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

초록

With regards to field testing under a CO2-ECBM project, phase transition of CO2 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 CO2, wellbore, and its surrounding rock hasn't yet been reached. In terms of the Canadian project CSEMP (CO2 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 CO2 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 CO2 injection test. Secondly, the influences on the CO2 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, CO2 may undergo a phase change process of "gas state-liquid state" at the bottom of the well; when the injection temperature is high, CO2 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 CO2 might have existed at the bottom of the wellbore during the 19-day CO2 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.

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