To study the effects of gas storage, long-term autoclave experiments with rock cores originating from a natural gas reservoir and brines taken from a gas well were conducted at 40 bar and 80o C in a CO_{2} atmosphere. While the main mineral composition of the sandstones was not affected due to CO_{2} exposure over a period of 31 months, the dissolution of rock-forming minerals such as feldspar, quartz or pore-filling anhydrite and calcite was noted. Changes in the brine composition were also detected. A release of iron and total organic carbon (TOC) from the rock samples to the saline fluid was observed. Temporary changes in the acetic acid concentration might have contributed to the dissolution processes. Precipitation and dissolution of minerals in sandstones were mostly related to diagenetic fluid-rock reactions and CO_{2} exposure, but in general the impact of CO_{2} on rock composition was only minor.