Study Investigates Effects of Cyclic Salt Precipitation on CO2 Injectivity

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 219993, “Effects of Cyclic Salt Precipitation on CO2 Injectivity: From Pore Scale to Near Wellbore,” by Andrew Fogden, Till Bollermann, and Yennyfer Barreto, SPE, Wintershall Dea, et al. The paper has not been peer reviewed. _ The rate and severity of potential loss of CO2 injectivity in saline aquifers as the result of brine dry-out and salt accumulation near the wellbore is one of the main uncertainties in carbon capture and storage project planning. Default parameters implemented in commercial reservoir simulators overestimate the degree of salt precipitation and the resulting permeability impairment. The current study used laboratory and digital core analyses of Berea sandstone to estimate the petrophysical and dynamic properties for adjustment of predicted precipitation and flow reduction in reservoir simulation models of intermittent CO2 injection with aquifer drive. Introduction Salt precipitation has multiple effects on CO2 flow, involving a complex interplay of physical and chemical mechanisms, dependent on length scale, time, and reservoir and injection history. The salt occludes pore volume to ultimately reduce absolute permeability, while formation brine dehydration initially increases the effective permeability to CO2 because of the increase of CO2 saturation. Precipitation is potentially more severe for intermittent injection. Backflow of “fresh” formation brine to the already-vaporized zone through aquifer drive and additional capillary suction created by the microporous and hydrophilic salt aggregates can lead to further salt accumulation. The current study focuses on cyclic injectivity impairment caused by salt precipitation caused by formation water evaporation in contact with dry gaseous or supercritical CO2. The experimental procedure is inspired by work in the literature in which salt was precipitated in core plugs by oven drying and vapor evacuation, analyzed by mass-balance and gas-permeability measurements. In this study, Berea sandstone plugs are used for brine infiltration/drying cyclic experiments and measurements of dried-state porosity and permeability, together with nondestructive X-ray imaging for correction of the effects of salt accumulation at plug boundaries. Analogous small-scale experiments and high-resolution imaging were performed on a Berea miniplug for pore-scale simulation of the effect of precipitated salt on porosity/permeability and two-phase flow properties of brine and CO2. Furthermore, these results were input to finite-difference simulations at plug- and reservoir-sector scales. While the complete paper provides a detailed discussion of experimental results, including a series of figures and graphs, this synopsis concentrates on the experimental setup of the study and concludes with a summary of the authors’ findings. The synopsis also retains a discussion of the reservoir simulations used by the authors.