Abhijit Y. Dandekar

Studies on hydrate film growth

The hydrate film properties and growth law at the interface between water phase and hydrate former (guest) phase are of significance to the overall study of hydrate formation kinetics and for developing methods to promote or inhibit the hydrate formation with respect to different industrial applications. In recent years, the experimental tools have become available to qualitatively observe the morphological nature of the film and quantitatively measure the growth of hydrate film. In this review, we provide an overview of the current state-of-the-art in the observation of hydrate film morphology, measurement and modeling of hydrate film growth rate. First, we review the morphological observations of hydrate formation at hydrate former/water interfaces occurring in different cases, such as bulk hydrate former phase contacting bulk water phase, small hydrate former droplets being exposed to bulk water phase, small water droplets being exposed to bulk hydrate former phase, and gas bubbles being suspended in bulk water phase. In the second section, the experimental determination of the lateral growth rate along the guest/water interface, the vertical growth rate normal to the guest/water interface and thickness of hydrate film are summarized. The mechanism and modeling of lateral growth and vertical growth of hydrate film are reviewed in the third section.

Measurement of the Wax Appearance Temperature of Gas-to-Liquids Products, Alaska North Slope Crude, and their Blends

As part of a major project on studying the operational challenges in gas-to-liquids (GTL) transportation through the Trans Alaska Pipeline System (TAPS), the wax appearance temperatures (WAT) of GTL products and the Alaska North Slope Crude (ANSC) and their blends were measured. The WAT measurements of GTL products were based on the American Society for Testing and Materials (ASTM) D3117 standard, whereas the WAT measurements of ANSC and its blends with the GTL products were measured by the viscometry technique. The reliability of the viscometry technique was ascertained by comparing the WATs of the colorless GTL products measured by the ASTM D 3117 method. The WATs measured by the viscometry technique and the ASTM D3117 method were found to be in excellent agreement.

Heat transfer analysis for gas-to-liquids transportation through trans Alaska Pipeline

Abstract Gas-to-liquids (GTL) technology involves the conversion of natural gas to liquid hydrocarbons. In this article, theoretical studies have been presented to determine the feasibility of transporting GTL products through the Trans-Alaska Pipeline System (TAPS). To successfully transport GTL through TAPS, heat loss along the route must be carefully determined. This study presents heat transfer and fluid dynamic calculations to evaluate this feasibility. Because of heat loss, the fluid temperature decreases in the direction of flow and this affects the fluid properties, which in turn influence convection coefficient and pumping power requirements. The temperature and heat loss distribution along the pipeline at different locations have been calculated. Fairly good agreement with measured oil temperatures is observed. The powers required to pump crude oil and GTL individually, against various losses have been calculated. Two GTL transportation modes have been considered; one as a pure stream of GTL and the second as a commingled mixture with crude oil. These results show that the pumping power and heat loss for GTL are less than that of the crude oil for the same volumetric flow rate. Therefore, GTL can be transported through TAPS using existing equipment at pump stations.

A New Modified Compositional Viscosity Model for Medium-heavy Oils

Viscosity of heavy oils is a key parameter in designing potential enhanced oil recovery methods. The recently proposed semiempirical viscosity model by Lindeloff et al. (2004) was studied in depth. This model is the simplest yet powerful tool for medium-heavy oil viscosities. Based on the analysis, appropriate adjustments have been suggested, with the major one being the division of the pressure-viscosity profile into three different regions. New modifications have improved the overall fit, including the saturated viscosities at low pressures. The %AAD for the data sets studied was found to be 1.37. However, with the limited geographically diverse data, it is not possible to make a comprehensive predictive model. A few limitations of the new model are also stated.

Comparative study of compositional viscosity prediction models for medium-heavy oils

This paper presents a comprehensive comparative study of composition-based Lohrenz-Bray-Clark (LBC) and principle of corresponding states (PCS) class viscosity prediction models for 11 different medium to heavy oils. The data sets span a wide range of pressures from 14.7 psia to 5000 psia; temperatures from 80 F to 180 F; and viscosities from 10 cP to 475 cP, typical of medium to heavy oils. The results show that all the LBC class models heavily underpredict the viscosities, whereas qualitatively PCS class models, in general, predict the viscosities in the range of the experimental values. However, none of the studied models were found to be truly predictive. [Received: June 2, 2007; Accepted: September 21, 2007]

Characterization of Gas-to-Liquid (GTL) and Alaska North Slope Crude (ANSC) Oil Blend Properties for Flow Through the Trans Alaska Pipeline System (TAPS) by Density and Viscosity Measurements

Abstract As part of a major project on studying the transportation of gas-to-liquids (GTL) through the existing Trans Alaska Pipeline System (TAPS), the density and viscosity of GTL and the Alaskan North Slope Crude (ANSC) and their blends were measured. All density and viscosity measurements were performed, using the Anton–Paar vibrating tube densitometer and a cone and plate viscometer respectively, at temperatures ranging from 20 oC to 60 °C at atmospheric pressure. The accuracy of the measured data has been ascertained. In addition to the density and viscosity data, other data reported for GTL and ANSC include compositions. Finally, generalized dens ty and viscosity correlations for the measured data are presented.

Evaluation of the modes of transporting gas-to-liquid (GTL) products through the trans Alaska pipeline system (TAPS)

ABSTRACT As part of a project on studying the transportation of gas-to-liquids (GTL) through the Trans Alaska Pipeline System (TAPS), two GTL transportation modes are evaluated: (i) as single slugs (batches) and (ii) commingled (mixed) with the Alaskan North Slope Crude (ANSC) oil. The pertinent energy equations are solved for both the batch and commingled flow modes. The solutions of these equations are analytically presented for determining among other parameters, the pressure gradient and the slug length required for batching. A comparison of the pressure gradient calculations is presented for the batching and the commingled flow cases.

A Probabilistic Economic Analysis of the Transportation of GTL Blends Through TAPS

Abstract The Alaska North Slope is a potential candidate for gas to liquid (GTL) technology. With over 38 trillion cubic feet (TCF) of natural gas reserves stranded on the Alaska North Slope, GTL technology is considered as a possible method of harnessing the abundant resources. The main objective of this study is to perform a complete economic evaluation using rate of return analysis and the net present value to identify the most favorable commingled mode for the transportation of the GTL products. Crystal Ball software was also used to run sensitivity analysis by using the probabilistic approach to give a clear view of the various scenarios on the project economics. Evaluating the options of transporting GTL products as a blend (commingled) with the Alaska North Slope crude oil through the existing Trans-Alaska Pipeline System is the main focus of this study.

Production Modeling of a Potential Methane Hydrate Accumulation on the North Slope of Alaska

Abstract The feasibility of a pilot production project on the North Slope of Alaska was computed to determine the production potential of a hydrate accumulation. The production of gas from a 1 mile by 4 mile reservoir block containing hydrate underlain by an accumulation of free gas was simulated and the resulting production profiles were analyzed. Results of the simulations indicate that depressurization of the free gas zone reduces the pressure at the gas-hydrate interface below that necessary for hydrate stability and causes the hydrate to dissociate into methane gas and water.

Economic Appraisal of Transporting Gas-to-Liquids Products through the Trans-Alaska Pipeline System (TAPS)

Abstract The Alaska North Slope oilfield is a potential candidate for the gas-to-liquid (GTL) technology. With over 38 Tcf of natural gas occurring on the slope, gas gathering for a GTL facility will be relatively easy. GTL fuels are environmentally friendly (sulfur free) with better ignition and burning properties than conventional petroleum products from crude oil. Evaluating the options of transporting GTL products through the existing Trans-Alaska Pipeline System (TAPS) together with crude oil, either as a blend of crude oil and GTL (commingled) or as alternate slugs of each product (batching), is the main focus of this study. Economic evaluation model was employed using Rate of Return analysis to identify the most favorable mode of transportation of the GTL products. Batching, using modern tracking and sensor techniques, was found to be a more economic mode, yielding the highest rate of return on the investment.