Flow Assurance in the Design and Operability of a CO2 Transportation System
Authors: Peters David , Lacy Rusty , Dykhno Leonid 
Published: April 2012

As part of potential work in a CO2 sequestration project, several flowassurance related issues were evaluated. Of particular importance were issuesthat impacted design decisions and operability for wells, pipeline, andfacilities. In this work, the major design impacts were compressor dischargepressure, dehydration requirements of the injected CO2, hydrate mitigation atthe wellhead, and placement of safety valves in the wells. In addition to thesemain design decisions, flow assurance also played a role in developing keyoperating strategies. These included:pipeline blowdown impacting materialintegrity,hydraulics analysis for required number of wells,thermalperformance of the system for both materials issues and hydrate management,fluid hammer impacts due to rapid valve closure. Finally, to complete thiseffort, various tools were employed which had to be assessed for theirapplicability and accuracy in both the steady state and transient systemperformance.

Introduction

As a part of a potential carbon sequestration project, CO2 is captured from acrude upgrading process and subsequently dehydrated, compressed, andtransported via pipeline (50 miles) before finally being injected into anunderground saline aquifer. The goal is to capture, transport, and sequesterseveral billion pounds of CO2 per year.

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

Modeling Background

In order to accurately design and operate the system, a sound understanding ofthe various thermodynamic and flow modeling software was required. Although thephase behavior of pure CO2 is well understood, it is relatively more difficultto model once impurities are introduced. These impurities can influence thephase behavior as well as the solubility of water in the CO2-rich fluid. Thephase behavior is important in the assessment of the potential operating regionwhere multiphase conditions may exist. The solubility of water into theCO2-rich fluid largely dictates the ice and hydrate stability region. Inaddition to the phase behavior aspects of the fluid, the physical properties ofthese different phases need to be accurately captured in a flow simulator toadequately model transient phenomena.

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