Elsevier

Magnetic Resonance Imaging

Volume 21, Issue 9, November 2003, Pages 1061-1069
Magnetic Resonance Imaging

Magnetic resonance imaging
Monitoring hydrate formation and dissociation in sandstone and bulk with magnetic resonance imaging

https://doi.org/10.1016/j.mri.2003.05.001Get rights and content

Abstract

Magnetic resonance imaging (MRI) has been shown to be a very effective tool for monitoring the formation and dissociation of hydrates because of the large intensity contrast between the images of the liquid components and the solid hydrate. Tetrahydrofuran/water hydrate was used because the two liquid components are miscible and form hydrate at ambient pressure. These properties made this feasibility study proceed much faster than using methane/water, which requires high pressure to form the hydrate. The formation and dissociation was monitored first in a THF/water-saturated Berea sandstone plug and second in the bulk. In both cases it appeared that nucleation was needed to begin the formation process, i.e., the presence of surfaces in the sandstone and shaking of the bulk solution. Dissociation appeared to be dominated by the rate of thermal energy transfer. The dissociation temperature of hydrate formed in the sandstone plug was not significantly different from the dissociation temperature in bulk.

Introduction

Hydrates are a curious phenomena that can have a major impact on the drilling, production and transport of hydrocarbons[1], [2], [3]. While drilling a well, primarily in the Gulf of Mexico, other deep waters or in the permafrost at the upper latitudes of the northern hemisphere, there can be a nearly explosive increase in pressure when the drill bit reaches and melts a layer of hydrate [3], [4], [5]. Thus, predicting and controlling hydrate properties is of significant safety and economic importance while drilling a well. During drilling and production, if the gas/water mixture reaches the appropriate temperature and pressure, hydrate will form in the wellbore or production and transfer lines, respectively. This hydrate can block or restrict the production of hydrocarbon and in the worst cases do significant damage by dissociation at the wrong time or place, often with an explosive increase in pressure leading to loss of equipment and lives [6], [7]. Pipelines, supported by the seafloor, are often used for transporting oil from offshore wells in the Gulf of Mexico. In some locations the stability of the seafloor is provided by hydrates in the seafloor bed. If the transport of warm oil initiated dissociation, the seafloor stability could be lost resulting in damaged platforms or broken pipelines [8], [9]. To date, most hydrate studies have consisted of evaluating the effects on an engineering scale or characterizing the molecular structure of hydrates on an atomic scale. The present work was undertaken to determine whether it was possible to develop a method, using magnetic resonance imaging (MRI), to monitor hydrate formation and dissociation. The ultimate goal is to develop a laboratory tool that can monitor hydrate formation and dissociation while conducting experiments devised to eliminate hydrate problems. After the experimental work for this paper was completed, several papers were presented that also used MRI to characterize properties of gas hydrates and hydrate systems [10], [11], [12], [13].

Section snippets

Experimental procedure

1H MRI produces images of hydrogen containing liquids, but does not image hydrogen containing solids such as crystals or ice because of their much shorter relaxation times. The rock is not imaged because it is solid and contains little, if any, hydrogen. This makes MRI a potent tool to distinguish between solid hydrate and the liquid mixture from which the hydrate forms. The images shown in this work were collected with a Varian CSI 30 cm MR imager (Varian Inc, Palo Alto, CA, USA), which has

Results and discussion

Fig. 5 shows the nonimaged dissociation cycles for the 1:16 molar ratio of THF/water in bulk and in a sandstone plug. The steep portion, at the left-hand side of the plot, represents warming of the solid hydrate, the flat portion is the temperature during dissociation and the steep final temperature increase, on the right-hand side, is the warming of the liquid mixture. The dissociation of the bulk sample took about five times longer than the sandstone/hydrate because there was about five times

Conclusions

A porous sandstone rock, which is representative of reservoir rock, had no measurable effect on the dissociation temperature of THF/water hydrate.

MRI was shown to be an effective tool for detecting the formation of hydrate with excellent contrast between the hydrate phase and the individual hydrate components.

MRI can be used to determine the spatial distribution of the hydrate and nonhydrate phases and quantify the rate of hydrate formation and dissociation.

Acknowledgements

Thanks to Dr. W.R. Parrish and Dr. R.D. Roadifer for many helpful discussions and speeding up the process by pointing us in the right direction.

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  • Experimental study on hydrate saturation evaluation based on complex electrical conductivity of porous media

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    Citation Excerpt :

    Furthermore, the differences in the hydrate formation procedure such as the cooling mode or temperature history would affect the cage occupancy as well as the final conversion rate of the THF to hydrate (Strauch et al., 2018; Yin et al., 2019a; Liu et al., 2019). Conversely, a complete transformation from the THF solution to hydrate was reported with the aid of magnetic resonance imaging (MRI) visualization (Baldwin et al., 2003). To summarize, it is still an open question to accurately quantify the conversion rate of the THF to hydrate and thus the actual hydrate saturation for the initial THF-water-sediment system.

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1

Current address: President, Green Country Petrophysics LLC, Dewey, OK, consultant to ConocoPhillips.

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