injection in depleted reservoirs*

Geological CO₂ storage in reservoirs where the ambient pressure is low poses significant challenges due to the decompression and concomitant Joule-Thomson cooling on injection of high-pressure, dense CO₂. The resulting low temperatures around the wellbore risk phase change, thermal fracturing and/or freezing of pore waters or precipitation of gas hydrates which would reduce injectivity and jeopardise near-well stability. 

 

Here we present models of non-isothermal flow of CO₂ in the near wellbore region, which describe the pressure and temperature within the reservoir. We show that during radial injection, with fixed injection rate, the process of transient Joule-Thomson cooling is self-similar. Pressure diffuses into the reservoir and temperature propagates as a sharp thermal front which lags the injected CO₂ front due to the heat capacity of the solid rock which heats up the fluid. The positions of the CO₂ and thermal fronts are described by self-similar scaling relations which we use to identify the parametric dependence of Joule-Thomson cooling. Our self-similar analysis presented a computationally efficient approach to assessing the degree of Joule-Thomson cooling expected during injection start-up, providing a complement and benchmark to full numerical simulations.  The approach also provides physical insight into the coupled processes of pressure and thermal diffusion alongside phase change within porous media.