A novel correlation approach for prediction of natural gas compressibility factor

Gas compressibility factor (z-Factor) is one of the most important parameters in upstream and downstream calculations of petroleum industries. The importance of z-Factor cannot be overemphasized in oil and gas engineering calculations. The experimental measurements, Equations of State (EoS) and empirical correlations are the most common sources of z-Factor calculations. There are more than twenty correlations available with two variables for calculating the z-Factor from fitting in an EoS or just through fitting techniques. However, these correlations are too complex, which require initial value and more complicated and longer computations or have magnitude error. The purpose of this study is to develop a new accurate correlation to rapidly estimate z-Factor. Result of this correlation is compared with large scale of database and experimental data also. Proposed correlation has 1.660 of Absolute Percent Relative Error (E_(ABS)) versus Standing and Katz chart and has also 3.221 of E_(ABS) versus experimental data. The output of this correlation can be directly assumed or be used as an initial value of other implicit correlations. This correlation is valid for gas coefficient of isothermal compressibility (c_g) calculations also.     

An efficient correlation for calculating compressibility factor of natural gases

Compressibility factor (z-factor) values of natural gases are necessary in most petroleum engineering calculations. Necessity arises when there are few available experimental data for the required composition, pressure and temperature conditions. One of the most common methods of calculating z-factor values is empirical correlation. Firstly, a new correlation based on the famous Standing-Katz (S-K) Chart is presented to predict z-factor values. The advantage of this correlation is that it is explicit inzand thus does not require an iterative solution as is required by other methods. Secondly, the comparison between new one and other correlations is carried out and the results indicate the superiority of the new correlation over the other correlations used to calculate z-factor.     

Prediction of gas condensate properties by Esmaeilzadeh–Roshanfekr equation of state

The Esmaeilzadeh–Roshanfekr (ER) equation of state (EOS) is used to predict the PVT properties of gas condensate reservoir fluids. Three gas condensate fluid samples taken from three wells in a real field in Iran, referred here as SA1, SA4 and SA8, as well as five samples from literature have been used to check the validity of the ER EOS in calculating the PVT properties of gas condensate mixtures. Some experiments such as constant composition expansion (CCE), constant volume depletion (CVD) and dew point pressures are carried out on these samples. In order to have an unbiased comparison between the ER and the Peng–Robinson (PR) equation of state, van der Waals mixing rules are used without using any adjustable parameters (k_ij = 0). Also, no pure component parameters are adjusted. The critical properties and acentric factor for plus-fraction are estimated by the Kesler–Lee, Pedersen et al. and Riazi–Daubert characterization methods. The results of dew point pressure calculations show that the ER EOS has smaller error than the PR EOS. For some mixtures, relative volume, gas compressibility factor and condensate drop-out in CVD and CCE test were also predicted. Comparison results between experimental and calculated data indicate that the ER EOS has smaller error than the PR EOS. The total average absolute deviation was found to be 0.82% and 2.97% for calculating gas compressibility factor and gas specific gravity in CVD test. Also, the total average absolute deviation was found to be 2.06% and 3.42% for calculating gas compressibility factor and relative volume in CCE test.     

An accurate empirical correlation for predicting natural gas compressibility factors

The compressibility factor of natural gas is an important parameter in many gas and petroleum engineering calculations. This study presents a new empirical model for quick calculation of natural gas compressibility factors. The model was derived from 5844 experimental data of compressibility factors for a range of pseudo reduced pressures from 0.01 to 15 and pseudo reduced temperatures from 1 to 3. The accuracy of the new empirical correlation has been compared with commonly used existing methods. The comparison indicates the superiority of the new empirical model over the other methods used to calculate compressibility factor of natural gas with average absolute relative deviation percent (AARD%) of 0.6535.     

Density and isothermal compressibility for two trialkylimidazolium-based ionic liquids at temperatures from (278 to 398) K and up to 120 MPa

In this paper, a densimeter based on vibrating tube principle is used to determine experimentally the density of 1-butyl-2,3-dimethylimidazolium tris(pentafluoroethyl)trifluorophosphate and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide at temperatures between (278.15 and 398.15) K and at pressures up to 120 MPa. The apparatus was calibrated by using water, vacuum and bromobenzene. The Tammann–Tait equation of state was used to correlate (p, T, ρ) results with standard deviations around 2 · 10⁻⁴ g · cm⁻³. Other volumetric properties, such as isothermal compressibility and isobaric thermal expansivity, were obtained from this equation. For each ionic liquid, the α_p isotherms present a crossing point within the experimental pressure range. Besides, the effect that the C2-methylation in the imidazolium cation provokes on density values is analyzed. The prediction ability of the group contribution methods of Gardas and Coutinho and Jacquemin et al. were tested with the experimental densities.     

Liquid density of 1-pentanol at pressures up to 140 MPa and from 293.15 to 403.15 K

This work reports new density data (180 points) of 1-pentanol at twelve temperatures between 293.15 and 403.15 K, and pressures up to 140 MPa (every 10 MPa). A new Anton Paar vibrating-tube densimeter, calibrated with an uncertainty of ±0.5 kg m⁻³ was used to perform these measurements. The experimental density data were fitted with the Tait-like equation with low standard deviations. In addition, the isobaric thermal expansivity and the isothermal compressibility have been derived from the Tait-like equation.     

Experimental determination and prediction of the compressibility factor of high CO_2 content natural gas with and without water vapor

In order to study the effect of different CO2 contents on gas compressibility factor(Z-factor),the JEFRI-PVT apparatus has been used to measure the Z-factor of dry natural gas with CO2 content range from 10.74 to 70.42 mol%at the temperature range from 301.2 to 407.3 K and pressure range from 7 to 44 MPa.The results show that Z-factor decreases with increasing CO2 content in natural gas at constant temperature and increases with increasing temperature for natural gas with the same CO2 content.In addition,the Z-factor of water-saturated natural gas with high CO2 content has been measured.A comparison of the Z-factor between natural gas with and without saturated water vapor indicates that the former shows a higher Z-factor than the latter.Furthermore,Peng-Robinson,Hall-Yarborough,and Soave-Benedict-Webb- Rubin equations of state(EoS)are used for the calculation of Z-factor of high CO2 content natural gas with and without water vapor.The optimal binary interaction parameters(BIP)for PR EoS are presented.The measured Z-factor is compared with the calculated Z-factor based on three models,which shows that PR EoS combined with van der Waals mixing rule for gas without water and Huron-Vidal mixing rule for water-saturated gas,are in good agreement with the experimental data.     

Liquid density of oxygenated additives to biofuels: 2-Butanol at pressures up to 140 MPa and temperatures from (293.15 to 393.27) K

This work reports new density data (159 points) of 2-butanol at seven temperatures between (293.15 and 393.27) K and 23 pressures from (0.1 to 140) MPa (every 5 or 10 MPa). An Anton Paar vibrating tube densimeter, calibrated with an uncertainty of ±0.7 · 10⁻³ g · cm⁻³, was used to perform these measurements. The experimental density data were fitted with the Tait-like equation with low standard deviations. In addition, the isobaric thermal expansivity and the isothermal compressibility have been derived from the Tait-like equation.     

Liquid density of biofuel mixtures: 1-Heptanol + heptane system at pressures up to 140 MPa and temperatures from 298.15 K to 393.15 K

This work reports new experimental density data (954 points) for binary mixtures of 1-heptanol + heptane over the composition range (seven compositions; 0 ⩽ 1-heptanol mole fraction x ⩽ 1), between 298.15 and 393.15 K, and for 23 pressures from 0.1 MPa up to 140 MPa. An Anton Paar vibrating tube densimeter, calibrated with an uncertainty of ±0.7 kg · m⁻³ was used to perform these measurements. The experimental density data were fitted with a Tait-like equation with low standard deviations. Excess volumes have been calculated from the experimental data. In addition, the isobaric thermal expansivity and the isothermal compressibility have been derived from the Tait-like equation, provided as supplementary material.     

Phase behavior of condensate gas and CO2 / CH4 re-injection performance on its retrograde condensation

Wenchang A Depression in Pearl River Mouth Basin is the largest hydrocarbon generating depression in the west of the area. After more than 30 years of exploitation, a large amount of gas condensate has been produced near the wellbore, which will cause gas condensate damage to the reservoir. It is planned to reinject the self-produced gas from Well WC9-2-X and the gas transported from the WC14-3 gas field to relieve the condensate damage in the near-wellbore area by means of retrograde condensation. In this article, the phase state change process of condensate gas in Well WC9-2-X with temperature and pressure was firstly investigated, and then the retrograde condensation effect of two types of gas on condensate was investigated. The research shows that when the reservoir temperature is 158.80 °C, the dew point pressure of condensate gas is 20.71 MPa, and the maximum amount of condensate is 1.28% (P = 9.01 MPa). Although Wenchang 9–2 is a low condensate reservoir, in the process of depressurization and production over the years, gas condensate has gradually accumulated, resulting in a large amount of gas condensate near the wellbore. With the increase of the gas re-injection amount, the two types of gas have a significant effect on the retrograde condensation of the gas condensate. From the variation trend of the gas and oil density released by the retrograde condensation, it can be seen that the re-injection gas preferentially dissolves the light components in the condensate, and then gradually dissolves the heavy components. The self-produced gas (gas No. 1) of Well WC9-2-X is dominated by CH₄ (78.33 mol%), and the CO₂ / CH₄ contents in the input waste gas (gas No. 2) of the WC14-3 gas field are 42.50 mol% / 41.60 mol%, respectively. The retrograde condensation effect of gas No. 2 is better than gas No. 1, mainly because the content of CO₂ in gas No. 2 is high, and it is easier to achieve the effect of miscible dissolution of condensate when mixed with condensate.It is recommended that gas No.2 should be preferentially used in WC9-2-X well for reinjection of retrograde condensation to relieve condensate damage. This article provides theoretical support for gas re-injection to relieve condensate damage in Wenchang 9–2 gas field, and has important significance for long-term exploitation of condensate gas reservoir.     

Liquid density of 1-butanol at pressures up to 140 MPa and from 293.15 K to 403.15 K

This work reports new density data (178 points) of 1-butanol at twelve temperatures between 293.15 and 403.15 K (every 10 K), and fifteen pressures from 0.1 up to 140 MPa (every 10 MPa). An Anton Paar vibrating tube densimeter, calibrated with an uncertainty of ±0.5 kg m⁻³ was used to perform these measurements. The experimental density data were fitted with the Tait-like equation with low standard deviations. In addition, the isobaric thermal expansivity and the isothermal compressibility have been derived from the Tait-like equation.     

Densities,excess molar volume,isothermal compressibility,and isobaric expansivity of (dimethyl carbonate + n-hexane) systems at temperatures (293.15 to 313.15) K and pressures from 0.1 MPa up to 40 MPa

The densities of dimethyl carbonate, n-hexane and their mixtures were measured for 12 compositions at five different temperatures varying from (293.15 to 313.15) K and over the pressure range of (0.1 to 40) MPa. The densities of pure substances and their mixtures at atmospheric pressure were measured by a vibrating-tube densimeter. The densities at high pressures were measured by a variable-volume autoclave and precise analytical balance. The excess molar volume, isothermal compressibility, and isobaric expansivity were derived from the experimental densities.     

Spatial average velocity of carrier gas is proportional to column pressure drop

Summary While the commonly known temporal average velocity of a carrier gas is approximately proportional to the pressure drop along a column, the spatial average velocity is exactly proportional to that pressure.     

Speed of sound in liquid cyclic alkanes at temperatures between (283 and 343) K and pressures up to 20 MPa

The speed of sound in liquid cyclopentane, cyclohexane, methylcyclopentane and methylcyclohexane were measured, with a sing-around technique operated at a frequency of 2 MHz, at temperatures from (283 to 343) K and pressures up to 20 MPa with an estimated error of less than ± 0.2 per cent in the high-density region. From these measurements the densities and isobaric and isentropic compressibilities of each compound were estimated. The behaviour on the temperature and pressure in these quantities is discussed based on the difference in molecular shape between cyclic and normal alkanes with those for n-alkanes.     

Thermophysical Properties of n-tridecane from 313.15 to 373.15 K and up to 100 MPa from Heat Capacity and Density Data

Isobaric heat capacities of liquid n-tridecane were measured at temperatures from 313.15 to 373.15 K and at pressures up to 100 MPa using a calorimetric device based on a Calvet calorimeter (Setaram C80). These experimental data combined with the additional knowledge of density data were used to calculate the following properties at pressures up to 100 MPa: isochoric heat capacity, isentropic compressibility and ultrasound velocity. This revised version was published online in August 2006 with corrections to the Cover Date.     

Study of the responses of a gas chromatography—reduction gas detector system to gaseous hydrocarbons under different conditions

The response of a reduction gas detector (RGD) to C₂-C₆ alkenes, C₂-C₆ alkanes, isoprene and benzene has been investigated using gas chromatography (GC) with a packed column. The RGD is considerably more sensitive to alkenes than is the flame ionization detector. The detection limit of this present GC/RGD system for alkenes is about 0.01 ng. It has much greater sensitivity to alkenes than to alkanes. Its sensitivity increases with increasing HgO bed temperature, but its selectivity towards alkenes decreases at the same time. The selectivity of the RGD may not be significant for much heavier molecules. The sensitivity of the RGD is inversely proportional to the carrier gas flow rate through the HgO bed. The baseline of the system increases significantly with increasing oven temperature.     

Enthalpy of solution of CO2 in aqueous solutions of methyldiethanolamine at T = 322.5 K and pressure up to 5 MPa

Enthalpies of solution of CO₂ in aqueous solution of methyldiethanolamine (MDEA) are determined for two different concentrations, respectively, 15 wt% and 30 wt%, at the temperature of 322.5 K and at pressures up to 5 MPa using a flow calorimetric technique. Gas solubilities were consequently deduced from the calorimetric data and compared to available literature values. Experimental enthalpies of solution were compared to direct measurements and to values derived from VLE data available using a simple model based on ideal property considerations.     

Investigation into the determination of trimethylarsine in natural gas and its partitioning into gas and condensate phases using (cryotrapping)/gas chromatography coupled to inductively coupled plasma mass spectrometry and liquid/solid sorption techniques

Speciation of trialkylated arsenic compunds in natural gas, pressurized and stable condensate samples from the same gas well was performed using (Cryotrapping) Gas Chromatography-Inductively Coupled Plasma Mass Spectrometry. The major species in all phases investigated was found to be trimethylarsine with a highest concentration of 17.8 ng/L (As) in the gas phase and 33.2 μg/L (As) in the stable condensate phase. The highest amount of trimethylarsine (121 μg/L (As)) was found in the pressurized condensate, along with trace amounts of non-identified higher alkylated arsines. Volatile arsenic species in natural gas and its related products cause concern with regards to environment, safety, occupational health and gas processing. Therefore, interest lies in a fast and simple field method for the determination of volatile arsenicals. Here, we use simple liquid and solid sorption techniques, namely absorption in silver nitrate solution and adsorption on silver nitrate impregnated silica gel tubes followed by total arsenic determination as a promising tool for field monitoring of volatile arsenicals in natural gas and gas condensates. Preliminary results obtained for the sorption-based methods show that around 70% of the arsenic is determined with these methods in comparison to volatile arsenic determination using GC-ICP-MS. Furthermore, an inter-laboratory- and inter-method comparison was performed using silver nitrate impregnated silica tubes on 14 different gas samples with concentrations varying from below 1 to 1000 μg As/m³ natural gas. The results obtained from the two laboratories differ in a range of 10 to 60%, but agree within the order of magnitude, which is satisfactory for our purposes.     

Measurement of the (pressure,density, temperature) relation of two (methane + nitrogen) gas mixtures at temperatures between 240 and 400 K and pressures up to 20 MPa using an accurate single-sinker densimeter

Comprehensive (p, ρ, T) measurements on two gas mixtures of (0.9CH₄ + 0.1N₂) and (0.8CH₄ + 0.2N₂) have been carried out at six temperatures between 240 and 400 K and at pressures up to 20 MPa. A total of 108 (p, ρ, T) data for the first mixture and 134 for the second one are given. These measurements were performed using a compact single-sinker densimeter based on Archimedes’ buoyancy principle. The overall uncertainty in density ρ is estimated to be (1.5 · 10⁻⁴ · ρ + 2 · 10⁻³ kg · m⁻³) (coverage factor k = 2), the uncertainty in temperature T is estimated to be 0.006 K (coverage factor k = 2), and the uncertainty in pressure p is estimated to be 1 · 10⁻⁴·p (coverage factor k = 2). The equipment has been previously checked with pure nitrogen over the whole temperature and pressure working ranges and experimental results (35 values) are given and a comparison with the reference equation of state for nitrogen is presented.     

Measurement and Calculation of Heat Capacity of Heavy Distillation Cuts Under Pressure up to 40 MPa

Isobaric heat capacities of natural mixtures were determined up to 40 MPa with a modified C-80 Setaram calorimeter equipped with cells designed for high pressures. The systems investigated were heavy distillation cuts with respective boiling points of 150, 200, 250 and 300°C. These experimental data were used as discriminatory values to test thermodynamic models and more precisely to choose among the great number of equations of state and mixing rules proposed in the literature, the most appropriate for the characterization of the heavy components.This revised version was published online in November 2005 with corrections to the Cover Date.