CO2-induced geochemical reactions in heterogeneous sandstone and potential conditions causing the tight cementation

Abstract

The contribution of tight gas, stored in sandstone reservoirs, to hydrocarbon production has been increasing. However, the formation mechanisms of these tight sandstone reservoirs remain unclear and the conditions leading to them are not well-understood. It is generally thought that early CO2 emission may have caused the sandstones of the Permian Shihezi Formation in the Ordos Basin, China, to become tight. In this study, we quantitatively investigate the effects of CO2 intrusion on reservoir evolution, choosing three factors thought to dominate this process (i.e. detrital mineral assemblage, formation water, and intruded CO2 volume) for sensitivity analyses. We constructed eight two-dimensional models to investigate CO2-water-rock reactions and reservoir porosity changes under various conditions. Subsequent to determination of the potential conditions leading to the formation of tight reservoirs, we then constructed a three-dimensional model applying given heterogeneities in porosity (Φ), permeability (K), and mineral assemblages to evaluate porosity distribution. Results show that the processes underlying the formation of tight reservoirs are controlled by various factors rather than any single one. Reservoirs containing Ca-rich minerals (i.e. calcite and anorthite) and Ca2+-rich formation water are apt to become tight as a result of CO2 intrusion because of large-scale carbonate precipitation. In addition, under primary geological and geochemically heterogeneous conditions, CO2-water-rock reactions vary spatially, which further enhances reservoir heterogeneities. Although minerals are heterogeneously distributed, they exhibit some similar characteristics as their common ions link them. Results show that it is possible to predict both high-quality and tight reservoirs. In the study area, favorable reservoirs are found in regions with initially high Φ, high K, and high feldspar mineral contents, while regions that are characterized by low Φ and K as well as Ca2+-rich formation water and a high content of Ca-rich minerals tend to become tight after CO2 intrusion.