Godwin A. Chukwu

Determination of Rheological Behavior of Aluminum Oxide Nanofluid and Development of New Viscosity Correlations

Abstract Experimental investigations have been carried out to study the rheological behavior of aluminum oxide nanofluid. Nanoparticles with average particle size of 53 nm were dispersed in a base fluid of 60% (by mass) of ethylene glycol and water. Nanofluids of volumetric concentrations 1 to 10% were tested for determining the viscous properties. It was found that this nanofluid behaved as non-Newtonian at lower temperatures (-35°C to 0°C) and Newtonian at higher temperatures (0°C to 90°C). The data showed that the viscosity increases with an increase in concentration and decreases with increase in temperature. Two new correlations were developed expressing viscosity as a function of temperature and concentration.

Evaluation of rheological models and application to flow regime determination

Abstract Rheological models are frequently used to predict the shear stress-shear rate behavior of drilling fluids. However, none of these models can accurately predict the rheological behavior of all drilling fluids over all ranges of shear rates. In this study, the following rheological models were used to predict shear stress as a function of shear rate in several mud samples: Bingham Plastic, Power Law (2 point method and least-square method), Herschel-Bulkley and Robertson-Stiff. The first objective of this study was to identify the model that provided the best overall match of the experimentally observed rheological data. This was done by democratic ranking of the rheological models based on error analysis. The Robertson-Stiff model provided the best, and the Bingham Plastic model provided the worst overall prediction of rheological behavior. The second objective of the study was to develop a new method for determination of flow regimes in pipe flow using the Herschel-Bulkley and Robertson-Stiff models. The proposed method has been shown to predict the onset of turbulence in pipe flow more accurately than the conventional method for Bingham Plastic and Power Law models.

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.

Application of unsteady couette flow of non-Newtonian power-law fluids in concentric annular wellbore

Abstract Fluid flow phenomena that occur when the fluid is confined between two co-axial cylinders, one of which is stationary and the other is moving at a specified velocity, is known as couette flow. Such flow is representative of that which occurs in the borehole annulus where the wall of the wellbore is represented by the stationary cylinder, and the drill string or casing is represented by the moving cylinder. The fluid local velocity is dependent on the velocity of the moving cylinder or pipe. Different mathematical relations have been developed for surge and swab pressure calculations for flow of power-law fluids in the wellbore annulus. The application of couette flow equations and relationships to evaluate the surge or swab pressure has been limited. These equations are either too complex for field use or require much computation for field application. In this study, the motion equations are analytically solved for non-Newtonian power-law fluids. The solutions of the equations are presented in both dimensionless and graphical forms, which are applied to predict the surge or swab pressure encountered when running tubular goods in liquid-filled boreholes. The technique presented here is easy to use and requires minimal computational efforts for determining the surge or swab pressure for an unsteady motion of a moving inner pipe in a concentric annulus. The results of this study show that the magnitude of surge or swab pressure increases with increase in the pipe acceleration.

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.