Formation dry‐out from CO2 injection into saline aquifers: 1. Effects of solids precipitation and their mitigation

Abstract

Injection of CO2 into saline aquifers may cause formation dry‐out and precipitation of salt near the injection well, which may reduce formation porosity, permeability, and injectivity. This paper uses numerical simulation to explore the role of different processes and parameters in the salt precipitation process and to examine injection strategies that could mitigate the effects. The main physical mechanisms affecting the dry‐out and salt precipitation process include (1) displacement of brine away from the injection well by injected CO2, (2) dissolution (evaporation) of brine into the flowing CO2 stream, (3) upflow of CO2 due to gravity effects (buoyancy), (4) backflow of brine toward the injection point due to capillary pressure gradients that oppose the pressure gradient in the CO2‐rich (“gas”) phase, and (5) molecular diffusion of dissolved salt. The different mechanisms operate on a range of spatial scales. CO2 injection at constant rate into a homogeneous reservoir with uniform initial conditions is simulated in 1‐D radial geometry, to resolve multiscale processes by taking advantage of the similarity property, i.e., the evolution of system conditions as a function of radial distance R and time t depends only on the similarity variable R2/t. Simulations in 2‐D vertical cross sections are used to examine the role of gravity effects. We find that counterflow of CO2 and brine can greatly increase aqueous phase salinity and can promote substantial salt precipitation even in formations with low dissolved solids. Salt precipitation can accentuate effects of gravity override. We find that injecting a slug of fresh water prior to commencement of CO2 injection can reduce salt precipitation and permeability loss near the injection well.