Pore Structure Changes Occur During CO2 Injection into Carbonate Reservoirs

Abstract

observations and modeling studies have shown that during co 2 injection into underground carbonate reservoirs, the dissolution of co 2 into formation water forms acidic brine, leading to fluid-rock interactions that can significantly impact the hydraulic properties of the host formation. However, the impacts of these interactions on the pore structure and macroscopic flow properties of host rock are poorly characterized both for the near-wellbore region and deeper into the reservoir. Little attention has been given to the influence of pressure drop from the near-wellbore region to reservoir body on disturbing the ionic equilibrium in the co 2-saturated brine and consequent mineral precipitation. In this paper, we present the results of a novel experimental procedure designed to address these issues in carbonate reservoirs. We injected CO 2-saturated brine into a composite core made of two matching grainstone carbonate core plugs with a tight disk placed between them to create a pressure profile of around 250 psi resembling that prevailing in reservoirs during CO 2 injection. We investigated the impacts of fluid-rock interactions at pore and continuum scale using medical X-ray CT, nuclear magnetic resonance, and scanning electron microscopy. We found that strong calcite dissolution occurs near to the injection point, which leads to an increase in primary intergranular porosity and permeability of the near injection region, and ultimately to wormhole formation. The strong heterogeneous dissolution of calcite grains leads to the formation of intra-granular micro-pores. At later stages of the dissolution, the internal regions of ooids become accessible to the carbonated brine, leading to the formation of moldic porosity. At distances far from the injection point, we observed minimal or no change in pore structure, pore roughness, pore populations, and rock hydraulic properties. The pressure drop of 250 psi slightly disturbed the chemical equilibrium of the system, which led to minor precipitation of sub-micron sized calcite crystals but due to the large pore throats of the rock, these deposits had no measurable impact on rock permeability. The trial illustrates that the new procedure is valuable for investigating fluid-rock interactions by reproducing the geochemical consequences of relatively steep pore pressure gradients during co 2 injection. Capturing CO 2 from large industrial sources and storing it in geological formations, such as saline aquifers, and depleted oil and gas fields, has become widely accepted as a viable solution for reducing high CO 2 levels in the atmosphere 1-3. Among underground formations, carbonate reservoirs are attractive CO 2 sequestration options, as the majority of the world's oil reserves (60%) are held in these types of rocks (especially in the Middle East), making them a primary storage target when combined with enhanced oil recovery (EOR) operations 4. Furthermore, since these reservoirs could hold the hydrocarbons for millions of years, it can be assumed that they can hold the CO ...

Effects of carbonated water injection on the pore system of a carbonate rock (coquina)

“…Most studies indicate that core flooding with carbonated water leads to higher permeabilities, sometimes by creating a wormhole at and near the core entrance. For example, Nowrouzi et al (2020) and Seyyedi et al (2020) observed higher porosity and permeability values after injection. However, Drexler et al (2019) noted a reduction in weight, porosity, pore volume and permeability of the sample after injecting carbonated water (desulphated seawater saturated with CO2), while Abbaszadeh et al (2016) reported a reduction in porosity and the sample's weight, but an increase in the permeability.…”

Dissolution of Carbonate Rocks in a Laboratory Setting: Rates and Textures

“…In light of continued climate change, CO 2 has been injected into various carbonate reservoirs to sequester the carbon (e.g., [67][68][69][70]). These reservoirs are subsequently modified due to the presence of the CO 2 (e.g., [71][72][73][74][75]). Studies have demonstrated that the injection of CO 2 into these deep-seated reservoirs results in reorganization of the porosity and permeability of these units (e.g., [71][72][73][74][75]).…”

“…These reservoirs are subsequently modified due to the presence of the CO 2 (e.g., [71][72][73][74][75]). Studies have demonstrated that the injection of CO 2 into these deep-seated reservoirs results in reorganization of the porosity and permeability of these units (e.g., [71][72][73][74][75]). The role of the petrology in this reorganization is poorly constrained, though based on a study by Seyyedi et al [75], it appears that ooids preferentially dissolve relative to their surrounding cements.…”

“…Studies have demonstrated that the injection of CO 2 into these deep-seated reservoirs results in reorganization of the porosity and permeability of these units (e.g., [71][72][73][74][75]). The role of the petrology in this reorganization is poorly constrained, though based on a study by Seyyedi et al [75], it appears that ooids preferentially dissolve relative to their surrounding cements. The work discussed in this paper may have implications for understanding how reservoirs that have been injected with CO 2 may respond and reorganize.…”

Porosity of the porous carbonate rocks in the Jingfengqiao–Baidiao area based on finite automata

“…Seyyedi et al [ 38 ] studied the pore structure changes that occur during the injection of CO 2 into carbonate reservoirs. They showed that the pores in carbonate rocks may indeed be affected by microbial chemical reactions.…”