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Abstract
Abu Dhabi has been importing natural gas for domestic consumption since 2007 and at the same time has been injecting natural gas into producing oilfields for reservoir management and sequestration. An opportunity exists to inject CO2 instead and release valuable supplies of natural gas for other uses while reducing greenhouse gas emissions. CO2 suitable for injection will be available as part of Masdar's Carbon Capture and Storage (CCS) project which will capture wet CO2 at atmospheric conditions from industrial plants and facilities, dehydrate it, compress it, and transport it by pipeline to producing oilfields for injection.

This paper presents options for dehydrating and compressing CO2 to achieve the optimum result while meeting all technical requirements. Technical and economic aspects of CO2 water content specification are analyzed and discussed along with current international practices. It focuses on challenges faced by the design team in developing water content specifications and selecting dehydration technology and methods for the Masdar CCS project.

Dense phase CO2 exhibits retrograde water condensation behaviour at pressures and temperatures used for pipeline transportation and injection. This means that CO2, unlike natural gas, can carry more water rather than less as pressure increases. Other products such as natural gas do not share this property with CO2 at pipeline operating conditions.

Pipelines in hot countries such as the United Arab Emirates (UAE) operate at higher temperatures than pipelines in North America and Europe and this enables them to carry product containing more water without it condensing. Therefore allowable water content specifications established by the pipeline industry in North America and Europe are unnecessarily restrictive for pipelines in hotter countries.

Retrograde water condensation in dense phase CO2 combined with higher pipeline operating temperatures in the UAE and other hot countries, permits higher allowable water content for pipelines carrying CO2 than is typical in other parts of the world. The specification of higher allowable water content can reduce both capital and operating costs of dehydration equipment leading to improved economics for CCS projects.

Introduction
In June 2007, Masdar announced the Abu Dhabi Carbon Capture and Storage (CCS) project. Front-end engineering design for the first phase of the project started in November 2008 and has now been completed. Figure 1 shows the proposed route, which will form part of the basic infrastructure that will allow a significant reduction of greenhouse gas emissions in the United Arab Emirates (UAE) from the year 2020 onwards.

CO2 captured from steel mills and industrial plants will be transported in a new national CO2 pipeline network to producing oil reservoirs throughout Abu Dhabi. Masdar is working closely with Abu Dhabi National Oil Company (ADNOC) and Abu Dhabi Company for Onshore Oil Operations (ADCO) who will inject the CO2 into the reserviors. The project will have a threefold benefit—it will reduce greenhouse gas emissions in the UAE, make CO2 available for enhanced oil recovery (EOR), and free up natural gas that is currently being injected to maintain pressure in some of the fields.

Several options exist for dehydrating wet CO2 captured at atmospheric pressure and for compressing it to pipeline conditions. To find the optimum solution it is necessary to consider all the technical and economic aspects of CO2 dehydration and compression. This requires special consideration of high ambient operating temperatures in the UAE as well as the ability of CO2 to hold water without precipitating it in either liquid (water) or solid (ice or hydrate) form at pipeline operating pressures and temperatures.