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Abstract
As part of potential work in a CO2 sequestration project, several flow assurance related issues were evaluated. Of particular importance were issues that impacted design decisions and operability for wells, pipeline, and facilities. In this work, the major design impacts were compressor discharge pressure, dehydration requirements of the injected CO2, hydrate mitigation at the wellhead, and placement of safety valves in the wells. In addition to these main design decisions, flow assurance also played a role in developing key operating strategies. These included: 1) pipeline blowdown impacting material integrity, 2) hydraulics analysis for required number of wells, 3) thermal performance of the system for both materials issues and hydrate management, 4) fluid hammer impacts due to rapid valve closure. Finally, to complete this effort, various tools were employed which had to be assessed for their applicability and accuracy in both the steady state and transient system performance.

Introduction
As a part of a potential carbon sequestration project, CO2 is captured from a crude upgrading process and subsequently dehydrated, compressed, and transported via pipeline (50 miles) before finally being injected into an underground saline aquifer. The goal is to capture, transport, and sequester several billion pounds of CO2 per year.

This work looks at many of the flow assurance issues that impact system design and the safe operation of the project. By evaluating these issues early in the project development phase, relatively simple design changes can be made that significantly reduce the operating complexity of the project. In this work, the flow assurance risks consisted of hydrate and ice formation, material integrity issues due to low operating temperatures, and potential issues regarding the multiphase flow operation of the pipeline and injection wells.

Modeling Background
In order to accurately design and operate the system, a sound understanding of the various thermodynamic and flow modeling software was required. Although the phase behavior of pure CO2 is well understood, it is relatively more difficult to model once impurities are introduced. These impurities can influence the phase behavior as well as the solubility of water in the CO2-rich fluid. The phase behavior is important in the assessment of the potential operating region where multiphase conditions may exist. The solubility of water into the CO2-rich fluid largely dictates the ice and hydrate stability region. In addition to the phase behavior aspects of the fluid, the physical properties of these different phases need to be accurately captured in a flow simulator to adequately model transient phenomena.

Given these modeling complexities, it was critical to understand how well each of the software packages predicted these various items and the intended range of applicability of the software. In cases where it was not possible to model the system precisely, it was necessary to understand what the critical input and output parameters were from the model so that a reasonable proxy could be used instead.