This paper investigates the effect of higher concentrations (0- 100%) of CO2, H2S, and N2 on natural gas well deliverability, reserve estimation, and pressure test analysis quantitatively. Physical properties of natural gases such as viscosity and compressibility are corrected according to the concentrations of the contaminant gases such as CO2, N2, and H2S present in it. These contaminant gases have profound impact on pressure test analysis. The Carr et all1 viscosity correction chart allows adjusting the viscosity up to 15% concentration of these contaminant gases. However, Wichert and Aziz2 compressibility correction chart allows up to 80% concentration of the CO2 and H2S.
Tiab3 developed an analytical method to estimate pseudopressure function for 0-100% combined-concentration of CO2, H2S, and N2. His pseudopressure was first re-plotted to simplify the procedure and then it was used to analyze the deliverability, pressure tests, and decline curves quantitatively. The analysis was performed with Carr et all1 viscosity correction chart, pure CO2 properties, and then with Tiab’s corrected pseudopressure. Pure CO2 properties were used due to the fact that the sample data has 98.256% CO2.
During this study it was observed that the compressibility factor has a little effect on analysis since it is a volume- related property. Viscosity, however, has the largest effect on the analysis since pressure is transmitted through the fluid in the porous media and viscosity works against it. It was also observed that the numerical method of calculating pseudopressure function introduced successive error in the analysis. Number of pressure data points also contributed to the error in numerical integration of the pseudo- pressure function.
Analysis of field as well as simulated examples resulted an absolute error range of 13-75% in the permeability estimation in pressure tests, 77% in deliverability tests, and 20-95% with pressure derivative. Error in AOF was observed as 15% and as high as 32 % in reserve estimation.