Research Activity Summary and Data Base

Flowloop experiments have given us fundamental insights into hydrate flow mechanisms and have been the basis for development of numerous models and hypotheses. The testing program has examined the impact of several parameters such as the pressure, temperature, liquid loading, water cut, cooling rate, flow pattern, salinity and many more on the formation and transportability of hydrates. Data analysis indicates that hydrate flow mechanisms may be explained from the differential pressure signature for a given transporting system. Data generated in this test program has been used to develop flow models and criteria describing hydrate slurry behavior in a wide range of flowing conditions. Conditions that lead to transportable hydrate slurries have been investigated and correlated to fluid properties and flowing conditions. Flow patterns of hydrate transporting systems have been investigated and efforts are ongoing. Several experiments are being conducted in an attempt to characterize the flowing fluid once hydrates are formed, especially to determine whether the slurry behaves as a Newtonian or non-Newtonian fluid, and to establish and express the apparent viscosities as a function of solid fraction.

The main objectives in these studies are to correlate carrier fluid properties to hydrate flow behavior and to establish safe hydrate transporting practices/operations. Many hydrate flow loop tests were performed using several model oils, crude oils, gas compositions, and salinities, both with and without inhibitors. A wide range of operating conditions such as velocity, liquid loading, watercut and gas oil ratios have been tested. General trends can be derived. Hydrates may flow in form of pumpable slurry, slush or pellets, or stick to the periphery of the pipe inner wall and form a bed deposit or a plug. By examining over 500 tests, a criterion was established to classify hydrate transporting systems as a PASS or FAIL. Factors/conditions that lead a PASS or FAIL system were established and correlated to testing conditions and fluid properties. A risk factor chart was developed that maps hydrate transporting risk to flowing conditions and fluid properties. Work in this area is ongoing to identify additional factors affecting hydrate transport and improve the existing chart.

This study is aimed at understanding the behavior of hydrate transport systems and the development of hydrate plugs during restart operations. Tests are conducted to validate the hypothesis that hydrate plugging can be prevented during restart operations through optimized system conditions. Parameters such as restart rate, phase distribution, salinity, watercut and anti-agglomerants (AA) are studied. Tests are conducted in three modes: rocking mode, horizontal pumping and low spots plugging to simulate the different restart scenarios. The rocking mode experiments are aimed at studying phase distribution without any forced circulation. The horizontal pumping mode experiments are aimed at investigating the effect of the restart rate. The low spots plugging experiments are designed to simulate the development of plugs in low spots. Parametric study is ongoing to evaluate the effects of salinity, restart rate, oil composition, watercut, AAs, gas composition and water in oil dispersions on the tendency to form restrictions in the system during restart.

The main purpose of these tests was to generate data to tune the PVTSim DEPOWAX model for the TAPS crude oil. Several operational temperature and flow rate conditions were evaluated in the twenty five wax tests run in the Arctic flow loops. The testing periods at steady state conditions to allow wax deposition were as short as 1 day and as long as 30 days. Nevertheless, most of the tests were run between 7 to 14 days. The DSC and pigging results clearly showed the impact of different operational conditions on the total amount of wax depleted from the flowing stream, as well as the expected amount of wax that can deposit on the pipe wall as a function of the oil temperature.

The main purpose of these tests was to generate data to understand the ice fouling phenomena in pipelines. Several temperature, flow rate, water cut, cooling ramps and test length conditions were evaluated in the forty nine ice tests run in the Arctic flow loops. The loop temperatures tested were as low as 10oF. The testing periods usually were within 4 to 10 days, only a few tests lasted longer than 10 days without showing signs of ice fouling. During the ice testing period, the following variables were studied: high differential pressure across the perforated plates, fouling phenomena for several loop configurations (with or without plates and/or clappers, mesh screens, etc.), ice onset for a set water cut, safe operating temperature while increasing water content, ice fouling inhibition/remediation with methanol and/or polyvinyl alcohol injection and water settling at different flow rates.

A 2-inch inner diameter carbon steel flow loop was developed to perform similar tests as the large cold flowloops. It uses less volume and less manpower. Tests were designed to investigate the effects of pressure, temperature, and differential pressure, geometry, fluid properties, and water fractions on ice deposition and ice morphology. Experimental results showed that ice formation can restrict flow at the low spot in front of the flow meter, the inserted thermocouples, and the perforated plate. Annular ice deposition was found at the pipe wall. The morphology of the deposition was rime ice, indicating the deposition was due to small ice crystals sticking to the pipe surface. In addition, the formation of annular deposition requires a negative temperature gradient.

Since wax appearance temperature was higher than the freezing point of water, wax precipitation and deposition occurred before water froze. Thus, the deposition layer in the crude oil tests is composed of wax and ice. The formed wax deposition layer is porous, which enables water droplets to fill in the pores. After water in the deposition layer freezes, an ice-wax layer will form. In addition, Brownian motion, particle interaction, and sloughing-deposition process also occur in the crude oil tests.

The overall objective of this work is to develop a tool aimed at enabling safe dissociation of hydrate plugs using DEH. The occurrence of hydrate plugs in the field poses acute operational and safety problems coupled with significant losses of revenue. Potential remediation techniques such as direct electrical heating (DEH) have been successfully installed and safely used in several fields in the North Sea for hydrate mitigation. However, it has not been widely considered for hydrate plug remediation strategies due to potential risks associated with this thermal method. One (1) cubic foot of hydrate, upon dissociation, can release 164 cubic feet of gas at standard conditions and, hence, there is a possible risk of exceeding the burst pressure of the pipe during hydrate dissociation. During operation, the pipe test section is filled with 70 vol% water followed with gas. The hydrate plug is formed by flowing gas into the test section at a pressure and temperature that meet hydrate formation conditions based on the equilibrium curve for the Tulsa city gas. During this process, the plug permeability is monitored by measuring pressure differential across the test section and the final plug effective permeability may be controlled by injecting additional water, oil or gas. Density traces are measured while the hydrate plug is being formed as well as during dissociation. Pressure and temperature profiles are also recorded.

An autoclave has been modified into in-house mixer-viscometer device that can operate at pressures up to 3000 psia and a maximum volume of 1400 mL. The most important parameter for slurry characterization on this system is torque measurements. Torque trends can give insights regarding solid distribution within the slurry. During shaft speed ramp down, if solids agglomerate towards the mixing blade, torque increases. However, if the agglomerating solids move away from the blades, i.e., toward the wall, torque decreases. The effects of pressure, salinity, AA type and AA concentration were studied and quantified in this device.

This study involves the experimental and computational study of the mixing and displacement phenomena that take place during hydrate inhibition of jumper type configurations using mono-ethyleneglycol (MEG) and methanol. All experiments were conducted in a 3” jumper system. The results were presented with respect to the effect of inhibitor type, injection rate, brine salinity and liquid loading. Different dispersion and partitioning mechanisms were observed for methanol and MEG, especially in the vertical sections and low spots. Simulations using 1D transient multiphase flow simulator OLGA were conducted to evaluate its capacity to predict the thermodynamic inhibitor dispersion by using the inhibitor tracking module. Large discrepancy between OLGA simulation results and experimental data exits for low injection rate cases. CFD simulations using FLUENT ® help optimize the amount and flowrates of chemicals required as well as to optimize the location of the injection ports. The results are presented for the miscible displacement of THI in the jumper configurations. Comparisons were made between the simulation results and experimental data from full fresh water loading jumper displacement tests with MEG and methanol. Both 2D and 3D CFD simulations provide reasonable prediction for THI distribution along the jumper after displacement test, except that neither was able to reproduce methanol overriding water phase at both low spots. The results obtained by Star-CCM+ 3D generally gave better agreement with the results from the experiment.

This study aims to describe the interaction of hydrate particles with the wall and aggregation into hydrate lumps. The scope of this project includes conducting experiments to investigate interface development in inhibited and under – inhibited systems. The input parameters in the test matrix include sub cooling temperature, water cut, hydrate forming procedure, jacket temperature and pressure, salinity, inhibitor concentration and hydrate former. The variables that are measured are the induction time, aggregate size, pattern of formation, percent hydrate formed and how the adhesion forces affect the hydrate formation, agglomeration and plugging. A better understanding of these factors will help in optimizing the design of inhibitor injection facilities and to reduce the capital and operational expenditures. The main objective of these experiments is to find the effect of sub cooling and inhibitor concentration on agglomeration of particles and the relation between the adhesion force measurements with the visual observations. A low pressure facility to make hydrates was constructed and experiments were conducted. Visual observations were made. Different operating modes were tested and an optimum operating procedure has been defined from experiments. Rocking or inclined bubbling is the preferred method since it was found difficult to form hydrates with vertical bubbling only. Following observations were made from the runs:  

 Vertical bubbling has longer induction time than the other mixing methods i.e. vigorous mixing method (rocking) promotes hydrate formation

 Initially hydrates form at the interface

 Small slush eventually compacts into a larger mass of hydrates

 Hydrates deposition on the wall is observed