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Abstract
Production of oil/gas to meet the increasing energy needs results in generation of significant amount of CO2. This study presents results of experimental and simulation work to understand opportunities and challenges in sequestering CO2 in carbonate rocks typical of aquifers and oil/gas reservoirs in the Middle East. CO2 corefloods were conducted in samples from carbonate analog outcrops to simulate sequestration process, and, changes in the pore-space due to dissolution/precipitation were quantified by time lapse CTscans and measurement of relative permeabilities. Density and compressibility of CO2 and Solubility in reservoir brines was measured to calibrate predictive models and used in simulation studies. Objective is to reliably calculate sequestration capacity available in aquifers and depleted reservoir in a region, and to evaluate the long-term stability of seals in such systems. Experimental results facilitate reliable simulation studies which are very important for determining how much CO2 can be sequestrated. Such studies also lead to understanding of potential leaks due to interaction of CO2 with carbonate minerals of sealing formations.

Density and compressibility measurements of CO2 are reported at up to 5000 psia and 250 F. The results show good agreements with available predictive models and were used to calibrate parameters in such models. CT-scans and petrophysical properties of core samples show dissolution in some pores but precipitation in other pores of carbonate matrix.  Simulation study reports ranges for storage capacity in tones/acreft.

The results of this study are directly applicable for evaluating CO2 sequestration opportunities in Qatar, Middle-East since the outcrop samples studied are analog for Arab and Khuff Formations, which hold most of the petroleum reserves in the Middle-East. CO2 properties and its interaction with carbonate matrix are significant for CO2 sequestration study anywhere.

Introduction
Combustion of fossil fuels such as oil, natural gas and coal generates in excess of 27 billion tons of carbon dioxide (CO2) world-wide per year. As a result, the concentration of CO2 in the atmosphere has risen from pre-industrial levels of about 280 parts per million (ppm) to over 365 ppm and is a major contributor to global warming. One of identified approaches to reducing carbon (CO2) concentration in atmosphere is to capture CO2 from concentrated sources such as power plants and storing it in subsurface geological formations (Schrag, 2007). Of particular interest in Middle East region are Gas-to-liquid complexes that convert natural gas to sulfur free diesel but in the process generate enormous amount of CO2 . Also, a number of aluminum smelters are being located in the region that generate large amounts of CO2 during power generation and in smelting process. Such concentrated sources present a great opportunity to capture CO2 and store it in subsurface geological formations. Carbon dioxide (CO2) sequestration/storage in aquifers and depleted oil and gas reservoirs is widely accepted as a feasible solution to provide substantial reduction in concentration of greenhouse gases in the atmosphere.

Heavily fractured carbonate aquifers and reservoirs in the middle-east may be good candidates for the storage of large volumes of carbon dioxide. However, process design requires careful analysis of integrity of the formation due to mechanical fracturing or chemical interactions of sealing formations. Therefore, there exists a need to understand the factors affecting the CO2 sequestration potential and capacity of particular formation/reservoir in carbonate environment.

According to Kaldi & Bachu (2009), Any Carbon dioxide sequestration site must satisfy at least the following basic requirements:
1. Economic capacity to store targeted quantity of CO2.
2. Injectivity to accept CO2 at the rate at which it is being generated by the target source, and
3. Containment assurance that their will not be a leakage from the site in long-term (thousands of years).