeatability of heavy oil viscosity measurements are not only affected by sample handling, storage, and cleaning procedures, but they are also affected by the selection of viscometers and the experimental procedures followed by different operators. A study published by Miller et al.7 indicated that heavy oil viscosities measured by different viscometers or by the same type of viscometer but measured in different laboratories following different operating procedures were found to be quite different. The inconsistency of the measured viscosities makes data interpretation and application to process designs difficult. However, in the current open literature, there has rarely been any evaluation reported on investigating the accuracy of dead and live heavy oil viscosity measurements made by different viscometers. Alkandari et al.8 reported on a comprehensive comparison of heavy crude oil viscosities measured by three different techniques, however, only dead heavy crudes were tested and the testing condition was only over
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the range of 298K to 353K (25°C to 80°C) at atmospheric pressure. Bitumens and/or extra heavy oils and low temperature measurements of dead heavy crudes also introduce the possibility of non-Newtonian behavior15, either in the form of stress dependent viscosity (i.e. viscoelastic fluid) or time dependent behavior (i.e. thixotropic fluid), which can give the perception of inconsistent viscosity measurement with routine laboratory techniques. A third challenge is the ability to reasonably predict the viscosity of heavy oil and heavy oilsolvent blends. The difficulties lie in obtaining reliable experimental viscosity data from properly prepared and representative samples and the lack of an ability to predict live and dead oil physical properties without case specific model tuning. Empirical models that depend on heavy oil-diluent property databases have been successfully applied9,10, although their application is limited to the samples and operating conditions captured in the database used to generate the models. Compositional based viscosity models11,12 offer the ability to predict blend viscosities based on tuning to a limited amount of data. However, the range and reliability of the predictions can only improve over time as heavy oil-diluent property databases expand. Thus, reliable and predictive heavy oil related fluid viscosity prediction depends, to a large extent, on developing a reliable property database covering a broad range of composition and operating conditions. To address these challenges in heavy oil viscosity measurements, the main objectives of the study being reported in this paper are to evaluate currently employed viscosity measurement techniques for both dead and live heavy oil samples, to compare the viscosity variations measured by the different techniques/viscometers, and as a result to recommend best practices and procedures for heavy oil viscosity measurements. One other objective of our study is to provide a reliable set of dead and live heavy oil viscosity data for future validation and improvement of heavy oil viscosity models.
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Viscometer Introduction
Capillary viscometers (CV), electromagnetic viscometers (EMV) and rheometers (Rh) are commercially available and widely used for heavy oil viscosity measurements. In this study, a systematic study will be presented covering evaluating these three devices, in addition to comparing their performances in heavy oil viscosity measurements. Note that other techniques, such as rolling ball, were not considered in this work since they are not commonly found in commercial laboratories. For each selected viscometer, calibration was performed prior to usage. The calibration was performed using viscosity standards (Cannon Instrument Company, US) with known viscosity values at atmospheric pressure while at various temperature conditions. To cover the range of viscosity to be measured, the viscosity standards utilized in viscometer calibration and validation processes included S8000, N5100, N4000, N1000, N750, N100, N75, N10, N7.5 and HT390. The key features and operating principles of these three viscometers are summarized in Table 1. The following sections will give a detailed introduction on these selected viscometers, including the viscometer’s setup, model, specification, working principle, calibration and validation procedures, as well as measurement procedures. Capillary Viscometers (CV) CVs have been widely used in heavy oil viscosity measurements by commercial laboratories and by various researchers13-16. A CV can be operated under high pressure with an operating temperature to 573 K (300°C). The CV employed in this study was developed and manufactured by Schlumberger. It had an operating temperature range of 283K to 473 K (10°C to 200°C) and a pressure range of atmospheric to 68.9 MPa (14.7 to 10,000 psia). This CV was able to measure in situ viscosities over the range of 1 to 500,000 mPa·s. Three coils were used in this study to
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cover the viscosity range being measured. The coils were 1.5 to 6.1 m (5 to 20 f) in length and 0.05 to 0.18 cm (0.02 to 0.07 in) in inner diameter. Each coil had a coil diameter of ~ 23 cm (9 in). As shown in Figure 1, a typical CV setup consists of two high-pressure cylinders connected to a capillary coil and a differential pressure transducer that monitors pressure drop across the capillary coil. The cylinders and the coil were set up in a temperature controlled oven and the temperature of the test fluid was measured by a temperature sensor, which was attached to the outside of the capillary coil. Two pumps were connected to the cylinders on the hydraulic side of the cylinder. Both the static system pressure and the flow rate of the fluid in the coil were controlled by these two pumps. One of the cylinders was charged with a single-phase fluid (live or dead) prior to the test. During viscosity measurements, the two pumps were used to transfer the fluid through a capillary coil, from one cylinder to the other, at a specified volumetric flow rate (Q). The corresponding pressure drop (∆P) across the coil is measured using the differential pressure transducer. For a Newtonian fluid, the viscosity can be calculated using the following equation for laminar flow as follows: =
∆ ×
(1)
where, µ
=
Fluid viscosity, mPa·s
∆P
=
Pressure drop across the capillary coil, Pa
Q
=
Flow rate of the fluid through the capillary coil, m3/s
k
=
Calibration coefficient of the capillary coil, m3.
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(k ∼ /128, D is inner diameter (ID) and L is length of the capillary coil. Due to variations in the coil ID, the orientation of the tube, and potential end effects (see below), this value must be determined by calibration.) When making viscosity measurements, several factors should be considered to ensure the fluid flow in the capillary coil obeys the fundamental assumptions of Eq. 1. The flow should be laminar as indicated by a Reynolds number ( = /) less than 2100. Also, the Dean number ( = ( ⁄ ))17 should be less than six to ensure that radial velocity components are negligible. In addition, the capillary coil should be sufficiently long to ensure end effects will be negligible. Prior to making viscosity measurements, the calibration coefficient of each coil, k, was determined by Eq. 1 using Cannon viscosity standards with known viscosity. During calibration, the flow rate and the corresponding pressure drop across the capillary coil for a viscosity standard were measured at three pressures for a given temperature; thus, obtaining the correlation between measured viscosities and pressures. This measured pressure-viscosity correlation, linear in general, was extrapolated to determine the viscosity of the viscosity standard at atmospheric pressure. The coefficient of the coil at the testing temperature was then determined by matching the measured viscosity to the given standard viscosity value at atmospheric pressure. Following this procedure, each coil was calibrated at a series of temperatures using various viscosity standards, and typically the coefficient, k, had a linear correlation with temperature. After calibration, the CV, equipped with one of the coils, was validated by a different viscosity standard that was not used in the calibration procedure at various temperatures ranging from 298K to 373K (25°C to 100°C). At a given temperature, viscosity measurements of these standards were performed at three pressures of 3.4, 6.9, and 10.3 MPa (500, 1,000 and 1,500
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psia). Because linear correlation between measured viscosity and pressure was obtained, viscosity at atmospheric pressure was calculated by linear extrapolation, which was then compared with the given viscosity of the standard at a given test temperature. Normally, 30 to 40 mL of dead or live sample was required to execute a measurement. To ensure the measured viscosity quality, each fluid sample viscosity at a given temperature and pressure condition was measured at three flow rates and average viscosity value of the three measurements was reported. After each viscosity measurement, the absolute relative difference among the three measurements was calculated and it was less than 2% in all cases. In addition, the Re and the De were calculated to ensure that the viscosity measurement prerequisites outlined previous were satisfied. In this study, the calculated Re were all found to be less than 50 with the minimum value of less than 0.01. For De, the maximum value was close to 0.2. Electromagnetic Viscometers (EMV) The EMV is another commercial viscometer widely used in heavy oil viscosity measurements16,18,19. A high-temperature, high-pressure viscometer, the EMV is designed to measure the viscosity of gas, gas condensate, and petroleum fluid samples. The EMV used in this study was the VISCOlab PVT with a Cambridge Viscosity SPL440 sensor developed by Cambridge Applied Systems. This EMV is capable of continuous operation up to 138 MPa (20,000 psia) over a temperature range from 273K to 458 K (0°C to 185°C). With a variety of available pistons, this EMV was capable of measuring viscosity over the range of 0.02 to 10,000 mPa·s. In this study, five pistons covering viscosity ranges of 0.5 to 10, 1 to 20, 10 to 200, 100 to 2000, and 500 to 10,000 mPa·s were used. The EMV sensor relies on electromagnets to drive the oscillation of a metal piston inside of a measurement chamber as shown in Figure 2. For a viscosity measurement, the test fluid was
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charged to the vessel and the piston was surrounded by fluid. Subsequently, the piston was moved inside of the measurement chamber by imparting a force on the piston with two electromagnetic coils inside of the sensor body. After traveling the length of the chamber, the piston returns to its starting location by reversing the magnetic field of the electromagnet. The motion of the piston inside the chamber was impeded by viscous flow around the annulus between the piston and the measurement chamber wall. Viscosity was determined by measuring the piston transit time for a complete cycle of piston movement and comparing the measured transit time with times obtained using calibration standards. Prior to usage, the EMV was calibrated with each piston at various temperatures using two Cannon viscosity standards following the procedures specified by the EMV manufacturer. Viscosity at ambient pressure was determined following the same procedure described in the previous CV calibration section. After calibration, the EMV was validated following the similar procedure used in validating the CV. The relative viscosity difference between the measured and reference viscosity was calculated to ensure it was less than 1.5% of the piston’s full scale as suggested by the EMV manufacture. The VISCOlab PVT requires approximately 12 mL of sample per measurement. The actual viscosity measurement was completed when the system was stable at the given test conditions indicated by the less than 1% variation in signal readings. Furthermore, if the viscosity measured under high-temperature and high-pressure conditions was less than 200 mPa·s, the raw data were corrected using the temperature and pressure correction correlation provided by the EMV manufacturer. Rh
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The Rh, commonly used in various rheology studies to measure the stress and strain relationships of fluids, has been applied to viscosity measurements of heavy oil, especially in extra heavy oil and bitumen viscosity measurements15, 20-22. In general, the Rh is used in dead oil viscosity measurements under ambient pressure and near ambient temperature conditions. The Rh utilized in this study was the ViscoAnalyser Var 100 manufactured by Reologica Instruments AB, which is shown in Figure 3. For this Rh type, a high-pressure cell was available for highpressure viscosity measurement, but a gas gap above the sample is required during the measurements. Any gas used in filling the gas cap could dissolve into the sample under pressure, thus, alter the sample composition and its corresponding viscosity. Therefore, this Rh was not used in live oil liquid viscosity measurements. The Rh has three operational modes at ambient pressure, open cup and bob (concentric cylinders), cone and plate, and parallel plates. The different modes are used to change the measuring stress and shear rate range and thus, the viscosity range. Usually cup and bob mode is used for liquids, cone and plate is used for highly viscous materials and liquids, and parallel plates mode is used for materials with extremely high viscosity. In this study, the cup and bob mode was selected in dead oil viscosity measurements based on the expected viscosity range, its ease of use and sample containment. This mode was also considered the most popular Rh set up due to its direct measurement of torque and requirement for a great range of constant shear rate23. The ViscoAnalyser Var 100 Rh had a testing temperature of 263K to 423 K (-10°C to 150°C). The applied torque could be controlled between 0.0005 and 50 mN·m, allowing for viscosity measurements as low as 2 to 3 mPa·s. The fundamental principle describing the Rh is as follows: τ = μ ∙ γ
(2)
where,
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µ
=
Fluid viscosity, mPa·s
τ
=
Shear stress, Pa
=
Shear rate, s-1
Equation 2 can be derived from the fluid moving along two parallel plates as shown in Figure 3. As shown in Figure 3, the upper plate moves and the lower plate is fixed. When a fluid is moving along the solid boundary, it will incur a shear stress ( = ⁄ ) on the boundary. For all Newtonian fluids in laminar flow, the shear stress is proportional to the shear rate ( = ! ⁄ " ) in the fluid and the viscosity is the constant of proportionality. However, for nonNewtonian fluids, this condition is no longer the case because the viscosity is not constant with shear rate changes. Past experience15 has shown that the heavy oil samples to be used in this study were expected to be Newtonian over the planned test conditions. The required sample volume for the open cup and bob arrangement is approximately 25 mL. The system motor calibrations were provided by the supplier. The temperature offset between the system temperature measured in the heating jacket of the cup and the measurement point located in the annular space between the cup and bob was checked and adjusted using viscosity standards at multiple temperatures over the temperature range of interest. Validation was also performed and the relative difference was controlled to be less than 5%. For all sample viscosity measurements with the Rh, a shear rate sweep measurement was conducted over a range of shear rates at a given test temperature, and the average of three repeated tests was used.
Experimental
Sample Information – Dead Oil Samples
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Three dead heavy oil samples with API values ranging from 20 to 11°API were used in this study and were categorized as follows: Low-viscous oil (LVO):
Dead oil viscosity @ 311K (38°C) = 50-200 mPa·s
Medium-viscous oil (MVO):
Dead oil viscosity @ 311K (38°C) = 500-1000 mPa·s
High-viscous oil (HVO):
Dead oil viscosity @ 311K (38°C) = 4,000-10,000 mPa·s
The dead oil samples as received were all “clean” oils (i.e. BS&W
