Joule-Thomson cooling and phase transitions during CO2 injection in depleted reservoirs

Depleted oil and gas reservoirs are attractive sites for CO₂ sequestration. However, the injection of CO₂ into depleted reservoirs carries the potential for significant Joule-Thomson cooling, when dense, supercritical CO₂ is injected into a low-pressure reservoir. The resulting low temperatures around the well-bore risk causing thermal fracturing and/or freezing of pore waters or precipitation of gas hydrates which would reduce injectivity and jeopardise near-well stability. Injection into reservoirs at subcritical pressure also leads to a phase transition from liquid to vapour CO₂. This is accompanied by cooling, due to the latent heat of vaporisation, and dramatic changes in fluid properties including density, compressibility and viscosity.

We present models of non-isothermal flow of CO₂ in the near well-bore region, which demonstrate the controls on cooling and constrain the different pressure-temperature regimes that can emerge. We show that during radial injection, with fixed injection rate, transient Joule-Thomson cooling can be described by similarity solutions at early times. The positions of the CO₂ and thermal fronts are described by self-similar scaling relations. The scaling analysis here identifies the parametric dependence of Joule-Thomson cooling. We present a sensitivity analysis which demonstrates that the primary controls on the degree of cooling are the injection rate and Joule-Thomson coefficient. The analysis presented provides a computationally efficient approach to assessing the degree of Joule-Thomson cooling expected during injection start-up, providing a complement to full numerical simulations.