Key findings:

The study on estimating compressibility factor values for the Wafa gas field, with non-hydrocarbon components, reveals that gas compressibility increases with non-hydrocarbon proportions. Additionally, the compressibility factor (Z-factor) rises with pseudo-reduced temperature (Tr), and pseudo-critical temperature increases with higher nitrogen (N2) content in the gas mix.

What is known and what is new?

Gas compressibility factor is a critical parameter in natural gas engineering, influencing various calculations. Established sources for z-factor values include experimental data, equations of state, and empirical correlations. However, natural gases with components like H2S and CO2 exhibit distinct compressibility behaviors compared to "sweet" gases. Wichert and Aziz developed a method to adjust pseudocritical properties for such cases. This study focuses on estimating compressibility factors for the Wafa gas field, which contains non-hydrocarbon components. 

What is the implication, and what should change now?

The implications of this study suggest the importance of considering non-hydrocarbon components in natural gas engineering calculations. Engineers and researchers should incorporate adjustments for these components when estimating gas compressibility factors. This approach could lead to more accurate predictions and improved efficiency in natural gas operations, benefiting the industry as a whole.

The natural gas is a mixture of hydrocarbon and nonhydrocarbon gases. The hydrocarbon gases that are normally found in a natural gas are methane, ethane, propane, butanes, pentanes, and small amounts of hexanes and heavier. The nonhydrocarbon gases (i.e., impurities) include carbon dioxide, hydrogen sulfide, and nitrogen. Centration of up to 5 percent of these nonhydrocarbon components will not seriously affect accuracy.  but occur Errors in compressibility factor calculations as large as 10 percent in higher concentrations of nonhydrocarbon components in gas mixtures [1] Gas compressibility factor also called or "z factor" is essential in necessary in most natural gas engineering calculations, it is in calculating some important gas properties such as volumetric formation coefficient (gas formation volume factor), density (density), compressibility and viscosity, design production pipes. The most common sources for z-factor values are empirical measurements and equation of empirical relationships and correlations, this research focuses on estimate compressibility factor values which calculate for Wafa gas field contains non-hydrocarbon components. It is found gas compressibility values increases with the increase in the proportion of non-hydrocarbon compounds in the gas mixture. Also, it is observed that the pseudo-critical temperature increases with the increase in nitrogen (N₂).

The input data for the study is secondary data. The gas component composition for the Wells no (131, 144 and 148) and reservoir formation temperature (T) and pressure (P) are 26.28 ℃ and 3.4 MPa have been used. All the data used for this study is given in table 1.

Table 1: Gas components produced from Wafa field

Methodology of estimation Calculation of the Z-factor values

The most common method is to use one of the forms of the principle of corresponding states. In this form, gas compressibility factor is expressed as function of pseudo reduced pressure and temperature (Ppr, Tpr). Compressibility factors are function of composition as well as temperature and pressure. Standing and Katz (SK) presented a chart for determining gas compressibility factor based on the principle of corresponding states. The SK chart was prepared for binary mixture of low molecular weight sweet gases. Several mathematical expressions fitting the SK chart have been proposed to calculate the gas compressibility factor. Dranchuk- Abou- Kassem (DK) correlation is the most accurate representation of SK chart. When dealing with gas mixture, the gas mixture is critical pressure (Ppc) and temperature (Tpc) are required. Critical properties of natural gas are calculated from either gas composition or gas gravity. Several Mixing rules have been proposed to calculate mixture critical properties of natural gases. Among these methods, Kay’s mixing rule and Stewart- Burkhardt-Voo (SBV) are the most widely used. Kay’s mixing rule is simple and provides an accurate determination of gas compressibility factor for sweet gases of low molecular weight. Satter and Campbell evaluated several mixing rules for calculating properties of natural gases [2-4].

The data are analyzed Kay’s mixing rule for predicting pseudo-reduce pressure and temperatures are used for these data with knowing composition. Also, Braille and Bugs method was used to calculate the Z-factor values Moreover, according to present of non-hydrocarbon on the data I used the correction method which are Wichert- Aziz, to Adjust the calculated P_pc and T_pc, and calculate the Z-factor values. The data of three wells (131,144,148) with carbon dioxide and Nitrogen are shown in Table 1, the gas composition is known and Kay’s mixing rule has been used to calculate the pseudo-critical temperature & pressure. From the data analysis, a detailed pseudo-critical temperature & pressure results are shown in Table-2, 3 and 4. Braille and Bugs method was used to calculate the Z-factor values, the calculation and result for three Wells are appear in Tables 5. From calculation, it is found that gas pseudo-critical temperature increases with increase of N2 as shown in Figure 1. In addition, gas Z-factor increases with increase in the pseudo critical temperatures shown in Figure 2. Also, it is observed that Z-factor values increase with the increase in the proportion of non-hydrocarbon compounds in the gas mixture.

Table 2: Kay’s mixing rule and critical properties of natural gas from well 131

Table 3: Kay’s mixing rule and critical properties of natural gas from well 144

Table 4: Kay’s mixing rule and critical properties of natural gas from well 148

Table 5:The values of the critical properties and the Z-factor 

Figure 1: The relationship between the pseudo-critical temperature and the Z-factor 

Figure 2: The relationship between the pseudo-critical temperature and the Z-factor

Natural gases, which contain H₂S and CO₂ frequently, exhibit different compressibility factor behavior than do sweet gases. Wichert and Aziz and others developed a simple procedure to account for these differences and adjust the pseudocritical properties of natural gases. During this study, this research focuses on estimate compressibility factor values which calculate for Wafa gas field contains non-hydrocarbon components. It is found gas compressibility values increases with the increase in the proportion of non-hydrocarbon compounds in the gas mixture, In addition, the value of Z-factor increases with the increase in the pseudo-reduced temperature (T_r).Also, it is observed that the pseudo-critical temperature increases with the increase in nitrogen (N₂)

Funding: No funding sources 

Conflict of interest: None declared

Ethical approval: The study was approved by the Institutional Ethics Committee of Zawia University

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