### Abstract

The purpose of this study is to develop general analytical equations and type curves for estimating the CO2 storage capacity of natural gas reservoirs Numerical simulations for different types of natural gas reservoirs were done to study the CO2 storage capacity and to validate the developed analytical solutions A simulation case study is implemented to calculate the CO2 storage capacity in a target storage site and the simulated result of CO2 storage capacity is compared by that from the derived p/zmixCO2 plot This study successfully derives general analytical equations and type curves This general solution is capable of analytically calculating CO2 storage capacity of dry-gas wet-gas and gas-condensate reservoirs Furthermore this method is useful for site screening of CO2 storage in depleted natural gas reservoirs In the gas-production stage the z-factor of natural gas (z) decreased with the decreasing formation pressure However in the CO2-injection stage the z-factor of mixed gases (zmixCO2) increased when the formation pressure was recovering Generally the value of the zmixCO2 was smaller than that of the z-factor of natural gas under a specific formation pressure If the initial formation pressure (pi) is considered the value of the pi/zmixCO2 when CO2 injection finished will be higher than that of the pi/zi of the gas-condensate reservoir More CO2 can be stored in a gas-condensate reservoir than the amount of natural gas produced Numerical simulations for different types of gas reservoirs were used to study their CO2 storage capacity Additionally the comparisons of CO2 storage capacity estimates showed that the outcomes of analytical solutions and numerical simulation were similar The accuracy of the derived general equation was validated For the case study the target site was the Y gas-condensate reservoir located in the Y gas field in northwestern Taiwan The original gas in place (OGIP) of the Y gas-condensate reservoir was about 45 540 million standard cubic feet (MMSCF) which was estimated from the p/z plot based on the measured productions formation pressures and corresponding z-factors The Y gas-condensate reservoir is a nearly depleted reservoir with a very weak water drive Geological and numerical models of the Y gas-condensate reservoir were constructed in this study Before the simulated CO2 injection started the numerical model was well tuned using history matching The simulations of CO2 injection showed that the total CO2 injected was 48 870 MMSCF (2 58 million tons) when the formation pressure was recovered to the initial pressure of 4 850 psi The injection/production ratio (IPR) calculated by the derived equation was 1 44 based on the estimates of the ratio of initial p/z and injected p/zmixCO2 (PZR) dimensionless total equivalent gas ratio (DTE) and dimensionless produced equivalent gas ratio (PEG) of 1 275 1 088 and 1 028 respectively The value of IPR from analytical method was identical to that derived using the numerical methodDate of Award | 2016 Feb 16 |
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Original language | English |

Supervisor | Bieng-Zih Hsieh (Supervisor) |