Measured Z-Factor of CO2–Dry Gas/Wet Gas/Gas Condensates for CO2 Storage in Depleted Gas Reservoirs

Abstract

In support of the research project at the Center for Applied Petrophysical and Reservoir Studies in geologic storage of carbon dioxide in depleted gas reservoirs, the PVT laboratory at Texas Tech University performed compressibility factor (z-factor) measurements at various composition of CO2 with hydrocarbon (HC) gas mixture. For the sole purpose of the measured Z-factor data, three temperatures of 100°F, 160°F and 220°F and pressures ranges from 50 psia to 5000 psia are selected as representative of the depleted gas reservoirs (DGRs). In order to analyze the various phase behavior to be encountered in gas reservoirs (dry gas, wet gas and retrograde gas), the median gas compositions for dry, wet and retrograde gases are specified by gas type. The gas types are categorized by representative compositional analysis for the three types of gas reservoirs (dry gas, wet gas and retrograde gas). The measurements of z-factors for CO2-hydrocarbon mixtures in varying proportions and at the three specified temperatures for various pressures are performed on the median gas compositions of the type gases. The results of the z-factor measurements of CO2-hydrocarbon mixtures are used to interpret the expected phase behavior to be encountered in the geologic storage of CO2 in gas reservoirs. Also, the z-factor measurements of CO2-retrograde gas mixtures are used to quality the benefits of enhanced gas and condensate recovery in gas reservoirs. Introduction Since the earlier 1970, carbon dioxide has been used for enhanced oil recovery through miscible and immiscible displacement of oil at high pressures and moderate temperatures. Recently, research is being directed for the use of carbon dioxide in the oil refining through supercritical extraction of hydrocarbons.1 Current activities are to find ways for geologic storage of CO2 in oil or gas reservoirs.2 Although one laboratory measurement of CO2-hydrocarbon mixture in the limited ranges of temperatures and pressures used in this project has been reported in Venezuela (Rojas-Requena, 1992),3 this paper presents experimental measurements of z-factors for CO2-hydrocarbon mixtures at three specified temperatures and pressures ranging from 50 psia to 5000 psia. The results of the experimental z-factors are used to quantify economic benefits (such as enhanced oil recovery (EOR) and enhanced condensate vaporization) of geologic storage of CO2 in gas reservoir. Procedure for Z-factor Determination The compressibility factor, or Z-factor, is determined by manipulating the Real Gas Law and assuming that reservoir gas will behaves as an ideal gas at ambient pressure and temperature (McCain, 1990, page 106). The Real Gas Law is defined as follows:Equation 1 For a constant composition system, the product of pressure and volume is constant, thusEquation 2 Where the subscripts are:Condition in the cellAmbient condition

Suitability of depleted gas reservoirs for geological CO2 storage: A simulation study

“…In the first step, the composition of dry, wet, and condensate gas at the depleted stage was taken from the literature . Adisoemarta et al .…”

“…In the first step, the composition of dry, wet, and condensate gas at the depleted stage was taken from the literature . Adisoemarta et al . used these compositions to calculate the compressibility factor for storage purposes in depleted gas reservoirs.…”

“…used these compositions to calculate the compressibility factor for storage purposes in depleted gas reservoirs. In the data preparation stage, by Adisoemarta et al ., the hydrocarbon components were normalized and analyzed to find the median composition for each category of gas reservoirs. Table gives the normalized median composition for each category of gas reservoir used for the purpose of this study.…”

“…This might be due to the complexity of key storage aspects of these reservoirs such as injectivity, storage capacity, trapping mechanisms, and containment, which have not been fully understood . For instance, some gas reservoirs may cause loss of injectivity, overpressurization or less CO 2 immobilization as the mixture of CO 2 with the residual gas may change the properties of the CO 2 and gas mixture . In the meantime, CO 2 mixing with resident gas depends on the reservoir geometry, anisotropy, heterogeneity, injection rate, time of injection, rock or fluid properties, and diffusion, which can only be studied through numerical simulation.…”

Impact of Carbon Dioxide Sequestration in Gas/Condensate Reservoirs

“…Adisoemarta et al (2004) performed measured compressibility factors, z-factor, for various mixtures of CO 2 and natural gas. They concluded that zfactor for the mixture is much lower than natural gas at the specified temperatures and pressures.…”