Numerical Modeling of Pressure and Temperature Profiles Including Phase Transitions in Carbon Dioxide Wells
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 115946, "Numerical Modeling of Pressure and Temperature Profiles Including Phase Transitions in Carbon Dioxide Wells," by Lincoln Paterson, SPE, CO2CRC, CSIRO; Meng Lu, SPE, and Luke D. Connell, SPE, CSIRO; and Jonathan Ennis-King, SPE, CO2CRC, CSIRO, prepared for the 2008 SPE Annual Technical Conference and Exhibition, Denver, 21-24 September. The paper has not been peer reviewed.
Geological storage of carbon dioxide (CO2) usually is at conditions above the critical temperature and pressure; therefore, the CO2 will exist as a single dense phase. However, conditions in the upper part of a CO2 well with surface temperatures below the critical point of 31°C can lead to boiling and condensation in the well. The consequences of this are most apparent when the flow rate changes. Density profiles have been calculated for wells experiencing different thermal conditions to determine how bottomhole pressures are related to wellhead pressures.

Introduction
CO2-injection wells are used in enhanced oil recovery (EOR). CO2-production wells, from underground natural accumulations, provide a source of CO2 for EOR and other industrial uses. Interest in CO2 wells has intensified for geological storage as a means of reducing atmospheric greenhouse gases.
Accurate determination of downhole pressures is important if pressure is used to monitor the performance of a geological-storage reservoir. Reliable knowledge of bottomhole pressure helps prevent injection above pressures that can damage the formation. While bottomhole pressure can be measured with gauges, it is possible that, over a long period of time, downhole gauges may fail. Hence, calculating downhole pressure from wellhead pressure is desired. CO2 has a critical pressure of 7.38 MPa and critical temperature of 31.0°C, so if the fluid is near the usual surface temperatures, conditions in the upper part of a well can cross the saturation line of CO2, with boiling and condensation in the well if fluid pressures are in the vicinity of the critical pressure. Also, near the saturation line of CO2, fluid properties display severely nonlinear behavior, challenging numerical simulation.
This phase-change issue usually is not of concern during injection of CO2 for EOR because EOR normally involves continuous columns of liquid to the surface with the reservoir pressures required for minimum miscibility.
Conditions for phase change are more likely in circumstances not involving EOR, including CO2 production from depleted CO2 reservoirs, CO2 injection into pressure-depleted gas reservoirs, and CO2 storage at depths less than 2000 m.
The mathematical treatment of well-bore flow has been described extensively. The full-length paper details the interpretation of flow by use of numerical methods. In the rock surrounding the well, phase change accompanying vertical migration of CO2 during hypothetical leakage has been studied. Leakage usually involves smaller flow rates and closer thermal coupling to the surrounding rock compared with wellbore flow.