Jefferson L. Creek

Search for memory effects in methane hydrate: Structure of water before hydrate formation and after hydrate decomposition

Neutron diffraction with HD isotope substitution has been used to study the formation and decomposition of the methane clathrate hydrate. Using this atomistic technique coupled with simultaneous gas consumption measurements, we have successfully tracked the formation of the sI methane hydrate from a water/gas mixture and then the subsequent decomposition of the hydrate from initiation to completion. These studies demonstrate that the application of neutron diffraction with simultaneous gas consumption measurements provides a powerful method for studying the clathrate hydrate crystal growth and decomposition. We have also used neutron diffraction to examine the water structure before the hydrate growth and after the hydrate decomposition. From the neutron-scattering curves and the empirical potential structure refinement analysis of the data, we find that there is no significant difference between the structure of water before the hydrate formation and the structure of water after the hydrate decomposition. Nor is there any significant change to the methane hydration shell. These results are discussed in the context of widely held views on the existence of memory effects after the hydrate decomposition.

Wax deposition in single phase flow

A series of experiments on wax deposition from oil have been performed in a 50-m long by 43.4 mm ID jacketed flow loop at Tulsa University. Tests were performed on a 35°API crude oil from the Gulf of Mexico with a wax appearance temperature (WAT) of 120°F. The series of tests were designed to determine temperature and flow rate effects on the deposition rate and fraction of oil in the deposit. The deposit thickness in the flow loop was determined using five methods; pressure difference, energy balance, the spool piece volume change (LDLD), ultrasonic transit time, and direct measurement in a test section. Samples of the deposits were analyzed at the conclusion of each test for included oil. The effect of the difference in temperature between the oil and pipe wall showed a simple increasing deposition rate with temperature difference. The change in deposition rate was a weak function of oil temperature relative to WAT. The variation in deposition rate with flow velocity gave large differences between laminar and turbulent flow. Deposit oil contents decreased with increasing flow velocity. The fraction of oil in the deposit decreased with time in turbulent flow tests but did not change in laminar tests.

Asphaltene Deposition on Metallic Surfaces

Abstract The potential for asphaltene deposition in wellbores and flowlines is a major concern during design of oil production and transportation facilities, especially in deep‐water environments. Understanding the processes that control asphaltene deposition, especially the relationship between precipitation and deposition, can help to reduce the risk and cost. Stainless steel capillary tubes were used to study the influences of factors including temperature, degree of asphaltene instability, and precipitant molar volume on asphaltene deposition from mixtures of stock‐tank oils and n‐alkanes. Temperature varied from 20°C to 60°C. Pressure drop across the capillary tube was used to estimate the amount and distribution of deposit formation. Existing asphaltic particles in stock‐tank oil samples did not create deposits. Below the wax appearance temperature, intermittent pressure spikes indicated deposition of wax. Above the wax appearance temperature, deposition occurred gradually from near‐onset mixtures created by co‐injection of oil and n‐alkane precipitants. Asphaltenes flocculated by addition of higher molar volume n‐alkanes deposited more material than those flocculated by lower molar volume n‐alkanes. Examination of the deposited materials showed that they contained not only asphaltenes but also waxes. Much different depositional characteristics were observed for solutions of asphaltenes in an aromatic solvent than for the stock‐tank oil from which they were derived.

Solubility of the Least-Soluble Asphaltenes

The key to understanding many asphaltene-related phenomena is a quantitative description of the solubility conditions at which the least-soluble asphaltenes begin to flocculate from a crude oil, often referred to as the onset of flocculation. Models that treat asphaltene flocculation as a liquid–liquid phase separation of large solute molecules dispersed in a solvent composed of much smaller molecules can successfully describe experimental observations in which solubility conditions vary due to changes in pressure and composition. Formation of small, well-dispersed asphaltene aggregates of colloidal dimensions (on the order of nanometers) does not invalidate the thermodynamic approach to modeling asphaltene phase behavior. The parameters needed to describe asphaltene phase behavior are solubility parameters and molar volumes of asphaltic and nonasphaltic portions of the oil. There are experimental barriers to accurate measurement of these important parameters, especially for the asphaltenes. We review several approaches to estimation of the solubility parameters of stock tank oil (STO) and mixtures with flocculating agents at the onset conditions, including the use of refractive index to estimate solubility parameters. We discuss the minimum data requirements for quantifying and predicting asphaltene instability from experiments with liquid alkane nonsolvents that define an asphaltene instability trend (ASIST) and we demonstrate application of STO ASIST data to prediction of asphaltene instability during depressurization of live oil. Finally, we apply the thermodynamic model to predict asphaltene instability in mixtures of petroleum fluids. Asphaltenes are defined, based on standardized tests, as the materials in petroleum products that are insoluble in n-heptane or n-pentane, but soluble in benzene or toluene (e.g., ASTM D2007). Asphaltene characterization techniques can be divided into two main groups: those based on determination of the amount of asphaltene using the standardized tests and those based on observations of

Predicting Downhole Fluid Analysis Logs to Investigate Reservoir Connectivity

This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an

Evaluation of Effects of Selected Wax Inhibitors on Paraffin Deposition

Abstract Deposition from decane solutions of model paraffins such as n-C24H50 (C24), and n-C36H74 (C36), as well as a mixture of n-alkanes (C21 to C44) was examined with and without chemical wax deposition inhibitors. The device used to produce the deposits investigated was a “Cold Disk” Wax Deposition Apparatus (CoDWaD) capable of producing field like deposits with relatively small volumes of oil in minutes. It was found that most of commercial wax inhibitors tested could decrease the deposition of low molecular weight paraffins (C34 and below), while having little effect on the wax deposition for high molecular weight paraffins (C35–C44). In many cases, although the total amount of wax formed on the cold plate was reduced, the absolute amount of deposition for high molecular wax was actually increased. Therefore, the net effect of many commercial inhibitors is to make even harder wax under the tests conditions studied here. One intriguing result was that the addition of an oleic imidazoline c rrosion inhibitor improved the performance of two wax inhibitors tested. It was also observed that there are subtle differences in inhibitor performance depending on whether the test solutions are binary mixtures, synthetic wax mixtures, or crude oil.

Verification of Asphaltene-Instability-Trend (ASIST) Predictions for Low-Molecular-Weight Alkanes

Summary Anticipating when and where asphaltenes may flocculate during oil production is a key step in successfully preventing or mitigating asphaltene problems in the field. Because there will be no deposition without precipitation, mapping of asphaltene stability over a wide range of temperature, pressure, and composition is required. The ASIST allows the determination of the onset of asphaltene instability to be established with a series of liquid n-alkanes. These data are used to predict asphaltene stability of live fluids by extrapolating the onset condition from the base data to reservoir conditions by use of a linear extrapolation of the onset solubility parameter vs. square root of the partial molar volume of the precipitant. This extrapolation has been demonstrated previously to be accurate for methane and a model oil. The present work verifies that such an extrapolation is valid for predicting the asphaltene instability for mixtures of methane, ethane, and propane with a representative stock-tank oil (STO). The STO was combined with known amounts of methane, ethane, or propane. The asphaltene onset pressure was determined by a combination of near-infrared (NIR) light scattering and microscopic observation. The onset conditions at ambient pressures were examined for flocculation periods ranging from 20 minutes to 24 hours. Onset pressures calculated with the 5-hour ASIST trends compared well with measured onset pressures.

Evaluation of Effects of Selected Wax Inhibitors on Wax Appearance and Disappearance Temperatures

Abstract In this investigation, a light transmittance method was used to evaluate the wax appearance temperatures (WAT) and wax disappearance temperatures (WDT) of model paraffin compounds (n-C24H50 (C24) and n-C36H74 (C36)) in n-decane (C10) solutions both with and without wax inhibitors. The change in WAT at different paraffin concentrations in the presence of an inhibitor behaves as though there is a constant amount of paraffin removed by the inhibitor. However, the amount of apparent paraffin reduction by an inhibitor (e.g. 160 g of C24 by one gram of an inhibitor) indicates that the inhibition mechanism cannot easily be explained by a simple “sequestering” effect. Wax inhibitors that decrease the WAT tend to also increase the WDT. Most of the wax inhibitors tested at a dosage of 100 ppm did suppress the WAT of lower molecular weight paraffin (C24) solutions, but had little or no effect for higher molecular weight paraffin (C36) solutions. Side-chain length of polymethacrylate wax inhibitors is an important performance parameter. Of the three polymethacrylate wax inhibitors tested, the one with the longest alkyl side-chain (C18) had the most effect on suppressing the WAT and increasing the WDT of the binary mixtures (n-C10–n-C24 solutions).

Investigating the performance of clathrate hydrate inhibitors using in situ Raman spectroscopy and differential scanning calorimetry

Abstract Raman spectroscopy and differential scanning calorimetry have been used to determine the kinetics and mode of action of a range of inhibitors on tetrahydrofuran (THF) hydrate formation. The results from these kinetic measurements were compared to those obtained from gas hydrate studies. The thermodynamic inhibitory nature of methanol was reflected in the findings as expected. However, some nucleation retardation was also observed at all concentrations studied, with higher concentrations of methanol resulting in ice formation. Addition of poly-N-vinylpyrrolidone (PVP) to THF/water mixtures caused nucleation times to increase and crystal growth to decelerate. No significant synergic effect was observed for THF hydrate inhibition using a combination of PVP and one of two glycol ethers, which were found to dramatically increase nucleation periods in natural gas hydrate formation. Only a slight synergic tendency was observed for the combination from the DSC results, while the Raman data gave no indication of a synergic effect for the combination. The results suggest that the nature of the guest species may be an important factor in the mechanism of hydrate inhibition.

A novel multiple cell photo-sensor instrument: principles and application to the study of THF hydrate formation

A new multiple cell photo-sensor instrument has been constructed to study the kinetic process of crystallization from solution. Tetrahydrofuran (THF) hydrate formation from water/THF solutions was chosen as an example application. The instrument simultaneously monitors the entire hydrate formation process in a group of 12 water/THF samples, by measuring four physical parameters: transmitted light intensity (Pi), the change in the polarizing state of emerging light (B), the half depth diameter of the vortex (Dv) and magnetic bar spinning rate (Vstr). The appearance and location of the new THF hydrate phase were identified by analysing the following parameters plotted as a function of time: transmission, vortex and change in the polarizing state of the emerging light. The macroscopic completion of hydrate crystal growth was determined as the time taken for the magnetic bar inside the sample tube to cease movement. These studies show that hydrate crystallization from 27:1 mole ratio solutions of water/THF commence at the vapour-liquid interface and near the internal wall of the glass sample tube. This was in contrast to stoichiometric mole ratio solutions of water/THF in which THF hydrate formation commenced in the bulk liquid phase.