Per Fotland

Pressure and Composition Effect on Wax Precipitation: Experimental Data and Model Results

Wax precipitation is often studied using the stock tank oil. However, precipitation may be very different in well tubing and production facilities due to the effects of pressure and composition. As an example, the cloudpoint temperature may decrease as much as 15 K from atmospheric pressure to the saturation pressure of 100 bar mostly due to the dissolution of light gases into the oil (i.e. due to composition changes). It is also often assumed that the addition of solvents such as C 5 and C 6 decreases the cloudpoint temperature. On the contrary, from our modeling results, we have found that the mixing of a crude with a solvent increases the cloudpoint temperature (i.e., enhances the wax precipitation). In this study, the cloundpoint temperature at live oil conditions and the amount of the precipitated wax at stock tank oil conditions are provided for three crudes. A modified multisolid wax precipitation model is used to study the effects of pressure and composition on wax precipitation. The modeling results reveal that an increase in methane and CO 2 concentration decreases the cloudpoint temperature while an increase in C 5 concentration increases the cloud point temperature.

Detection of asphaltene precipitation and amounts precipitated by measurement of electrical conductivity

Abstract Asphaltene deposition during oil production might lead to expensive clean up operations. The production plans may also have to be reviewed. It is therefore necessary to investigate the potential for asphaltene deposition prior to field development. Reliable techniques for detection of asphaltene precipitation are needed in order to test existing theoretical models. The available experimental techniques are elaborate and not suited for high pressure work. A novel technique to determine asphaltene precipitation and relative amounts precipitated is presented. The technique is based on measurement of electrical conductivity, of the crude oil. It is seen that the electrical conductivity changes abruptly at the point of precipitation. Both gravimetric analysis and microscopy has been used to verify the results obtained by conductivity. The principle has been checked for a number of oils with varying asphaltene content from 0.3wt% to 9wt%. Results from four different oils are reported.

Effect of pressure and temperature on the phase behavior of microemulsions

This paper reports that optimal salinity of three different anionic microemulsions was found to increase as a function of increased hydrostatic pressure. This is equivalent to a phase transition from an upper Winsor II(+)(WII(+)) microemulsion to a lower Winsor II({minus})(WII({minus})) microemulsion. Increased pressure induces a compressibility effect that is consistent with the observed phase transition. Increasing temperature also leads to increasing optimal salinity. Prediction of temperature effects is complicated by temperature-dependent interactions and entopic contributions caused by dispersion. Fluid models that account for temperature effects are needed; therefore, no attempt was made to develop a theoretical interpretation of this effect. The temperature range is 0 to 100{degrees}C, and the pressure was varied from 0.1 to 50 MPa.

PRECIPITATION OF ASPHALTENES AT HIGH PRESSURES; EXPERIMENTAL TECHNIQUE AND RESULTS

ABSTRACT Precipitation of asphaltenes in crude oil has been measured both as a function of pressure and composition. The onset of precipitation has been determined by the measurement of conductivity. A specially designed high pressure conductivity cell has been constructed. Due to the low conductivity of most crude oils the electrode spacing has to be very narrow and the effective electrode area as large as practical. The cell has a coaxial design with inlet and outlet on opposite sides on the cylinder. The cell is rated to 700 bar and 120 deg.C but can be constructed for both higher pressure and temperature. The onset of precipitation is detected by a maximum in the weight normalized conductivity, i.e. the conductivity divided by the weight fraction of oil By using normal alkanes from ethane to pentane the precipitation onset is shown to increase linearly as a function of n-alkane carbon number. A smaller but significant increase in the onset is found by increasing the hydrostatic pressure. For monophasi...

ELECTRICAL CONDUCTIVITY OF ASPHALTENES IN ORGANIC SOLVENTS

ABSTRACT Conductivities of asphaltenes were measured as a function of concentration in solvents of varying permittivity. The molar conductivity was calculated assuming the molecular weight of asphaltenes to be 1000 g/mol The Fuoss conductivity model was fitted to the experimental data with two free parameters i.e. the association constant and the molar conductivity at infinite dilution. The experimental data show indication of ion-pairs and strong dipole-ion interaction. Accordingly, the measurements were consistent with the model for ion pairs as given by Fuoss and multibody interaction theory given by Petrucci At low concentration the asphaltenes consist mainly of free ions, as the concentration increases ion pairing leads to a decrease of the molar conductivity. At higher concentration ion-dipole interaction may lead to an increase of the molar conductivity, as is seen in some of the solvents. At even higher concentrations some indications of aggregation are observed The results indicate that part of t...

ISOLATION AND MOLECULAR IDENTIFICATION OF HYDRATE SURFACE ACTIVE COMPONENTS IN PETROLEUM ACID FRACTIONS

The anti-agglomerating hydrate behavior observed for some crude oils has previously been related to crude oil composition and to surface adsorption mechanisms. Petroleum acids derived from some crude oils have been found able to convert systems with initially high risk of plugging into easily flowable dispersions. In this work, acid fractions are isolated from three oils with low tendency to form hydrate plugs and from two oils associated with high risk of hydrate plugging by using an ion-exchange resin. The extracts are further separated into four sub-fractions by solid phase extraction (SPE). The chemical composition of the fractions is studied by means of HPLC, GPC, FTIRand UV/VIS spectroscopy and elemental analysis. The distribution of chemical compound classes in the fractions differs between the non-plugging and plugging oils, and the differences are most distinctive in one of the sub-fractions. The results imply that acid sub-fractions holding a significant proportion of more weakly polar compounds, like ester functionalities, are important for how the hydrate surfaces and the oil phase interact.

Conductivity of Asphaltenes

Asphaltenes have been studied extensively over the past 50–60 years.1–2 A number of experimental techniques have been employed. The simplest and most widely used technique is the measurement of the solubilities of isolated asphaltenes in various organic liquids, both aromatic solvents and flocculants like pentane.3–4 These studies yield valuable information with regard to the solubility parameters of asphaltenes. Advanced scattering techniques like neutron (SANS) and x-ray (SAXS) scattering have been used in order to elucidate the size and shape of asphaltene particles in solution.5–7 The focus of the different studies has varied depending on the area of application. Information with regard to the structure of asphaltenes is required in areas like crude oil production, paving asphalts, refinery processes, etc.8–12 This chapter will focus on the area of oil production.

CRITICAL DESCRIPTORS FOR HYDRATE PROPERTIES OF OILS: COMPOSITIONAL FEATURES

In petroleum production systems, hydrate morphology is observed to be influenced by the crude oil composition. This work is aimed at identifying which crude oil compositional parameters that need to be determined in order to evaluate natural anti-agglomerating properties of crude oils, i.e. the critical compositional descriptors. The compositional features of 22 crude oils have been studied, and multivariate data analysis has been used to investigate the possibility for correlations between several crude oil properties. The results show that biodegradation together with a relatively large amount of acids are characteristic for non-plugging crude oils, while excess of basic compounds is characteristic for plugging crude oils. The multivariate data analysis shows a division of the nonbiodegraded oils, which are all plugging, and the biodegraded oils. In addition, the biodegraded oils seem to be divided into two groups, one with plugging oils and one with mostly non-plugging oils. The results show that the wettability can be predicted from the variables biodegradation level, density, asphaltene content and TAN.