Analysis of CO2 phase transition within well-bore during injection into low-rank but relatively shallow coal seams

• Published in: Geomechanics and geophysics for geo-energy and geo-resources. Vol. 8; no. 3
• Main Authors: Ma, Qianqian, Gou, Shuaiwu, Li, Hong, Cao, Zhilin, Shin, Hyundon
• Format: Journal Article
• Language: English
• Published: Cham Springer International Publishing 01.06.2022; Springer Nature B.V
• Subjects: Boring; Carbon dioxide; Carbon sequestration; Coal; Compressibility; Energy; Engineering; Environmental Science and Engineering; Field study; Foundations; Geoengineering; Geophysics/Geodesy; Geotechnical Engineering & Applied Earth Sciences; Heat transfer; High temperature; Hydraulics; Injection; Intake temperature; Mathematical models; Model accuracy; Numerical analysis; Original Article; Phase change; Phase transitions; Potential energy; Pressure; Simulation; Storage; Tanks; Testing; Thermodynamic equilibrium; Wellbore; CO; sequestration; Enhanced coalbed methane recovery; Phase transition
• ISSN: 2363-8419; 2363-8427
• DOI: 10.1007/s40948-022-00408-z

With regards to field testing under a CO 2 -ECBM project, phase transition of CO 2 inevitably occurs under particular conditions during the injection process, which is obviously entangled with pressure and temperature distributions inside the wellbore. Nevertheless, due to that phase transition within wellbore was hard to be controlled and monitored, conventionally its transient influence had been neglected when reservoir simulations were undertaken. Numerical analysis studies on this topic still rarely exist, especially on the initial stage of injection during the stage when thermal equilibrium among CO 2 , wellbore, and its surrounding rock hasn’t yet been reached. In terms of the Canadian project CSEMP (CO 2 storage and enhanced methane production), referring to its engineering background and field data, a “1D + Pseudo-1D” model was established and then the associated numerical simulation on its wellbore dynamics was undertaken using software OLGA. The specific features of CO 2 behavior within the wellbore were analyzed for transient pressure build-up during the initial stage of the operation. The accuracy of the modelling was first verified by the downhole data with regards to the 4-day CO 2 injection test. Secondly, the influences on the CO 2 phase state inside the wellbore from different cases of injection temperatures were analyzed. The results revealed that: in the short term, when the injection temperature is low, CO 2 may undergo a phase change process of “gas state–liquid state” at the bottom of the well; when the injection temperature is high, CO 2 at the well's bottom may go through a phase change process of “gas state–liquid state–gas state”. As well, the simulation predicted that supercritical CO 2 might have existed at the bottom of the wellbore during the 19-day CO 2 injection. This is due to the work done by both gravitational potential energy and the compressibility of gas, which should have been considered in studying the field testing of the CSEMP project. Article highlights Most existing studies concerned deep non-mineable coal seams, their CO 2 usually presents in the supercritical state under high temperature and pressure. However, the buried depth of target coal seams under this project is shallow. The down-hole temperature and pressure did not necessarily attain to their critical values. The state of the CO 2 phase at the bottom of the well-bore had not been probed. Based on the field monitoring data, this paper analyzes the phase change of CO 2 within the wellbore at its initial stage of injection into relatively shallow coal seams. In the field testing of most pilot projects, CO 2 from tanks was injected directly as the liquid. However, our project was rare due to its gaseous CO 2 injection. This paper analyzes transient behaviors of phase change as well as its reasonable heat transfer during the short-term injection process. The influences of various injection temperatures on CO 2 phase states inside wellbore were studied through numerical analysis, especially at the bottom-hole level.