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Abstract
The material balance techniques have been used in the oil and gas industry for estimating hydrocarbon reserves for a long time. The objective of this paper is to introduce a fairly simple and fast material balance technique that can provide a fairly good estimate of CO2 storage capacity in depleted gas reservoirs.

Sequestration of CO2 in geological formations is a strategy currently being considered for decreasing CO2 emissions to the atmosphere. These geological formations can be either depleted oil and gas reservoirs or saline reservoirs. A depleted gas reservoir can store significantly more gas than a depleted oil reservoir due to the fact that gas is more compressible than oil and the ultimate recovery in gas reservoirs is higher than that in oil reservoirs. Many researchers have published reservoir simulation studies of CO2 sequestration in depleted gas fields, however, a reservoir simulation study, depending on its complexity, can take several months to perform. In this paper, a fairly simple and fast material balance technique, combined with nodal analysis, is presented that can provide a fairly good estimate of CO2 storage capacity in depleted gas reservoirs.

A depleted gas reservoir, for which the production and pressure history data were available, was selected as a candidate to perform this material balance study. First the material balance calculations were performed to estimate the size of the gas reservoir, aquifer and reservoir pressure. The formation parting pressure was estimated based on basic rock mechanics principles as a function of reservoir pressure. The bottom hole injection pressure was maintained below the formation parting pressure, until the surface facilities limitations were reached. As a result of this study, the amount of CO2 that can be stored in this depleted gas reservoir was estimated within a few weeks.

Introduction
Increased CO2 emissions into the atmosphere can trap solar thermal radiation on the earth’s atmosphere instead of letting it get reflected back toward the sun. Consequently, CO2 in the atmosphere can behave similar to a green house and contribute to global warming. Many governments around the world are concerned about the role that CO2 has on global warming and want to reduce CO2 emissions into the atmosphere.

One way to reduce future CO2 emissions into the atmosphere would be to capture CO2 from major emission sources and inject it into underground reservoirs. Injection of CO2 into oil reservoirs has been done for many years to improve oil recovery, a technique that also retains portion of the injected CO2 in the reservoir. Two SPE monographs, (Stalkup 1984) and (Jarrell et al. 2002) discuss the process of CO2 injection in oil reservoirs in great detail. Other types of reservoirs that CO2 may be injected into are coal beds, depleted gas reservoirs, and saline reservoirs. These latter three types of reservoirs have had only limited application of CO2 injection, mostly because the economics are not necessarily favorable. However, future governmental regulations could provide incentives for CO2 sequestration in these reservoirs in order to reduce CO2 emissions into the atmosphere.