Michael Volk

Investigation of Paraffin Deposition During Multiphase Flow in Pipelines and Wellbores—Part 2: Modeling

Results are presented from two-phase flow wax deposition tests using a state-of-the-art, high-pressure, multiphase flow test facility. Wax deposition was found to be flow pattern specific and dependent on the flow velocities of the two-phase fluids. Wax deposition occurs only along the pipe wall in contact with a waxy crude oil. An increase in mixture velocity results in harder deposits, but with a lower deposit thickness. The wax buildup trend at low mixture velocities is similar to that observed in laminar single-phase flow tests. The wax buildup trend at high mixture velocities is similar to that observed in turbulent single-phase flow tests. Thinner and harder deposits at the bottom than at the top of the pipe were observed in horizontal and near-horizontal intermittent flow tests. For annular flow tests, thicker and harder deposits were observed at low superficial liquid velocity than at high superficial liquid velocity. In stratified flow tests, no wax deposition was observed along the upper portion of the pipe.

Improvements in Single-Phase Paraffin Deposition Modeling

Paraffin deposition under single-phase flow conditions was investigated to determine its dependence on shear stripping, deposit aging, flow regime, temperature gradient, and fluid properties. In this study, a new model for the prediction of single-phase wax deposition has been developed. Most of the models previously used assume that equilibrium exists at the deposit-fluid interface. A kinetic resistance of the fluid is considered in the new model. Therefore, the interfacial-wax concentration might be different from the equilibrium-wax concentration. The model also includes continuous diffusion of wax into the deposit. We believe that this enrichment of the deposit is responsible for the increasing hardness of the deposit with time—a process known as “aging.” The effect of shear stripping may also be incorporated in the prediction. The model predictions are compared with predictions from previous models, as well as with experimental data gathered at the Tulsa U. Paraffin Deposition Projects, with two different oils: a black oil and a condensate. Even though some tuning is required for each type of oil, the new model is based on physical phenomena, reducing the empiricism of previous models.

Development of a mixer-viscometer for studying rheological behavior of settling and non-settling slurries

Abstract Slurry transport has become a subject of interest in several industries, including oil and gas. The importance of slurry/solid transport in the oil and gas industry is evident in areas of cuttings transport, sand transport and, lately, hydrates. There is therefore a great need to develop instrumentation capable of characterizing fluids with high solid content. Presence of solids in fluids makes the rheological characterization of these systems difficult. This is because available rheometers are designed with a narrow gap and cannot prevent solids from settling. The main aim of this paper is to present a step-by-step procedure of converting torque and shaft speed into viscosity information by applying the Couette analogy, equivalent diameter and inverse line concepts. The use of traditional impeller geometries such as cone and plate may be challenging due to their narrow gap and inability to prevent settling. Therefore, the use of non-conventional impeller geometry is imperative when dealing with settling slurries and suspensions. The most challenging task using complex geometry impeller is data interpretation especially when dealing with complex rheology fluids. In this work, an autoclave is transformed into a mixer-type viscometer by modifying its mixing, cooling and data acquisition systems. Mathematical models relating the measured torque to shear stress and the measured shaft speed to shear rate were developed and expressed in terms of the equivalent diameter. The shear rate and shear stress constants were expressed in terms of equivalent diameter and measureable parameters such as impeller speed and torque. The mixer-type viscometer was calibrated using four Newtonian and four Power-Law fluids to determine the rheological constants (equivalent diameter, shear rate and shear stress constants). The concept of inverse line was used to identify the laminar flow regime. The calibrated instrument was used to characterize two Power-Law fluids. This procedure can be extended to any rheological model. Methods developed in this work can be used to characterize fluids with high solid content. This is particularly important when dealing with complex rheology slurries such as those encountered in food processing, oil and gas and pharmaceuticals.

TULSA UNIVERSITY PARAFFIN DEPOSITION PROJECTS

As oil and gas production moves to deeper and colder water, subsea multiphase production systems become critical for economic feasibility. It will also become increasingly imperative to adequately identify the conditions for paraffin precipitation and predict paraffin deposition rates to optimize the design and operation of these multi-phase production systems. Although several oil companies have paraffin deposition predictive capabilities for single-phase oil flow, these predictive capabilities are not suitable for the multiphase flow conditions encountered in most flowlines and wellbores. For deepwater applications in the Gulf of Mexico, it is likely that multiphase production streams consisting of crude oil, produced water and gas will be transported in a single multiphase pipeline to minimize capital cost and complexity at the mudline. Existing single-phase (crude oil) paraffin deposition predictive tools are clearly inadequate to accurately design these pipelines, because they do not account for the second and third phases, namely, produced water and gas. The objective of this program is to utilize the current test facilities at The University of Tulsa, as well as member company expertise, to accomplish the following: enhance our understanding of paraffin deposition in single and two-phase (gas-oil) flows; conduct focused experiments to better understand various aspects of deposition physics; and, utilize knowledge gained from experimental modeling studies to enhance the computer programs developed in the previous JIP for predicting paraffin deposition in single and two-phase flow environments. These refined computer models will then be tested against field data from member company pipelines.

Fundamentals of Delayed Coking Joint Industry Project

The coking test facilities include three reactors (or cokers) and ten utilities. Experiments were conducted using the micro-coker, pilot-coker, and stirred-batch coker. Gas products were analyzed using an on-line gas chromatograph. Liquid properties were analyzed in-house using simulated distillation (HP 5880a), high temperature gas chromatography (6890a), detailed hydrocarbon analysis, and ASTM fractionation. Coke analyses as well as feedstock analyses and some additional liquid analyses (including elemental analyses) were done off-site.

Experimental investigation of hydrate formation, plugging and flow properties using a high-pressure viscometer with helical impeller

Slurry transport has become a subject of interest in several industries, including oil and gas. The importance of slurry/solid transport in the oil and gas industry is evident in areas of cuttings transport, sand transport and, lately, hydrates. Hydrate formation, if not properly monitored and controlled, may lead to pipeline blockage. To avoid pipeline blockage and other hydrate formation risks, chemical additives are added to the system. Additives such as anti-agglomerants help improve hydrate transportability by dispersing the formed hydrates into slurries and preventing them from sticking to the pipe wall. This enables transportation of highly concentrated slurries. However, the high hydrate volume fractions (HVF) slurries may exhibit complex rheology. There is therefore a great need to correlate flow properties such as friction factor and viscosity to HVF. Hydrate slurry transport is important whether hydrates are deliberately generated for energy storage purposes or hydrates formed because of the prevailing flow conditions. However, when determining the viscosity of a fluid containing solid particles, the conventional viscometer types such as concentric cylinders and cone and plate are often not suitable. This is because either the narrow gap would not accommodate the particle size or their inability to maintain the particles suspended leading to bed formation. In this work, a high-pressure mixer-type viscometer was used to generate and characterize hydrate slurries. This work aims to generate a significant amount of hydrate slurry characterization data that may be used as basis for better rheometer designs, hydrate slurry flow properties modeling or integration of hydrate transportability into general multiphase modeling. Results showed that intermediate watercuts posed the greatest pipeline plugging risk for all the oils tested. The amount of transportable hydrates increased with oil viscosity. Generally, hydrate slurries generated exhibited shear thinning behavior that increased with increasing hydrate volume fraction. However, the overall rheology of these slurries is a complex function of the oil used, watercut, gas added to the system and hydrate solid fraction. Lowering shear rates for high HVF systems resulted in separation. Results in this work further suggest that hydrate transportation may be possible with minimum risk if anti-agglomerants are used and high enough shear is applied. On the other hand, if no anti-agglomerant is used, severe aggregation may result in flow line plugging.

An Experimental Study on Wax Removal in Pipes With Oil Flow

Pigging is recognized as one of the most used techniques for removing wax deposits in pipelines. In an earlier paper, the mechanics of the wax removal was studied using an experimental setup under dry conditions, i.e., no oil presence. In this study, the pigging experiments are conducted for both regular disc and by-pass disc pigs under flowing conditions. A new test facility was designed and constructed. The test section is 6.1 m (20-ft) long Schedule 40 steel pipe with an inner diameter of 0.0762 m (3-in.). A mixture of a commercial wax and a mineral oil is cast inside the spool pieces for different wax thicknesses and wax oil contents. The wax breaking and plug transportation forces are investigated separately. The results indicated that the wax breaking force increases as wax thickness increases, and the wax plug transportation force gradient is independent of the wax plug length. In comparison to previous test results, presence of oil reduced the wax plug transportation force. Experimental results also showed that the wax transport behavior of the by-pass pig is significantly different than that of the regular pig. The by-pass pig allows the oil to flow through the by-pass holes and mobilizes the removed wax in front of the pig resulting in no discernible wax accumulation in front of the pig. Therefore, no measurable transportation force was observed for the by-pass pig tests.Copyright